volume 4 – hydrology and water resources

Volume 4 – Hydrology and Water Resources

Jabatan Pengairan dan Saliran Malaysia Jalan Sultan Salahuddin 50626 KUALA LUMPUR

GOVERNMENT OF MALAYSIA DEPARTMENT OF IRRIGATION

AND DRAINAGE

DID MANUAL Volume 4

March 2009 i

Disclaimer

Every effort and care has been taken in selecting methods and recommendations that are appropriate to Malaysian conditions. Notwithstanding these efforts, no warranty or guarantee, express, implied or statutory is made as to the accuracy, reliability, suitability or results of the methods or recommendations. The use of this Manual requires professional interpretation and judgment. Appropriate design procedures and assessment must be applied, to suit the particular circumstances under consideration. The government shall have no liability or responsibility to the user or any other person or entity with respect to any liability, loss or damage caused or alleged to be caused, directly or indirectly, by the adoption and use of the methods and recommendations of this Manual, including but not limited to, any interruption of service, loss of business or anticipatory profits, or consequential damages resulting from the use of this Manual.

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Foreword

The first edition of the Manual was published in 1960 and was actually based on the experiences and knowledge of DID engineers in planning, design, construction, operations and maintenance of large volume water management systems for irrigation, drainage, floods and river conservancy. The manual became invaluable references for both practising as well as officers newly posted to an unfamiliar engineering environment. Over these years the role and experience of the DID has expanded beyond an agriculture-based environment to cover urbanisation needs but the principle role of being the country’s leading expert in large volume water management remains. The challenges are also wider covering issues of environment and its sustainability. Recognising this, the Department decided that it is timely for the DID Manual be reviewed and updated. Continuing the spirit of our predecessors, this Manual is not only about the fundamentals of related engineering knowledge but also based on the concept of sharing experience and knowledge of practising engineers. This new version now includes the latest standards and practices, technologies, best engineering practices that are applicable and useful for the country. This Manual consists of eleven separate volumes covering Flood Management; River Management; Coastal Management; Hydrology and Water Resources; Irrigation and Agricultural Drainage; Geotechnical, Site Investigation and Engineering Survey; Engineering Modelling; Mechanical and Electrical Services; Dam Safety, Inspections and Monitoring; Contract Administration; and Construction Management. Within each Volume is a wide range of related topics including topics on future concerns that should put on record our care for the future generations. This DID Manual is developed through contributions from nearly 200 professionals from the Government as well as private sectors who are very experienced and experts in their respective fields. It has not been an easy exercise and the success in publishing this is the results of hard work and tenacity of all those involved. The Manual has been written to serve as a source of information and to provide guidance and reference pertaining to the latest information, knowledge and best practices for DID engineers and personnel. The Manual would enable new DID engineers and personnel to have a jump-start in carrying out their duties. This is one of the many initiatives undertaken by DID to improve its delivery system and to achieve the mission of the Department in providing an efficient and effective service. This Manual will also be useful reference for non-DID Engineers, other non-engineering professionals, Contractors, Consultants, the Academia, Developers and students involved and interested in water-related development and management. Just as it was before, this DID Manual is, in a way, a record of the history of engineering knowledge and development in the water and water resources engineering applications in Malaysia. There are just too many to name and congratulate individually, all those involved in preparing this Manual. Most of them are my fellow professionals and well-respected within the profession. I wish to record my sincere thanks and appreciation to all of them and I am confident that their contributions will be truly appreciated by the readers for many years to come.

Dato’ Ir. Hj. Ahmad Husaini bin Sulaiman, Director General, Department of Irrigation and Drainage Malaysia

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Acknowledgements

Steering Committee: Dato’ Ir. Hj. Ahmad Husaini bin Sulaiman, Dato’ Nordin bin Hamdan, Dato’ Ir. K. J. Abraham, Dato’ Ong Siew Heng, Dato’ Ir. Lim Chow Hock, Ir. Lee Loke Chong, Tuan Hj. Abu Bakar bin Mohd Yusof, Ir. Zainor Rahim bin Ibrahim, En. Leong Tak Meng, En. Ziauddin bin Abdul Latiff, Pn. Hjh. Wardiah bte Abd. Muttalib, En. Wahid Anuar bin Ahmad, Tn. Hj. Zulkefli bin Hassan, Ir. Dr. Hj. Mohd. Nor bin Hj. Mohd. Desa, En. Low Koon Seng, En. Wan Marhafidz Shah bin Wan Mohd. Omar, Sr. Md Fauzi bin Md Rejab, En. Khairuddin bin Mat Yunus, Cik Khairiah bt Ahmad Coordination Committee: Dato’ Nordin bin Hamdan, Dato’ Ir. Hj. Ahmad Fuad bin Embi, Dato’ Ong Siew Heng, Ir. Lee Loke Chong, Tuan Hj. Abu Bakar bin Mohd Yusof, Ir. Zainor Rahim bin Ibrahim, Ir. Cho Weng Keong, En. Leong Tak Meng, Dr. Mohamed Roseli Zainal Abidin, En. Zainal Akamar bin Harun, Pn. Norazia Ibrahim, Ir. Mohd. Zaki, En. Sazali Osman, Pn. Rosnelawati Hj. Ismail, En. Ng Kim Hoy, Ir. Lim See Tian, Sr. Mohd. Fauzi bin Rejab, Ir. Hj. Daud Mohd Lep, Tn. Hj. Muhamad Khosim Ikhsan, En. Roslan Ahmad, En. Tan Teow Soon, Tn. Hj. Ahmad Darus, En. Adnan Othman, Ir. Hapida Ghazali, En. Sukemi Hj. Sidek, Pn. Hjh. Fadzilah Abdul Samad, Pn. Hjh. Salmah Mohd. Som, Ir. Sahak Che Abdullah, Pn. Sofiah Mat, En. Mohd. Shafawi Alwi, En. Ooi Soon Lee, En. Muhammad Khairudin Khalil, , Tn. Hj. Azmi Md Jafri, Ir. Nor Hisham Ghazali, En. Gunasegaran M., En. Rajaselvam G., Cik Nur Hareza Redzuan, Ir. Chia Chong Wing, Pn Norlida Mohd. Dom, , Ir. Lee Bea Leang, Dr. Hj. Md. Nasir Md. Noh, Pn Paridah Anum Tahir, Pn. Nurazlina Mohd Zaid, PWM Associates Sdn. Bhd., Institut Penyelidikan Hidraulik Kebangsaan Malaysia (NAHRIM), RPM Engineers Sdn. Bhd., J.U.B.M. Sdn. Bhd. Working Group: Ir Mohd Zaki bin Mat Amin, Tn. Haji Azmi bin Mat Jafri, En.Sazali bin Osman, Pn. Yuhaslin Binti Yusof, Ir. Hapida binti Ghazali, En. Adnan bin Ab Latif, En. Asmadi bin Ahmad, Pn Noorhazilah binti Baharin, En Ng Kim Hoy, Pn Haliana binti Hamid, Tn. Hj Faahkaruddin bin Hj Tahir, Ir Liam We Lin, Ir. Mohd. Adnan Mohd Nor, Dr Heng Hock Hwee, En. Lee Yew Jin, En Jamal Abdullah, En. Ahmad Ashrin Abdul Jalil, Cik Nurulziana binti Jaidin.

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Registrations of Amendments

Amend

No

Page No

Date of Amendment Amend

No Page No

Date of Amendment

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Table of Contents Disclaimer....................................................................................................................................i

Foreword....................................................................................................................................ii

Acknowledgements.....................................................................................................................iii

Registration of Amendments........................................................................................................iv

Table of Contents........................................................................................................................v

List of Abbreviations....................................................................................................................vi

Glossary....................................................................................................................................vii

List of Volume

Chapter 1 Introduction

Chapter 2 Precipitation

Chapter 3 Water Losses

Chapter 4 River Discharge

Chapter 5 Statistical Hydrology

Chapter 6 Low Flows, Drought Analysis and Monitoring

Chapter 7 River Sedimentation

Chapter 8 River Water Quality

Chapter 9 Flood Forecasting and Warning Services

Chapter 10 Catchment Modelling

Chapter 11 Safety Considerations

Chapter 12 Emerging Technologies in Hydrological Observations and Instrumentation

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List of Abbreviations

DAD Depth; Area; Duration Curves ARF Areal Reduction Factor IDF Intensity Duration Frequency PMP Probable Maximum Precipitation PMF Probable Maximum Flood MOM Method of Moments MLM Method of L-Moments HP Hydrological Procedure NAHRIM National Hydraulic Research Institute of Malaysia FAO Food and Agriculture Organisation AAF Annual Average Flood FWB Flood Warning Board SOP Standard Operating Procedure FFC Flood Forecasting Centre SCADA Supervisory Control And Data Acquisition QPF Quantitative Precipitation Forecast ANN Artificial Neural Network MSE Mean Square Error MA Moving Average PAR Periodic Auto Regressive OSHA Occupational Safety Health Act DOSH Department of Occupational Safety Health OSH Occupational Safety Health FMA Factories and Machinery Act TCP Traffic Control Plan ADCP Acoustic Doppler Current Profilers RTU Remote Terminal unit GSM Global System for Mobile communication UHF Ultra High Frequency PSTN Public Switched Telephone Network GPRS General Packet Radio Service PFD Personal Flotation Device CPR Cardiopulmonary Resuscitation UH Unit Hydrograph DRF Direct Runoff Hydrograph MASMA Manual Saliran Mesra Alam DRH Direct Runoff Hydrograph DPT Dew Point Temperature UH Unit Hydrograph

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Glossary

Term A-Frame ACM Acoustic method ADCP Backwater Bank, right or left Bench mark

Definition The portable steel contraption which allows for a hand-operated winch to lower and raise a current meter for gauging work. Usually deployed from either a bridge or a boat. An acronym for acoustic current meter. It is a sound-based submersible instrument designed to measure velocity at a programmable trajectory and point in the water column A means to measure velocity (and flow) using sound signals passed underwater. System usually integrates a transmitter, receiver, temperature sensor and data logger. Flow is then derived using the velocity-area method. Also see ACM and ADCP An advanced generation of acoustic current meters which uses Doppler principles to simultaneously measure current velocities at multiple points along the water column. Latest model with bottom tracking feature can be incorporated into the moving boat method in lieu of the propeller type current meter and echosounder A rise in stage produced by an obstruction in the stream channel caused by ice, weeds, control structure, etc. It may be caused by channel storage for which the reservoir properties vary with the depth of flow at the given location. The difference between the observed stage for a certain discharge and the stage as indicated by the stage-discharge relation for the same discharge is reported as the backwater at the station. The margin of a channel as viewed facing downstream. The expression "right" or "left" applies similarly to right or left abutments, cableway towers, etc. A permanent, fixed reference point for which the elevation is known. It may when practicable, be related to mean sea level, MSL datum

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Term Broad-crested weir Cableway Control Cardiopulmonary Resuscitation (CPR) Crest gauge Critical flow Cross section of a stream Current meter Data logger

Definition A weir of such crest length in the direction of flow that critical flow occurs on the crest of the weir. Consist of a pair of suspension cables strung across the river and used in conjunction with gauging winch when deploying a current meter The condition downstream from a gauging station that determines the stage discharge relation. It may be a stretch of rapids, a weir or other artificial structure. In the absence of such features, the control may be a less obvious condition such as a convergence of the channel or even simply the resistance to flow through a downstream reach. A shifting control exists where the stage-discharge relation tends to change because of impermanent bed or banks. Is an emergency medical procedure for a victimof cardiac arrest or, in some circumstances, respiratory arrest. This is associated with the application of mouth-to-mouth ventilation, combined with chest compressions based on the assumption that active ventilation is necessary to keep circulating oxygenated blood in the lung. A gauge, usually vertical, used to indicate a peak stage that has occurred since the previous setting. The flow in which specific energy (depth of flow + velocity head) is a minimum for a given discharge; under this condition a small surface disturbance can not travel upstream. The ratio of inertia to gravity forces (Froude Number) is equal to unity. A specified vertical plane through a stream bounded by the wetted perimeter and the free surface. Instrument to measure discretely velocity of water flow in the water column. Can be of propeller, electromagnetic or acoustic type. Is an electronic device that records data over time and is usually integrated to either a built in instrument or sensor or connected to external instruments and sensors. A multi-parameter data logger has many channels to accommodate a repertoire of measurements e.g. temperature, humidity, rainfall, water level etc.

http://en.wikipedia.org/wiki/Emergency

http://en.wikipedia.org/wiki/Medical

http://en.wikipedia.org/wiki/Victim

http://en.wikipedia.org/wiki/Cardiac_arrest

http://en.wikipedia.org/wiki/Respiratory_arrest

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Term Digital counter Digital counter Discharge coefficient Discharge, Q Discharge measurement Double-drum winch Energy slope Float and stilling well gauge Flood mark Flow

Definition Is an electronic device that records data over time and is usually integrated to either a built in instrument or sensor or connected to external instruments and sensors. A multi-parameter data logger has many channels to accommodate a repertoire of measurements e.g. temperature, humidity, rainfall, water level etc. A counting device to record the number of revolutions in a propeller-type current meter during gauging A coefficient in the discharge equation, in general relating the actual discharge to a theoretical discharge. The volume of liquid flowing through a cross section per unit of time. It is not synonymous with "flow". The determination of the rate of discharge at a gauging station on a stream, including an observation of `no flow', which is classed as a discharge measurement A winch with two drums, one of which controls and measures the vertical displacement ofhydrometric instruments and the other of which controls and measures the horizontal displacement of an unmanned cableway carriage. Refers to the slope between the upstream and downstream cross sections of the high water marks plus the velocity head. Used as part of the slope-area method equation to calculate peak discharge. A manual gauge consisting of a float that rides on the water surface, rising and falling with the surface. The float's movements are transmitted to a paper chart recorder or encoded further onto a electronic data logger. A trace of any kind left by a flood on the banks, obstacles or flood plain. It may be used to determine the highest level attained by the water surface during the flood The movement of water in a channel without reference to rate, depth, etc

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Term Flume Fluorometer Free flow; modular flow Gas bubbler method Gauge correction Gauge datum Gauge height Gauge observation; Gauge reading Gauging section; measuring section Gauging station Gauging winch

Definition A specially shaped open channel flow section that may be installed in a channel to measure discharge. Depending on the shape of the section, flumes may be termed Parshall, Montana H-flumes, cut-throat, etc. An optically-based instrument used in detecting and measuring minute concentrations of fluorescent substances or molecules e.g. Fluoroscein, Rhodamine WT dye, chlorophyll a, hydrocarbons etc. A flow which is not influenced by the level of water downstream of the measuring device A system which measures water depth i.e. level by the amount of pressure required to force nitrogen gas bubbles into the water column from bubbler unit mounted at a fixed position from the streambed. Any correction that must be applied to the gauge observation or gauge reading to obtain the correct gauge height. The elevation of the zero of the gauge (referenced to bench marks, or MSL datum) to which the level of the water surface is related. The height of the water surface above the "Gauge datum"; it is used interchangeably with the terms "stage" and "water level". An actual notation of the height of the water surface as indicated by a gauge, it is the same as a "gauge height" only when the 0.000 metre mark of the gauge is set at the "gauge datum". The cross section of an open channel in the plane of which measurements of depth and velocity are made. Also referred to as river discharge station. Refers to specially designed winches mechanical or motorized used to suspend current meters in the water column for gauging purposes. See also cableway.

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Term Head on (or over) the weir Hydraulic jump Hydraulic mean depth; hydraulic radius Inclined gauge; ramp gauge Inspection Left [right] bank Level check Manual gauge Mean velocity at a cross section Mean velocity depth Moving boat method

Definition Elevation of the water above the lowest point of the crest, measured at a point upstream. The distance upstream for the point of measurement depends on the type of weir used but is upstream of the transition zone from sub- to supercritical flow at full weir flow The sudden passage of water in an open channel from super-critical depth to sub-critical depth, accompanied by energy dissipation. The quotient of the wetted cross sectional area and the wetted perimeter. A gauge on a slope, generally graduated directly to indicate vertical gauge height. Inspectorial works carried out twice yearly on river stage stations to document any physical changes taking place at the site itself, both upstream and downstream of the site as well as within the catchment concerned. Equipment are also checked and site personnel competency evaluated. The bank to the left [right) of an observer looking downstream. The procedure followed to determine the movement of a gauge with respect to the gauge datum. A non-recording type of gauge from which observations of stage are obtained. The velocity at a given cross section of a stream, obtained by dividing the discharge by the cross sectional area of the stream at that section. The depth below the surface at which the mean velocity on a vertical occurs. Refers to a gauging technique where a boat equipped with either a current meter or ADCP, depth finder and positioning equipment e.g. GPS transects across the river while collecting the necessary information relating to the flow calculation.

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Term Observations Panel Peak stage Personal Flotation Device (PFD) Pless system Point method (one-; two-; three-; five-; six-) Propeller-type current meter Open channel Rating curve Reach Reference point

Definition The practice to ensure that the entries are properly documented and identified with river name, station number, location, date and time. The area at a vertical defined by the depth at that vertical multiplied by one-half of the distance between the preceding and the succeeding verticals. The maximum instantaneous stage during a given period. Device such as the life jacket worn to keep one afloat when in water An improved technique devised by Russians to the moving boat method (Klein et al, 1993). Method of measuring the velocity in a vertical by placing a current-meter at a number of designated points in the vertical. A device with vanes that spin in proportion with the water flow velocity. The observed number of revolution by an accompanying counting device within a set time period is converted into velocity using a specific calibration formula which comes with instrument. The longitudinal boundary surface consisting of the bed and banks or sides within which the liquid flows with a free surface. The term "channel" generally means the deep part of a river or other waterway, and its meaning is normally made clear by a descriptive term, either stated or implied, such as "low water" channel, "main" channel, "artificial" channel. The relationship between stage and discharge which is developed by pairing stage data with individual point-in-time discharge measurements A length of open channel between two defined cross sections. A point of known elevation from which measurements may be made to a water surface. It is also known as a "measuring point"

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Term Remote sensing Rhodamine WT dye River Stage Station River Discharge Station SCADA Shift Slope-area method Sounding Staff gauge Stage; gauge height; water level Stage

Definition Is the science of deriving information about the earth's land and water areas from images acquired at a distance. It usually relies upon measurement of electromagnetic energy reflected or emitted from the features of interest. A water soluble dye, red in color used as a tracer in gauging studies. Detected and measured at parts per billion level with aid of a fluorometer. A location, temporary or otherwise where records of gauge height are obtained from a series of systematic readings (all reduced to mean sea level) from stick gauges or from automatic water level recorders. The complete installation at a measuring site where systematic records of water level and discharge are obtained. or Supervisory Control and Data Acquisition refers to the process by which real-time information is gathered from remote locations for processing and analysis; and the process by which equipment is controlled. A change in the station control which alters the stage-discharge relationship. This change can be either temporary or permanent. An indirect method of peak discharge estimation in a reach based on the surface slope, the reach roughness, the wetted perimeter and flow areas of the various cross sections within the reach. The operation of measuring the depth from the free surface to the bed. A manual gauge consisting of a graduated plate or rod that is normally set vertically in streambed or attached to a solid structure. The elevation of the free surface of a stream, lake or reservoir relative to a gauge-datum. A general term used to describe the height of a water surface and, in a particular application, may be either a gauge height or a water elevation.

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Term Stage-discharge relation Station control Steady [unsteady] flow Stick gauge Stilling well Stream Submersible pressure transducer Sub-surface float Surface float Telemetry Tracer techniques Triangular-profile weir Ultrasonic Sensor Method

Definition A curve, equation or table which expresses the relation between the stage and the discharge in an open channel at a given stream cross section. Refers to the physical features, natural or otherwise found downstream which exerts influence over the existing rating curve for the gauging station. Condition in which the discharge does not change [changes] in magnitude with respect to time. Also known as staff gauge. A well [tube] connected with the stream in such a way as to permit the measurement of the stage in relatively still conditions (natural surging dampened). The generic term for water flowing in an open channel, e.g., including creeks and rivers. a sensor which incorporates piezo-resistive strain gauge technology to detect and measure the subtle change in water depth hence level. A float with its greatest drag below the surface for measuring sub-surface velocities. A float with its greatest drag near the surface for measuring surface velocities. Refers to the use of telecommunications for automatically sending/receiving data at a distance from the measuring instrument. Use of detectable and quantitative solutes such as salt, dye, radio-isotopes etc. to determine discharge. Flow is computed using either velocity-area or dilution methods. A long-base weir having a triangular longitudinal profile. Consists of a transducer mounted at a fixed distance above the water surface which transmits and receives acoustics signals which invariably change with the stage thus giving the water level reading.

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Term Velocity floats Uniform flow Velocity-area method Vertical Vertical velocity coefficient Wading rod Water level recorder Water surface slope Weir Wetted perimeter, P

Definition Usually used with a stopwatch within a straight reach to roughly and quickly determine flow velocities. Can be in the form of surface or sub-surface float; usually the float material is made of PVC. Flow in which the depth and velocity remain constant with respect to distance along the channel. Uniform flow is possible only in a channel of constant cross section Method of discharge determination deduced from the area of the cross section, bounded by the wetted perimeter and the free surface, and the integration of the component velocities in the cross section. The vertical line in which velocity measurements or depth measurements are made. The coefficient applied to a single, or an equivalent single, velocity determination at any depth in a vertical to infer the mean velocity on that vertical. A light, hand-held, graduated, rigid rod, for sounding the depth and positioning the current meter in order to measure the velocity in shallow streams suitable for wading. It may also be used from boats at shallow depths. An instrument that records water levels in an analogue or digital form. The recorder may be actuated by a float or by any one of several other sensor types. Refers to the gradient at the water surface between two high water marks, one upstream the other downstream of a gauging site. It is used to derive the energy slope, which is ultimately the slope that the slope-area method relies on. An overflow structure built across an open channel to measure the discharge in the channel. Depending on the shape of the opening, weirs may be termed rectangular, trapezoidal, triangular, etc. The wetted boundary of an open channel at a specified section.

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CHAPTER 1 INTRODUCTION

Chapter 1 INTRODUCTION

March 2009 1-i

Table of Contents Table of Contents .................................................................................................................... 1-i

List of Tables .................................................................................................................. 1-iii

List of Figures .................................................................................................................. 1-iii

1 INTRODUCTION ......................................................................................................... 1-1

1.1 DEFINITION AND IMPORTANCE OF HYDROLOGY .......................................................... 1-1

1.2 USER NEEDS FOR HYDROLOGICAL DATA AND INFORMATION ....................................... 1-2

1.3 HYDROLOGICAL ACTIVITIES IN MALAYSIA ................................................................... 1-3

1.4 OUTLINE OF ACTIVITIES ............................................................................................. 1-3

1.5 FIELD COVERED BY DIVISION OF HYDROLOGY AND WATER RESOURCES ...................... 1-4

1.5.1 Operational Hydrology ................................................................................. 1-6

1.5.1.1 Field Operation ........................................................................... 1-6

1.5.2 Hydrological Data Management .................................................................... 1-7

1.5.2.1 Collection of data ....................................................................... 1-7

1.5.2.2 Submission and Processing of Hydrological Data .......................... 1-7

1.5.2.3 Dissemination of Hydrological Data ............................................. 1-8

1.5.4 Drought Identification and Monitoring Services .............................................. 1-8

1.5.5 Hydrological Application .............................................................................. 1-9

1.5.6 Technical Advice and Consultancy ................................................................ 1-9

1.5.7 Water Resources Assessment....................................................................... 1-9

1.5.8 Applied Researches, Local, Regional and International Collaboration ............. 1-10

1.5.8.1 Experimental and Representative Basin Studies ........................... 1-10

1.5.8.2 IHP Participation in Collaborative Researches .............................. 1-10

1.6 EVALUATION AND DESIGN OF HYDROLOGICAL NETWORK .......................................... 1-11

1.6.1 General Considerations .............................................................................. 1-12

1.6.2 Hydrological Station Network Design .......................................................... 1-13

1.6.3 Selection and Training of Observers ........................................................... 1-14

1.6.4 Data Error Correction ................................................................................ 1-14

1.6.5 Minimum network ..................................................................................... 1-14

1.6.5.1 Network for Rainfall .................................................................. 1-14

1.6.5.2 Network for Stream flow stations ............................................... 1-15

1.7 DISSEMINATION OF DATA AND INFORMATION .......................................................... 1-16

REFERENCES ................................................................................................................. 1-26


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APPENDIX 1.A LIST OF DID HYDROLOGICAL PROCEDURES PUBLICATION ............................ 1A-1

APPENDIX 1.B LIST OF DID WATER RESOURCES PUBLICATION ........................................... 1A-2

APPENDIX 1.C OPERATING PROCEDURE OF THE MALAYSIAN NATIONAL COMMITTEE FOR INTERNATIONAL HYDROLOGICAL PROGRAMME ............................................ 1A-3


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List of Tables

Table Description Page 1.1 National Hydrology Station 2005 1-12

1.2 Charges for Hydrological Data (Fees Order 1966) 1-22

List of Figures

Figure Description Page 1.1 The Hydrological Cycle 1-1

1.2 Flowchart of the hydrological data management system by DID Malaysia. 1-5

1.3 Daily Rainfall at Stn 5204048 in PDAY Format 1-17

1.4 Monthly Water Level Data at Stn 5505412 in PMONTH format 1-18

1.5 Annual Maximum and Minimum Water Levels atStn. 5506416 Retrieved Using PEXTREME 1-19

1.6 Maximum Rainfall Over 3 Hours at Stn. 5105051 Retrieved Using PMOVE 1-19

1.7 DID’s Real-Time Flood Information System 1-21

1.8 Flow Chart for Processing Hydrological Data Applications 1-25


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1 INTRODUCTION

1.1 DEFINITION AND IMPORTANCE OF HYDROLOGY

Hydrology deals with the study of the occurrence and processes of water. Processes in hydrology would include precipitation, infiltration, surface runoff, percolation, ground water flow, evaporation, transpiration. Water on earth occurs in many forms such as water vapour, rainfall, surface water storage and ground water storage. The occurrence of water in its various forms and the processes water undergoes as it move from on form to another is best illustrated by the hydrological cycle (see Figure 1.1)

(cloud)

Surface runoff

Infiltration

Precipitation[976 bil m3/yr]

Total Runoff[527 bil m3/yr]

Evapotranspiration[385 bil. m3/yr]

Evaporation

Precipitation

(ocean)

(cloud)

(estuary) Groundwater Recharge

[64 bil m3/yr]

Interflow

groundwater flow

(vapour, dew)

(groundwater storage)

Streamflow

(river)

(pond)

*Figure in [] indicates average values for Malaysia (Source: NWRS 2000)

Figure 1.1 The Hydrological Cycle

As a general guide, the average annual rainfall for Malaysia is about 3000mm; evaporation, 1700mm; surface runoff 1000 mm.

Practical Applications of Hydrology

Knowledge and understanding of hydrology is important in many applications:

• The flood mitigation engineer has to estimate the flood discharge and volume to size a flood storage pond and to determine channel capacity.

• The irrigation engineer needs to understand the rainfall pattern and its temporal variation, the evaporation and infiltration losses peculiar to his project area to estimate irrigation requirements.


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• The water resources engineer needs to estimate the amount of flow available from the

catchment of his proposed dam site, losses due to evaporation and seepage so that he can work out the dam storage capacity required to support a given flow abstraction rate.

• The urban drainage engineer needs to estimate the intensity of rainstorm the proposed urban drainage system needs to cope with.

• Irrigation or hydropower engineers, (for run-of-the river diversion scheme) need to project the dry weather flow in a river under extreme drought conditions to derive the dependable supply.

• The hydropower engineer uses the flow duration curve to determine hydropower generation capacity.

Besides design applications mentioned above, there are other aspects such as flood forecasting, water resources modelling, real-time monitoring and management of water resources systems and flood risk mapping where hydrology is applied.

• Developing a rainfall-runoff model for flood forecasting requires hydrological data and expertise in configuring and calibrating the model.

• In managing an irrigation project, model simulation of water resources systems are increasingly being applied to aid decision making and setting up a simulation model requires substantial hydrological input.

• As technology becomes available and affordable, water resources systems can now be managed based on remote and real-time monitoring of hydrological parameters such as rainfall, water levels and flows at various critical locations in their systems. Input in terms of hydrological data collection, processing and analyses is required.

• Flood risk is now given greater emphasis in urban planning as it is now accepted that proper planning with due consideration to flooding is more effective and economical compared to solving flood problems due to poor planning when it occurs. Flood risk maps are prepared based on hydrological data, hydrological modelling techniques and statistics.

1.2 USER NEEDS FOR HYDROLOGICAL DATA AND INFORMATION

To arrive at realistic estimates of the hydrological parameters, data and information is needed. DID has set up a network of hydrological stations to collect such data and the data collected are processed and stored in a hydrological database so that they can be easily located and retrieved when needed.

Hydrological data are records of natural phenomena i.e. rainfall and streamflow. The building up of sufficient data for the statistical analyses or the capture of extreme events of interest to flood studies (high flow, high rainfall) or for drought analyses (low flow, consecutive days of zero rainfall) is slow and it takes years to get a useful and reliable database. Hydrological data collection is expensive and is a long term investment for the government as representative and reliable hydrological data is important to water resources development and management.


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1.3 HYDROLOGICAL ACTIVITIES IN MALAYSIA

The government agency most active in hydrology is the Department of Irrigation and Drainage and the hydrological activities carried out can be classified into three major areas:

• Operational hydrology which covers

o data collection and management, o maintenance of a flood forecasting/warning service and o maintenance of a drought monitoring service.

• Applied hydrology, covering

o development of hydrological procedures (HPs) which are guidelines on how to estimate hydrological parameters required in design of water projects,

o processes hydrological data into useful information in the area of water resources which the public can refer to and use and publish the information in water resources publications (WRPs)

o providing in-house(within DID) consultancy and advise on matters related to hydrology

• Hydrological research which covers

o Experimental and representative river basin studies o IHP (International Hydrological Programme) participation in collaborative research

DID is the agency most active in area of hydrology. Most of the hydrological stations under the principal hydrological network in the country are maintained by DID. The principal network also contains some rainfall stations maintained by Malaysian Meteorological Department (MMD) and some by Department of Forestry (DOF).

Department of Environment (DOE) contracts out regular river water quality samplings at DOE’s network of water quality sampling points. But DID also maintain river stations where water quality parameters are sampled regularly.

DID maintains a hydrological database where data can be retrieved in various formats by users. The public can also purchase the data (Fees Order 1966) and most purchasers are contractors using the rainfall data as prove of bad weather and basis for extension of time (EOT). Data is given free to those working on government projects.

1.4 OUTLINE OF ACTIVITIES

The main activities of the Division of Hydrology and Water Resources are:

• Hydrological data collection, processing, storage and retrieval • Periodic publication of hydrological data and dissemination of information via the internet • Regional and country-wide water resources assessment and water resources research • Development of procedures and guidelines for hydrologic design • Operation of flood warning and forecasting services • Operation of drought monitoring services • Hydrological instrument support and maintenance • Participation and collaboration in international hydrological programmes


1-4 March 2009

1.5 FIELD COVERED BY DIVISION OF HYDROLOGY AND WATER RESOURCES

The Department of Irrigation and Drainage through the Hydrological Division as a whole is responsible for the setting up and maintaining the hydrological stations throughout Malaysia, collecting, processing, archiving and dissemination of the hydrological data such as rainfall, river stage and discharge (streamflow), evaporation, river suspended sediment, river water quality and agro-hydrological data. As for the ground water data, this is carried out in conjunction with the Geological Survey Department as the latter is responsible for the long term groundwater resources survey and exploratory in the country.

In the case of rainfall, the data collection is supplemented by the Malaysian Meteorological Service which operates a network of climatological stations, mainly to facilitate aircraft movement and weather forecast.

The Division of Hydrology and Water Resources in collaboration with the Malaysian Meteorological Services (MMD) Department are also responsible in providing flood forecasting and warning services on the main rivers and also to monitor the flood and drought events. MMD provide the daily weather forecast to State DID engineers during monsoon season and particularly special warning of heavy rain exceeding 125 mm and their duration. The data are then used as inputs to the various flood forecasting models available in DID for predicting the future lag time flood levels.

The Hydrological Department is also active in providing consultancy services on matters pertaining to hydrological requirements and in updating the various hydrological procedures, manuals and in carrying out water resources studies where necessary.

DID is also actively participating in the international arena such as undertaking international collaborative research under the UNESCO’s International Hydrological Program (IHP) and actively participating in activities pertaining to flood disaster management under the Typhoon Committee which in turn is under the auspices of the ESCAP and WMO.

To fulfil the activities in an orderly and standardised manner, a quality manual based on the MS ISO 9001:2000 had been formed by the Hydrological Division. Figure 1.2 shows the flowchart of the hydrological data management system by DID Malaysia.


March 2009 1-5

Management Responsible

Resources Management

Analysis & Improvement

Figure 1.2 Flowchart of the hydrological data management system by DID Malaysia.


1-6 March 2009

1.5.1 Operational Hydrology

The operational hydrology conducted under the DID Hydrological Division are as follows:

1.5.1.1 Field Operation

The field operation constitutes of the setting up, maintain and collecting the various hydrological data. In order that the quality of the hydrological data collected is acceptable, the following criteria with standard types of equipment are adopted for use in the Department:-

(a) Rainfall: -

(i) Manual measurements taken at fixed time of 8.00 a.m. daily from 203 mm (8") diameter daily raingauge or taken once monthly from storage raingauge by observers for Secondary Stations.

(ii) Self-recording from either Hattori type weekly or long-term graphical rainfall recorders for Principal Stations.

(iii) Telemetric real-time recording at .any intervals by teleprinter (in addition to self-recording by rainfall recorder) for flood forecasting purpose only.

(b) Evaporation:-

(i) Manual measurements taken at fixed time of 8.00 a.m. daily from modified US Class A Aluminum pan by observers for Secondary Stations.

(ii) Self-recording (will be locally developed and manufactured) for Principal stations.

(c) River Stage:-

(i) Manual stick gauges read at fixed time of 6.00 a.m. and 6.00 p.m. daily by observers for Secondary Stations.

(ii) Self-recording from Ott float-type week1y or Ott or SEBA long-term graphical water level recorder. This is for permanent (Principal) stations. In special case, if the construction of a float-type water level recorder well is prohibited, an Ott type pneumatic or Sher1ock DP 30 pressure sensing unit is used in conjunction with the recorder.

(iii) Self-recording from Negretti and Zambra type weekly pressure-bulb water level recorder for temporary stations (e.g. investigation for tide level).

(iv) Telemetric real-time recording at any interval (in additiol1 to self-recording) for food forecasting purpose only.

(d) River Discharge:-

(i) Regular measurements at least once or preferably twice a month (in order to define the rating curve for the station at all times) by Ott type current meters.

(ii) Measurements by weir or flume (for small basin)


March 2009 1-7

(e) River Suspended Sediment:-

(i) Regular measurements at least once a month by US DR4-8, DH59 Or D49

suspended sediment samplers. (ii) Laboratory analysis of samples by Research Station, DJ.D. Ampang.

(f) River Chemical Water Quality:-

(i) Regular measurements at fortnightly intervals by automatic depth sampler (Kahlsico type but modified locally to 2-litre capacity).

(ii) Laboratory analysis of standard 4-litre capacity samples by Chemistry Department.

(g) Agro-hydrological Data:-

Agro-hydrological stations network is recently established by the Department to collect the required data for the planning and monitoring of major agricultural projects. The data required are rainfall, evaporation, air temperature, relative humidity, wind run, sunshine duration, soil temperature and soil moisture. The measurements are taken by observers one to three times daily depending on the types of parameters.

The data under items (d), (e) and (f) are being observed at the same site for data under item (iii) so that relationships between stage and discharge, suspended sediment and, discharge, quality and discharge can be established.

As for standardization on observation and instrumentation, technical regulations and guides on hydrological practices, the World Meteorological Organisation (WMO) is to be closely followed.

1.5.2 Hydrological Data Management 1.5.2.1 Collection of data The collection of the hydrological data is in accordance with the standards specified in 'Technical Regulations - Hydrology and International Hydrological Codes" published by the World Meteorological Organization in 1980. The accuracies of the measurements for some of the hydrological data are as follows:-

(a) River stage, 1.0 cm.

(b) River discharge and suspended sediment 5%

(c) Rainfall, 0.5 _ for tropical condition.

(d) Evaporation, 0.1 mm.

1.5.2.2 Submission and Processing of Hydrological Data

In the States, the hydrological data are collected by the field parties or local observers under the supervision of the State Hydrological Officer. The data are scrutinised for any errors or discrepancies before submission to the hydrology Branch of the Department at monthly intervals for centralised processing.


1-8 March 2009

On receipt of the data, they are checked, recorded in the registers and then processed using the Electronic Data Processing (E.D.P.) System which includes a computer. for the States of Sarawak and Sabah in the eastern part of. Malaysia, the data are partly processed by using an electronic digitizer with one stationed at Kuching and another at Kota Kinabalu, before being submitted to the Hydrology Branch for further processing. After the data are processed by the EDP System, the 'clean' processed data are temporarily stored on magnetic disk and then merged into the Hydrological Databank (comprising of magnetic tapes) set up in the computer section. The Databank was implemented in 1974 using the Time Dependent Data Processing System (TIDEDA) originally developed by the Ministry of Works, New Zealand.

1.5.2.3 Dissemination of Hydrological Data

To fulfil one of the functions of the Hydrology Branch in the Department, the hydrological data from the Databank have to be analysed and published. This is done with the aid of a computer. Computer printouts containing daily readings of the hydrological e1ements such as rainfall, river discharger, river suspended sediment, evaporation, river water qua1ity, etc., for all the hydrological stations in the States are distributed to the States concerned information. Due to the large amount of hydrological data, the publication is based on the summaries of the data for each two to five-year periods. The publications are sold at nominal costs to the users.

On special request, the hydrological data for any period can also be retrieved in any desired format directly from the computer. However, the use of the computer time involved is borne by the users themselves. In general, the Department is authorized to charge the users for the supply of hydrological data unless proved to be used for Government purposes. A data bulletin containing the availability and contents of the Hydrological Databank is normally issued to the users for information.

1.5.3 Flood Forecasting and Warning Services

Structural flood mitigation alone only provides protection levels of normally 10 to 100 years average recurrence intervals (ARI). One of the economically effective non-structural solution in flood mitigating and flood management is to carry out the flood forecasting and warning services to the flood prone areas.

Early warnings of the incoming flood events will enable the flood victims to prepare themselves before the flooding occurs. Properties and lives could be saved by keeping their movable properties above the flood levels and if necessary, early evacuation from the area. With respect to flood fighting, early warning to the approaching flood should alert the organization in charge of the flood fighting actions, the authority for making the necessary decisions, and the general public to be aware of the pending danger.

Detailed discussion on this topic will be covered in chapter 9.

1.5.4 Drought Identification and Monitoring Services Unlike flood, drought is more difficult to forecast. When rainfall pattern shows an abnormally lower record than usual caused by prolong dry periods over a long period of time (more than 14 days), it will cause adverse depletion in water supply reserve. Drought causes inadequate water supply to irrigate crops which will result in withering and eventually kills the crop.


March 2009 1-9

Prolong drought can result in extensive bush and forest fires which could engulf life and property if unchecked. Fires especially in peat areas, causes the air quality to deteriorate because of the smoke generated. Such a situation can persist for months and poses a serious threat to the health of the people. In some places, schools had to be closed down temporarily, poor vision due to the thick haze causes problems in traffic drivability including problems to the pilots in landing their aircrafts. Hence droughts can cause extensive impact to the environment, economic and social activities of the whole nation. The Hydrological Division is responsible to monitor drought occurrences and if it happens, they will monitor the situation more closely and to inform the relevant agencies involved in preparation for the disaster such as water rationing by the water authority and stringent control of irrigation water in irrigation schemes like the MUDA Irrigation Project. Drought monitoring is being done continuously throughout the year in the Department of Irrigation and Drainage to keep track of rainfall deficits happening in the country. This would enable appropriate drought management options to be undertaken in a timely manner to reduce the impact of droughts. In addition to rainfall deficits, the trend of river low flow water levels and dam impoundment water surface levels are also being monitored in real time measurements to enhance the effectiveness of drought monitoring and decision support system. From the DID hydrological data, Malaysia has experienced some degree of drought starting from the month of January until August each year. During this time, dams will be carefully regulated so that the water will be made available throughout the dry months.

1.5.5 Hydrological Application The Hydrology Branch of the Department also analyses the available hydrological records and reduces them into a simple format such, as chart or map and presents it in the publication series on "Hydrological Procedure" or "Water Resources Publication". This is for the convenience or handy use of practicing engineers and planners in solving water problems. The publications are also sold at nominal costs to the users. Some of these publications will be updated or reviewed from time to time when additional hydrological data at 5 or 10 years intervals are analysed. A list of publications prepared by the Department to date is shown in Appendix 1.A– List of HP and 1.B– List of WP

1.5.6 Technical Advice and Consultancy

The Division is responsible in providing technical advice, consultancy services for in house projects and being part of technical examiners on outsourced consultative works pertaining to the hydrological components. This includes in explaining and assisting users on the methodologies and procedures in utilizing the various hydrological data. The Division is also responsible in providing the necessary trainings and seminars for the DID staff on utilizing the various hydrological procedures and water resources publications and to receive feedbacks on its inadequacies for further improvement works.

1.5.7 Water Resources Assessment

In the urge to understand the hydrological scenarios, its impact and effects to our localized catchment areas, DID is responsible in carrying out the necessary Water Resources Studies (WRS). Results of the findings are then published as the DID Water Resources Publication for the Departmental and public user to refer. Lists of the publication are shown in Appendix 1.B. They could also be accessed through the internet at DID Portal: http://www.water.gov.my under Water resources information of the Resource Centre.


1-10 March 2009

1.5.8 Applied Researches, Local, Regional and International Collaboration Hydrological research works are to be carried out on the Malaysian Soil to obtain hydrological parameters that are more representative to our local and regional consumptions. Some of these parameters are inputs to many of the empirical formulas in which their related parametric coefficients are to be derived and verified from observation or experiment. As an example, the run-off coefficient used in the rational method to calculate flood design discharge and the areal reduction factors in reducing rainfall amount due to area distribution. In this perspective, DID Hydrology, undertakes research works from time to time which includes Experimental and Representative Basin Studies as part of the Water Resource Studies. These studies can also be part of the local, regional and international Collaborative works under the auspices of the United Nations such as WMO and UNESCO’s International Hydrological Programme (IHP) Collaborative Researches. 1.5.8.1 Experimental and Representative Basin Studies DID Hydrology conducts research works to gain the understanding of specific hydrological processes and characteristic through experimental and representative basin studies. It also includes the basic and continuous water resources assessment studies, representative to our local conditions. Examples are the Sg. Lui and Sg. Tekam experimental Representative Basin. The works are published in the DID Water Resources Publication. (Refer Appendix 1.B) 1.5.8.2 IHP Participation in Collaborative Researches The aim for DID involvement in the IHP is to play a leading role in promoting and advancing the field of hydrological science in Malaysia and the region. Its primary objectives are:

• To represent Malaysia on all issues related to the programmes of IHP UNESCO and participate actively in those programmes.

• To promote and coordinate research programmes on hydrology and water resources in the country and region.

• To promote and coordinate practices on hydrology and water resources.

• To promote and coordinate programmes on education, training and public information on hydrology and water resources.

The UNESCO Regional Offices are responsible for the implementation of IHP at the regional level. Regional Hydrologists are posted in the field and serve as IHP focal points for all issues relating to the Programme, both at regional and national levels. Two Regional Hydrologist posts have been set up to cover the Asia and Pacific region, one for Central and South Asia, based at the UNESCO New Delhi Office, and another for Southeast Asia and the Pacific, based at the UNESCO Jakarta Office. As a programme with scientific and educational goals, IHP is a cooperative effort, relying on the worldwide efforts of Member States and their designated IHP National Committees to function efficiently. For Malaysia, the national IHP Committee is called the Malaysian National Committee for IHP or MIHP and the operating procedures of MIHP is presented in Appendix 1.C. The National Committees present reports on their activities in the framework of the Programme. These national reports are submitted to the IHP Intergovernmental Council and cover the activities for the inter-seasonal period between Council sessions.


March 2009 1-11

1.6 EVALUATION AND DESIGN OF HYDROLOGICAL NETWORK

Hydrological data are the most important overlooked element in water resources planning and design of projects. So many times dams, irrigation projects, embankments, and other water related programs are design and built using scanty poor quality data.

In view of the need for more adequate and good quality hydrological data for planning and operation of the various water resources development projects, hydrological data collection had to be carried out in an organized and integrated manner, nationwide. Hydrological data collections in Malaysia have been undertaken by four government agencies namely: - the Department of Irrigation and Drainage (DID), Malaysian Meteorological Service (MMD), Public Works Department (PWD) and the National Electric Board (NEB).

DID undertake to maintain and operate a network of hydrological stations on a long-term basis, whilst the other agencies restrict their hydrological investigations to meet their specific requirements, mainly on a short term programme. Table 1.1 Shows the Distribution of the National Hydrological Stations for each States until the year 2005, operated and maintained by DID.

Hydrological data collection in DID is carried out by the state personnel in accordance with a national hydrological observation program co-ordinated at Federal Level. The establishment of the national network of hydrological stations is financed by the Federal Government, whilst the State Government provide for the operation and maintenance costs.

DID as a whole, is responsible for the collection, processing and publication of the long-term hydrological data such as rainfall, river stage and discharge (stream flow), evaporation, river suspended sediment, river water quality and agro-hydrological data. As for groundwater data, the geological survey department is responsible for the long term groundwater resources survey and exploration in the country.

The hydrological network in Malaysia consists of principal and secondary stations. Principal stations are permanent or fixed stations and are equipped with self-recording gauge. Secondary stations are short-term or project station which is subjected to review after continuous operation for 5 to 10 years. They are equipped with either manual gauges or self-recording gauges but may have the equal priority to principal stations for data processing and analysis.


1-12 March 2009

Table 1.1 National Hydrology Station 2005

STATE STATION

RAINFALL EVAPORATIONRIVER STAGE & DISCHARGE

SUSPENDED SDIMENT

WATER QUALITY

PERLIS 14 1 3 2 0 KEDAH 46 1 3 1 1 PULAU PINANG 21 0 2 0 0 PERAK 90 1 14 11 16 SELANGOR 70 2 8 8 7 WILAYAH PERSEKUTUAN 12 0 3 3 5

NEGERI SEMBILAN 47 1 9 3 3 MELAKA 19 0 3 2 2 JOHOR 89 5 9 4 4 PAHANG 120 3 19 18 12 TERENGGANU 55 2 10 9 4 KELANTAN 50 2 9 9 8 TOTAL 633 18

92 70 62

1.6.1 General Considerations

Though it would be best to set up the hydrological stations everywhere possible, in practice it would be too expensive and laborious to maintain the lot. The networks for the various inter-related elements should be designed to provide a maximum of information for the budget allocated. It is most important that networks be operated in a manner to assure reliable and trustworthy measurements which are homogeneous, spatially and temporally. Factors which should be considered in the establishment of new networks as well as in the review of those in existence are:

a. Minimum density of gauges

b. Gauge type

i. Manual readings / non recording

ii. Automatic / semi automatic recording / self recording (analogue – charts and paper punch tapes or digital logging system)

c. Gauge site location

d. Observers

i. Trainable

ii. Conscientious

iii. Reliable


March 2009 1-13

e. Standards of observations

i. Time and frequency of observations

ii. Limit of measurement and degree of accuracy

• Climatological uses

• Hydrological uses

iii. Substation inspection and quality control

• Gauge maintenance standards

• Statistical analysis for error detection and correction

f. Method of observing and communicating the data.

g. Data processing, storage, retrieval, and dissemination to users.

1.6.2 Hydrological Station Network Design

In planning hydrological networks, it is essential to keep their principal purpose always in view; i.e., to delineate the hydrology of the area. For this reason, stations should be so located that they sample all physiographical characteristics and so that the data will be useful in determining the functional relationships involved. To assure that the programme fully meets these requirements, it is customary to include a number of representative catchments – relatively well instrumented basins. Hydrological bench-marks (catchments) and climatological references stations are included to provide long term, stationary time series.

Among the hydrological data that needs the most attention is rainfall. This is because in the study of hydrology, it mainly deals with the land phase of the hydrological cycle. Rainfall Stations network need to be established and organized properly. Poor quality data can result in bad and even catastrophic design failure. Related climatological observations data such as the evaporation, moisture condition, sunshine and wind, usually will be incorporated in the rainfall station where necessary to form an integrated hydrological observation station location.

River stage and discharge station, which is equally important, will be setup depending on their necessity and functions such as for flood forecasting and warning, water resource project planning and catchment study and monitoring. In almost all Rainfall Runoff computer model, observed streamflow data is required to calibrate or compare the model simulated discharge output to the actual observed/gauged values. Where real time data are required, the use of telemetric system of data teleporting will be required

A principal difficulty in the proper planning of basic data networks concern funding. Although planning can reflect inadequate funding, it must be based on projected funds for many years in advance – hopefully an orderly increase with time. Wide fluctuations in funding, reflecting economic and other conditions in our country can result in gross inefficiencies. An important consideration in the location of a gauge is, once installed, how long can it be maintained at that location and what will be the quality of the observation.

A certain number of the network stations should be designated as ‘Benchmark’ or primary stations which will be long term sites for statistical or climatological purposes. These stations should be at sites where there will be little or no chance of distributions for 50 years or more.


1-14 March 2009

They should be at site where attendance is by quality observers with responsible supervision. The other stations in the network can be designated “temporary or secondary” and are established for special purpose uses such as project design and operation. WMO has set up standards of gauge location and exposure which should be referred to and adhered to as closely as possible. The station inspector should always be aware of any changes in or near the site that will tend to affect the observation, such as nearby trees or shrubs which grow so large that they will interfere with, or the construction of buildings which will shield the gauge unduly or change the wind patterns in a detrimental way.

It is most important that networks be operated in a manner to assure reliable and trustworthy measurements which are homogeneous, spatially and temporally. Standardization of instruments and methods of observations must be achieved through wise decisions and close supervision by headquarters staff backed up by an adequate and well trained corps of observers, inspectors and maintenance staff in the field. Since the goal of a basic data programme is to serve the users, quality control must be exercised also in processing, publication, storage and retrieval; and the information must be readily available in a suitable form at all times.

1.6.3 Selection and Training of Observers

The selection of an observer can often be more important than any other factor. The quality of the data can rise or fall depending upon how well the observer’s attitude and responsibility is. The best observers are generally those that have a technical background or training. Police stations, fire stations, hospitals and other more or less permanently located public institutions are the best source of supply. The observation can be considered a part of their daily routine. These people most often will have a good sense of responsibility and knowledge of the value of good reliable data. Another good source is the farmers or others who can make use of the data themselves.

1.6.4 Data Error Correction

Aside from improper location and poor maintenance the largest source of error is due to the lack of proper standards of observation, observer training and a follow-up station inspection program. A well-organized observer training and sub-station inspection program is absolutely essential since unreliable data is the final result of poor training and improper follow-up and correction of errors.

1.6.5 Minimum network

1.6.5.1 Network for Rainfall

For rainfall, the World Meteorological Organization guidelines for determining the minimum densities of precipitation networks are: a) For flat regions – ideally, 1 station per 600 – 900Km2 (minimum - 1station per 900 –

3000Km2)

b) For mountainous areas – ideally, 1 station per 100 – 250 Km2 (minimum 1 station per 250 – 1000 Km2 )


March 2009 1-15

The minimum density network should consist of three kinds of gauges.

a) Standard gauges – non-recording to be read daily or at other predetermined intervals depending upon operational requirements. A standard time of observation should be established and adhered to. The recommended is near 8 am local time with the data entered on the day of observation. Some countries have varied this by entering the data on the day previous. This has led to considerable confusion when the data is to be used for statistical studies or for procedure development as this method of entry has not been consistent between agencies or even between observes in the same agency.

b) Recording gauges – It is recommended that at least 10% of the total gauges in the network

be of the recording type. These may be of the siphon, tipping bucket or weighing mechanism depending upon availability and operational requirements.

c) Storage gauges - In many areas it is important to obtain data but due to a lack of population or communication a standard gauge cannot be installed. A gauge with large enough capacity so it can be read on a monthly, semi annual or annual basis. This data can be very useful in filling gaps in the data field for climatological and hydrometeorological analysis.

For operational or other high priority purposes, it may be advisable to concentrate the precipitation network in those areas of greatest importance at first rather than to try to get an even distribution. Later, as funds become available, the network can be filled in to achieve the desired distribution.

1.6.5.2 Network for Stream flow stations

The river stage, discharge, suspended sediment and water quality data are normally observed at the same site so that relationships between stage and discharge, suspended sediment and discharge, river water quality and discharge can be established.

a) Network for Suspended Sediment stations

Detail description on the Determination of Suspended Discharge can be referred to HP No.19: “The Determination Of Suspended Sediment Discharge”

b) Network for River Water Quality stations

Guidelines for the sampling of surface water involving in chemical and bacteriological sampling of stream river lakes and ponds are discussed in DID’s Hydrological Procedure No.2: “Water Quality Sampling for Surface Water” – 1973.

c) Agro-hydrological stations network

Agro-hydrological stations network established by the Department is to collect the required data for the planning and monitoring of major agricultural projects. The data required are rainfall, evaporation, air temperature, relative humidity, wind run, sunshine duration, soil temperature and soil moisture. HP 24. Establishment of agro-hydrological station – 1982, discuss in details on its establishment.


1-16 March 2009

1.7 DISSEMINATION OF DATA AND INFORMATION Data stored in the hydrological database is disseminated to users in various forms. Data can be retrieved in various standard formats:

• PDAY format for those interested in tabulations of daily rainfall, daily evaporation, average daily water levels, average daily discharge, average daily suspended sediment load (see Figure 1.3)

• PMONTH format (see Fig. 1.4)

• PEXTREME format for those interested in extreme high or extreme low readings in each year.

o PEXTREME for river level and flow data will give the instantaneous high/low water level/discharge for the station and for the year the user is interested in. (see Figure 1.5)

• PMOVE format (see Figure 1.6)

o PMOVE for rainfall will require a duration parameter (for example: 5 minutes, 15 minutes, 3 hours, 24 hours) and the output of PEXTREME presents the maximum rainfall recorded

Data can be supplied in hard copies or in digital form (ASCII text format)

Besides direct retrieval of data on request from the hydrological database, once in a few years, DID releases publications of hydrological data as listed below:

1. Hydrological Data - Rainfall Records 1879-1958(1961) 2. Hydrological Data - Rainfall Records 1959-1965 (1970) 3. Hydrological Data - Rainfall Records 1965-1970 (1974) 4. Hydrological Data - Rainfall Records 1970-1975 (1977) 5. Hydrological Data - Rainfall Records 1975-1980 (1983) 6. Hydrological Data - Rainfall and Evaporation Records 1981-1985 (1987) 7. Hydrological Data - Rainfall and Evaporation Records 1986-1990 (1991) 8. Hydrological Data - Streamflow Records 1910-1940 (1962) 9. Hydrological Data - Streamflow Records 1941-1960 (1969) 10. Hydrological Data - Streamflow Records 1941-1960 (1969) 11. Hydrological Data - Streamflow Records 1960-1965 (1972) 12. Hydrological Data - Streamflow Records 1965-1970 (1975) 13. Hydrological Data - Streamflow Records 1970-1975 (1978) 14. Hydrological Data - Streamflow and Suspended Sediment Records 1975-1980 (1985) 15. Hydrological Data - Streamflow and Suspended Sediment Records 1981-1985 (1988) 16. Hydrological Data - Streamflow and Suspended Sediment Records 1986-1990 (1995) 17. Hydrological Data - Water Quality Records 1974 (1975) 18. Hydrological Data - Water Quality Records 1975 (1976) 19. Hydrological Data - Water Quality Records 1976 (1977) 20. Hydrological Data - Water Quality Records 1977 (1979) 21. Hydrological Data - Water Quality Records 1978 (1981) 22. Hydrological Data - Water Quality Records 1979-1980 (1982)


March 2009 1-17

Daily totals Year 1954 site 5204048 Item 1 Day Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1 213 31 7 46 0 19 0 44 0 0 390 0 2 526 0 0 0 0 0 0 0 155 0 326 0 3 227 0 0 23 0 0 0 203 78 289 264 0 4 0 0 0 26 0 0 17 119 0 161 127 296 5 0 0 0 7 55 0 82 9 0 47 43 148 6 7 0 0 0 70 0 37 0 18 48 12 0 7 3 0 127 35 304 0 0 107 9 14 465 0 8 0 0 163 56 141 0 0 53 0 0 466 0 9 0 0 50 19 29 15 0 0 240 23 117 35 10 0 0 121 22 107 7 0 0 120 274 0 18 11 0 0 61 263 46 0 0 7 0 359 0 0 12 0 0 0 126 64 145 0 3 0 300 0 25 13 0 0 301 0 32 73 0 0 0 232 0 13 14 140 0 151 37 33 193 132 0 25 187 5 0 15 249 84 0 18 17 96 259 0 13 394 2 0 16 90 50 0 0 0 0 96 0 0 167 0 0 17 76 15 155 0 0 181 0 0 0 0 45 0 18 38 6 82 129 129 229 152 0 215 0 23 0 19 0 0 2 64 64 69 172 66 107 25 11 0 20 7 0 32 33 42 0 48 122 20 13 6 0 21 3 0 170 75 21 0 0 45 10 40 0 0 22 33 79 77 29 0 61 25 22 0 294 0 0 23 17 90 0 0 186 30 60 11 0 137 0 0 24 0 52 0 262 93 0 24 0 0 0 22 0 25 0 13 0 131 0 0 381 0 0 0 94 0 26 0 318 0 15 0 209 190 0 0 0 117 0 27 0 159 129 7 0 105 0 0 115 8 38 0 28 0 15 64 0 186 0 0 0 277 4 10 0 29 0 121 0 93 0 195 0 161 91 5 0 30 0 61 0 37 0 444 0 25 46 0 0 31 62 93 57 261 0 199 0 Min 0 0 0 0 0 0 0 0 0 0 0 0 0 Tot 1691 913 1967 1426 1806 1432 2575 811 1588 3353 2589 535 20686 Max 526 318 301 263 304 229 444 203 277 394 466 296 526 NO>0.0 15 12 19 21 21 14 17 13 16 23 21 6 198

Figure 1.3 Daily Rainfall at Stn 5204048 in PDAY Format


1-18 March 2009

~~~ NIWA Tideda ~~~ JPS - Limited Functionality Version 24-JUL-2008 ~~~ PCAL ~~~ VER 2.1 Source is C:\Documents and Settings\Engineers\Desktop\P.Pinang\Wlevel.mtd Monthly means 1965 to 1974 site 5505412 Item 1 Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean 1965 5087 4816 4749 5535 5952 5042 5237 5978 6345 7496 7759 7369 5954 1966 6070 5639 5617 5838 6134 6049 5778 5526 5858 7312 7183 7208 6188 1967 7228 5798 5336 5483 6387 5907 5791 5568 5913 6950 6862 6242 6126 1968 5167 4828 4632 4964 5398 5346 5594 6069 5415 6606 6088 5425 5464 1969 5578 4872 4979 5224 5947 5913 5440 5775 5440 7103 6976 6420 5812 1970 5517 4889 4651 5078 5653 5394 5689 ? ? ? 7553 6364 5647? 1971 5815 5232 5718 5135 5244 5492 5140 5753 ? ? ? ? 5445? 1972 ? ? 4563 5419 5193 5095 4760 4671 5628 6589 7872 6542 5629? 1973 5285 4659 4553 5475 5799 5785 5334 5932 5701 6399 6761 7154 5743 1974 5436 5110 4691 5132 5831 5291 4893 5242 5851 5822 6012 ? 5392? Min. 5087 4659 4553 4964 5193 5042 4760 4671 5415 5822 6012 5425 5464 Mean 5687 5094 4949 5328 5754 5532 5366 5613 5769 6785 7007 6591 5881 Max. 7228 5798 5718 5838 6387 6049 5791 6069 6345 7496 7872 7369 6188 The Min Mean and Max of Annual means are for complete years only. End of process

Figure 1.4 Monthly Water Level Data at Stn 5505412 in PMONTH format


March 2009 1-19

~~~ NIWA Tideda ~~~ JPS - Limited Functionality Version 24-JUL-2008 ~~~ PEXTREME ~~~ Source is Wlevel.mtd Site 5506416 From 600701 60000 to 721231 180000 Interval = 0 Item 1 Coeff. Year Mean of Var. Minimum Date Maximum Date ---- ---- ------- ------- ---- ------- ---- *1960 9571.3 0.06 8839.0 600828 180000 11277. 600911 60000 1961 9421.6 0.06 8778.0 610708 60000 11277. 610418 180000 1962 9380.3 0.07 8564.0 620323 60000 11551. 621012 180000 1963 9189.7 0.07 8473.0 630513 180000 11338. 631115 60000 1964 9335.2 0.08 8503.0 640322 180000 11582. 640909 60000 1965 9489.4 0.09 8473.0 650129 180000 12466. 651031 180000 1966 9634.2 0.06 8748.0 660216 60000 11400. 660930 180000 1967 9386.1 0.07 8565.0 670323 60000 11674. 670107 60000 1968 9079.3 0.06 8352.0 680309 180000 11369. 681031 180000 1969 9376.6 0.08 8534.0 690212 180000 11521. 691017 60000 1970 9467.9 0.08 8443.0 700304 180000 11521. 701009 180000 1971 9345.2 0.08 8229.0 710328 60000 11704. 710920 180000 *1972 9315.2 0.08 8534.0 720314 60000 11460. 721111 60000 Average Annual Minimum 8514.7 Maximum 11582. (complete yrs) '*' denotes years with gaps in the data or incomplete years Coeff. of Var. = sd/mean Minimum is 8229.00 at 710328 60000 Maximum is 12466.0 at 651031 180000 Mean is 9376.49 Std. Dev. is 702.797 Coeff. of Var. is 7.495E-02 End of process

Figure 1.5 Annual Maximum and Minimum Water Levels atStn. 5506416 Retrieved Using PEXTREME

~~~ NIWA Tideda ~~~ JPS - Limited Functionality Version 24-JUL-2008 ~~~ PMOVE ~~~ VER 1.6 Source is Rfoto.mtd Site 5105051 < Untitled > From 720701 80000 to 750707 75036 Maximum rainfall intensities over 3.000hrs interval Year Ending Maximum Value (Item 1) 730701 80000 675.0 at interval beginning 730527 130248 740701 80000 588.0 at interval beginning 740503 163254 750701 80000 644.0 at interval beginning 750325 13702 750707 75036(PARTIAL) 390.0 at interval beginning 750707 25317 Mean annual Maximum = 635.7 (For complete years only) End of process

Figure 1.6 Maximum Rainfall Over 3 Hours at Stn. 5105051 Retrieved Using PMOVE


1-20 March 2009

With internet being a convenient and easily available channel where the public can search and browse for information publishing data and information via a website is an effective and probably economical mode of information dissemination. The Hydrology and Water Resources Division of the Department of Irrigation and Drainage (DID), Malaysia maintains an online hydrological data website at http://infobanjir.water.gov.my/ or popularly known as InfoBanjir. The hydrological data in InfoBanjir is updated at regular intervals (hourly to daily) from over 300 hydrological stations equipped with Remote Telemetry Units (RTUs). These stations also known as telemetric stations are capable of automatically sending at regular intervals hydrological data recorded at the stations to a central station (See Figure 1.7) InfoBanjir provides the public with online rainfall and water level data. Heavy rainfall is an event that could trigger flood or landslide and therefore InfoBanjir’s rainfall data are useful indicators of potential flooding or landslides. InfoBanjir’s river level data is a direct measurement of flood level available online to the public. Another online information made available to the public is InfoKemarau. InfoKemarau provides the public the results of DID’s drought monitoring effort. The results currently published on the web are only for Peninsular Malaysia and is updated monthly. Besides data DID also publishes two series of publications – Hydrological Procedures (HPs) and Water Resources Publications (WRPs) (see Appendices 1.A and 1.B) The HPs provides guidelines on hydrological design and are often used by consultants to estimate hydrological design parameters such as the 100-year flood discharge, 5 year low flow or the 20-year 3-hour rainstorm. Besides hydrological design, the HPs also provide guidelines on operation hydrology such as installation of hydrological recorders, gauging methodology and data processing procedures. The WRPs are documentations of water resources research carried out by the Hydrological Division of DID and they are useful references in hydrological design and research.

http://infobanjir.water.gov.my/

http://infobanjir.water.gov.my/diagram.htm


March 2009 1-21

Figure 1.7 DID’s Real-Time Flood Information System (source: DID’s InfoBanjir website)


1-22 March 2009

The public can apply for hydrological data from the Hydrology and Water Resources Division of DID. The application for data should be directed to:

Director Hydrology and Water Resources Division Department of Irrigation and Drainage Malaysia Km 7, Jalan Ampang, 68000 Ampang, Kuala Lumpur. Fax: 03 - 42601279 (u.p: Unit Pengurusan Maklumat) E-mail :[email protected]

Request for data can be made using appropriate application forms (Form DI.1) or (Form DI.2) below. Applicants can either fax the appropriate completed form to the above address or send it via E-mail. The flow chart for processing hydrological data applications is shown in Figure 1.8. DID will charge applicants for the data unless the data is used for the Government projects and for research and academic purposes. The charges involved (Fees Order 1966) are as tabulated below.

Table 1.2 Charges for Hydrological Data (Fees Order 1966)

Types of Data For 3 months

and below

3 months to 1 year or any part thereof

For any additional year up to 10 years

of any part thereof

For any additional year in excess of 10 years or any part thereof

Rainfall 50.00 100.00 50.00 per year or part thereof

25.00 per year or part thereof

Water Level 155.00 310.00 155.00 72.50

Discharge Data (Stage-Discharge Curve of Derived

Maps)

35.00 35.00 35.00 17.50

Discharge Data (Discharge Readings)

165.00 330.00 165.00 82.50

Evaporation 35.00 70.00 35.00 17.50

mailto:[email protected]

http://h2o.water.gov.my/appdata/FormDI1.doc

http://h2o.water.gov.my/appdata/FormDI2.doc


March 2009 1-23

FORM DI.1


1-24 March 2009

FORM DI.2


March 2009 1-25

Explain to data user on the types of hydrological data available and data

application procedure

User applies for data

Classify Application

Data to be used for private projects

Data to be used for government projects or for research and academic purpose

Figure 1.8 Flow Chart for Processing Hydrological Data Applications

Retrieves data and supply data to applicant free of

charge

Retrieve data and supply data to

applicant

Applicant fills Form DI.1

Applicant fills Form DI.2

Check availability of data and collect

charges as indicated in Table 1.2


1-26 March 2009

REFERENCES

[1] Klein, G.S., Yufit, G.A. & Shkurko, V.K., A new moving boat method for the measurement of discharge in large rivers, 1993.

[2] McCuen, R.H., Hydrologic Design and Analysis; Prentice Hall, New Jersey, 1998, 814 pages.

[3] Riggs, H.C., “A simplified slope-area method for estimation flood discharges in natural channels”, Journal Research US Geological Survey, 4(3), 1976.

[4] R. Imai, Health & Safety Alert – US Wildlife Operations, Department of Fish and Game (DFG) and the Office of Spill Prevention and Response (OSPR), 2004.

[5] D.K. Yobbi, T.H. Yorke and R.T. MYCYK, A Guide to Safe Field Operation, U.S. Geological Survey Open-File Report, Tallahassee, Florida 1996, 95-777.

[6] SMHB Sdn Bhd, Ranhill Bersekutu Sdn Bhd and Jurutera Perunding Zaaba, Kajian Sumber Air Negara ( Semenanjung Malaysia ) , 2000 -2050, March 2000.


January 2009 1A-1

APPENDIX 1.A LIST OF DID HYDROLOGICAL PROCEDURES PUBLICATION

HP 1. Estimation of design rainstorm - 1982.

HP 2. Water quality sampling for surface water - 1973.

HP 3. A general purpose event water-level recorder (Capricorder Model 1598) - 1973.

HP 4. Magnitude and frequency of floods in Peninsular Malaysia - 1987.

HP 5. Rational method of flood estimation for rural catchment in Peninsular Malaysia - 1989.

HP 6. Hydrological station numbering system - 1974.

HP 7. Hydrological station registers - 1974.

HP 8. Field installation and maintainence of capricorder 1599 - 1974.

HP 9. Field installation and maintenance of capricorder 1598 digital event water level recorder - 1974.

HP 10. Stage discharge curves - 1977.

HP 11. Design flood hydrograph estimation for rural catchment in Peninsular Malaysia (1976)

HP 12. Magnitude and frequency of low flows in Peninsular Malaysia - 1985.

HP 13. The estimation of storage - Draft rate characteristics for rivers in Peninsular Malaysia - 1976.

HP 14. Graphical recorders instructions for chart changing and annotation - 1976.

HP 15. River discharge measurement by current meter - 1976.

HP 16. Flood Estimation for urban areas in Peninsular Malaysia - 1976.

HP 17. Estimating potential evapotranspiration using the Penman Procedure - 1991.

HP 18. Hydrological Design of agriculture drainage system - 1977.

HP 19. The determination of suspended sediment discharge.

HP 20. Hydrological aspects of agricultural planning and irrigation design - 1977.

HP 21. Evaporation date collection using class 'A' aluminium pan - 1981.

HP 22. River water quality sampling - 1981.

HP 23. Operation and maintenance of cableway Installations - 1981.

HP 24. Establishment of agro-hydrological station - 1982.

HP 25. Standard stick gauge for river station - 1982.

HP 26. Estimation of design rainstorm in Sabah and Sarawak - 1984.


1A-2 March 2009

APPENDIX 1.B LIST OF DID WATER RESOURCES PUBLICATION

WR 1. Surface water resources map (Provisional of Peninsular Malaysia). (1974)

WR 2. Hydrological regions of Peninsular Malaysia (1974)

WR 3. Sungai Tekam experimental basin annual report No.1 for 1973-1974. (1975)

WR 4. Notes on some hydrological effects on the influence of land use changes in Peninsular Malaysia. (1975)

WR 5. Evaporation in Peninsular Malaysia. (1976)

WR 6. Average annual surface water resources of Peninsular Malaysia. (1976)

WR 7. Sg. Lui representative basin report no.1 - for 1971/72 to 1973/74.(1977)

WR 8. Water resources for irrigation of upland crops in South Kelantan. (1977)

WR 9. Sg. Lui representative basin report no.2 for 1974/75 to 1975/76.(1978)

WR 10. Sungai Tekam experimental basin report no.2 for Sept. 1974 to March 1977. (1978)

WR 11. Comparison of performance of U.S.Class 'A' evaporation galvanised iron pan and aluminium pan. (1982)

WR 12. Average annual and monthly surface water resources of Peninsular Malaysia. (1982)

WR 13. Sungai Tekam experimental basin calibration report from July 1977 to June 1980.

WR 14. Comparison of raingouge performance under tropical climate conditions. (1984)

WR 15. Average annual surface water resources of Sabah and Sarawak. (1984)

WR 16. Sungai Tekam experimental basin transition report July 1980 to June 1983. (1986)

WR 17. Variation of rainfall with area in Peninsular Malaysia. (1986)

WR 18. Tanjung Karang evapotranspiration study 1987.

WR 19. Mean monthly, mean seasonal and mean annual rainfall maps for Peninsular Malaysia 1988.

WR 20. Sungai Tekam experimental basin final report July 1977 to June 1986. (1989)


January 2009 1A-3

APPENDIX 1.C OPERATING PROCEDURE OF THE MALAYSIAN NATIONAL COMMITTEE

FOR INTERNATIONAL HYDROLOGICAL PROGRAMME OPERATING PROCEDURE OF THE MALAYSIAN NATIONAL COMMITTEE FOR INTERNATIONAL HYDROLOGICAL PROGRAMME

1. Name

The Committee shall be known as the Malaysian National Committee for International Hydrological Programme and shall be referred here after as the MIHP.

2. Objectives

The objectives of the MIHP are:

(a) To represent Malaysia on all issues related to the programmers of IHP under the UNESCO and participate actively in those programmers.

(b) To promote and coordinate research programmers on hydrology and water resources in the country and region.

(c) To promote and coordinate practices on hydrology and water resources.

(d) To promote and coordinate programmers on education, training and public information on hydrology and water resources.

3. Membership

3.1 The membership of MIHP shall be opened to any governmental organization involved in hydrology and water resources activities.

3.2 Subject to the approval of MIHP Executive Committee (EXCO), non-governmental organisations may be accepted as Associate Members of MIHP.

3.3 Associate members shall have no voting rights.

4. Organisation Structure

4.1 The Annual General Meeting (AGM) and the Extra-Ordinary General Meeting (EGM) of the MIHP shall be the highest decision making body within the MIHP.

4.2 The Director General of the Department of Irrigation and Drainage (DID) shall be the Chairperson of MIHP.

4.3 The Director of Division of Hydrology and Water Resources of DID shall be the secretary of MIHP.

4.4 The Executive Committee (ECXO) of MIHP shall consist of the Chairperson, Secretary, eight other members and the Chairpersons for the standing committees. The EXCO shall meet regularly to plan and implement programmers related to hydrology and water resources.


1A-4 March 2009

4.5 The Chairperson and Secretary of the MIHP shall also be the chairperson and Secretary

of the EXCO.

4.6 The other eight EXCO members shall consist of four permanent members and four elected members.

4.7 The four permanent members are:

(i) Malaysian National Commission for UNESCO (SKUM)

(ii) Malaysian Meteorological Department (MMD)

(iii) Department of Mineral and Geosciences (DMG)

(iv) The Department of Irrigation and Drainage (DID)

4.8 The four elected members shall be elected at the AGM and shall serve for a term of two years. They are eligible to be re-elected at the end of the term.

4.9 The AGM is empowered to set up standing committees to fulfill the objectives of MIHP. The EXCO is empowered to appoint the chairpersons of the standing committees.

4.10 The EXCO may from time to time establish sub-committees or working groups to handle issues pertaining to hydrology and water resources. Non governmental organizations may be co-opted into this sub-committees or working groups. The EXCO is also empowered to dissolve the sub-committees and working groups when deemed appropriate.

4.11 DID shall provide secretariat support for the MIHP and the EXCO.

5. Financial Management

5.1 The MIHP Chairperson shall operate the IHP Trust Account, which was approved by the Ministry of Finance since 1988, Under Section 9.3. of the Financial Procedure Act 1957.


January 2009 1A-5

FINANCIAL PROCEDURE ACT 1957

DIRECTION UNDER SECTION 9(3)

Trust Account for Malaysian National Committee For International Hydrological Programme

1. There is hereby established a Trust Account in the Consolidated Trust Account to be known as “Trust Account for Malaysian National Committee for International Hydrological Programmer” hereinafter referred to as the “Account”.

2. The Account is established for the purpose of receiving and meeting all expenses related to the activities of the Malaysian National Committee for International Hydrological Programmed.

3. The Account shall be controlled by the Director General, Department of Irrigation and Drainage or by an officer duly appointed by him in writing. The Senior Treasury Accountant, Ministry of Agriculture Malaysia, shall account for all receipts and payments for the purpose of the Account.

4. The Account shall be operated in accordance with the provisions of the Treasury Instructions and any other financial regulations issued from time to time under the Financial Procedure Act, 1957.

5. The Account shall be credited with contributions or donations from UNESCO, Malaysia National Commission of UNESCO, or members of the public or institutions, which are not wholly financed by the Government of Malaysia.

6. Moneys standing to the credit of the Account shall be expended for the following purposes:

(a) Expenditure on local meetings, courses, seminar and workshops related to International Hydrological Programme activities;

(b) Publication costs and expenditure for printing materials like articles, brochures, invitation cards etc;

(c) Travelling expenses and per diem allowance for Malaysian delegates attending meetings, seminars, conferences and workshops overseas;

(d) Expenditure for transportation and cost of living allowance for consultants;

(e) Incidental expenditures approved by the Director-General, Department of Irrigation and Drainage. All payments made from the Account shall be on vouchers duly certified by the Director General, Drainage and Irrigation Department or by officers duly appointed by him in writing for the purpose.

7. All payments made from the Account shall be on vouchers duly certified by the Director General, Drainage and Irrigation Department or by officers duly appointed by him in writing for the purpose.

8. The Account shall always be in credit and shall not be overdrawn.


1A-6 March 2009

9. (a) The Director-General, Department of Irrigation and Drainage shall as soon as

possible after 31st. December of each year, but in any case not later than 31st. May of the year following the financial year, submit to the Auditor-General statements of accounts showing the opening balance, detailed classifications or receipts and payments for the previous year, and the closing balance of the Account.

(b) The Director-General, Department of Irrigation and Drainage shall forward a copy of the account duly examined by the Auditor-General to the Secretary –General of the Treasury, Secretary-General Ministry of Agriculture and the Accountant-General Malaysia.

(c) The Director-General, Department of Irrigation and Drainage shall as soon as possible after 31st. December of each year, but in any case not later than 31st. May of the year following the financial year, submit to the Auditor-General statements of accounts showing the opening balance, detailed classifications or receipts and payments for the previous year, and the closing balance of the Account.

10. The Account shall be closed when it is no longer required and its credit balance, if any, shall be credited to the Federal Revenue.

11. This Direction shall be deemed to have effect as from 1st. January 1988.

CHAPTER 2 PRECIPITATION

Chapter 2 PRECIPITATION

March 2009 2-i

Table of Contents Table of Contents........................................................................ ............................................. 2-i

List of Tables ........................................................................................................................ 2-ii

List of Figures ....................................................................................................................... 2-ii

2.1 MEASUREMENT OF RAINFALL ..................................................................................... 2-1

2.1.1 Non Recording Raingauge ............................................................................... 2-2

2.1.2 Recording Rain Gauges .................................................................................. 2-2

2.2 SITING OF RAINFALL STATIONS ................................................................................. 2-3

2.3 ERRORS IN MEASUREMENT .................................................................................... 2-7

2.4 WIND SHIELDS ................................. .. ....................................................................... 2-9

2.5 ANALYSIS OF RAINFALL DATA .................................................................................. 2-10

2.5.1 Records and their interpretation .................................................................... 2-10

2.5.2 Adjustment and Interpolation of Observations ................................................ 2-10

2.5.3 Variation of Rainfall with Area ....................................................................... 2-11

2.5.4 Rainfall Depth-Area-Duration Curves ............................................................. 2-13

2.5.5 Areal Reduction Factors ................................................................................ 2-14

2.5.6 Variation of Rainfall with Time ....................................................................... 2-15

2.5.7 Rainfall Intensity-Duration-Frequency Curves ................................................. 2-16

2.5.8 Estimation of PMP ........................................................................................ 2-23

2.5.8.1 The Storm Maximisation Method ..................................................... 2-23

2.5.8.2 Example of PMP Estimation by Storm Maximisation Method ............... 2-29

2.5.8.3 The Statistical Method .................................................................... 2-33

2.5.8.4 Example of PMP Derived Using Statistical Method ............................. 2-36

2.5.8.5 Temporal and Spatial Distribution of PMP ......................................... 2-38

REFERENCE ........................................................................................................................ 2-39


2-ii March 2009

List of Tables

Table Description page

2. 1 Rainfall Temporal Patterns – West Coast of Peninsular Malaysia 2-16

2.2 Rainfall Temporal Patterns – East Coast of Peninsular Malaysia 2-16

2.3 Coefficients of IDF Equations for Various Towns in Malaysia 2-17

2.4 Precipitable Water (in mm) as a Function of1,000 mb DPT (oC) 2-24

2.5 Maximum Recorded Rainfalls in Peninsular Malaysia 2-25

2.6 Historical Storms Selected for PMP Estimation 2-29

2.7 Computation of Equivalent 12-hr DPT at Mean Sea Level 2-29

2.8 Estimated Precipitable Water Above The Station Level Given the DPT 2-30

2.9 Storm Maximisation Factors Derived for the Various Storm Events 2-30

2.10 PMPs Derived from the Various Historical Storms 2-31

2.11 Complete Tabulation of PMP Estimation Using Storm Maximisation 2-32

2.12 24- hour Rainfall Statistics 2-36

2.13 Determination of Adjustment Factors f1 to f4 2-36

2.14 Computed Statistical PMP 2-37

2.15 Temporal Rainfall Distributions of a 24-hour Rainstorm 2-39

List of Figures Figure Description Page 2.1 Standard 203 mm Diameter Rain Gauge 2-4

2.2 Measuring Cylinder 2-5

2.3 Standard Rainfall Station with Manual Gauge 2-6

2.4 Standard Rainfall Station with Weekly Rainfall Recorder 2-7

2.5 Standard Rainfall Station with Long-Term Rainfall Recorder 2-7

2.6 Variation of gauge catch with height for a given set of wind conditions as report by Symons (1881) 2-8

2.7 Expected Undercatch due to Wind 2-9

2.8 Double Mass Plot of Station A Against Stations B, C and D 2-11

2.9 An Example of DAD Curves 2-14

2.10 Areal Reduction Factor Curves of USWB 2-15

2.11 Intensity-duration-Frequency Curves of Wilayah Persekutuan 2-23

2.12 Pseudo-adiabatic Chart for Equivalent DPT at MSL (1000 mb atms pressure) 2-24

2.13 Maximum 24-hour Persistent Dew Point Temperature in Peninsular Malaysia 2-27

2.14 Maximum 24-hour Persistent Dew Point Temperature in East Malaysia 2-28


March 2009 2-iii

Figure Description Page

2.15 Km as a Function of Rainfall Duration and Mean Annual Series (after Hershfield) 2-33

2.16 f1 – the Adjustment Factor for Xn for to cater for Maximum Observed Rainfall 2-34

2.17 f3 – the Adjustment factor for Sn to cater for Maximum Observed Rainfall 2-34

2.18 f3 and f4, Factors to Adjust Xn and Sn Based on Length of Records 2-35

2.19 f5 Factor to to adjust PMP based on number of readings made over fixed interval records 2-35

2.20 Comparison of Various Temporal Rainfall Distributions 2-38


2-iv March 2009

(This page is intentionally blank)


March 2009 2-1

2 PRECIPITATION

Precipitation is water falling from the atmosphere to the ground and precipitation can take the form of rain, dew, snow, sleet or hailstone. Of concern to DID is rainfall and precipitation is referred to, it is rainfall most of the time. Rainfall is important to DID as projects such as flood mitigation, irrigation, drainage and dams, rainfall is critical in deciding their size, capacity, configuration and complexity. The average annual rainfall (AAR) in Peninsular Malaysia is 2500 mm, it is higher (about 3500 mm) in East Malaysia (Sarawak and Sabah). These are averaged figures and actual rainfall varies with area and time. Kuala Pilah and Perlis receive the lowest (1800 mm) AAR while Besut and Taiping receive the highest (4000 mm). The seasonal distribution of rainfall also varies from place to place. While Besut receives the highest AAR, most of the heavy rain occurs during the North East Monsoon months of November and December which is bad as far as flooding is concern and in the case of Taiping, the higher than normal annual rainfall is more evenly distributed throughout the year which is good for water resources development. Kuala Pilah receives low AAR but it does not have a distinct drought season experienced in Kedah and Perlis during January and February. Accordingly agricultural practices have evolved to meet the areal and temporal distribution of rainfall. In Kedah-Perlis area for instance, the harvesting season is timed such that the dry season of January and February is the harvesting season when dry weather is beneficial. The monsoon season brings heavy rain during November and December which would be a good time for pre-saturation of the paddy areas and that is if the floods brought about by the monsoon does not damage the paddy areas. Rainfall bring beneficial water resources but also causes flood when it comes pouring down with intensity so high that rivers and urban areas get flooded.

DID was initially an engineering branch under the Agriculture Ministry and was focused on designing irrigation and drainage systems for agricultural areas. Agriculture depends to a great extent on direct rainfall falling over the cropped areas for its crop water requirements and irrigation is meant to supplement crop water requirements when rainfall does not occur over extended periods of time. Hence the need to study rainfall, its amount and its distribution and to do this DID has set up a network of rainfall stations all over the country. Rainfall is also an important parameter in the study of water resources development for irrigation.

Besides irrigation DID also designs drainage systems initially for the agricultural areas but eventually extended the scope to cover urban drainage and flood mitigation. Rainfall is again an important parameter in designing urban drainage and flood mitigation projects. Under urban drainage, short term rainfall intensity data is important and for large river basin flood studies the temporal and areal distribution of rainfall must be considered. 2.1 MEASUREMENT OF RAINFALL Rainfall is measured using a raingauge installed at allocation of interest. Rainfall collected at this point would represent the rainfall over the area around the raingauge. DID has been recording point rainfall data for more than 40 years using rain gauges. For many years rainfall data were manually read by technicians who visited the raingauge stations regularly to measure the rainfall accumulated since the last visit. The rainfall is manually read and we refer them as non-recording raingauges. These raingauges are gradually replaced or supplemented with automatic recording raingauges so that variations in rainfall intensity can also be recorded. In an automatic recording raingauge there are recording devices added to register rainfall and its time of occurrence.


2-2 March 2009

2.1.1 Non Recording Raingauge Two types of non-recording raingauges are used by DID.

o 203 mm diameter raingauge and they are referred to as manual gauges and readings are taken daily or weekly

o 127 mm diameter raingauge with capacity to collect 4000 mm of rainfall and they are used as check gauge and are read monthly. This is usually installed in stations located in remote areas.

The standard rain gauge, shown in Figure 2.1, has a 203mm or 127mm diameter receiver cap on top to catch and funnel the rain into a can. The receiving funnel has a knife edge to catch rain falling precisely in the surface area of 203 mm or 127 mm diameter. Measurements are by pouring the rainwater collected in the raingauge into a measuring cylinder. The standard measuring cylinder (see Figure 2.2) is about 381mm high and has an internal diameter of 76.2mm. It is developed by DID for use with the 203 mm diameter raingauge. The cylinder is graduated in 1 and 0.5mm and is intended to read directly the depth of the rain collected without any need to apply any factors. The cylinder has a convex base to allow reading of rainfall less than 0.5mm. The level of water in the measuring cylinder is the level of the bottom of the meniscus. The raingauge is usually installed at a standard height of 1350m above the ground and has a windshield to reduce the effect of wind turbulence from affecting the catch of the raingauge. Figure 2.3 shows the standard layout of a DID rainfall station equipped with non-recording raingauge (or normally referred to as a manual rainfall station)

2.1.2 Recording Rain Gauges Daily read rainfall data do not provide information on the temporal distribution of the rain and it does not give details like how intense was the rainfall over short periods of time, information critical to design of urban drainage systems and to flood simulation. The tipping bucket type of rainfall recorder solves the problem of recording rainfall intensity and temporal distribution and DID’s rainfall stations were gradually replaced with this type of recorder. The description of the tipping bucket recorder is given in detail in the Hydrology Manual - Revised and Updated 1988. Tipping bucket recorder has a bucket system that catches rainfall and once rainfall amount exceeds a certain value (DID adopts 0.2mm or 0.5mm tipping bucket), the weight of the rainfall in the bucket causes it to tip. In the case of a 0.5mm tipping bucket, the bucket system collects 0.5mm rainfall and it tips and empties the bucket and waits for the next 0.5 mm of rain. Likewise the 0.2mm tipping bucket tips when rainfall collected reaches 0.2mm. Each time it tips the time is recorded. So if a series of tips is recorded, the total amount of rain that occurred can be computed and the tipping rate gives the intensity of rainfall. In the 80s, the tipping was recorded on paper charts but this soon gave way to digital recordings on memory chips. Paper charts have to be digitized to convert it to digital form and this is a time consuming effort. The digital recordings have the advantage that data is already in digital form.

Figure 2.4 shows the standard layout of a weekly recording rainfall station. The weekly recording rainfall station earns its name during the early days of automatic recording stations when the rainfall tips were recorded on paper charts. The weekly recording stations have charts which lasts one week.


March 2009 2-3

To record more than one week (this is necessary for remote stations with difficult access, for example the rainfall station at Gunung Gagau, Kelantan which is accessed by helicopter for maintenance and data downloading) there are recorders with long paper scrolls to last months of data recording. With digital recordings on memory chips the differentiation between weekly and long term recording equipments is no longer there as storage of data is no longer a constraint and all automatic recorders are now long term recorders. There is some difference still maintained between weekly and long term rainfall stations. All automatic recording stations have a manual station acting as a check gauge. In the case of a weekly rainfall station (Figure 2.4) the check gauge has a smaller capacity. The long-term recording station (Figure 2.5) has a large storage gauge as a check gauge.

2.2 SITING OF RAINFALL STATIONS There are several conditions to consider when locating rainfall stations and these are mentioned in detailed in DID’s hydrological manual –Revised and Updated 1988. The rainfall station must be located such that the rainfall collected is representative of the rainfall of the area of interest. Basically, the rain gauge must not be covered by trees and the guideline is the distance, d of the rain gauge must at least be 4 times the height, h of the tree. The same ruling goes for building, structures, etc. in the vicinity of the rain gauge. In practice we have to allow a more generous distance as trees grows with time. Ideally the “d ≥ 4h” exposure criteria should be met but in practice, it is sometimes not easy to get a suitable site with the exposure requirements. In some cases the exposure “h ≥ 2h” are accepted. There are other considerations besides the “4h” criteria such as availability of land for the station (need to acquire land for the station), easy access to the station (near a road), security from vandalism (in a school compound, in a plantation office compound), station not subjected to floods, etc. Another consideration would be the availability of readers to regularly read the rain gauge especially for the manually stations. However, DID is gradually converting the rainfall stations from manual stations to automatic recording stations. However, so far experience shows that the recording stations are still do need human input as the recording mechanism do malfunction and there is still a need to read the check gauge results to cross-check readings, a part of DID’s quality assurance and quality control measure. For accurate determination of rainfall over a catchment, the coverage of rainfall stations is important. If is not feasible economically to have sufficient density of rainfall stations to accurately the measure area variation of localised thunderstorms often seen as the main cause of flash floods in urban catchments. The rainfall stations network of DID is planned based on the recommendations of the World Meteorological Organisation (WMO)


2-4 March 2009

Figure 2.1 Standard 203 mm Diameter Rain Gauge

COLLECTINGFUNNEL

RIM WELDED ON TO TRAP WATER

COLLECTING CAN

POURING CAN

381 mm

160 mm

126 mm Ø

203 mm Ø

£ SHARP EDGE

U TUBE TO TRAP WATER


March 2009 2-5

Figure 2.2 Measuring Cylinder

Depth of rainfall is read directly and the correct level is at the bottom of the meniscus


2-6 March 2009

Figure 2.3 Standard Rainfall Station with Manual Gauge

(a) Plan View

(b) Isometric View


March 2009 2-7

Figure 2.4 Standard Rainfall Station with Weekly Rainfall Recorder

Figure 2.5 Standard Rainfall Station with Long-Term Rainfall Recorder 2.3 ERRORS IN MEASUREMENT

Several sources of measurement error is associated with the raingauge such as:

• Height of gauge above the ground level

• Wind

• Instrument mechanism – tipping bucket size


2-8 March 2009

It was demonstrated that a raingauge 30 feet (about 9m) above the ground level caught 80% of the rain and 50 feet above ground level caught 50% of the rain

Figure 2.6 Variation of gauge catch with height for a given set of wind conditions as report by Symons (1881)

(source: Curtis and Burnash 1996) That rainfall catch reduces with height is actually due to another factor - wind or turbulent airflow around the gauges when strong wind occurs.

0

1

2

3

4

5

6

86889092949698100

Gau

ge Height (m)

Percent of Catch at 2" Elevation


March 2009 2-9

Figure 2.7 Expected Undercatch due to Wind (source: Curtis and Burnash 1996)

The other factor is related to the tipping bucket mechanism. It has been shown by researchers that the tipping bucket raingauge underestimates higher intensity rainfall because of rainwater during the tipping movement is not captured by the recorder. Marsalek 1981 reported that the loss is in the range of 10 to 15 % for rainfall intensity greater than 200 mm/h. If tipping movements lead to loss of rainfall records then smaller bucket size leading to more tip movements for the same storm will lead to more under reporting of rainfall.

Size of tipping bucket also affects the rainfall recorded. At the end of a storm the remaining water in the tipping bucket evaporates before the next tip. So in this sense a larger tipping bucket induces more error.

2.4 WIND SHIELDS

Experiments have been carried out by the Hydrology Division to study the impact of DID’s standard wind shield on rainfall catch since wind turbulence is the major factor affecting the catch of rainfall by raingauges. However, the results show that the undercatch is only 1% for raingauge without windshield for raingauge installed at the standard DID raingauge height of 1350 mm.

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45

Height (m)

Velocity (kph)

24 km/h Rain Gauge 15% undercatch

Velocity Profile

Ground Conditions: Mowed grass and

Wet Soil

12% undercatch

Relative positions of 2 main rain gauges in a wind profile with winds of 24 kph at the height of raingauge

Velocity ( km/h)


2-10 March 2009

2.5 ANALYSIS OF RAINFALL DATA

2.5.1 Records and their interpretation

Rainfall data are usually recorded as cumulative or incremental data.

• Cumulative data – the data represents the cumulative value of rainfall measured or calculated up to a given instant of time

• Incremental data – the value represents the incremental value of a variable over a time interval ∆t

In DID’s hydrological database, rainfall is stored as incremental data. A further variation in data recording is the time interval for incremental data can be regular or irregular. In the case of DID’s hydrological database, the incremental rainfall data has irregular time intervals

The reason for irregular time interval is that it reduces data storage space for observations such as rainfall. The nature of rainfall is such that there will be many days without rainfall and if it rains in any of the days, more often the rain only lasts a few hours within the day. By adopting regular time intervals then the question would be what time interval? If the interval is too short, for instance, 5 minutes then for days when there are no rain there will be a lot of 5 minute intervals records with zero rainfall. If the regular time interval is too long then the database would miss out short duration intensity records. But with irregular time interval it can be set such that each time the incremental rainfall exceeds 0.5mm the data is recorded. Very detailed record of temporal distribution of rainfall is possible during very intense storm. If there is no rainfall for 2 weeks nothing is recorded. This is the advantage of irregular time interval recording. Incidentally this also how the tipping bucket recorder records rainfall, i.e. at irregular time intervals.

Another issue with rainfall data storage is the treatment of missing data. In DID’s hydrological database missing data is given a code “GAP”. So the value of rainfall during a particular interval is assigned the value “GAP” it means that between the interval there is no records. It is important to differentiate between an interval with zero rainfall and an interval with no data (“GAP”)

When extracting data for analyses, the user must be aware of the various types of rainfall stations. To get rainfall intensities less than 24 hours, only data from automatic recording rainfall stations can be used. If the user specifies a 3-hour maximum intensity data from a manual station, the data retrieval software merely interpolates the intensity from a daily read database and the results will be erroneous.

DID has longer records of manually read rainfall (daily data) records and it is not possible to ignore the information and statistics that could be derived from these manually read records. Automatic recording stations may be able to give more detail short duration intensity information but being introduced more recently, the records are not long enough and moreover during the early days of automatic rainfall recording there were a lot of problems with regard to instrument reliability.

2.5.2 Adjustment and Interpolation of Observations

As discussed above, the tipping bucket recorder has a tendency to record less rainfall due to loss during the tipping action and the evaporation of rainfall in the tipping bucket. In many DID’s rainfall stations there is another raingauge that is used as a check gauge. The check gauge does not use any tipping bucket mechanism. It stores all the rainfall collected and the total rainfall can be measured using a measuring cylinder. The check gauge data is used to adjust the tipping bucket rainfall data.


March 2009 2-11

There is the issue of rainfall records not being consistent because through the years trees in the surrounding may have grown or buildings constructed affecting the catch. The double mass analysis is a method recommended for detecting such changes and for correcting the data.

Figure 2.8 shows a double mass plot of station A against Stations B, C and D. Station A is the station whose data is being investigated because the changing conditions have resulted in changes in catch. Stations B, C and D are surrounding stations whose data are deemed consistent through the years. The y-axis is the cumulative annual rainfall of Station A. The x-axis is the cumulative of the average annual rainfall of B, C and D. The plot detects that a change in catch of station A occurs in the year 1990 and that the rainfall at A from 1991 to 1996 must be adjusted to bring the annual cumulative rainfall to the dotted line.

Figure 2.8 Double Mass Plot of Station A Against Stations B, C and D 2.5.3 Variation of Rainfall with Area

Rainfall varies with area and several methods of estimating areal rainfall are described below.

Arithmetic Mean Method

This is the simplest method for determining areal average rainfall over a catchment. The computation of arithmetic mean is shown below.

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

Cumulative Rainfall (mm) Average of Stations B, C and D

Cumulative Ra

infall (m

m) o

f Statio

n A

Actual double mass plot To adjust rainfall at Station A to this line


2-12 March 2009

P= 1

N∑ Pi

Ni=1 (2.1)

P= 10+20+30

3=20

Thiessen polygon method

The assumption in the Thiessen Polygon Method is that any point in the watershed receives the same amount of rainfall as that at the nearest gauge

p= 1

A∑ Ai

Ni=1 P

i (2.2)

P = 10 x 17+20 x 13+30 x 3565

=22.8 mm

For the storm of interest over the catchment shown the following point rainfalls were recorded

o P1 = 10 mm o P2 = 20 mm o P3 = 30 mm

The arithmetic mean of the rainfall P would be

• Steps in Thiessen polygon method 1. Draw lines joining adjacent gages 2. Draw perpendicular bisectors to the

lines created in step 1 3. Extend the lines created in step 2 in

both directions to form representative areas for gauges

4. Compute representative area Ai for each gauge

5. Compute the areal average using the following formula


March 2009 2-13

Isohyetal method

If many point rainfall data are or if radar raingauge data is available it is possible to plot a isohyetal map of the variation of rainfall over the catchment during a particular time interval. The isohyetal method of estimating areal rain fall is presented below.

P= 1

A ∑ Ai

Ni=1 Pi

p = 5 x 5+30 x 15 x 15 x 25+15 x 3565

= 21.2mm

2.5.4 Rainfall Depth-Area-Duration Curves

Point rainfall cannot be used to represent areal rainfall as the area of interest increases. In natural occurrence of a storm, there is a storm centre where the rainfall is highest and the depth of storm decreases with distance from the storm centre.

A typical Depth-Area-Duration (DAD) curve is shown in Figure 2.9.

– Compute area between each pair of adjacent isohyets (Ai)

– Compute average precipitation for each pair of adjacent isohyets (pi)

– Compute areal average using the following formula


2-14 March 2009

Figure 2.9 An Example of DAD Curves

The Depth-Area-Duration (DAD) curves show the maximum depth of precipitation occurring over various size of catchment area for various storm durations. The curves can be derived using (1) the mass curve method or (2) the isohyetal method. DAD curves are used in estimation of areal PMP which is described in further detail in section 2.6.9.

2.5.5 Areal Reduction Factors

A variation of the DAD curves is the areal reduction factor (ARF) curves.

In many hydrological studies, the interest is to estimate the extreme areal rainfall over a catchment (e.g the 20-, or 100-year ARI storm). Many do not have the resources to study areal distribution of design storm in detail and would merely depend on point rainfall records to estimate areal rainfall. There are also intensity-duration-frequency curves (IDF curves) and design rainfall estimation estimation procedures (HP1 and HP26). However, these design rainfall estimates are derived based on point rainfall data and applying the estimates for large catchment will tend to overestimate the rainfall. Rainfall does not occur evenly over large catchment and this is particularly true for thunderstorm type of rainfall which tends to be very localized. The other factor governing the areal distribution of storm is the duration of the storm. Over a short duration, the areal variability of storm is high. The longer the time duration considered the more even the distribution of storm depth. Hence the shape of ARF curves (see Figure 2.10)

Figure 2.10 shows the ARF curves taken from US Weather Bureau. An attempt was made to derive local ARFs. Details of ARFs obtained are presented in DID’s Water Resources Publication No.17.


March 2009 2-15

Figure 2.10 Areal Reduction Factor Curves of USWB

2.5.6 Variation of Rainfall with Time

HP1 and HP23 recommend standard temporal rainfall patterns (see Tables 2.1 and 2.2) for various regions in the country. Design rainfall obtained using frequency analysis or read from intensity duration frequency curves gives the total rainfall amount for a given storm duration. The above temporal patterns can be applied to distribute the design rainfall over the storm duration. Another approach would be to distribute the design rainfall according to the temporal pattern of a selected storm event recorded by a nearby rainfall station. The selected storm event would be a major storm of significant intensity and with the same storm duration. A third method of distributing the design rainfall over the storm duration would be to use the synthetic bell-shaped temporal distribution derived from the intensity-duration-frequency (IDF) curve. Being derived from the IDF curve, it contains within it the design storm of shorter storm duration and is often used to distribute the probable maximum precipitation (PMP) for derivation of probable maximum flood (PMF).


2-16 March 2009

Table 2.1 Rainfall Temporal Patterns – West Coast of Peninsular Malaysia

Duration No. of Fraction of Rainfall in Each Time Period (min) Time

Periods 10 2 0.57 0.43 - - - - - - - - - - 15 3 0.32 0.5 0.18 - - - - - - - - - 30 6 0.16 0.25 0.33 0.09 0.11 0.06 - - - - - - 60 12 0.039 0.07 0.168 0.12 0.232 0.101 0.089 0.057 0.048 0.031 0.028 0.017120 8 0.03 0.119 0.31 0.208 0.09 0.119 0.094 0.03 - - - - 180 6 0.06 0.22 0.34 0.22 0.12 0.04 - - - - - - 360 6 0.32 0.41 0.11 0.08 0.05 0.03 - - - - - -

Table 2.2 Rainfall Temporal Patterns – East Coast of Peninsular Malaysia

Duration No. of Fraction of Rainfall in Each Time Period (min) Time

Periods 10 2 0.57 0.43 - - - - - - - - - - 15 3 0.32 0.5 0.18 - - - - - - - - - 30 6 0.16 0.25 0.33 0.09 0.11 0.06 - - - - - - 60 12 0.039 0.07 0.168 0.12 0.232 0.101 0.089 0.057 0.048 0.031 0.028 0.017120 8 0.03 0.119 0.31 0.208 0.09 0.119 0.094 0.03 - - - - 180 6 0.19 0.23 0.19 0.16 0.13 0.1 - - - - - - 360 6 0.29 0.2 0.16 0.12 0.14 0.09 - - - - - -

2.5.7 Rainfall Intensity-Duration-Frequency Curves

Intensity-Duration-Frequency (IDF) curves are sets of curves showing the relationship between the intensity (I) and duration (D) of a rainfall event with exceedance frequency (F). Figure 2.11 shows the rainfall intensity-duration-frequency IDF curves derived for Wilayah Persekutuan. From this curve the user can determine the design rainfall intensity given the storm duration and given the frequency of occurrence (the average recurrence interval or ARI)

DID has developed, for 35 main cities/towns in Malaysia, polynomial equations of IDF curves of the form:

ln( RI t ) = a + b ln( t ) + c (ln( t )) 2 + d (ln( t ))3 (2.3) where, RI t = the average rainfall intensity (mm/hr) for ARI = R years and Duration = t minutes R = average return interval (years) t = duration (minutes) a to d are fitting constants for ARI = R years


March 2009 2-17

Table 2.3 Coefficients of IDF Equations for Various Towns in Malaysia

Location ARI (year)

Coefficients of IDF Equation a b C d

Kangar

2 4.6800 0.4719 -0.1915 0.0093 5 5.7949 -0.1944 -0.0413 -0.0008 10 6.5896 -0.6048 0.0445 -0.0064 20 6.8710 -0.6670 0.0478 -0.0059 50 7.1137 -0.7419 0.0621 -0.0067 100 6.5715 -0.2462 -0.0518 0.0016

Alor Setar

2 5.6790 -0.0276 -0.0993 0.0033 5 4.9709 0.5460 -0.2176 0.0113 10 5.6422 0.1575 -0.1329 0.0056 20 5.8203 0.1093 -0.1248 0.0053 50 5.7420 0.2273 -0.1481 0.0068 100 6.3202 -0.0778 -0.0849 0.0026

Pulau Pinang

2 4.5140 0.6729 -0.2311 0.0118 5 3.9599 1.1284 -0.3240 0.0180 10 3.7277 1.4393 -0.4023 0.0241 20 3.3255 1.7689 -0.4703 0.0286 50 2.8429 2.1456 -0.5469 0.0335 100 2.7512 2.2417 -0.5610 0.0341

Ipoh

2 5.2244 0.3853 -0.1970 0.0100 5 5.0007 0.6149 -0.2406 0.0127 10 5.0707 0.6515 -0.2522 0.0138 20 5.1150 0.6895 -0.2631 0.0147 50 4.9627 0.8489 -0.2966 0.0169 100 5.1068 0.8168 -0.2905 0.0165

Bagan Serai

2 4.1689 0.8160 -0.2726 0.0149 5 4.7867 0.4919 -0.1993 0.0099 10 5.2760 0.2436 -0.1436 0.0059 20 5.6661 0.0329 -0.0944 0.0024 50 5.3431 0.3538 -0.1686 0.0078 100 5.3299 0.4357 -0.1857 0.0089

Teluk Intan

2 5.6134 -0.1209 -0.0651 0.0000 5 6.1025 -0.2240 -0.0484 -0.0008 10 6.3160 -0.2756 -0.0390 -0.0012 20 6.3504 -0.2498 -0.0377 -0.0016 50 6.7638 -0.4595 0.0094 -0.0050 100 6.7375 -0.3572 -0.0070 -0.0043


2-18 March 2009

Table 2.3 Coefficients of IDF Equations for Various Towns in Malaysia (contd)

Location ARI (year)

Coefficients of IDF Equation a b c d

Kuala Kangsar

2 4.2114 0.9483 -0.3154 0.0179 5 4.7986 0.5803 -0.2202 0.0107 10 5.3916 0.2993 -0.1640 0.0071 20 5.7854 0.1175 -0.1244 0.0044 50 6.5736 -0.2903 -0.0482 0.0000 100 6.0681 0.1478 -0.1435 0.0065

Setiawan

2 5.0790 0.3724 -0.1796 0.0081 5 5.2320 0.3330 -0.1635 0.0068 10 5.5868 0.0964 -0.1014 0.0021 20 5.5294 0.2189 -0.1349 0.0051 50 5.2993 0.4270 -0.1780 0.0082 100 5.5575 0.3005 -0.1465 0.0058

Kuala Kubu Bahru

2 4.2095 0.5056 -0.1551 0.0044 5 5.1943 -0.0350 -0.0392 -0.0034 10 5.5074 -0.1637 -0.0116 -0.0053 20 5.6772 -0.1562 -0.0229 -0.0040 50 6.0934 -0.3710 0.0239 -0.0073 100 6.3094 -0.4087 0.0229 -0.0068

Kuala Lumpur

2 5.3255 0.1806 -0.1322 0.0047 5 5.1086 0.5037 -0.2155 0.0112 10 4.9696 0.6796 -0.2584 0.0147 20 4.9781 0.7533 -0.2796 0.0166 50 4.8047 0.9399 -0.3218 0.0197 100 5.0064 0.8709 -0.3070 0.0186

Malacca

2 3.7091 1.1622 -0.3289 0.0176 5 4.3987 0.7725 -0.2381 0.0112 10 4.9930 0.4661 -0.1740 0.0069 20 5.0856 0.5048 -0.1875 0.0082 50 4.8506 0.7398 -0.2388 0.0117 100 5.3796 0.4628 -0.1826 0.0081

Seremban

2 5.2565 0.0719 -0.1306 0.0065 5 5.4663 0.0586 -0.1269 0.0062 10 6.1240 -0.2191 -0.0820 0.0039 20 6.3733 -0.2451 -0.0888 0.0051 50 6.9932 -0.5087 -0.0479 0.0031 100 7.0782 -0.4277 -0.0731 0.0051


March 2009 2-19


Location ARI (year)


Kuala Pilah

2 3.9982 0.9722 -0.3215 0.0185 5 3.7967 1.2904 -0.4012 0.0247 10 4.5287 0.8474 -0.3008 0.0175 20 4.9287 0.6897 -0.2753 0.0163 50 4.7768 0.8716 -0.3158 0.0191 100 4.6588 1.0163 -0.3471 0.0213

Kluang

2 4.5860 0.7083 -0.2761 0.0170 5 5.0571 0.4815 -0.2220 0.0133 10 5.2665 0.4284 -0.2131 0.0129 20 5.4813 0.3471 -0.1945 0.0116 50 5.8808 0.1412 -0.1498 0.0086 100 6.3369 -0.0789 -0.1066 0.0059

Mersing

2 5.1028 0.2883 -0.1627 0.0095 5 5.7048 -0.0635 -0.0771 0.0036 10 5.8489 -0.0890 -0.0705 0.0032 20 4.8420 0.7395 -0.2579 0.0165 50 6.2257 -0.1499 -0.0631 0.0032 100 6.7796 -0.4104 -0.0160 0.0005

Batu Pahat

2 4.5023 0.6159 -0.2289 0.0119 5 4.9886 0.3883 -0.1769 0.0085 10 5.2470 0.2916 -0.1575 0.0074 20 5.7407 0.0204 -0.0979 0.0032 50 6.2276 -0.2278 -0.0474 0.0000 100 6.5443 -0.3840 -0.0135 -0.0022

Johor Bahru

2 3.8645 1.1150 -0.3272 0.0182 5 4.3251 1.0147 -0.3308 0.0205 10 4.4896 0.9971 -0.3279 0.0205 20 4.7656 0.8922 -0.3060 0.0192 50 4.5463 1.1612 -0.3758 0.0249 100 5.0532 0.8998 -0.3222 0.0215

Segamat

2 3.0293 1.4428 -0.3924 0.0232 5 4.2804 0.9393 -0.3161 0.0200 10 6.2961 -0.1466 -0.1145 0.0080 20 7.3616 -0.6982 -0.0131 0.0021 50 7.4417 -0.6247 -0.0364 0.0041 100 8.1159 -0.9379 0.0176 0.0013


2-20 March 2009


Location ARI (year)


Raub

2 4.3716 0.3725 -0.1274 0.0026 5 4.5461 0.4017 -0.1348 0.0036 10 5.4226 -0.1521 -0.0063 -0.0056 20 5.2525 0.0125 -0.0371 -0.0035 50 4.8654 0.3420 -0.1058 0.0012 100 5.1818 0.2173 -0.0834 0.0001

Cameron Highland

2 4.9396 0.2645 -0.1638 0.0082 5 4.6471 0.4968 -0.2002 0.0099 10 4.3258 0.7684 -0.2549 0.0134 20 4.8178 0.5093 -0.2022 0.0100 50 5.3234 0.2213 -0.1402 0.0059 100 5.0166 0.4675 -0.1887 0.0089

Kuantan

2 5.1899 0.2562 -0.1612 0.0096 5 4.7566 0.6589 -0.2529 0.0167 10 4.3754 0.9634 -0.3068 0.0198 20 4.8517 0.7649 -0.2697 0.0176 50 5.0350 0.7267 -0.2589 0.0167 100 5.2158 0.6752 -0.2450 0.0155

Temerloh

2 4.6023 0.4622 -0.1729 0.0066 5 5.3044 0.0115 -0.0590 -0.0019 10 4.5881 0.5465 -0.1646 0.0049 20 4.4378 0.7118 -0.1960 0.0068 50 4.4823 0.8403 -0.2288 0.0095 100 4.5261 0.7210 -0.1988 0.0071

Kuala Dungun

2 5.2577 0.0572 -0.1091 0.0057 5 5.5077 -0.0310 -0.0899 0.0050 10 5.4881 0.0698 -0.1169 0.0074 20 5.6842 -0.0393 -0.0862 0.0051 50 5.5773 0.1111 -0.1231 0.0081 100 6.1013 -0.1960 -0.0557 0.0035

Kuala Terengganu

2 4.6684 0.3966 -0.1700 0.0096 5 4.4916 0.6583 -0.2292 0.0143 10 5.2985 0.2024 -0.1380 0.0089 20 5.8299 -0.0935 -0.0739 0.0046 50 6.1694 -0.2513 -0.0382 0.0021 100 6.1524 -0.1630 -0.0575 0.0035


March 2009 2-21


Location ARI (year)


Kota Bahru

2 5.4683 0.0499 -0.1171 0.0070 5 5.7507 -0.0132 -0.1117 0.0078 10 5.2497 0.4280 -0.2033 0.0139 20 5.4724 0.3591 -0.1810 0.0119 50 5.3578 0.5094 -0.2056 0.0131 100 5.0646 0.7917 -0.2583 0.0161

Gua Musang

2 4.6132 0.6009 -0.2250 0.0114 5 3.8834 1.2174 -0.3624 0.0213 10 4.6080 0.8347 -0.2848 0.0161 20 4.7584 0.7946 -0.2749 0.0154 50 4.6406 0.9382 -0.3059 0.0176 100 4.6734 0.9782 -0.3152 0.0183

Kota Kinabalu

2 5.1968 0.0414 -0.0712 -0.0002 5 5.6093 -0.1034 -0.0359 -0.0027 10 5.9468 -0.2595 -0.0012 -0.0050 20 5.2150 0.3033 -0.1164 0.0026 50 5.1922 0.3652 -0.1224 0.0027

Sandakan

2 3.7427 1.2253 -0.3396 0.0191 5 4.9246 0.5151 -0.1886 0.0095 10 5.2728 0.3693 -0.1624 0.0083 20 4.9397 0.6675 -0.2292 0.0133 50 5.0022 0.6587 -0.2195 0.0123

Tawau

2 4.1091 0.6758 -0.2122 0.0093 5 3.1066 1.7041 -0.4717 0.0298 10 4.1419 1.1244 -0.3517 0.0220 20 4.4639 1.0439 -0.3427 0.0220

Kuamut

2 4.1878 0.9320 -0.3115 0.0183 5 3.7522 1.3976 -0.4086 0.0249 10 4.1594 1.2539 -0.3837 0.0236 20 3.8422 1.5659 -0.4505 0.0282 50 5.6274 0.3053 -0.1644 0.0079 100 6.3202 -0.0778 -0.0849 0.0026

Simanggang

2 4.3333 0.7773 -0.2644 0.0144 5 4.9834 0.4624 -0.1985 0.0100 10 5.6753 0.0623 -0.1097 0.0038 20 5.9006 -0.0189 -0.0922 0.0027


2-22 March 2009


Location ARI (year)


Sibu

2 3.0879 1.6430 -0.4472 0.0262 5 3.4519 1.4161 -0.3754 0.0200 10 3.6423 1.3388 -0.3509 0.0177 20 3.3170 1.5906 -0.3955 0.0202

Kapit

2 5.2707 0.1314 -0.0976 0.0025 5 5.5722 0.0563 -0.0919 0.0031 10 6.1060 -0.2520 -0.0253 -0.0012 20 6.0081 -0.1173 -0.0574 0.0014 50 6.2652 -0.2584 -0.0244 -0.0008

Kapit

2 3.2235 1.2714 -0.3268 0.0164 5 4.5416 0.2745 -0.0700 -0.0032 10 4.5184 0.2886 -0.0600 -0.0045 20 5.0785 -0.0820 0.0296 -0.0110

Kuching

2 5.1719 0.1558 -0.1093 0.0043 5 4.8825 0.3871 -0.1455 0.0068 10 5.1635 0.2268 -0.1039 0.0039 20 5.2479 0.2107 -0.0968 0.0035 50 5.2780 0.2240 -0.0932 0.0031

Miri

2 4.9302 0.2564 -0.1240 0.0038 5 5.8216 -0.2152 -0.0276 -0.0021 10 6.1841 -0.3856 0.0114 -0.0048 20 6.1591 -0.3188 0.0021 -0.0044 50 6.3582 -0.3823 0.0170 -0.0054


March 2009 2-23

Figure 2.11 Intensity-duration-Frequency Curves of Wilayah Persekutuan 2.5.8 Estimation of PMP

Probable maximum precipitation (PMP) is an estimated maximum rainfall that can ever occur and is thus an ultimate extreme event to be used for designing spillways of large dams. Failure of large dams are disastrous and thus the requirement that the spillway which is an overflow structure be designed to cater for such an extreme event. PMP is defined in the WMO Manual for Estimation of Probable Maximum Precipitation as:

"...the greatest depth of precipitation for a given duration meteorologically possible for a given size storm area at a particular location at a particular time of the year, with no allowance made for long-term climatic trends."

There are two commonly adopted methods of estimating PMP: (1) the storm maximization (hydro-meteorological) and transposition method and (2) the statistical (Hershfield) method.

2.5.8.1 The Storm Maximisation Method

The storm maximization method requires the identification of a major storm event. For this major storm event the following must be known:

• The storm depth Po (mm) • The storm duration (hours/days) • The maximum 12-hour dew point temperature (DPT) during the storm event (°C) • The elevation of the raingauge recording the major storm event (m. MSL)

Storm maximisation can be carried out to maximised the storm at its current location and the transposing the maximised storm to the location of interest.


2-24 March 2009

At the target location, the following must be known:

• The maximum 12-hour DPT (°C) • The general elevation of the catchment (m MSL)

Moisture maximization involves increasing the observed storm precipitation by a factor Fm., where

Fm = (Wpm / Wps) (2.4)

where Wps is precipitable water at maximum 12-hour DPT observed during the major storm event and Wpm is precipitable water at expected maximum DPT at the location of interest.

The precipitable water as a function of 1000 mb DPT is given in Table 2.4.

Table 2.4 Precipitable Water (in mm) as a Function of1,000 mb DPT (oC)

Height (m)

Precipitable Water 9in mm) at 1,000 mb Level for Equivalent Dew Point Tempertaure of

16 17 18 29 20 21 22 23 24 25 26 27 28 29 30 200 3 3 3 3 3 4 4 4 4 4 5 5 5 6 6

12,000# 37 40 44 48 52 57 63 68 74 81 88 96 105 114 123 # assumed altitude of the cloud surface. Since Table 2.4 only gives precipitable water at 1,000 mb atmospheric pressure which is the atmostpheric pressure at MSL (mean sea level), there is a need to convert to equivalent DPT if DPT is recorded at higher elevations. Figure 2.12 presents the conversion chart. As an example, a DPT of 20.8 °C recorded at 600m MSL lies exactly on the 23 °C line in Figure 2.12 and is equal to 23 °C at MSL

Figure 2.12 Pseudo-adiabatic Chart for Equivalent DPT at MSL (1000 mb atms pressure) Wps and Wpm can be obtained from Table 2.4, if the highest persisting 12-hour dew point at the location is known.


March 2009 2-25

The above factor Fm applies for storm maximization at the location of the observed storm. In practical applications, the observed storm is not available at the location of interest or the locally observed storms are events considered not extreme enough (i.e the precipitation depth is not high enough). Usually the observed storm is taken from another place. For many years PMP estimation

in Malaysia were based on the storm (29 Dec 1970 to 3 Jan 1971) in Mersing. Table 2.5 below shows the maximum recorded storms in Malaysia. If storms from moisture maximization is taken from another place then a transposition factor Ft is applied and Ft is given by:

Ft = (Wpt / Wpm) (2.5)

where, Wpm is precipitable water at maximum DPT at storm location and Wpt is precipitable water at maximum DPT at transposed location.

PMP is computed as follows:

PMP = Po Fm Ft (2.6)

Note that

Fm Ft = (Wpm / Wps). (Wpt / Wpm) = (Wpt / Wps) (2.7)

In the above the factor Ft merely accounts for the difference in maximum DPT between the storm location and the location of interest, i.e moisture adjustment for relocation. There is also the need to take into account elevation differences. Table 2.4 gives the precipitable water from MSL up to 200m height and up to 12000 m height.

Table 2.5 Maximum Recorded Rainfalls in Peninsular Malaysia


2-26 March 2009

NAHRIM has recently developed a standardized approach to PMP estimation in Peninsular Malaysia and the PMP. A country wide analyses have been carried out to maximize and transpose storms to all areas in Peninsular Malaysia and PMP isohyets have been prepared for PMPs of 1-day, 3-day and 5-day durations.

Maps (see Fig 2.13 and 2.14) showing the maximum DPT for Peninsular Malaysia and East Malaysia extracted from NAHRIM’s report is useful reference as it is an important parameter for storm maximisation.


March 2009 2-27

Figure 2.13 Maximum 24-hour Persistent Dew Point Temperature in Peninsular Malaysia (Source: NAHRIM 2007)


2-28 March 2009

Figure 2.14M

aximum

24-hour Persistent Dew

Point Temperature in East M

alaysia(Source: N

AHRIM

2007)


March 2009 2-29

2.5.8.2 Example of PMP Estimation by Storm Maximisation Method

The PMP for Timah Tasoh Dam in Perlis is to be estimated. Several selected historical storms for storm maximization were selected. The historical storms are as shown in Table 2.6

Table 2.6 Historical Storms Selected for PMP Estimation

Stations Air Tawar Mersing K Bahru K Krai Pdg Besar B'Worth

from 29-Dis-70 06-Dis-60 02-Jan-67 18-Nov-88 18-Nov-88 16-Sep-95

to 04-Jan-71 10-Dis-60 06-Jan-67 21-Nov-88 21-Nov-88 18-Sep-95

Observed Rainfall 24-hr rainfall (mm) 605 434 608 357 233 350 72-hr Rainfall (mm) 1143 843 1185 655 374 360

Moisture Adjustments Altitude of Stns (m) sea level 43.6 4.6 68.3 60 2.8 12-hr persistent DPT (deg C) during observed storm 23.3 23.9 22 22 22 23.5

Table 2.6 shows the selected 24-hour and 72-hour storms at Kg Air Tawar, Johor, Mersing, Kota Bahru, Padang Besar and Butterworth and the parameters to note are the recorded rainfall amount and the corresponding 12-hour persistent dew point temperature occurring during the storm events. Of interest too is the altitude of the station recording the dew point temperature.

The DPT was first converted to equivalent DPT at mean sea-level and for this purpose the chart shown in Figure 2.12 was used. The results are presented in Table 2.7.

Table 2.7 Computation of Equivalent 12-hr DPT at Mean Sea Level


Moisture Adjustments Altitude of Stns (m) sea level 43.6 4.6 68.3 60 2.8 12-hr persistent DPT (deg C)

observed storm 23.3 23.9 22 22 22 23.5 at sea level (from Fig 2.12) 23.3 24.1 22 22.3 22.2 23.5

Precipitable water was then computed and reference was made to Table 2.4. Table 2.4 indicates the precipitable water at 1000 mb which is at mean sea level to the height indicated. Only two heights are indicated in Table 2.4 and for these two heights namely 200 m and 12,000 m above mean sea level. It is assumed that 12,000 m above mean sea level is the level of the cloud surface and whatever precipitable water is water up to this level. The computed precipitable water are presented in Table 2.8


2-30 March 2009

Table 2.8 Estimated Precipitable Water Above The Station Level Given the DPT

During the Storm Event


Precipitable water (mm) (from Table 2.4)

above sea level 69.8 74.7 63 64.5 64 71 below station level 0 0.9 0 1.5 1 0 above station level (WPs) 69.8 73.8 63 63 63 71

The precipitable water for the various storm events must be compared to the precipitable water at the location of interest which is Timah Tasoh Dam Catchment. The precipitable water at Timah Tasoh Dam Catchment is maximum when the corresponding DPT is maximum and from Fig 2.13 it is indicated that the maximum 24 hour dew point temperature at Timah Tasoh Dam Catchment is 30 deg C. Another parameter of interest is the average elevation of Timah Tasoh Dam Catchment which is estimated to be 195m.

From Table 2.4, the precipitable water corresponding to 30 deg C at mean sea level is 123mm. But as the Timah Tasoh Dam Catchment level is about 200m, the precipitable water for the same dew point temperature up to level 200m must be deducted. From Table 2.4, the precipitable water up to level 200m is 6mm. Therefore the precipitable water WPt above Timah Tasoh catchment is

WPt = 123-6 = 117mm.

Therefore the Storm-Maximisation Factor is WPm/WPs for the respective stations . the computed factors are presented in Table 2.9.

Table 2.9 Storm Maximisation Factors Derived for the Various Storm Events

Stations Air Tawar Mersing K Bahru K Krai Pdg Besar B'Worth WPt 117 117 117 117 117 117 WPs 69.8 73.8 63 63 63 71 Moisture Adjustment and Transposition Factor WPt / WPs

1.67 1.58 1.85 1.85 1.85 1.64

The Point PMP estimated based on the various historical storms can then be estimated by multiplying the observed rainfall Po by WPt/WPs

PMP = Po x WPt/WPs (2.8)

Finally to convert the Point PMP to Catchment PMP, the ARF (Areal Reduction Factor) was applied. From Fig 2.10 the ARFs for a catchemt area of 191 km2 are:

• 0.94 for a 24-hour duration rainstorm • 0.96 for a 72-hour duration rainstorm


March 2009 2-31

The results of PMP computations are presented in Table 2.10

Table 2.10 PMPs Derived from the Various Historical Storms

Stations Air Tawar Mersing K Bahru K Krai Pdg Besar B'Worth PMP at Timah Tasoh Dam (mm)

24- hr Point PMP 1010 686 1125 660 431 574 24-hr ARF 0.94 0.94 0.94 0.94 0.94 0.94 24-hr Catchment PMP 950 645 1057 621 405 540 72-hr Point PMP 1909 1332 2192 1212 692 590 72- hr ARF 0.96 0.96 0.96 0.96 0.96 0.96 72-hr Catchment PMP 1832 1279 2105 1163 664 567

From Table 2.10 it can be seen that the maximum 24-hr catchment PMP is 1057 mm and the critical 72-hr catchment PMP is 2105 mm both derived from the Kota Bahru storm of 1967. The overall computation is presented in Table 2.10. It has been the practice to estimate PMP for many dam projects in the country based on the East Coast storms in particular the Mersing and Air Tawar storms and of late the Kota Bahru and Kuala Terengganu Storms (See Table 2.5) regardless of whether the nature of storm experienced in the target area (in this case – the Timah Tasoh catchment) is the same or not with the East Coast storms. The East Coast of Peninsular Malaysia experiences monsoonal rain due to rain brought by the north easterly wind. Timah Tasoh catchment is separated from the east coast by a high mountain range and the validity of the transposition may be questionable. However, storms from Padang Besar and Butterworth which lies on the same side of the main range as the Timah Tasoh Dam catchment is acceptable. If the Padang Besar and Butterworth storms are adopted for storm maximization and transposition the PMPs would be:

• the 24-hour catchment PMP = 540 mm and • the 72-hour catchment PMP = 664 mm


2-32 March 2009

Table 2.11 Complete Tabulation of PMP Estimation Using Storm Maximisation

Approach for Tamah Tasoh Dam

maximum persisting dew point at Timah Tasoh Dam Catchment(deg C) (from Fig 2.13)

30

Average elevation of Timah-Tasoh Catchment (m) 195 Dam catchment area (km2) 191 Areal Reduction Factor - ARF(from Fig. 2.10) 0.94 Precipitable water (mm)

Above sea level (Table 2.4) 123 Below EL 195m (Table 2.4) 6 Above Timah-Tasoh catchment (WPt) 117


From 29-Dis-70 06-Dis-60 02-Jan-67 18-Nov-88 18-Nov-88 16-Sep-95

To 04-Jan-71 10-Dis-60 06-Jan-67 21-Nov-88 21-Nov-88 18-Sep-95

Observed Rainfall 24-hr rainfall (mm) 605 434 608 357 233 350 72-hr Rainfall (mm) 1143 843 1185 655 374 360

Moisture Adjustments Altitude of Stns (m) sea level 43.6 4.6 68.3 60 2.8 12-hr persistent dew point (deg C)

observed storm 23.3 23.9 22 22 22 23.5 at sea level (from Fig 2.12) 23.3 24.1 22 22.3 22.2 23.5

Precipitable water (mm) (from Table 2.4)

above sea level 69.8 74.7 63 64.5 64 71 below station level 0 0.9 0 1.5 1 0 above station level (WPs) 69.8 73.8 63 63 63 71

Moisture Adjustment and Transposition Factor

1.67 1.58 1.85 1.85 1.85 1.64

PMP at Timah Tasoh Dam (mm)

24- hr Point PMP 1010 686 1125 660 431 574 24-hr ARF 0.94 0.94 0.94 0.94 0.94 0.94 24-hr Catchment PMP 950 645 1057 621 405 540 72-hr Point PMP 1909 1332 2192 1212 692 590 72- hr ARF 0.96 0.96 0.96 0.96 0.96 0.96 72-hr Catchment PMP 1832 1279 2105 1163 664 567


March 2009 2-33

2.5.8.3 The Statistical Method

The statistical method developed by Hershfield is a more simplified method and is adopted if the other data such as dew points and wind records are not available. The estimation of the statistical PMP is similar to the frequency factor approach (described in section 2.6.8) carried out to estimate statistical extreme precipitation such as the 100-year ARI precipitation. The equation is a follows:

Xt = Xn + Km . Sn (2.9)

Where:

• Km is the frequency factor whose value depends on the frequency distribution selected to fit the annual maximum rainfall series. Values of Km depends on storm duration and the mean annual maximum rainfall and can be obtained from Figure 2.14

• Xn, the mean of the annual maximum rainfall records • Sn, the standard deviation of the annual maximum rainfall data.

Figure 2.15 Km as a Function of Rainfall Duration and Mean Annual Series (after Hershfield)

Hershfield also developed the charts to facilitate determination of various adjustment factors in an attempt to correct bias in datasets for outliers, record length and manually read rainfall data.

• f1 : to adjust Xn based on length of records n and Xn-m/Xn (see Figure 2.16) • f2 : to adjust Sn based on length of records and Sn-m/Xn (see Figure 2.17) • f3 and f4: to adjust Xn and Sn based on length of records n (see Figure 2.18) • f5 : to adjust PMP based on number of readings made over fixed interval records (see Figure

2.19)

2-34

F

Figure 2.16 f

Figure 2.17

f1 – the Adjus

7 f3 – the Adj

Chapt

stment Facto

justment fact

ter 2 PRECIPIT

or for Xn for t

tor for Sn to

TATION

to cater for M

cater for Max

Maximum Ob

ximum Obse

M

bserved Rainf

erved Rainfal

March 2009

fall

l

March 2

Figure

2009

Figure 2

e 2.19 f5 Fac

2.18 f3 and f4

ctor to adjust

Chapt

4, Factors to

t PMP based

ter 2 PRECIPIT

Adjust Xn an

on number o

Length of Record (y

TATION

nd Sn Based

of readings m

years)

on Length o

made over fix

of Records

xed interval

2-35

records


2-36 March 2009

2.5.8.4 Example of PMP Derived Using Statistical Method

The example below shows the determination of statistical PMP.

To estimate the PMP at Timah Tasoh dam catchment , the 24-hour annual maximum rainfall time series were extracted for four rainfall stations namely

• Kaki Bukit Station (DID station 6602002) • Tasoh Station (DID station 6502003) • Kangar (MMS station 41614)

The relevant statistics for PMP estimation are shown in Table 2.12.

Table 2.12 24- hour Rainfall Statistics

Stations Kaki Bukit Tasoh Kangar A Setar Mean (mm) Xn 103.4 95.4 94.9 100.1 Maximum (mm) Xm 212 200 137.6 178.8 mean w/o max (mm) Xn-m 100.7 92.4 92.8 98.2 mean ratio Xn-m/Xn 0.97 0.97 0.98 0.98 std deviation (mm) Sn 30.8 33.4 21.3 27.1 SD w/o max (mm) Sn-m 26.1 28.7 19.5 24.5 SD ratio Sn-m/Sn 0.85 0.86 0.92 0.90

Mean ratio Xn-m/Xn and SD ratio Sn-m/Sn are required for the determination of adjustment factors f1, f2, f3 and f4 . The factors are obtained using charts in Figure 2.16, 2.17 and 2.18. Table 2.13 presents the adjustment factors and the adjusted Xn and Sn values.

Xn' = f1. f2. Xn (2.10)

Sn' = f3. f4. Sn (2.11)

Table 2.13 Determination of Adjustment Factors f1 to f4

Stations Kaki Bukit Tasoh Kangar A Setar length of records (years) n 42 36 21 42 Adjustment factors for mean

max obs rainfall (fig 2.15) f1 0.99 1 1.03 1 length of record (fig 2.17) f2 1 1.01 1.02 1 Adjusted Mean (mm) Xn' 102 96 100 100

Adjustment factors for SD max obs rainfall (fig 2.16) f3 0.92 0.95 1.04 1 length of record (fig 2.17) f4 1.01 1.02 1.08 1.01 Adjusted SD (mm) Sn' 28.62 32.36 23.92 27.37


March 2009 2-37

Next the PMP frequency factor Km were determined from Figure 2.15. Km = 15 for all stations. Another factor f5 was determined for all the stations and this relates to the number of observations made during the 24-hour time interval. All the data are from manually read rainfall stations and it is therefore 1 reading per 24-hour interval. From Figure 2.19, f5 = 1.13 for all stations. The computed 24-hour PMPs for all four stations are presented in Table 2.14.

Table 2.14 Computed Statistical PMP

Stations Kaki Bukit Tasoh Kangar A Setar Adjusted Mean (mm) Xn' 102 96 100 100 Adjusted SD (mm) Sn' 28.62 32.36 23.92 27.37

PMP frequency factor (fig 2.14) Km 15 15 15 15 Factor for observation

1 reading per day (fig 2.18) f5 1.13 1.13 1.13 1.13 24-hr Point PMP (mm) 601 657 518 577 catchment area (km2) 191 191 191 191 Areal Reduction Factor ARF 0.94 0.94 0.94 0.94 24-hr Catchment PMP (mm) 565 618 487 542

The 24-hour point PMPs were computed using Point

PMP =( Xn' + Km. Sn'). f5 (2.12)

The 24-hour catchment PMP is

Catchment PMP = ARF x Point PMP (2.13)

The highest 24-hour catchment PMP is the one derived from the Tasoh Station and is equal to 618mm. Compare this to the 24-hour point PMP of 540 mm derived using Storm Maximisation and Transposition approach (see Example in Section 2.5.8.2)


2-38 March 2009

2.5.8.5 Temporal and Spatial Distribution of PMP

Having determined the PMP, the next step is to distribute the PMP spatially and temporally.. This is done by computing the DAD curves for a typical large storm and the temporal and spatial distribuition shown in the DAD curves can be applied. Derivation of DAD curves is a time consuming process and in many projects the spatial distribution is taken care by simply applying the areal reduction factor to the point PMP as have been done in the above PMP examples and a bell shaped temporal distribution adopted. There are several temporal rainfall distribution that could be adopted and Figure 2.20 presents several popularly used distribution (Moore and Riley)

Figure 2.20 Comparison of Various Temporal Rainfall Distributions (Moore and Riley )

Moore, et al conclude that the World Curve Distribution was found to provide a valid basis for design of high hazard structures and the World Curve Distribution for a 24-hour storm together with some other distributions considered in their study is presented in Table 2.15.


March 2009 2-39

Table 2.15 Temporal Rainfall Distributions of a 24-hour Rainstorm

Duration (hr)

Rainfall distribution (%) World curve

ESFB HMR‐52 5‐Point

0 0 0 0 0 1 21.6 19.2 1.2 1.5 2 30.1 35.3 2.4 3 3 36.6 43.3 3.7 4.5 4 42.1 48.7 5.2 5.9 6 51.2 57.4 8.6 8.9 9 62.3 67.4 28.4 42.9 12 71.5 75.8 77.6 76.9 15 79.7 82.9 87.2 84 18 87 89.2 94.2 91.1 21 93.8 94.8 97.3 95.5 24 100 100 100 100

REFERENCE

[1]Curtis, D.C. and Burnash, R.J.C., “Inadvertent rain gauge inconsistencies and their effect on hydrologic analysis”, 1996 California-Nevada ALERT Users Group Conference, Ventura, CA, May 15-17, 1996. [2] DID, Varieties of rainfall with area in peninsular Malaysia, water resources Publication No 17.

[3]World Meteorological Organization-No.322,Manual for Estimation of Probable Maximum Precipitation, Geneva, 1973 [4]NAHRIM, Technical Research Publication (TRP) No.1-Derivation of probable Maximum Precipitation for design Floods in Malaysia, August 2007. [5] James N Moore and Ray C Riley, “Comparison of Temporal Rainfall Distributions for Near Probable Maximum Precipitation Storm Events for Dam Design” , National Water Management Center, NRCS [6] Hershfield, D. M. (1965) Method for Estimating Maximum Probable Precipitation, Journal of American Waterworks Association, Vol. 57, pp. 965-972


2-40 March 2009

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CHAPTER 3 WATER LOSSES

Chapter 3 WATER LOSSES

March 2009 3-i

Table of Contents

Table of Contents .................................................................................................................... 3-i

List of Tables ......................................................................................................................... 3-ii

List of Figures ........................................................................................................................ 3-ii

3.1 LOSSES TEMPORARY AND PERMANENT ........................................................................ 3-1

3.2 EVAPORATION ........................................................................................................... 3-1

3.3 MEASUREMENT OF EVAPORATION ............................................................................... 3-2

3.4 EVAPORATION FROM OPEN WATER SURFACES ............................................................. 3-3

3.5 TRANSPIRATION ........................................................................................................ 3-3

3.6 EVAPOTRANSPIRATION .............................................................................................. 3-3

3.7 FAO’s PENMAN-MONTIETH METHOD ............................................................................ 3-3

3.8 INTERCEPTION .......................................................................................................... 3-4

3.9 SURFACE RETENTION ................................................................................................. 3-4

3.10 INFILTRATION ........................................................................................................... 3-4

3.11 SUMMARY .................................................................................................................. 3-7

REFERENCE ........................................................................................................................... 3-8


3-ii March 2009

List of Tables

Table Description Page

3.1 Pan Coefficients 3-3

3.2 Parameter Estimates for Horton Infiltration Model (ASCE 1996) 3-5

3.3 Green-Ampt Parameters for Various Soil Classes 3-7

List of Figures Figure Description Page

3.1 Evaporation Pan –Class A Aluminium Pan 3-2

3.2 The Green-Ampt Equation concepts 3-6


March 2009 3-1

3 WATER LOSSES

3.1 LOSSES TEMPORARY AND PERMANENT

The rainfall-runoff process is complicated. Not all rainfall on a catchment results in runoff. In addition even runoff can be subdivided into surface and subsurface runoff. Surface runoff reaches the river rapidly and is also known as rapid runoff while sub-surface runoff flows to the river slowly. In a simplistic manner, the relation between rainfall, water losses and volume of water reaching a river may be expressed: VT=P-LP (3.1) where P is Rainfall. Lp is that portion of the rainfall permanently lost (deep percolation and Evapotranspiration VT is quantity or water reaching the river. Rainfall is lost through infiltration into the ground and evapotranspiration. The part that infiltrates may emerge again to form surface runoff or may result in deep percolation. In flood simulation where the interest is in rapid suface runoff and the relation between the rainfall, water losses and volume of water rapidly reaching a river may be expressed as:

VR=P-Lp-LT (3.2)

where LT is that portion of the rainfall that infiltrates and slowly emerges and enters the river by subsurface flow

VR is quantity or water rapidly reaching the river by surface runoff (rapid runoff) During extended periods of no rainfall, it is the sub-surface flow which maintains the river discharge and this discharge is usually referred to as the base flow. 3.2 EVAPORATION Evaporation is water lost to the atmosphere. Open water evaporation depends on two main factors: energy (mainly solar energy) to provide latent heat of vaporization and wind to transport the vapour from the water surface. Evaporation can be obtained by direct measurement (from evaporation pans) or computed using empirical equations. Evaporation from a catchment comes from open water surfaces, from the soil and from vegetative surface (also known as transpiration). For saturated soils the evaporation is expected to be essentially the same as open water evaporation. As the water table drops, the evaporation from soil surface drops drastically. In general, the rate of evaporation from soil surfaces usually is less than that from an open water surface. The rate of evaporation from a soil surface depends on the moisture of the soil at the surface. The actual evaporation from a catchment is therefore less than the potential evaporation which is the open water evaporation. DID Water Resources Publication No; 5: “Evaporation in Peninsular Malaysia, 1976" gives more details on this subject.


3-2 March 2009

3.3 MEASUREMENT OF EVAPORATION Evaporation pans are used to measure evaporation. A typical evaporation pan is as shown in Figure 3.1

Figure 3.1 Evaporation Pan –Class A Aluminium Pan The rate of evaporation from a pan is dependent on its exposure conditions, material of construction and colour and accordingly there are different pan coefficients for different types of pans. The coefficient relates the rate of evaporation from the pan to that from a large open water surface. The U.S. Weather Bureau Class A galvanised iron pan has been adopted as standard pan for use in DID. Due to maintenance problems the material for making the pan was changed to unpainted aluminium pans in 1980. The pan coefficients for various types of Class A pans is reported in DID Water Resources Publication (WRP No 11) "Comparison of Performance of U.S. Class A Evaporation Galvanised Iron Pan and Aluminium Pan, 1982". The U.S. Weather Bureau Class A pan is approximately 1,210 mm. internal diameter and 255mm. deep (See Figure 3.1) placed on a raised timber platform on open level ground. The pan is made of No. 20 gauge aluminium plate and is unpainted. On one side of the pan is a stilling chamber complete with fixed point gauge. The water surface in the pan is maintained at 190mm. from the bottom of the pan. Details on installation, maintenance and observation procedure can be obtained from DID Hydrological Procedure No. 21: "Evaporation Data Collection using Class A Aluminium Pan, 1981". Evaporation is measured daily at about 8.00 a.m. and recorded against the previous day's date, as in the case of recording daily rainfall observations. Water in the pan is set to 190mm depth. After a period evaporation will reduce the water depth and this is the pan evaporation. But because the order of daily evaporation is in millimeters (average daily evaporation is about 5mm in Malaysia) accurate measurement id difficult. Measurement is aided by appoint gauge in the stilling chamber. If rain occurs, depth of water in the pan may be more than 190mm in which case, water from the pan will have to be removed in order to lower the water level to the top of the of the point gauge. The estimation of evaporation rates may also be carried out by the energy balance method, aerodynamic method or the combined energy balance and aerodynamic method.


March 2009 3-3

3.4 EVAPORATION FROM OPEN WATER SURFACES To convert pan evaporation to open water surface evaporation the observed pan evaporation must be multiplied with a pan coefficient. A study using evaporation data obtained from three types of Class A pans installed at the DID Research Station at Ampang from 1977 to 1980 yields the following pan coefficients:

Table 3.1 Pan Coefficients

Pan Coefficient

Surface Black painted Unpainted G.I. Aluminium

G.I. Pan Pan Unpainted Pan

Open Water 0.90 0.99 1.03

3.5 TRANSPIRATION

Transpiration is the vaporisation of water through the tissues of vegetation. The vegetation takes up water from the soil and returns part of it to the atmosphere. The rate of transpiration depends on the type of plant, its stage of development, temperature, solar radiation, sunshine hours, moisture available, wind speed, etc. 3.6 EVAPOTRANSPIRATION

Evapotranspiration or consumptive use of water is the amount of water transpired by vegetation during growth for building of plant tissue including water evaporated from adjacent soil and water surface and water intercepted during rainfall. The rate of evapotranspiration varies with the stage of development of the vegetation and also depends on the availability of water. Consequently the term "Potential evapotranspiration” is used for the loss which would occur if sufficient water is available When water is applied to the soil surface, the upper layers absorb moisture which is held by capillary attraction while some passes downwards to make up the free groundwater. The maximum water held in the upper layers is called the "field capacity" and is available for absorption by the vegetation until a stage is reached (wilting point) when the surface tension in the water equals the suction power of the plants and no further water may be removed. If crops are to grow this capillary moisture in the upper layers must be replenished by irrigation or rainfall to ensure that sufficient water is available for transpiration. Many equations have been developed for the computation of evapotranspiration, all of which are empirical since it has not been possible to derive a purely theoretical relation which includes all factors. 3.7 FAO’s PENMAN-MONTIETH METHOD The Food and Agriculture Organisation (FAO) of the United Nations recommends a standardized method to compute reference evapotranspiration (ETo) from meteorological data i.e. the FAO Penman-Monteith method as this method as “It is a method with strong likelihood of correctly predicting ETo in a wide range of locations and climates and has provision for application in data-short situations”


3-4 March 2009

Penman-Montieth Equation

In 1948, Penman derived an equation to compute the evaporation from an open water surface from standard climatological records of sunshine, temperature, humidity and wind speed. This method was further developed by many researchers and extended to cropped surfaces by introducing resistance factors and under FAO, the Penman-Montieth Method evolved. The Penman-Monteith equation is given by:

ETo =

Δ Rn-G +ρaCp(es-ea)

ra

Δ+γ 1+rsra

(3.3)

where Rn is the net radiation, G is the soil heat flux, (es - ea) represents the vapour pressure deficit of the air, r a is the mean air density at constant pressure, cp is the specific heat of the air, D represents the slope of the saturation vapour pressure temperature relationship, g is the psychrometric constant, and rs and ra are the (bulk) surface and aerodynamic resistances. Details of the FAO Penman-Montieth Method for computing reference crop evapotranspiration is given in FAO Irrigation and Drainage Paper 56.

FAO also distributes a software CROPWAT which assist the user in estimation of reference crop water requirement ETo using the Penman-Montieth method,

3.8 INTERCEPTION When rain falls, a portion of it is caught by vegetation and is later evaporated; thus never reaching the ground. This interception loss is highest at the start of a storm, but as the capacity is satisfied, a greater proportion of the rain reaches the ground. Thus the amount of rain collected in a correctly exposed, rain gauge is not the same as the amount reaching the ground in nearby areas under vegetation. In light storm no rain may teach the ground at all. 3.9 SURFACE RETENTION

The surface retention includes interception and depression storage. It is that part of the rain which does not appear as stream flow during or immediately after a storm, The depression storage is the water which collects in depressions in the ground surface. 3.10 INFILTRATION Infiltration is the movement of water into the soil surface and is the source of water to sustain vegetation growth and groundwater supply. The main factors influencing rate of infiltration are the vegetation cover, the soil structure and soil moisture. Vegetation cover prevents surface sealing due to impact of raindrops and has a major influence in maintaining infiltration rate. Soil structure determines the size and continuity of pore space in the soil and is influenced by the size of the particles that make up the soil the degree of aggregation of soil particles. When rain falls on a soil surface some or all of it passes through into the underlying soil. At the beginning of a storm the rate of infiltration high, but it slowly decreases to a limiting value.

The decrease in the rate of infiltration is due to the compaction of the soil surface by the impact of the rain drops, the clogging of the small pore in the soil, swelling of clay particles due to the absorption of moisture, and the filling of the pores with capillary moisture thus creating resistance to flow to lower levels.


March 2009 3-5

Infiltration rate will achieve its potential if the rate of water becoming available for infiltration (i.e the rainfal intensity less the rate of surface retention) is equal to or greater than the existing rate of infiltration. There are various commonly used infiltration equations. One of the simplest equations is the Horton’s infiltration equation. Horton developed the following infiltration equation: fp = fc + (fo – fc) e-Kt (3.4)

fp is infiltration capacity which is the maximum rate at which water can enter the soil under given conditions.

Fo is maximum rate of infiltration existing at the beginning of the storm, i.e. when t = 0 fc is constant value of rate of infiltration to which the infiltration tends with time.

Table 3.2 Parameter Estimates for Horton Infiltration Model (ASCE 1996)

Soil and Cover fc mm/h

f0

mm/h Kt

min-1 Standard Agricultural(bare) 280 6-220 1.6 Standard Agricultural(turfed) 900 20-290 0.8 Peat 325 2-29 1.8 Fine sandy clay (bare) 210 2-25 2.0

Fine sandy clay (turfed) 670 10-30

1.4

Infiltration is essentially vertical flow of water through the unsaturated zone of the soil to the ground water table (the saturated zone). Darcy’s Law of flow through a porous medium describe flow rate per unit area of soil q as:

q= -K ∂h∂z

(3.5)

Where K: is the hydraulic conductivity of the soil, dh: the differential head and dz, the differential + K) vertical distance. The total head at any point in the unsaturated soil is the sum of suction head ψ and the gravity head z. h= ψ+z (3.6)

hence

q= -K ∂(ψ+z)∂z

(3.7)

The continuity equation for one-dimensional unsteady and unsaturated flow in a soil is given by:

∂θ∂t

+ ∂q∂z

=0 (3.8)

where θ is the soil moisture content.


3-6 March 2009

Combining the flow rate equation with the continuity equation yields the one-dimensional Richard’s Equation:

∂y∂x

= ∂y∂x

(D ∂y∂x

+ K) (3.9)

Richard’s Equation in its general form cannot be solved analytically and therefore Richard’s equation is often solved using numerical approximations. Another infiltration equation often adopted is the Green-Ampt infiltration equation. The Green-Ampt infiltration method simplifies the infiltration concept so that an analytical solution to the infiltration equation can be obtained. The method assumes that the wetting front as water infiltrates into the soil is a sharp boundary dividing soil of moisture content = θi with the saturated soil (moisture content = θs) above. The concept is as illustrated below.

Figure 3.2 The Green-Ampt equation concepts The Green-Ampt Equation is derived based on Darcy’s equation:

q=-K5h2-h1

z2-z1=-K5

ψf+Zf - H+0

Zf-0=-K5

ψf+Zf -H

Zf (3.10)

where, H = the depth of ponding, cm, Ks = saturated hydraulic conductivity (cm/s), q = flux at the surface (cm/h) and is negative, ψf = suction at wetting front (negative pressure head), θi = initial moisture content (dimensionless) and θs = saturated moisture content (dimensionless) zf =distance of wetting front from soil surface. However, the following simplifying assumptions are made: a) The wetting front is a sharp boundary with constant volumetric water contents above and below

the wetting front. b) The soil-water suction immediately below the wetting front remains constant as the wetting front

advances. The Green Ampt equation for cumulative infiltration is given by:

K5t=F-ψf θs-θi ln 1+ F

θs-θi ψf (3.11)

H=ψ

soil

water

z (negative direction)

sθ iθ θ

fzz =

0z = 01 +=Hh

wetting frontff zh +=ψ2

fψψ =


March 2009 3-7

The Green-Ampt equation for infiltration rate is:

f=Ks 1+θs-θiψf

F (3.12)

Table 3.3 shows typical values of porosity, effective porosity, wetting front soil suction head and hydraulic conductivity for various soil classes.

Table 3.3 Green-Ampt Parameters for Various Soil Classes

Soil class Porosity Effective Wetting front Hydraulic ŋ Porosity soil suction conductivity θc Ψ (cm) k (cm/h) Sand 0.437 0.417 4.95 11.78 (0.374-0.500) (0.354-0.480) (0.97-25.36) Loamy Sand 0.437 0.401 6.13 2.99 (0.363-0.506) (0.329-0.473) (1.35-27.94) Loam 0.463 0.434 8.89 0.34 (0.375-0.551) (0.334-0.534) (1.33-59.38) Silt Loam 0.501 0.486 16.68 0.65 (0.420-0.582) (0.394-0.578) (2.92-95.39) Sandy clay 0.398 0.33 21.85 0.15 loam (0.332-0.464) (0.235-0.425) (4.42-108.0) Clay Loan 0.464 0.309 20.88 0.1 (0.409-0.519) (0.279-0.501) (4.79-91.10) Silty Clay 0.471 0.432 27.30 0.1 loam (0.418-0.524) (0.347-0.517) (5.67-131.50) Sandy clay 0.430 0.321 23.90 0.06 (0.370-0.490) (0.207-0.435) (4.08-140.2) Silty Clay 0.479 0.423 29.22 0.05 (0.425-0.533) (0.334-0.512) (6.13-139.4) Clay 0.475 0.385 31.63 0.03 (0.427-0.523) (0.269-0.501) (6.39-156.5) The number in parentheses below each parameter are one standard deviation around the

parameter value given. Source: Rawls, Brakensiek, and miller, 1983. 3.11 SUMMARY Summarising the above, therefore, the relation between volume of water reaching the river, rainfall, losses etc., may be expressed as follows. The total volume of runoff VT is given by precipitation P minus the permanent losses Lp:

VT = P - Lp (3.13) The losses which together, make up the permanent losses Lp are interception, evaporation and transpiration plus that part of infiltrated water reaching the sea or area outside the limits of the catchment under consideration. The total volume of rapid runoff, VR, which is the surface runoff component is precipitation P minus the permanent losses Lp and minus the temporary loss LT

VR = P - Lp - LT (3.14)


3-8 March 2009

The temporary loss LT is that water lost by infiltration, but ultimately reaching the river within the catchment area via subsurface flow through a slower route. In general the flow rate Q is made up of two main components the rapid surface runoff QR and the subsurface flow rate Qs. Q = QR + Qs (3.15) REFERENCE [1] DID Water Resources Publication No.5, Evaporation in Peninsular Malaysia, Department of Irrigation and Drainage (1976). [2] Rawls, W.J., Brakensiek, D.L., and Miller, N, “Green-Ampt Infiltration Parameters from Soils Data”, Journal of Hydraulic Engineering, Vol. 109, No. 1, American Society of Civil Engineers, 1983.

CHAPTER 4 RIVER DISCHARGE

Chapter 4 RIVER DISCHARGE

March 2009 4-i

Table of Contents


List of Tables ......................................................................................................................... 4-ii

List of Figures ........................................................................................................................ 4-ii

4.1 RAINFALL – RIVER DISCHARGE RELATION ................................................................... 4-1

4.2 DATA REQUIRED ........................................................................................................ 4-1

4.2.1 Stage Discharge Relationship ........................................................................ 4-3

4.2.2 Permanent Control ....................................................................................... 4-3

4.3 EQUIPMENT ............................................................................................................... 4-8

4.3.1 Equipment for River Stage Measurements ...................................................... 4-8

4.3.2 Equipment for Measuring Current Velocities .................................................. 4-23

4.3.3 Discharge Measurement Method IV ............................................................. 4-37

4.4 CAUSES OF FLOOD AND THEIR ESTIMATION .............................................................. 4-45

4.5 ESTIMATION OF DESIGN FLOODS ............................................................................. 4-46

4.5.1 Empirical Methods and Regional Equations ................................................... 4-47

4.5.2 Hydrograph Method ................................................................................... 4-51

4.6 ESTIMATION OF PMF ................................................................................................ 4-58

REFERENCE ......................................................................................................................... 4-65

APPENDIX 4A MOVING BOAT METHOD BY KLEIN, ET. AL. ....................................................... 4A-1


4-ii March 2009

List of Tables


4.1a Calculations Output of Example 4-7

4.2 Summarizes the Discharge Measurement Methods Described Above, Their Typical Applications and Required Equipment 4-38

4.3 Coefficients of the MAF Regional Equations 4-48

4.4 Constants of the FF Regional Equations 4-49

4.5 Catchment Groups of HP11 and Parameters 4-49

4.6 Parameters of Dimensionless Triangular Flood Hydrographs 4-50

4.7 Incremental Area, Rainfall and Effective Rainfall Hyetographs 4-52

4.8 Derivation of Runoff Hydrograph Using the Time-Area Method 4-53

4.9 Methods of Estimating Loss Rates Recommended in MSMA Manual 4-54

4.10 4-hourly Effective Rainfall Hyetograph 4-55

4.11 Derivation of the DRH by Convolution Process 4-56

4.12 Computation of Lag Time Lg 4-58

4.13 Computation of Unit Hydrograph Parameters 4-59

4.14 Ordinates of the Synthetic 2-hour UHs for Unit Rainfall of 1cm 4-60

4.15 2- Hourly Effective Rainfall Hyetographs 4-61

4.16 Derivation of DRH for Catchment A 4-61

4.17 Derivation of DRH for Catchment B 4-62

4.18 Derivation of DRH for Catchment C 4-63

4.19 Combining the Catchment hydrographs to form the PMF 4-64


4.1a, 4.1b Stage-Discharge Curve 4-3

4.2 a–Stage Discharge Curve: Arithmetic Plot and 4.2b Stage Discharge Curve Logarithmic Plot 4-4

4.3 Selections of Q1, Q2, Q3 4-5

4.4 Typical Graduated Plastic Plate Used in Building a Stick Gauge 4-9

4.5 Multiple Stick Gauge Installation at K. Jemakah, Endau. 4-10

4.6 Float & Stilling Well Method for Water Level Monitoring 4-11

4.7 Typical Drawing for Float and Stilling Well Installation 4-12

4.8 Submersible-Type Pressure Sensor with External Standalone Data Logger 4-13

4.9 A Sherlock DP20 Gas Bubbler System with Gas Tank in Background. 4-15

4.10 Typical Single Line Installation of Gas Bubbler System 4-16


March 2009 4-iii


4.11 Ultrasonic Water Level Sensor Installed from a Custom Arm Rest Overlooking the Water Surface. 4-17

4.12 Regular Maintenance Includes Silt Removal from Stilling Well. 4-21

4.13 Submersible Pressure Sensor and Stilling Pipe before Cleaning 4-22

4. 14 Submersible Pressure Sensor and Stilling Pipe after Cleaning. 4-22

4.15a Ott C31 Propeller-Type Universal Current Velocity Meter 4-24

4.15b. Ott C2 Propeller-Type Current Velocity Meter for Small Streams 4-24

4.16 Pigmy Cup-Type Current Velocity Meter, also for Small Streams 4-25

4.17 Ott C2 Current Meter with Wading Rod and Counter 4-26

4.18 Current Meter and Sinker Weight (Yellow) Being Lowered Down from a Cableway 4-27

4.19 Type SDW-ES Electric Double Drum Winch with Invariable Motor 4-27

4.20 Hand-Operated A-Frame with Russian which Developed from a Bridge 4-28

4.21 Portable Gauging Winch with Boom Mounted to a Static Boat. 4-28

4.22 Electrical double drum gauging winch firmly anchored to secured cableway shelter 4-29

4.23 Typical Cableway Installation Drawing 4-30

4.24 Cableway with Weight Attached for Deployment 4-30

4.25 Rod Floats on Standby for Deployment. 4-31

4.26 Equally Sub-Divided Vertical Areas along River Cross-Section 4-33

4.27 One-Point and Two-Point Gauging VA Method 4-33

4.28 Coefficient to Convert Measured Float Velocity to Stream Velocity. 4-34

4.29 Float Deployment from a Bridge. 4-36

4.30 Profile Drawing of a Sharp-Crested Weir 4-37

4.31 Upstream Velocity Head would be much higher than the Downstream 4-40

4.32 A Single Point Acoustic Current Meter or ACM 4-44

4.33 Triangular Direct Runoff Hydrograph of HP11 4-50

4.34 Time-Area Method 4-53

4.35 4-hour UH for 1 cm Effective Rain and Tabulated UH Ordinates 4-54

4.36 Convolution of UH of Figure 5.4 with Effective Rain (ER) Hyetograph of Table 4.9 4-57

4.37 Triangular Unit Hydrograph 4-59

4.38 The 24-Hour PMF and the Component Catchment Hydrographs 4-65


4-iv March 2009



March 2009 4-1

4 RIVER DISCHARGE

4.1 RAINFALL – RIVER DISCHARGE RELATION The entry of water into a river as a result of a particular storm occurs over a long period of time; in large catchments it may take several years before all the water from a particular storm has reached the rivers. It is therefore necessary to appreciate that the river discharge at any given time at a point in a river is not simple related to any one storm of series of storms. Analyses of the rate of discharge are usually referred to as Intensity Studies. The relation between precipitation and runoff, referred to as the rainfall – runoff relation since rainfall is practically the only source of precipitation in Malaysia, is complex and dependent on many factors. The factors are: a) Precipitation Characteristics.

(i) Amount of rain (ii) Intensity of rainfall (iii) Extent of the storm (iv) Seasonal distribution of rain (v) Type of rainfall (vi) Temperature (vii) Relative humidity (viii) Winds.

b) Catchment Characteristics.

(i) Size of catchment (ii) Shape of catchment (iii) Location of catchment in relation to storm path (iv) Topography (v) Geology (vi) Vegetation (vii) Extent and type of development (viii) Lakes and swamps.

c) Storage Characteristics. (i) Surface storage (depression storage and surface detention) (ii) Reservoir storage (natural and artificial) (iii) Ground Storage.

It is not proposed to deal at this stage with the effect of each factor on the runoff but it is obvious that the relation is so complex that the only reliable manner in which data can be obtained for design purposes is by accurate direct measurement. For this reason it is essential to pay great attention to the methods and equipment used in such investigations. And to check continually the results obtained. The need to check cannot be over emphasized as unreliable observations cannot be rectified and may prove extremely expensive by rendering a scheme partly or completely valueless in spite of great care in topographic surveys, design and construction. Such failures must not be permitted and hence constant checking is essential to eliminate unsatisfactory results and to limit the amount of time wasted on such observations. 4.2 DATA REQUIRED The hydrological information generally required for design purposes comprises regular water level observations and corresponding values of discharge over the entire range of flow. All heights should be expressed in meter above mean sea level.


4-2 March 2009

It is thus necessary to measure at regular intervals, or continuously. The water levels at selected points on certain rivers. These are tabulated and should be prepared in" a graphical form relating water level and time of observation, usually referred to as a Stage Hydrograph. It is also necessary to measure the velocity of the stream, its water level at time of observation, the cross section at the gauging station, and to note whether the water level is rising or falling. From these measurements the discharges are computed corresponding to different stages of flow and hence the Stage-Discharge Curve is prepared, which relates the water level and discharge. The stage-discharge curve is used to compute the discharges from the regularly observed water levels. The corresponding discharges are then both tabulated and presented in graphical form as a Discharge Hydrograph. This relates the discharge and the time. Thus offices operating river discharge stations should have the following information immediately available for all stations:

a) Reduced water levels and time of observation in tabular form. b) Stage – hydrograph. c) Discharges and time of occurrence in tabular form. d) Discharge – hydrograph. e) Cross section of stream. f) Stage and corresponding discharge observations with time of observation. g) Stage - discharge curve. h) Field books containing details of field measurements of velocities. i) Notes on changes in catchment, dredging of rivers, construction of structures in the river,

and details of water level equipment e.g. changes in stick gauge datum, damage to recorders etc.

From the above information, analyses can be made to supply the designer with any information required regarding peak flows, duration of flows, frequency of occurrence of flows, etc. In the case of water level stations, the information which should be immediately available at all times is:

a) Reduced water levels and time of observation in tabular form. b) Stage hydrograph.

There are four main types of river observation stations operated by the Department:-

a) Water level stations. Here, water levels only are recorded. b) Discharge Stations. In addition to reading water levels, velocity measurements are carried

out. This provides the information necessary to prepare a stage-discharge curve which is generally produced for the entire range of stages at the discharge station. There are, however, two exceptions to this.

c) High Stage discharge stations are generally established where interest is centered on flood protection works or the minimum permissible waterway in the case of a new structure such as a bridge. Observations are then restricted to the higher levels and discharges and no routine measurements are undertaken.

d) Low stage discharge stations are usually operated for the purpose of investigating the value of the stream as a source of water supply, particularly where the supply is directly from the river without any impounding facilities. Provided the station is suitable for observation over the necessary range of low stages, it does not matter if higher stages overspill and bypass this type of station.

In all cases, methods of observation are the same and it is necessary to obtain an up to date survey of the cross section.


March 2009 4-3

4.2.1 Stage Discharge Relationship The aim of all current-meter and other direct discharge measurements is to prepare a stage-discharge relationship which is also known as the rating curve. The measured value of the discharges when plotted against the corresponding stages will enable us to estimate the discharges with respect to the water depth. Plots of the observed discharges Q at a gauging station versus the simultaneously observed gauged water level G, one obtains a so-called “scatter diagram” of observed scatter points (Q,G). The relationship could be:

i. Well-defined – Fig 4.1a ii. Complicated – Fig 4.1b

The deviation from a well-defined relationship can becaused by different combined effects of a wide range of channel and flow parameters such as:

i) Unsteady flow ii) Backwater effects iii) Unstable controls iv) Change in roughness of the river bed condition

The combined effect of these parameters is termed control. If the stage discharge (G-Q) relationship for a gauging section is well-defined, then the control is said to be permanent. Otherwise it is called shifting control. 4.2.2 Permanent Control In cases where the relationship between the stage and the discharge is almost well-defined, the discharge (Q) in relation to the water gauged height (G), can be expressed by the rating equation as follows: Q = Cr (G –a) β (4.1)

Figure 4.1 a and 4.1b: Stage-Discharge Curve


4-4 March 2009

In which: Q = stream discharge (m3/s) G = gauge height (stage, m) a = the gauge reading corresponding to zero discharge Cr and β are rating curve constants. This relationship can be expressed graphically by plotting the observed stage (G) against the corresponding discharge (Q) values in arithmetic (Figure 4.2a) or logarithmic plot (Figure 4.2b). Logarithmic plot is advantageous as the equation above gives a straight line plot. The coefficient a, Cr and β need not be the same for the full range of stages.

The best values of Cr and β for a given range of stage are obtained by the least-square error method. Thus by taking logarithms, ( )β= − +log loglog loglog rQ G a C (4.2)

β= + or Y X b (4.3) where:

Y = log Q(4.4) X =log (G-a) (4.5) b =log Cr (4.6)

For the best-fit straight line of N observations of X and Y, by regressing X = log (G - a) on Y = log Q

β=Ν ∑ XY - ∑ X ∑ Y

N ∑ X2 - ∑ X 2 (4.7)

Figure 4.2a–Stage Discharge Curve: Arithmetic Plot and 4.2b Stage Discharge Curve Logarithmic Plot


March 2009 4-5

b= ∑Y-β ∑XN

(4.8)

The coefficient of correlation r is given by:

r= N ∑XY -(∑X) ∑Y

N ∑X2 -(∑X)22

N ∑Y2 - ∑Y 22 (4.9)

For a perfect correlation, r = 1.0. If r is between 0.6 and 1.0 it is generally taken as a good correlation. Calculating a, Stage for Zero Discharge The stage for zero discharge in the stream “a” is a hypothetical parameter and cannot be measured in the field but can be obtained from the Running,s graphical method. In this method, the Q vs G data are plotted to an arithmetic scale graph paper and a smooth curve through the plotted points are drawn. Three points A, B and C on the curve are selected as in figure 4.3 such that their respective discharges Q1, Q2 and Q3 are in geometric progression. i.e.

Q1Q2

=Q2Q3

(4.10)

or Q2

2=Q1Q3 (4.11)

Figure 4.3 Selections of Q1, Q2, Q3

A

B

Gau

ge H

eigh

t G

(m)

Discharge Q (m3/s)Q 1 Q 2 Q 3

G3

G2

G1

C

D

E

Rating Curve

Fa


4-6 March 2009

At A and B, vertical lines are drawn and then horizontal lines a drawn at B and C to get D and E as intersection points with the verticals. Two straight line ED and BA are drawn to intersect at F. The ordinate at F is the required value of a, that is the gauge height corresponding to zero discharge. This method assumes that the lower part of the stage-discharge curve to be a parabola. The value of the above can also be calculated based on the equation below:

G1-a

G2-a= G2-a

G3-a (4.12)

Therefore:

a= G1G3-G2

2

G1+G3 -2G2 (4.13)

Example: Following are the data of gauge and discharge collected at a particular river by stream gauging operation.

a) Develop a stage discharge relationship for this section of the river b) What is the coefficient of correlation of the derived relationship? c) Estimate the discharge corresponding to a gauge reading of 4 m

Gauge readings (m) Discharge

(m3/s)

Gauge readings (m) Discharge (m3/s)

4.18 95 2.20 22 4.48 114 2.01 16 4.18 105 2.68 35 3.90 87 1.92 13 1.43 3 3.11 48 1.80 10 4.33 109 1.65 7 4.63 123 1.49 5


March 2009 4-7

Calculation Using the EXCEL Spreadsheet, establish the Table 4.1 below

Table 4.1a Calculations Output of Example

N Gauge G(m) G-a

Q

(m3/s)

X= log(G-a) X2 Y=logQ Y2 (XY)

1 4.18 3.08 95 0.4881 0.2382 1.9772 3.9092 0.9651

2 4.48 3.38 114 0.5291 0.2800 2.0577 4.2341 1.0888

3 4.18 3.08 105 0.4881 0.2382 2.0194 4.0780 0.9857

4 3.90 2.80 87 0.4475 0.2003 1.9382 3.7568 0.8674

5 1.43 0.33 3 -0.4777 0.2282 0.4652 0.2164 -0.2222

6 1.80 0.70 10 -0.1557 0.0242 1.0123 1.0247 -0.1576

7 1.65 0.55 7 -0.2625 0.0689 0.8623 0.7436 -0.2264

8 1.49 0.39 5 -0.4046 0.1637 0.6979 0.4871 -0.2824

9 2.20 1.10 22 0.0395 0.0016 1.3343 1.7804 0.0527

10 2.01 0.91 16 -0.0399 0.0016 1.1975 1.4339 -0.0478

11 2.68 1.58 35 0.1995 0.0398 1.5444 2.3852 0.3080

12 1.92 0.82 13 -0.0858 0.0074 1.1151 1.2435 -0.0957

13 3.11 2.01 48 0.3031 0.0919 1.6769 2.8120 0.5083

14 4.33 3.23 109 0.5091 0.2592 2.0368 4.1486 1.0370

15 4.63 3.53 123 0.5483 0.3006 2.0887 4.3625 1.1452


4-8 March 2009

Table 4.1b Calculations Output of Example

N = 15 Sum X = 2.1262 Sum Y = 22.0239 Sum(XY) = 5.9261

SumX2 = 2.1438

SumY2 = 36.6160

(SumX)2 = 4.5206

(SumY)2 = 485.05 β = 1.522 b = 1.2525 Cr = 17.89 r = 0.93 a = 1.1

G = 4 m

Q = Cr ( G - a ) β m3/s

Q = 90.44 m3/s Answer: The stage discharge relationship for this section of the river is:

Q = 17.89 (4 – 1.1)1.522 (4.14)

The coefficient of correlation of the derived relationship = r = 0.93 The discharge corresponding to a gauge reading of 4 m = 90.4 m3/s 4.3 EQUIPMENT This section covers the equipment involved in obtaining river discharge which includes the recording of water level or river stage and measuring current velocities. Their typical installation, maintenance and calibration and proper storage are also discussed. 4.3.1 Equipment for River Stage Measurements The basic types of equipment currently used to measure water level or river stage in Malaysia are generally confined to: A. Stick Gauge Method The traditional and manual method of measuring water level in an open channel is with a stick gauge. The stick gauge must be graduated in decimetres and centimetres and the values read thereinafter must refer to the elevation above mean sea level.


March 2009 4-9

The use of stick gauges graduated in any other manner should not be permitted as experience has shown that it is a source of many errors in water level records. The Department has created and adapted for use standard two-meter graduated phenolic, plastic, aluminum or fibreglass plate. To determine the river discharge over a period of time, the gauge must be read at least twice daily. More than one reading provides a means for checking the readings and also provides vital information on any drastic changes in the river stage. Readings should be taken more frequently when dynamic changes in stage e.g. high flow are expected or occur in the river body. Operation and maintenance of the stick gauge is discussed later in this chapter. Figure 4.4 shows a typical graduated plastic plate used by the Department while Figure. 4.5 represent a multiple-stick gauge installation on a river bank. For details on specification and installation procedures please refer to Hydrological Procedure (HP) No. 25. D.I.D. Hydrology “Standard Stick Gauge for River Station (1982)”. Stick gauges may be installed either vertical or inclined. The inclined type should be carefully graduated and accurately installed to ensure correct stage readings. Care should be taken to install the gages solidly to prevent errors caused by changes in elevation of the supporting structure.

Figure 4.4 Typical Graduated Plastic Plate Used in Building a Stick Gauge


4-10 March 2009

B. Float and Stilling Well Method One of the earliest automated equipment used to measure and record water level is a mechanical recorder which used a moving ink pen and long roll of graph paper to record changes in water level. A float is attached to one end of a steel cable and lowered into a stilling well where it rests on the surface of the water. The cable then passes over a float pulley on the recorder and back into the well. A counterweight is then attached to the opposite end of the cable to keep it taut. When the water level changes, it causes the float to rise and fall. That movement is transferred to the pulley on the recorder by the cable. As the pulley rotates it moves the pen back and forth, drawing a line on the paper (Appendix 4A, Figure21/II/V). A battery-driven clock governs the speed at which the chart paper is transported through the recorder. The biggest drawback when using mechanical recorders is the time required to process the data that they collected. It takes more than 100 m of paper and many reversals to match to record a year of data. Since the data is analog, processing has to be done manually and results are then transferred onto long-term paper graphs and subsequently digitized onto computers. Figure 4.6 shows a typical float and stilling well system installed by the Department while Figure 4.7 represents a typical drawing of a float and stilling well installation. The current use of digital shaft encoders is a natural evolution from the mechanical chart recorders. The existing mounted float and pulley arrangement could be removed from the mechanical recorder and mounted directly to the shaft of the encoder. The encoder would typically provide a pulse corresponding to the smallest measured increment (1 mm). As the float rises and lowers the encoder would provide a positive or negative count depending upon the direction of movement. The information is then recorded via a binary coded decimal onto a digital data logger. The main advantages of the shaft encoder are the ease to which it can be retrofitted to stations already using a float and well set-up and the excellent linearity and accuracy provided. The main disadvantage would be the installation costs as float sensors do require a stilling well to operate.

Figure 4.5 Multiple Stick Gauge Installation at K. Jemakah, Endau.


March 2009 4-11

Water Intake Pipes

Paper Chart Recorder

Stilling Well

weight

Float

Water Intake Pipes

Paper Chart Recorder

Stilling Well

weight

Float

These stilling wells are usually much larger in size than the ones used when deploying pressure sensors or gas bubblers. The bottom of the stilling well should be at least 45 cm below the lowest water level in the river. The recording mechanism should be easily accessible even during flood times so that charts can be changed.

Figure 4.6 Float & Stilling Well Method for Water Level Monitoring


4-12 March 2009

Figure 4.7 Typical Drawing for Float and Stilling Well Installation

C. Pressure-based Systems Typically there are two pressure-based systems in the market to measure and record water level. One is the (i) submersible pressure transducer system while the other is the (ii) gas bubbler system. (i) Submersible Pressure Sensor Submersible pressure sensor systems are commonly used where site logistics and related costs discourage the use of large stilling wells and float recorders. These pressure sensors featuring the latest silicon, micro-machined, piezo-resistive, strain gauge technology. The new generation of pressure sensors is very sensitive and accurate and some are even serviceable. The sensor gives out analog ouput i.e. 4 - 20 mA and is normally connected via a robust data cable to a data logger which is powered by an external battery with a solar-panel charger (www.stevenswater.com). Some pressure sensors such as the Diver (www.seba-hydrometrie.de), Level Troll (www.in-situ.com) come with an onboard datalogger and power supply while others e.g. the WL16 (www.globalw.com) depends on the power supply from their external data logger. Included amongst the signal wires of a conventional pressure sensor is a vent tube. The vent tube allows the sensor to equilibrate itself to changes in atmospheric pressure. The sensor measures the pressure head at the point in the water column where it is mounted, and this pressure value is converted to water depth above the sensor (pressure head is directly related to water depth by the


March 2009 4-13

unit weight of water). Submersible analog sensors can have accuracy as good as 0.1% of Full Scale Output (FSO), while digital sensors are available with accuracy's of 0.02% FSO or better, depending on the make and model. Disadvantages include variances in accuracy (again depending on make and model). Inaccuracy can be caused by non-linearity, when the sensor's signal curve deviates from that of a straight line. It can also be caused by sensors with poor repeatability and hysteresis, where the difference in value for the same measured point when pressure is first increased, then decreased past the point. Calibration drift in analog sensors can also introduce error in water level measurements. Digital submersible stand-alone sensors, like the Diver offer high accuracy and excellent long-term stability, but usually at a substantial cost over their analog counterparts. A small stilling pipe is usually recommended since the pressure sensor is subject to damage by debris including bedload in the channel during high flow events. The stilling pipe serves to protect the sensor while providing more conducive water conditions for reliable stage readings. Figure 4.7 shows a conventional submersible pressure sensor while Figs. 4.8a, 4.8b and 4.8c represent the types of submersible pressure sensor usually installed.

Figure 4.8 Submersible-Type Pressure Sensor with External Standalone Data Logger


4-14 March 2009

Stand alone datalogger inside secured housing

PVC or GI pipe

Figure 4.8a: Submersible pressure sensor connected to external datalogger, battery and solar

panel.

Figure 4.8b: Submersible pressure sensor with onboard logger and USB/RS-232 output.

Figure 4.8c: Submersible pressure sensor with standalone datalogger.

PVC or GI pipe

PVC or GI pipe

Only USB/RS-232 data cable inside secured housing.


March 2009 4-15

(ii) Gas Bubbler System Gas bubblers or pneumatic water level recorders consist of the bubbler unit, typically located in weatherproof housing or enclosure some distance away from the water body, with an orifice line that connects the bubbler unit to the water column in the channel. These "bubbler" gauges are similar to the submersible digital sensors, with the exception that they are typically mounted away from the water body in a walk-in shelter along with the pressure source (nitrogen tank or battery compressor) and pressure regulator. The bubbler unit constantly releases gas bubbles at a predetermined rate through the pressurized orifice line into the water. The pressure required to force the bubble into the water is converted to river stage using the same mathematical relationships described for pressure transducers. The river stage values are then recorded to either a paper chart recorder or a data logger. A significant advantage of using gas bubblers is that the sensor itself can be located quite a distance from the water body therefore circumventing any damage or loss which may occur during peak flow event. Only the orifice line is at risk and this is easily replaced at minimal cost. Both approaches provide a similar level of accuracy, generally about 2.1 mm. However the cost of a gas bubbler system is typically two to three times the cost of a pressure transducer instrument and the requirement of a replacement bulky pressure tank precludes its use in very remote and inaccessible sites. Figure 4.9 shows a Department employed gas bubbler system with a paper chart recorder while Figure 4.10 depicts a typical installation drawing of the system.

Figure 4.9 A Sherlock DP20 Gas Bubbler System with Gas Tank in Background.


4-16 March 2009

D. Ultrasonic Sensor Method Ultrasonic or acoustic sensors are used to measure river stage in cases where it is not feasible to install any portion of the measurement device below the high water elevation. These non-contact systems are installed above the highest water line and depend on the acoustic returns of ultra-high frequency sound waves transmitted from a sensor mounted above the water body to determine the river stage value. Sensors are typically installed from existing structures e.g. bridges. Because they are generally more costly than the other sensor options described above and more exposed when deployed, their use for measuring river stage at locations vulnerable to vandalism and theft is strongly discouraged. Figure 4.11 shows an actual ultrasonic sensor installed by the side of a riverbank.

Figure 4.10 Typical Single Line Installation of Gas Bubbler System


March 2009 4-17

Figure 4.11 Ultrasonic Water Level Sensor Installed from a Custom Arm Rest Overlooking the Water

Surface. Crest Gauge for Flood Level Measurements A crest stage gauge is usually vertical, used to indicate the highest peak stage that has occurred since the previous setting. It is normally made of standard 40mm. diameter galvanised iron pipes with timber rode cut to the lengths of approximately 1,165mm. Each pipe is threaded of a length of 25mm. on the top end to accommodate a galvanized iron cap. A hole is drilled at the top end of the pipe just below the threaded length to allow trapped air to escape when water level rises. A hole is also drilled about 40mm. above the bottom end of the pipe to allow a galvanized coachscrew to be fixed there as a datum pin. A rectangular timber rod with graduated notches at 100 mm apart is inserted into the pipe and seated on the coachscrew pin such that the lowest notch tallies with the zero of the stick gauge while the second highest notch tallies with the meter mark of the stick gauge. Prior to placing of the roll inside the pipe, the timber rod is coated all round with water soluble paint. After placing the rod in the pipe, the entrance is closed with the cap. Two pieces of 65 mm diameter disc galvanized wire mesh is forced up the bottom of the pipe using a suitable mandrell. The whole crest gauge assembly is then attached vertically to trees or H.W. timber post of sufficient length by means of the coachscrew and brackets. Figure24/II/V in Appendix 4A shows the design and specifications of a typical crest gauge used by the Department. When water reaches the highest mark in the pipe during a flood the soluble paint will dissolve accordingly leaving a record of the highest level occurred. To record this flood level the galvanized cap is removed and using the top locating clip, the timber rod is extracted from within the pipe and


4-18 March 2009

placed alongside the relevant stick gauge such that the lowest notch tallies again with the zero of the stick gauge. The flood mark on the rod is then read off against the stick gauge which will give the reduced level value of the maximum flood level. The rod is then recoated with the soluble paint and placed in its former position to recording the next flood. The crest gauge should also be inspected for damage and checked for level errors. Several stages of crest gauges are usually installed to cover the maximum range of water levels at a particular river section where discharge measurements are not required. With this information and an estimation of peak discharge from nearby gauging stations a rating curve can be prepared for use in design and construction of works at this river section. lf a series of crest gauges with several barges are installed on a suitable over reach, the slope of the water surface can be used to estimate the peak discharge. Depth Gauge for Ground Water Level Measurements The electric contact gauge is commonly used for measuring well water levels. It consists of a weight suspended on #16 or #18 stranded insulated wire with depth markings and an ammeter or indicator lamp to indicate a closed circuit. Current flows through the circuit when the end of the wire touches the water surface. Current is supplied by a small 9- or 12-volt battery. To make a reading, lower the electric wire or sounding line until the lamp lights up or needle (ammeter) deflects. Read the distance from the water to the top of casing on the line. Mark the reference point on the casing where you measured the depth. Most commercial models use two conductors and work in conjunction with a standard polyamid tape measure to measure the distance between the marks on the line. For low electrical conductivity in some groundwater, use a meter in the circuit rather than a light bulb. Electrical sounders, which include a reel of wire, meter, and battery, are available commercially. Operation and Maintenance of River Stage Stations The stage of a stream, canal, or lake is the height of the water surface above an established datum. The head in a water measurement structure or device can be defined similarly. The stage, or gage height, of the water is usually expressed in meter and centimeter. Records of stage are important in stream gauging because the rate of flow is plotted against stage in preparing discharge or rating curves. After a curve has been established for a stable channel, rate of flow can be directly determined from stage reading. Reliability of the stage reading is, therefore, of great importance. Records of gage height may be obtained from a series of systematic readings of stick gauges or from automatic water-stage recorders. Normally mean sea level, MSL datum is selected for the station. The operating datum for the station should be set below the water-stage elevation for zero flow. The datum should be permanent for the expected life of the station and should be referenced to at least two or three other benchmarks that are independent of the gauging structure. Two basic philosophies can be used to determine stage or gauge height- direct and indirect. Direct methods involve a measurement of the height from the water level to a datum line; an indirect method infers the stage level from some other characteristic, such as the head read by a pressure transducer. In most cases both are deployed as the latter allows for time-series data to be collected.


March 2009 4-19

Selection of Site The purpose in establishing a river stage station is to obtain a truly representative record of river levels whether for the direct use of this information or as a step towards the measurements of the river discharge. Therefore the selection criteria must generally conform to certain ideals as advocated by the World Meteorological Organization (WMO) for the degree of reliability of observations will depend in part on these conditions being met. One of the main criteria when selecting a suitable site for establishing a river stage station is would be to choose a stretch that has both firm and stable banks and bed, resistant to any scouring or erosion. The bed should also be free of any obstructions including vegetation and there should not be any structures e.g. bridges in proximity to the gauging station, both upstream and downstream. The flow should not under any circumstances by pass the station e.g. via outflow stream(s) upstream. The water surface in this reach should be steeper than the gradient, downstream of this reach. There should not also be any periodic changes in downstream conditions which might affect the relation between stage and discharge at the gauging site. Ideally there should be a straight section upstream at least five times the width of the channel and for downstream twice the width. The site should also be accessible during all stages of flow. Choice of telemetry system depends on the prevailing terrain and topography and if real-time data e.g. for early flood warning system is required. Due consideration must be given towards planning and designing the water level recording system so as to allow seamless integration into the Departments existing network. Observation and Maintenance of Stick Gauge Reading of stick gauges should always be done as close as possible to the gauge itself and at eye level to avoid errors. Observation should only be done when the gauge is cleared of any debris or dirt. Normal procedure dictates the observation to be taken to the nearest centimetre or 0.01 m. All readings are recorded in metres with a qualifying “m” after the value. To improve rating curves gauges should be read more frequent when the water level exceeds certain elevations especially during high flooding events. Toward the highest stages, observations should be taken at short intervals, sometimes of the order of about fifteen minutes. Maintenance include routine inspection of the base to ensure it is still properly secured, cleaning of any dirt or debris left by a high flow event and replacing the plastic plate if the numberings are obliterated. The timber backing should be repainted to restore its condition or replaced if damaged beyond repair. All bolts and nuts must be inspected and tightened if necessary or replaced if damaged. Checking for the zero level at the base of the stick gauge in reference to a permanent benchmark must be conducted on an annual basis or more often if the base is suspected of shifting, especially after flooding events. In the case where a stick gauge is lost and replaced a temporary benchmark (TBM) is used to relocate the zero level for the stick gauge. The establishment details of the TBM can be found in HP. No.25, under Appendix A. Operation and Maintenance of Float Type Water Level Recorder All water level recording stations, be it float type or otherwise must also be equipped with stick gauges. Every week when the chart is changed a record of the water level on the stick gauge must be made and immediately compared with the level appearing on the chart. Remedial action should be taken if any difference in the readings is observed. An inspection of the recorder will indicate the course of action to adopt. If a recorder is out of action stick gauge readings should be taken so that the record of water levels at the station is still maintained until the recorder is fixed and reinstalled. Besides checking the water level on the stick gauge and chart, the time recorded on the chart should be compared with standard time. If a discrepancy is found the clock should be checked by a competent technician observer and either adjusted or sent to the Instrumentation Division for repairs.


4-20 March 2009

The paper chart must be replaced with the correct type when necessary with the name of the station and the date already written. The clock is wound, the drum replaced and the chart set to standard time by rotating it in an opposite sense to the normal direction of rotation. The pen too should be refilled with the special ink provided. Each time the maintenance team visits the station the shelter and site must be inspected for any potential problems such as damaged flooring or intake pipes, inoperative shutoff valves, and faulty ladders. Any observed faults should be repaired immediately if possible, or scheduled for repair at the earliest opportunity. Check for obstructions in the stilling well that may impede the float, float line, or clock weight travel. Check the well for silt deposition by sounding with a weighted tape; compare this measurement to the original clean depth of the well. Note: Remember, the lower intake pipe may be only 0.15 m above the well bottom. Silt deposits can be removed by using a pump and discharge hose to place the accumulated silt into a suspension and to build the water head inside the well. The water head is discharged through the intake pipes to remove the suspended silt. Flushing is continued until the plume of silt being discharged through the intakes is no longer visible. Stilling well system accuracy and reliability depend upon the free flow of water through the intakes, so these inlets must be cleaned thoroughly (Figure 4.12). Flush all intakes immediately before, during and immediately following seasonal high stage i.e. rainy season. For the rest of the year, flush the intakes during each station visit, or as required. Frequent flushing may be necessary where silting is an obvious problem. Make sure the intake valves are open after flushing and before you leave the site. Also ensure that the float tape or line has not been knocked out of position Depending on the locality high flow season in Peninsular Malaysia occurs around March – April and again from October - November. During high flows the water surface in the well may appear to be excessively turbulent. To dampen the oscillation, close one of the upper intake valves and look for improvement. Slowly open the valve until a tolerable level of disturbance is achieved. For in-stream installations, when the water level exceeds the elevation of the top vent holes, block the holes to avoid further velocity turbulence. Note these actions on the station record or log. If the water level should exceed the upper limit of float travel in the well, cut a hole in the floor to allow passage of the float. This measure will provide additional distance for float travel which will in turn add to record collection. Manually pull the float through the hole in the floor to its upper limit of travel. Check to see if the counterweight hits the well bottom. If it does, shorten the float line proportionately. Remove all buoyant material from the floor to prevent it from obstructing the float line or restricting the float's return passage through the hole when the water level recedes. Inspect the streambank for possible cave-in or excessive erosion. If the stilling well is endangered, arrange for emergency corrective action. Until proper corrective actions can be taken, the stilling well should be secured with a strong cable which can be anchored to inshore trees or other stable objects. This will prevent tilting or loss of the well.


March 2009 4-21

Figure 4.12 Regular Maintenance Includes Silt Removal from Stilling Well. Operation and Maintenance of Submersible Pressure Sensor Type Water Level Recorder The pressure sensor is normally installed in a stilling pipe and biological growth besides silt built-up can normally be observed on both the sensor and pipe after a period of time. Therefore periodic e.g. monthly cleaning is necessary (Figs. 4.13 & 4.14). The power supply to the data logger and sensor is also checked with a multi-tester during the cleaning exercise. The connections at the battery terminals are also inspected for corrosion and/or oxidation and these too are cleaned with hot water and a steel brush if necessary. If solar power is utilized, the panel must also be checked and cleaned of any debris including bird droppings and leaves. Data is normally downloaded using a notebook PC via a RS-232 connection. If no data is detected, then the battery power level has to be checked first followed by the pressure sensor and the data logger itself. Many data loggers come with wrap-around type of memory; therefore the frequency of downloading has to be timely enough to prevent unnecessary loss of data.


4-22 March 2009

Figure 4.13 Submersible Pressure Sensor and Stilling Pipe before Cleaning

Figure 4.14 Submersible Pressure Sensor and Stilling Pipe after Cleaning. Inspection In order to obtain accurate and reliable records thorough inspections of the station, its ancillary equipment and river conditions including both upstream and downstream activities must be carried out on at least twice a year. Particular attention should be paid to the construction of bridges, weirs, dams or any other structure in the river. Dredging, desilting and clearing operations in the river and any alterations to the catchment should also be noted.


March 2009 4-23

Each river stage station has a log book in which a record should be kept of all inspections and all changes in the river and conditions which affect in any way the water levels occurring or to the equipment used. Periodic test should be conducted on gauge readers and their respective records to determine their current competency level. Checks and maintenance care should also be conducted on stick gauges and the water level recorder as described in the previous section while recording any anomalies observed. Observations On each record of stick gauge observation the date and time of observation must be included together with the readings obtained. Gauge readings should be in meters and centimeters above mean sea level. On the top of each sheet the name of river, the site of the station and the station number should also be included. For recording stick gauge readings, standard DID Form No. J.P.T. 7A should be used. Care too has to be taken to ensure that the records of float type water level recorders have the station number, station name and period of observation appear on the chart. They should be carefully stored in a dry place to prevent the ink running. 4.3.2 Equipment for Measuring Current Velocities A. Current Meter Current meters are instruments to measure velocity of water flow in the water column. A conventional current meter measures the velocity and direction of the water flow at a discrete point. Each measured point velocity is then related to its respective sub-area of the entire cross section passing flow. The velocity-area formula is then used to compute discharge from current-meter data. Total discharge is then determined by the summation of partial discharges. Types of Current Meters Several types of current meters are used in measuring current velocities in stream and rivers with the majority falling into two categories:

• Propeller and cup type velocity meters • Doppler velocity meters and Doppler current profilers

Propeller and Cup Type Current Meters One of the most commonly used instruments to measure stream flows is the propeller current meter (Figs. 4.15a & 4.15b) which operates on a horizontal axis, while the cup type is based on a rotating vertical axis. In order to rate or calibrate these current meters, they are dragged through tanks of still water (also known as rating car) at known speeds. A velocity formula table with a constant is derived by calculating the number of revolutions of the cups or propeller in relation to the tow speed. The propeller type current meters in use in the Department are Ott C31 Universal Current Meter, Ott C2 Small Current Meter and Amsler 505, while cup type current meters are usual by Pigmy or Watts Mark IV. Cup type current meters are usually applicable only for shallow low flowing rivers while the propeller type is more suitable for all round flow conditions save flooding events. The Ott C31 Universal Current Meter is used for all river conditions except in very shallow streams where the dry-weather flows can only be measured by Ott C2 small meter or the Pigmy. Propeller and cup type current meters are unsuitable in extremely high velocity or very low velocity situations. In high velocity turbulent situations, it would be difficult to maintain the face of the propeller in a perpendicular orientation to the current and the presence of debris, either floating or submerged could also interfere with the gauging operation. In low velocity situations, the speed of the current is below the minimum threshold that enables the propellers to spin properly.


4-24 March 2009

It has been observed that both types do not function well when the water velocity drops below 0.05 m/s or when it exceeds 4.0 m/s or 80% of the given maximum range of the instrument, whichever greater. The current meter is deployed by clamped to the wading rod at the required depth (usually 0.2d & 0.8d) and the counter records the number of revolutions of the propeller over a given period of time. Reliability and accuracy of measurement with these meters can be determined by checking to see if there are any damaged parts and conducting spin time tests for excess change in bearing friction. Usually an experienced field technician is able to check and verify the performance of the instrument to see if further overhaul and calibration is required. As these high precision instruments are very sensitive, due care should be exercise during operation, transit and storage. An improved version of the H.P. No. 15 with Bahasa Malaysia translation provides complete details for proper operation, instrument selection and gauging techniques while for care, diagnostics and calibration the operating manual which comes with respective instrument should be consulted.

Figure 4.15a Ott C31 Propeller-Type Universal Current Velocity Meter

Figure 4.15b. Ott C2 Propeller-Type Current Velocity Meter for Small Streams


March 2009 4-25

Figure 4.16 Pigmy Cup-Type Current Velocity Meter, also for Small Streams Gauging Accessories for Propeller-type Current Meters Wading Rods To securely position a propeller-type current meter to any desired depth, a wading rod is used when gauging a stream. The depth of the measurement can also be adjusted by the operator depending on the water depth at a certain location. Conventional round wading rods are used when working in shallow and medium depth streams. The rod requires a base plate that is rested on the bottom of the channel. The base is to prevent the current meter from striking the stream bed which could damage the current meter and to stop the current meter from being placed too low near the bottom of the wading rod. When in use, an appropriate number of graduated rods usually in 1 cm are assembled based on the water depth. The double end hanger is fixed to the wading rod with a simple thumb screw, and the current meter is attached to the double end hanger. The wiring assembly is attached to the binding post of the current meter and to the top of the wading rod. The plug at the end of the wiring assembly attaches directly to a counting device e.g. headset, counter, Digimeter. The rod is placed vertically into the water until the base rests on the bottom, and the water depth is read off the graduations. Figure 4.17 shows a typical wading rod set up with an Ott C2 current meter and counter.


4-26 March 2009

Figure 4.17 Ott C2 Current Meter with Wading Rod and Counter Gauging Winches For deeper rivers where wading is impractical and dangerous, a winch is used instead if a bridge is conveniently located near the gauging site or if the conditions on both sides of the banks permit the installation of stanchions for a cable to be stretched across. The current meter is then together with a weight suspended from the cable (see Figure 4.18) and raised and lowered to the desired depth by way of a motorized winch installed on one side of the bank or in the case of an available bridge or a boat directly from a portable hand-operated winch. Winches for gauging purposes range from portable hand operated models to the electric-powered versions such as the double drum SEBA SDW-ES gauging winch for heavy duty gauging purposes (Figure 4.19). The corresponding cableway system is designed to bear a maximum of 100 kg sinker weight. The double drums allow the vertical (for actual gauging work) and horizontal movement of the current meter and two separate cable counters account for the precise positioning of the current meter throughout the cross-section of the channel and also in the water column (0.2d, 0.6d & 0.8d). The portable versions are usually mounted on an A-Frame and deployed from bridges (Figure 4.20), boat boards (Figure 4.21) and cable way. These gauging winches can handle loads of up to 140kg and using the spring ratchet stop, the reel can be locked at any desired depth. The winches usually come with counters and display either digital or analog cable readings. In the case of electrical winches, by employing two-conductor suspension cables, signal pulses from the suspended current meter can be transmitted through an electrical brush arrangement to the slip rings at the reel terminals which is then converted to meaningful values i.e. revolution counts and obtained via a display unit integrated in the winch itself. This configuration is also known as a cableway system (see following section for more details on cableway). Since the cables are highly expensive and not easily replaced due consideration should be given towards preventive maintenance. These electrical winches are normally installed in a permanent and lockable cableway shelter with properly secured anchorage (Figure 4.22).


March 2009 4-27

Figure 4.18 Current Meter and Sinker Weight (Yellow) Being Lowered Down from a Cableway

Figure 4.18 Type SDW-ES Electric Double Drum Winch with Invariable Motor


4-28 March 2009

Figure 4.20 Hand-Operated A-Frame with Russian which Developed from a Bridge

Figure 4.21 Portable Gauging Winch with Boom Mounted to a Static Boat.


March 2009 4-29

Figure 4.22 Electrical Double Drum Gauging Winch Firmly Anchored to Secured Cableway Shelter

Cableways There are two-types of cableway systems, one which is manned and operated from a cable car across a river while the other is remotely operated from one side of the river bank. The former approach has been discarded by the Department for the latter for safety reasons. Bank-operated cableways are normally employed where regular discharge measurements need to be made at suitable cross-sections of rivers or canals which are incidentally too deep to wade in, or where no bridge exists and/or the currents in the river are too swift and dangerous for a static boat operation to be used. A typical cableway system is one where a steel A-frame (galvanized) is installed on one bank of a river. This frame is used as a bearing post. The frame is driven into the ground and firmly anchored by two guide wires running from the post to two steel stakes at the rear. On the opposite bank, the double winch rests on a folding skid made of steel. This also is held in position by guide wires at the rear as previously shown in Figure 4.22. A traveller carriage running on the main cable is used to bring the current meter and sinker weight across the water body. The traveller carriage is moved either by the friction guide method or ‘haul out’ system. The former permits bi-directional traversing of the carriage while in the latter the carriage must first be hauled back to the operator end each time before the next gauging run. Both current meter and sinker are suspended from the traveler carriage via a conducting suspension cable as described in the previous section. Figure 4.23 represents a typical cableway installation drawing while Figure 4.24 shows a cableway gauging station with the weight already attached for deployment. The advantages of using the cableway method for gauging is personnel safety as no manpower is required on the water while some disadvantages are encountered when deployed over severely polluted rivers and where there are ongoing upstream logging activities. The cableway system too is limited to a cross-section distance of approximately 400 m.

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4-30 March 2009

Figure 4.23 Typical Cableway Installation Drawing

Figure 4.24 Cableway with Weight Attached for Deployment

The basic components required in a cableway system include the following:

a. Current meter, usually Ott C31; b. Counter and stopwatch; c. Electrical leads, connecting current meter to cable and subsequently to the counter; d. Traversing winch, usually electric with manual operation as back-up; e. Gauging winch with conducting cable; f. Grab bar; and g. Sinker weight; either Columbus 14, 23, 35 or 46 kg or Ott 50 or 75 kg;

Selecting a suitable site for a cableway system refers to a set of criteria similar to those previously stated for a river stage station. For safe operation the surroundings on either side of the bank including upstream must also be clear of any trees, buildings etc. so as to allow the operator a wide


March 2009 4-31

field of view. Maintenance includes bi-annual inspection where bolts, cables, motors, anchors, carriage, connectors etc. are inspected and serviced or replaced where necessary. Clearing the riverbed and banks of any lodged debris is also part of the routine maintenance. H.P. No. 23 describes in detail the installation, operation and maintenance of the cableway system. Other Gauging Apparatus Velocity Floats This method should be used only when other methods are impractical or impossible or where a quick but rough estimate of the discharge rate is desired. Site selection is critical with only a stretch that is straight and uniform (in cross section and grade) and with a minimum of surface waves, be chosen for this method. Surface velocity measurements should only be attempted on windless days to avoid wind-caused deflection of the floats. Surface floats should immerse one-fourth or less of the flow depth while rod floats are submerged more than one-fourth of the depth but do not touch the bottom. Figure 4.25 show some of the rod floats usually used by the Department.

Figure 4.19 Rod Floats on Standby for Deployment. River Discharge Stations Selection of Site In planning a discharge measurement system, a site selection process should be undertaken to determine the most effective and accessible location for the final discharge point. Besides the criteria already mentioned for selecting suitable locations for stage monitoring and cableway systems, the following items should be also be considered during this process:

• nature and frequency of the effluent discharge measurement (i.e., year round or seasonal); • requirements for upstream water level control (e.g., minimum and maximum operating

water levels); • vehicle and personnel access to the site during inclement weather; • personnel access to equipment for operation and maintenance (e.g., minimization of

confined spaces requiring specialized entry and safety procedures);


4-32 March 2009

• power supply to the site (remote locations may require solar-powered systems); • protection of the equipment and instrumentation from weather, wildlife and vandalism; and • existing soil conditions and their effects on stability and cost of the installation e.g., proximity

to bedrock.

Equipment Selection and Installation Once a site has been selected, discharge measuring techniques and equipment should be researched to determine the best system to accurately measure discharge rate at the selected location. The selection of gauging equipment should take into consideration the following:

• the range of flows to be measured; • accuracy requirements over the range of flows to be measured; • potential changes in flow with future expansion or changes to the site; • environmental conditions that may affect the operation of the equipment; • power requirements; • water quality of the effluent body (suspended solids, various chemical or even floating or

submerged logs and other debris) parameters could have negative effects on the operation of the equipment);

• personnel and equipment safety, from loss or damages; • access to equipment for operation, maintenance and inspection; and • space available for the installation

The detailed design of flow measurement systems should be undertaken by qualified personnel with experience in the design, calibration, inspection and maintenance of the gauging equipment. Any equipment shelter considered for protecting or enclosing the system or parts thereof should have adequate space for any necessary or appropriate health and safety equipment in addition to any water quality instrumentation and sampling devices. The site selection, system design and calibration should be thoroughly documented in report form and be updated as necessary to reflect “as built” conditions. This document should be kept available for review by the Department staff and other compliance authorities. Discharge Measurement in Remote Locations Measuring flow in remote locations is often problematic due to site access and power restrictions. Design considerations for remote flow measurement systems include the following:

• provision of a reliable power supply; • provision of a reliable data logging system to record and/or transmit flow measurement data;

and • minimal maintenance requirement.

With the advent of ADCPs, ultrasonic systems etc. real-time discharge rate can now actually be obtained via various telecommunication means including fixed lines, GPRS, GSM and even satellite network. If all means of communication are unavailable and access is restricted then the option of downloading regularly from the data logger must be exercised. Details of the latest ADCP and their various ways in measuring discharge are further discussed in Chapter 12. Routine inspections of remote flow measurement systems would also include monitoring of the power supply. Measurement of Discharge The following describes the four main discharge measurement methods usually adopted by the Department. Other non-traditional approaches such as moving-boat, tracer techniques and acoustic are also mentioned in passing with the last i.e. ADCPs further elaborated in Chapter 12.


March 2009 4-33

Discharge Measurement Method I – Velocity-Area Method This method is the standard approach employed by the Department to measure discharge and it depends on the measurement of velocities at various points across a stream or river, using a current meter. Depending on the prevailing conditions at the gauging station, the method can be employed using via four different ways i.e. wading gauging, bridge gauging, cableway gauging and stationary boat gauging which in part were discussed earlier. The velocity-area method is built around the premises that the discharge, Q can be derived if the vertical surface area and its respective flow velocity are known:

Q = vA, (4.15)

Where v is the mean velocity as measured by the current meter and A is the surface area of the equally subdivided portion of the cross-section in the river (Figure 4.26). By measuring the velocity at different depths in a sub-divided area, the mean velocity for the vertical area can be calculated. Various approaches could be utilized in obtaining the mean velocity, the most common being the one-point and two-point methods. The one-point method places the current meter at a distance of 0.6 the depth or 0.6d from the water surface whilst the two-point method measures both 0.2d and 0.8d from the surface (Figure 4.27)

Figure 4.26 Equally Sub-Divided Vertical Areas along River Cross-Section

Figure 4.20 One-Point and Two-Point Gauging VA Method


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Discharge Measurement Method II – Surface Float Method Where site conditions preclude the use of current meter e.g. during periods of floods when it may be necessary to sacrifice accuracy for speed in order to measure the discharge of a particular river on several occasions to ascertain the shape of the flood hydrograph then the surface float method may be employed although it must not be considered as a standard method to replace measurement by current meter. Cross sections are established along the straight reach of the channel at a beginning, midpoint, and end. The cross sections should be located far enough apart so the time interval required for the float to travel from one cross section to another can be accurately measured. The midpoint cross section provides a check on the velocity measurements made between the beginning and end sections. The channel width across the sections should be divided into at least three, and preferably five, segments of equal width. The average depth of each segment must then be determined. The float must be released far enough upstream from the first cross section to attain stream velocity before reaching the cross section. The times at which the float passes each section should be observed by stopwatch and recorded. The procedure is repeated with floats in each of the segments across the canal, and several measurements should be made in each segment. The more segments there are, the more accurate the measurement. However, it means that the degree of difficulty in doing the measurements will increase also. For flows in canals and reasonably smooth streams, the measured surface float velocities should be multiplied by the coefficients as listed in Figure 4.28.

Correction table, k

d/h k=Vmin/Vf 0.1 0.86 0.25 0.88 0.5 0.90 0.75 0.94 0.95 0.98

where : d = Float depth h = Water depth Vmin = min Velocity Vf = Float velocity

The corrected velocities should then be multiplied by the cross-sectional area of the corresponding stream segments to obtain the segment discharges. The sum of the segment discharges will be the total discharge. The criteria on site selection, float design and the ideal conditions in which to deploy them are already discussed earlier. The following procedures describe the rules-of-thumb subscribed by the Department to measuring discharge rates by surface float:-

(a) Measure at the river discharge station the water surface width and divide this into 10 to 20 sections each of equal width. The minimum number of sections may be used only in those rivers with fairly level beds and symmetrical cross sections. In all order cases more sections are necessary.

Figure 4.28 Coefficient to Convert Measured Float Velocity to Stream Velocity.


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(b) Note in the gauging card (Form No. JPT. IIA) the name of the river, station name, station

number, type of float and its dimensions, date and time, and whether the river stage is rising, steady or falling.

(c) Set out a measured distance along the river bank not less than twice the width of the river. At each end of the measured reach two stakes are set at right angles to the direction of flow.

(d) Note the elevation of the water surface on the stick gauge and the water level recorder if one exists.

(e) Note the distance of the first section between verticals 0 and 1. (f) Measure the depth of water at vertical l using either a sounding rod or lead weight on a

wire. Note the value in the gauging card. (g) Place the float on the water surface about 10 meter upstream of the first pair of stakes. (h) The time to travel the distance between the first and second pair of stakes is measured

and noted in the gauging cards. (i) Repeat steps (g) and (h) and if the difference between the two measurements is less

than 10% of the average, the observations may be accepted otherwise another observation should be taken.

(j) Note the distance between verticals 0 and 2 and carry out the depth and velocity measurements described in paras. (f) to (i) inclusive. Similarly observations are taken at the other verticals.

(k) The water level is again observed and noted with the time of observation (1) The mean velocity at each vertical is the measured velocity multiplied by 0.85.

One of the main uses of the above method will be for the measurement of flood flows when several sets of observations are required or several rivers must be gauged. Where depth readings are unobtainable, preparations must be made in anticipation of such observations and the necessary measurements of the cross section made immediately before and immediately after the flood, taking into consideration that bed scouring occurs during floods and the cross section of the river at the time of measuring the depth will not be the same as when measuring the velocity. Figure 4.29 shows a tubular float about to be discarded from a bridge.


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Figure 4.21 Float Deployment from a Bridge. Discharge Measurement Method III For small stream and rivers the challenge is to obtain discharge rates during dry season or low flow period. The accuracy with which discharge is estimated can be improved by using a weir to create an area of the channel where hydraulics are controlled (control section). Each type of weir needs to be calibrated either in the laboratory or by the manufacturer such that the stage at a predetermined point in the control section is related to the discharge rate using a known empirical equation. Weir is an obstruction (usually a vertical plane) built or placed across an open channel (or within a pipe under open channel flow) so that water flows over the weir's top edge (or through a well defined opening in the plane). Many types of weirs can be used to measure discharge; the most common being the broad crested or rectangular weir. Specific discharge equations are used for each type of weir. Weirs are simple structures and relatively easy to install, although the cost may be expensive depending on the site location (Figure 4.30). A weir can be used to regulate flow in a natural channel with irregular geometry, a situation where the Manning equation, for example, would not provide reliable estimates for the discharge rate. However, a weir will back water up in channels by creating a partial dam. During large storm events, backed-up water may cause or worsen flooding upstream. When evaluating the suitability of a monitoring site for a weir, it is important to determine whether the system is "over designed." That is, will the conveyance be able to move the design capacity after weir installation? In the case where the downstream depth of flow is greater than the crest of the weir, a different stage-discharge relationship for the weir shall apply. Another potential problem that weirs introduce to a channel is that sediments and/or debris may accumulate behind the weir, which can alter the hydraulic environment. By altering the hydraulic environment, these materials also change the empirical relationship between depth of flow and


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discharge rate. Therefore, weirs must be inspected regularly and all the accumulated sediment and debris removed.

Figure 4.30 Profile Drawing of a Sharp-Crested Weir 4.3.3 Discharge Measurement Method IV

A single float is deployed for quick estimate (± 25%) of discharge rate, during normal flow or even high flow. The stream or canal must be deep enough that the float does not touch the bottom and a straight stretch is then selected. The float is deployed in midstream usually from a bridge or a boat and the surface velocity measured with the aid of a stopwatch. The mean velocity in the channel is usually 0.6 times of the observed velocity, which is then multiplied by the area of cross section to determine the discharge.


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Table 4.2 Summarizes the Discharge Measurement Methods Described Above, Their Typical

Applications and Required Equipment

TABLE 4.2 DISCHARGE MEASUREMENT METHODS

Type Normal Applications Typical Site Required Equipment

Discharge Measurement I.

Current Meter w/wading rod

• Low and normal discharge rates

• Shallow streams and canals • Manual sampling

Propeller-type current meter with wading rod, fiberglass measuring tape, field forms/notebook

Current Meter w/portable winch from bridge

• Normal to high flow

• Streams and rivers too deep and/or dangerous to wade; bridge present

Propeller-type current meter with portable winch, fiberglass measuring tape, sounding weight and line

Current Meter from Cableway

• Normal to high flow

• Large and deep rivers with no bridge present Propeller-type current meter with

permanent double drum winch and shelter

Current Meter from Stationary Boat

• Normal flow

• Rivers too deep to wade and no bridge present and terrain unsuitable for cableway installation

Propeller-type current meter with portable winch, fiberglass measuring tape, handheld depth finder, cableway and stakes

Discharge Measurement II

Surface Float • High flow

• Homogeneous cross-sections and stretches of deep and fast flowing rivers

Suitable float, stopwatch, stakes, fiberglass measuring tape, sounding lead and line

Discharge Measurement III

Weir Installation • Low flow • Small streams and rivers Broad-crested weir and water level station.

Calibration is necessary.

Discharge Measurement IV

Single Float • Normal to

high flow

• Any streams or canal deep enough, with straight reach, well-define banks and a bridge nearby.

Single float, stopwatch, sounding weight and line and fiberglass measuring tape

Other Discharge Measurement Methods Indirect Method of Peak Discharge Measurements The discharge of a stream or river is typically measured directly by stream gauging or a rating curve. However, conditions sometimes preclude direct measurement of discharge, such as during flooding events, or in physically challenging sites. The slope-area method, also referred to as the stage fall discharge method is used to determine peak discharge along sections of a river or stream where gauges are not present. It is particularly applicable in determining the peak discharge during flooding along a particular reach of stream or to estimate the discharge necessary to cause flooding along a section of river. The slope-area method is based on the Manning's equation for determining discharge, Q = (AR2/3S1/2)/n] (4.16)


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where A is the cross-sectional area (m2), R is the hydraulic radius (cross sectional area/wetted perimeter (m), S is the slope (drop in elevation/length (dimensionless)) and n is the Manning roughness coefficient (dimensionless). Thus, the slope-area method is a function of (1) slope, (2) channel dimensions and (3) channel roughness, and therefore field data are required for estimation of peak discharge. These data include determining the elevation and location of high-water marks along the stream, measurement of channel cross section and wetted perimeter by surveying, tape and compass, or GPS, and selection of a roughness coefficient for the section of stream in question. Procedure A. Selection of Cross Sections Select two cross sections along a reach that have at least 15 cm of elevation difference between their high water marks (McCuen, 1998). We recommend a minimum of 100 m feet separation of these cross sections along a reach, and 600 m is optimal in low slope areas. a. High water marks must be clearly evident on both sides of the river, at both cross sections. b. The reach of river between the cross sections must have similar roughness characteristics c. There must not be any bridges or other "disruptions" to the stream course between the cross

sections. d. If GPS is used to acquire cross section measurements, there must be no barriers that will block

or degrade the satellite signals at both cross sections. B. Water Surface Slope Determination One of the most difficult concepts to understand when using the slope-area method is that 2 different slopes are calculated. The first is the water surface slope ((hu-hd)/L), which is simply the slope of the high water marks between the upstream (hu) and downstream (hd) sections, and would be the slope of the water surface during peak discharge. The water surface slope is used as a first approximation for the energy slope, which is ultimately the slope that the slope-area method relies on. The energy slope ((Hu-Hd)/L) is the slope between the upstream and downstream cross sections of the high water marks PLUS the velocity head (energy, or v2/(2g)), such that

Se = ((hu+ uvu2/(2g)) - (hd+ dvd

2/(2g)))/L (4.17) where Se is the energy slope, hu and hd are the high water marks, is a correction factor that accounts for expanding or contracting reaches (and is typically ignored by giving it a value of 1 for both upstream and downstream sections), L is the distance between the two sections, and g is gravity. For example, if the upstream section had a significant higher velocity than the downstream section, then the upstream velocity head (Hu = hu+ vu

2/(2g)) would be much higher than the downstream head (Hd = hd+ vd

2/(2g)) and the energy slope would be much greater (Figure 4.31). Alternatively, if the downstream section had a significantly higher velocity than the upstream section, than the energy slope would be lower (less difference between the upstream and downstream total heads). The diagram below shows the relationships between the water surface slope ((hu-hd)/L) and the energy slope ((Hu-Hd)/L).


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Figure 4.31 Upstream Velocity Head would be much higher than the Downstream The slope-area method ultimately uses the energy slope to determine peak discharge, but the energy slope is first approximated using the water surface slope (the "Calculation" section will explain this more in depth). Therefore, to determine the water surface slope, first identify the high water marks at each cross section (these marks should be at essentially the same elevation on both banks of the same cross section). Using GPS or surveying equipment, measure the elevation difference (hu-hd) and distance between the high water marks of the two cross sections (L) on the right edge (looking downstream), then repeat for the left edge. Calculate the slope between these water marks

S = (hu-hd)/L (4.18) Compare the calculated slope for the right and left edges. If these differ by greater than 5%, recheck your high water mark interpretation and your measurement of elevation differences and height. If high water marks are not evident, or if there is a large discrepancy between your right edge and left edge slopes, it is possible to estimate the water surface slope from a topographic map, realizing that this method can introduce significant error in your estimate (the water surface slope and bed slope may be very different during flooding events). Select two topographic lines that cut across the streambed on the topographic map, and measure the distance the stream travels between these lines (not straight-line distance), then divide the elevation difference by the distance. Remember that using water surface slope is recommended when possible, but bed slope can be used when other methods of determining water surface slope fail. Note if using GPS. Make sure you measure the distance along the banks of the stream in order to determine L, and not just the straight line distance between the high water marks. Thus, it is better to select a "line" rather than 2 "points to determine the elevation difference and distance between the cross sections. C. Measurement of the Hydraulic Radius (R) The hydraulic radius (R) is the cross-sectional area divided by the wetted perimeter of the section, and thus has units of ft. A simple topographic profile of the cross-section measured by GPS, surveying, or tape and compass provide the distance along the stream bed from the left high water mark to the right high water mark (wetted perimeter), and cross-sectional area is easily determine by multiplying the average depth of the section (elevation of the high water mark minus the average stream bed elevation along the profile) by the width of the profile (straight-line distance between the high water marks. Therefore, one must measure the following to obtain the necessary data at both the upstream and downstream cross-sections.


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a. Measure elevation (relative or absolute) of the high water marks (they should be essentially

the same elevation). b. Measure the straight line distance between the high water marks (width). c. Measure depth of the river bed below the high water marks at set intervals (every 1 m – 1.5

m) and calculate the average depth. The area (A) is calculated by multiplying the width of the river by the average depth.

d. Measure the wetted perimeter (P) by measuring the distance along the river bed from one high water mark to the other. The hydraulic radius

(R) = A/P (4.19)

Note if using GPS. Choosing a "line" setting rather than a "point" setting will enable you to easily dump the data into an Excel Spreadsheet and calculate both the wetted perimeter (3-dimensional line) and the straight line distance between the high water marks (2-dimensional line). You will also be able to reasonably estimate cross-sectional area IF you walk across the section with a constant velocity (straight line positions are fairly evenly spaced). See the GPS tutorial for calculating 2 and 3-d distances and cross-sectional areas for more help. D. Estimation of Manning's Roughness Coefficient (n) Inspection of the riverbed will reveal characteristics related to roughness. An excellent treatment of the use of Manning's coefficients is found on pages 128-136 of McCuen (1998). Below is a first-approximation of Manning's coefficients for some widely observed beds. n = 0.04 - 0.05 Mountain streams n = 0.035 Winding, weedy streams n = 0.028 - 0.035 Major streams with widths > 30 m at flood stage n = 0.015 Clean, earthen channels E. Calculations

i. Compute values of cross-sectional area (A), hydraulic radius (R), and roughness (n) for each cross section, and water surface slope (S) between the cross sections.

ii. Calculate the upstream and downstream conveyance values (Ku and Kd), such that Ku = AuRu

2/3/nu and Kd = AdRd2/3/nd (4.20)

iii. Calculate the average conveyance of the reach of river between the cross-sections

K = (KuKd)0.5 (4.21)

iv. Calculate the first estimation of peak discharge by multiplying the average conveyance (K) by the square root of the water surface slope. Qp = KS0.5 (4.22)

This answer is in m3/s, and gives a rough approximation of the peak discharge. However, a closer approximation is calculated if the slope includes the velocity heads, which can now be estimated from this initial discharge calculation.


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v. Calculate the difference in the velocity heads (dhv) of the upstream and downstream section

according to

dhv=Qp

2

2gαu

Au2 - αd

Ad2 (4.23)

where g is gravity (9.81 m/s2), and Au and Ad are the upstream and downstream cross-sectional areas. Also remember that α= 1. Note that the area values (and units) are squared, giving dhv in m. Note: If the downstream area (Ad) is less than the upstream area (Au) then dhv will be a negative number. Simply put, if the downstream area is less than the upstream, the downstream velocity and its velocity head will be greater than upstream. Thus, the greater downstream velocity head will reduce the energy slope (thus the negative number, see below).

vi. Compute a new energy slope Se = ((hu - hd) + dhv)/L (4.24)

vii. If the dhv is negative (downstream velocity is greater) than the energy slope is less than the

water surface slope. viii. Compute a new peak discharge using the energy slope

Qp = KSe0.5 (4.25)

ix. Repeat steps 6 and 7 above using this new peak discharge until Qp does not vary.

Non-Traditional Methods of Discharge Measurements Other discharge measuring methods and ancillaries not previously mentioned are the moving boat method, tracer methods and the use of acoustic (ADCP) technology. The following describes in brief each of these methods with further elaboration of the ADCP, the various types available and their discharge measurement approach in Chapter 12. Moving-Boat Method The moving-boat method is used when conditions in the river such as flooding events do not permit the conventional wading rod velocity-area approach or when no bridge or cableway system is available within the desired stretch. This method is also employed when there is a need to measure rapidly changing flow (and stage) in a wide river e.g. 400 m or more across where time is of essence. THEORY OF THE MOVING-BOAT METHOD The moving-boat method measurement is made by traversing the river along a pre-selected path that is normal to the flow. The parameters measured are flow velocity, vessel position and speed over ground, bottom depth and the angle at which the vessel deviates from the traverse line. Data is continuously collected at specified intervals along the path without stopping. A weighted hydrodynamic-shape platform is used to carry the various instruments involved and is lowered with aid of a winch. Flow velocity is measured by a propeller-type current meter, while a hydro-acoustic transducers of an acoustic Doppler log orientated perpendicular to the axis of the current meter and at an angle of 30° relative to the vertical axis is used to measure vessel velocity along the transect. An echo sounder is used to measure and record the bottom depth across the river. During a traverse of the cross section, the boat operator maintains course by “crabbing” into the direction of the flow sufficiently to remain on line. The step-by-step details describing the Pless system, an


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improved approach to the same method, its operation, formulas and calculations and field results and comparison as described by Klein et al (1993) are found in Appendix 4A. Tracer Techniques Tracer methods can be used to determine discharge with accuracies that can vary considerably from about +/-1 percent to over 30 percent, depending on the equipment used and the care in applying the techniques. A tracer is considered anything that mixes with or travels with the flow and is detectable. A detectable tracer can be timed as it passes through a reach, or tracer concentration profiles can be measured in a reach. However, there are certain difficulties which cause errors when using the tracer technique. Salt tracer used to be popular back in the 60’s but with the advent of cheap and reliable dyes this method is now obsolete. Radio-isotope tracers are very reliable but they warrant special care and handling by trained and certified personnel with special gear which makes the gauging task arduous. Therefore the contemporary choice of tracers for gauging is still dye tracer. Dye concentrations are measured by fluorimetry or color comparison standards. Sometimes, visual observation of an exiting dye cloud is used, but considerable loss of accuracy occurs. Dyes that are commonly used are Fluorescein, Rhodamine B, Rhodamine WT, or Pontacyl Pink B because they are easily visible in very dilute solutions and also easily measured with optical instruments such as fluorometer and spectrophotometer. Rhodamine WT have been cleared as nontoxic by the U.S. Food and Drug Administration and can be used in potable water to a maximum limit of 10 ppb. Rhodamine and Pontacyl Pink B are also quite stable with respect to fading by sunlight and to changes caused by waterborne chemicals. They do not tend to deposit on flow surfaces, sediments, or weeds. These dyes are usually available in liquid form, and solutions are easily prepared. Before conducting a discharge measurement program, selected dyes should be tested with water samples or earth canal embankment material samples and exposed to check for possible adsorption, chemical reaction, and fading effects on dye stability. Dye tracers are used to determine discharge in two basic ways: (1) the velocity-area method, in which time of tracer travel through a known channel length and average cross-sectional area determine discharge and (2) the dilution method, in which discharge is determined by the downstream concentration of fully mixed tracer, which has been added upstream at a constant rate, and by accounting for the amount of tracer substance found downstream. Detailed procedures of methods (1) & (2) can be obtained from www.turnerdesigns.com/applications/flurorescentdye tracing/998-5000.html Acoustic Method In transit time acoustic meters, the velocity of sound pulses in the direction of flow is compared to the velocity of sound pulses opposite to the direction of flow to determine the mean velocity and, thus, discharge. With Doppler acoustic meters, sound pulses are reflected from moving particles at different frequencies within the layers in the water column, similar to radar principle. A transit time acoustical sensor emits a sound wave under water across a channel and measures the time required for the signal's return. Transit time is correlated with channel width. The relative positions of the emitting and receiving sensors are used to estimate velocity. A minimum depth of flow is required. This type of sensor can only be used at sites with sufficient baseflow to provide the medium in which the sound wave travels. If there is no baseflow, the lower portions of the rising and falling limbs of the hydrograph will be lost. A drawback of this technique is that it requires almost permanent installation of both transmitter and receiver units including power supply thus exposing the system to vandalism and theft.

http://www.turnerdesigns.com/applications/flurorescentdye tracing/998-5000.html�

http://www.turnerdesigns.com/applications/flurorescentdye tracing/998-5000.html�


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Acoustic technique, too can be used to measure velocities at equally divided points across a stream the discharge rate can be calculated using the velocity-area equation. By measuring the change in frequency from the acoustic reflections from moving particles in the water body like small sediments and air bubbles, an acoustic type current meter can be used to determine the velocity. An Acoustic Current Meter or ACM (Figure 4.32) is akin to single point current meters in that it measures velocity in the water column one point at a time. By sending out an acoustic signal of a certain frequency which bounces off moving particles at a point in the water column and is returned to a receiver, the instrument is able to analyse the signals for frequency changes and the mean value of the frequency shifts then can be related to the mean velocity of the moving particles in the water. ACMs are typically used when only one point measurement in the water column is required over a period of time. Acoustic Doppler Current Profilers (ADCP) are natural progression from ACMs. These instruments measure average velocities in cells of selected size in a vertical series along the water column. ADCPs have long been used to measure current velocity and flow in deep waters in reservoirs, oceans, and large rivers and new models with very high frequencies are now available for very shallow rivers and streams. Most of the meters in this class are multidimensional or can simultaneously measure more than a single directional component of velocity at a time. Both ACMs and ADCPs are usually self-contained instruments; they come with tilt, temperature and pressure sensors, compass and on-board battery packs and internal data logger for long term deployment. Further details of ADCPs and their various approaches to discharge measurements are discussed in Chapter 12.

Figure 4.32 A Single Point Acoustic Current Meter or ACM

Frequency of Discharge Measurement The usual practice is to conduct gauging on a monthly basis to obtain an accurate rating curve. This is true for steady state or normal flow situations for most period of the year. However more frequent e.g. weekly basis gauging exercise are recommended for during drought months and also during rainy seasons where the both extreme ends of the curve may vary depending on the changes in river bank and bed morphology. For very low flow conditions dye tracer technique may be necessary in addition to the usual current meters whilst for very high flow events the use of unmanned ADCP (for small rivers and streams) or from a moving boat (manned ADCP) may be applied at locations where cableway are not available. Chapter describes further the use of ADCP in environmentally-challenging situations. During the low and high flow periods where manpower is short, it is suggested that the stations are gauged on a rotation basis.


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Station Control The pre-selected cross section of a river or artificial channel is usually referred to as a control. A control is usually dictated by the geometry of the cross-section and also the physical features of the river downstream of the section. Station control then refers to the physical features found downstream, natural or otherwise influencing the existing rating curve for the gauging station at hand. The control can be in the form of a weir, gravel bar, rapid channel, contracted passage etc. that would cause backwater effect onto the stage and discharge curve. There are mainly three types of station control:

a. Permanent station control whereby the effects of the control is steady and unchanged over the whole rating curve, from low flow to high. Therefore no adjustments or adaptations are required.

b. Partial station control refers to the situation where a simple stage discharge curve is only applicable under normal flow conditions but more empirical and frequent measurements would be required in order to extend the rating curve beyond the applicable limits.

c. Shifting station control poses more challenge as the stage discharge curve is constantly subjected to fluctuation over time caused by the dynamics in scouring, silting, vegetative growth etc. as reflected in a two stage-discharge curves, one for a rising flood and the other for a falling flood.

For many stations, a shift in the station control or a backwater condition may occur at certain times during the year as a result of vegetative growth, a jammed log and/or changes to riverbed and bank morphology from high floods. During such periods, shift or backwater corrections can be determined from available discharge measurements. However, apart from these measurements which plot off the curve for reasons indicated above, most of the measurements will plot somewhat off the curve as a result of normal scatter. For these, no correction is computed; however, it is normally found useful for purposes of expressing mathematically the degree of scatter to indicate for each measurement the percentage difference between measured discharge and the discharge indicated by the stage-discharge relation. For discharges less than about 0.005m³/s the differences may be expressed in cubic meters per second instead of percentage. A discharge measurement made during the computation period may plot substantially off the stage-discharge curve. It is recommended that discharge measurements be computed and plotted on site and redone if it plots off the curve. This can often determine if it is a bad measurement or if a shift has occurred. However, sometimes the second measurement cannot be done, or sometimes it is done and the departure cannot be explained. If, after careful analysis and review, no satisfactory cause of its departure from the stage-discharge curve can be determined, the measurement should be eliminated from use in the computation 4.4 CAUSES OF FLOOD AND THEIR ESTIMATION An area is said to be flooded if stormwater runoff cause a significant rise in water level above the ground level inflicting damage to properties or crops or disrupting the normal activities in the area. The source of stormwater runoff can be from a catchment upstream or it could be from an adjacent river overspilling its banks or bunds. Flood is of concern if it causes substantial property damage or it causes significant disruption to normal activities. Therefore a swampy area or an undeveloped open space experiencing a 0.3 m of rise in water level above the ground may not be viewed as flooding by some and that’s the reason some floods in undeveloped areas are not recorded as flood prone areas. On the other hand the same 0.3 m inundation along a highway would cause havoc to traffic and be given lots of media attention. Damage to properties will occur if flood rises above the floor level of properties and


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damage increases drastically as flood level rises. Beyond 1.0m flood becomes potential danger to lives. Swift flowing flood waters of 0.6m can carry away most vehicles due to buoyancy of the vehicle in water and momentum of flowing water. Agricultural areas are more tolerant to flooding than urban areas. Most crops especially tree crops (rubber, fruits) can withstand shallow inundation for periods of 48 hours. Oil palms are more resistant to flooding and can stand longer periods of inundation and the loss is often attributed to loss of harvest when access is cut off during a flood. Localised floods experienced in many urban catchments such as Kuala Lumpur, Penang and Ipoh are due to water brought about by short-duration and intense thunderstorms while the larger basin floods such as the widespread flooding that occurs in Kelantan and Pahang River Basins are attributed to longer duration and widespread rainfall brought about by the North-East Monsoon. It is of interest to estimate flood events in terms of flood peak discharges and flood volumes as they form the basis of design of flood mitigation works and flood management strategies. 4.5 ESTIMATION OF DESIGN FLOODS Various techniques can be adopted in estimating design flood discharges and the techniques are broadly categorized as follows:

• Empirical formula such as the Rational Formula and regional equations such as those described in DID’s hydrological procedures or HPs

• Flood frequency analysis of observed floods which can be extracted from DID’s hydrological database.

• Rainfall-runoff models simulating the behaviour of catchment in receiving rainfall, storing part of the rainfall and transforming the excess rain into runoff hydrographs

The choice of techniques depends on the availability of data, tools and expertise available (especially in flood modelling), objectives of project and the complexity and importance of the project. DID publishes Hydrological Procedures or HPs and many of the HPs describe techniques for estimating design floods. The HPs related to flood estimation are HP1, HP4, HP5, HP11, HP16 and HP18. The HPs resulted from studies carried out by DID using local data extracted from JPS’s hydrological database.

• HP1 provides a method for estimating design rainfall for various storm durations and ARIs. • HP4 allows the user to estimate design flood discharge based on the regional flood

frequency method. • HP5 describes a procedure for estimating design flood discharge based on the Rational

Formula. • HP16 describes a procedure for estimating flood discharge in urban catchments based on a

modified form of the Rational Formula. • HP11’s synthetic triangular hydrograph method of estimating floods in rural catchments is

based the Synder’s synthetic triangular flood hydrograph method but the equations used for estimating some of the hydrograph parameters were derived based on local data.

• HP18 is also a procedure for estimating floods but is meant to be applied to agricultural drainage design.

HPs are often used by consultants for developers to design urban drains and ponds. They provide the consultants with a uniform approach towards flood discharge estimation.


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In major flood studies such as development of urban stormwater master plans or river basin flood studies, it is usually expected that effort be made to use whatever available recorded data from DID’s hydrological database to estimate design flood discharges, flood volumes or flood hydrographs. Depending on the design objectives, there are various approaches in hydrological data analyses in flood estimation:

• Where only designed peak discharge is required, i.e for sizing channel capacity, then methods that yield design peak discharges such as the regional flood frequency method of HP4, the Rational and modified Rational Formula of HP5 and HP16 respectively can be applied.

• Statistical analysis of annual peak discharges yields design peak discharges and thus can be the adopted approach if historical peak flood discharge data are available.

• In design of ponds, pumping schemes and tidal control gates, the volume of flood discharge and the temporal distribution of flood discharge is an important consideration and therefore estimation of peak flood discharge only is insufficient. The shape of the design flood hydrograph is also important. For instance in the design of flood detention ponds the flood hydrograph with lower peak discharge but with higher flood volumes may be more critical than the flood hydrograph with higher peak discharge.

• The HP that yields design flood hydrographs is HP11 which estimates flood based on the synthetic triangular flood hydrograph method. But HPs which adopt the Rational Formula i.e. HP5, HP11 and HP16 which is usually meant for estimating peak flood discharge also allow the user to plot a simplified hydrograph (also known as the Rational Method Hydrograph Method)

• Where data is available and the complexity of the project justify the effort, then attempt should be made to estimate design floods using rainfall-runoff modelling.

• Many rainfall-runoff models: notable ones being as Hec-HMS of US Army Corps of Engineers, RORB Model of Monash University and NAM Model of Danish Hydraulic Institute allow user to break the catchments into sub-catchments and specify for each subcatchment, parameters such as lag-time or its equivalent and loss rates (infiltration loss). This allows the modeller to study impact of urbanisation of flood runoff as urbanisation affects infiltration loss and hydrograph lag time.

• Many models also allow the user to include storage dams, detention ponds, flood diversions and such changes are cannot be estimated with statistical analyses of historical discharge data.

• Another instance where historical discharge data is of limited use is when there is a need to estimate future urbanised discharges in a catchment where the current landuse is predominantly rural in nature.

• Many modelling effort are also carried out for catchments with limited runoff data for calibration and verification. The main reason for modelling despite the lack of data is the flexibility which the model accords to the user in terms of studying the impact of urbanisation, the impact of proposed dams and river diversions, etc. Values estimated from HPs are sometimes used to cross-check the results of models. This is not calibration in the true sense of the word but more of ensuring the models results are reasonable.

The various common flood estimation techniques are described below. 4.5.1 Empirical Methods and Regional Equations The Rational Method is an empirical flood estimation method which is described in detail in Hydrological Procedures HP5 and HP16 and DID’s Stormwater Management Manual (2000). The Rational Formula is given by:

Q y= C.I. A / 360 (4.26)


4-48 March 2009

Where Qy = y year ARI peak discharge (m3/s) C = dimensionless runoff coefficient I = y year ARI rainfall intensity over time of concentration tc (mm/hr) A = catchment area (ha) Catchment area A can be measured from maps, intensity data obtained from the IDF curves developed by DID or derived from recorded rainfall data. User needs to determine the time of concentration tc. Runoff coefficient C depends on catchment landuse and rainfall intensity. More developed areas will have more impervious areas leading to higher C values. Higher rainfall intensity also leads to higher C values, the reason being catchment becomes more impervious when the ground in ponded with water during intense rainstorms. The Rational Method is simple and is popularly used in urban drainage design. Critics of the Rational Method claim that the method is simplistic, does not take into account catchment storage and flood routing effects and as such the method tends to overestimate the peak flood discharge. The general opinion is that the Rational Method is acceptable for small catchment areas. DID’s Stormwater Management Manual (2000) says that method gives satisfactory results for catchments up to 80 ha. HP4 provides regional equations for estimating design floods for rural catchments greater than 20 km2. Catchments were categorized into 2 sets of regions.

• One set of regions, the MAF regions (MAF 1 to MAF 6), is for mean annual flood (MAF) determination.

• Another set of regions, the FF regions (FF 1 to FF 6), is meant for flood frequency determination.

The forms of the regional equations are as follows:

MAF = c . Aa . Rb (4.27) Where MAF = mean annual flood (m3/s) A = catchment area (km2) R = mean annual catchment rainfall (m) and a, b, and c are constants unique to each MAF region

Table 4.3 Coefficients of the MAF Regional Equations

MAF Regions a b C

MAF 1 0.6528 0.7901 0.1980 MAF 2 0.9630 0.6541 0.8093 MAF 3 0.1192 0.6175 3.0571 MAF 4 0.1048 0.7177 3.0224 MAF 5 0.0140 0.7954 5.0354 MAF 6 0.4783 0.9066 0.9463

In deriving the regional equations flood frequency analyses was carried out on flood discharge records extracted from DID’s river stations and the flood frequency curves obtained were converted to dimensionless curves by dividing the discharge by the MAF. Z = Qt / MAF (4.28)


March 2009 4-49

where Z = dimensionless variable MAF = Mean Annual Flood Qt = t year ARI flood discharge

Catchments located adjacent to one another and exhibiting similar dimensionless curves (Z versus ARI) were grouped into flood frequency (FF) regions. The general form of the dimensionless flood frequency curves is a s follows:

Z = m log10(t) + c (4.29) Where t = ARI (years) m and c are constants unique to each FF region

Table 4.4 Constants of the FF Regional Equations

FF Regions m c FF 1 0.77 0.72 FF 2 0.82 0.70 FF 3 0.92 0.66 FF 4 1.33 0.51 FF 5 1.49 0.45 FF 6 1.69 0.38

The regional frequency method allows the results of individual station flood frequency analyses to be extended to any catchment within the same region. Experience shows that HP4 gives quite low estimates of flood discharge. HP11 describes a procedure for flood estimation which adopts the Synder’s synthetic triangular flood hydrograph method (see Figure 4.33) and the parameters of the synthetic flood hydrograph are presented in Tables 4.5 and 4.6. Three groups of catchments, Group1, Group 2 and Group 3 were defined.

Table 4.5 Catchment Groups of HP11 and Parameters

Catchment Type Ct

n imperial metric

Group 1 Whole catchment very steep and covered in virgin jungle 2 0.32 0.35

Group 2

Upper catchment very steep and jungle covered, lower

catchment hilly and covered predominantly with rubber

4 0.64 0.35

Group 3

Whole catchment undulation with variable vegetation

including jungle, rubber and agricutural development

8 1.28 0.35


4-50 March 2009

Figure 4.33 Triangular Direct Runoff Hydrograph of HP11 The catchment lag time Lg (hours) is given by:

Lg=CtL*Lc

√S

n (4.30)

Where Ct ,n = catchment group parameters from Table 5.3

L = main stream length from outlet to catchment boundary (miles) Lc = main stream length from outlet to catchment centroid (miles) S = weighted mean stream slope (ft/mile)

Parameter C (hrs) is computed based on the storm duration D (hrs) and the lag time Lg (hours) i.e:

C = D/2 + Lg (4.31)

and based on C, the parameters Dp, Tb and Tp can be determined from Table 4.6

Table 4.6 Parameters of Dimensionless Triangular Flood Hydrographs

Catchment

Group Dp Tb Tp Tb/Tp

Group 1 1.06 1.89C 0.94C 0.50 Group 2 0.89 2.24C 0.87C 0.39 Group 3 0.75 2.67C 0.58C 0.22

The peak discharge is computed as

qp=Dp*A*640*Q

Lg+D2

(4.32)

Rainfall Excess

Q

Q/2 Q/2

D

C

Tb

D /2 Lg

TrTp

qp


March 2009 4-51

Local adaptation of the method is reflected in the locally derived parameters of the lag time equation and dimensionless flood hydrographs (Table4.5 and 4.6)

Feedback from many users of DID’s HPs indicates that HP11 gives the most realistic flood estimates for rural catchments compared to other flood estimation procedures described in DID’s Hydrological Procedures. HP11 contains a statement recommending that the procedure not be used for catchments larger than 200 sq miles (km2) because the assumption of uniform area distribution of rainfall becomes unrealistic for large catchments. The synthetic triangular hydrograph method of HP11 assumes uniform rainfall distribution. But it should be noted that HP11 contains an alternative procedure i.e. the triangular unit hydrograph method (note the difference between triangular hydrograph and triangular unit hydrograph) which allows the user to derive triangular unit hydrograph for any unit interval, (d hours) and thereafter the user can apply unit hydrograph procedure to derive the runoff resulting from any regular interval (d-hour interval) storms with varying temporal pattern.

Storage attenuation in larger catchments seems to be the issue in both the Rational Method and HP11 and the implication is that both methods will yield discharge estimates on the high side if applied to catchments larger than recommended. Catchment size is an issue if the whole catchment is viewed as one single catchment. This need not be the case if the catchment is sub divided into smaller sub catchments and each modeled separately and linked by channel and routing through channels carried out using methods such as the Nash cascade or the Muskingum method. This process would elevate the procedure to modeling techniques which is possible with powerful spreadsheets software such as Microsoft Excel. There are also models such as Hec-HMS which actually has the Synder Triangular Hydrograph Technique incorporated as one of its rainfall-runoff technique. 4.5.2 Hydrograph Method Time-Area Method In the design of flood storage ponds and pumping schemes, the designer is interested not just in the peak flood discharge values but also the shape of the flood hydrographs. Several flood hydrograph methods are often adopted. The time-area method described in DID’s Urban Stormwater Management Manual 2000 is an example of design flood estimation using the hydrograph method. The time area method is a hydrograph method for flood estimation. The example below illustrates the time-area method. The incremental time step for hydrograph computation is set and in this example it is 10 minutes. Isochrones or line of equal travel time to the catchment outlet of 10, 20, 30 and 40 minutes are drawn and the areas between the isochrones A1, A2, A3 and A4 determined. The hyetograph of rainstorm to be used in the time-area method is determined. In this example the hyetograph is as shown in Figure 5.3. Losses due to infiltration must be taken off to determine the effective rain. Table 5.10 taken from MSMA manual recommends various approaches in estimating values for loss rate. For this example, an initial loss of 5 mm for the first 10 minutes and constant loss rate of 1 mm per time interval for the remaining time intervals were assumed. This loss is subtracted from the rainfall hyetograph to yield the effective rainfall hyetograph (see Table 4.7)


4-52 March 2009

Table 4.7 Incremental Area, Rainfall and Effective Rainfall Hyetographs

A B C D E

Time (mins)

Incremental area (ha)

Rainfall (mm)

Losses (mm)

Effective rainfall (mm)

0 0.0

10 2.0 15.0 5 10

20 5.0 20.0 2 18

30 6.0 8.0 2 6

40 4.0 10.0 2 8 To determine the runoff hydrograph attributed to the effective rainfall a computation process known as convolution is applied. For the first 10 minutes the effective rainfall is 10 mm. This 10 mm over an area of 17 ha will yield a total runoff of 170 mm-ha and the runoff will be distributed in accordance with the time area curve as shown in column F of Table 4.8 which according to the principle of proportionality, is the product of the effective rain of 10 mm with the incremental area values in column B. The runoff attributed to the second 10-minute effective rainfall of 18 mm will likewise be the product of 18 mm with the incremental area values in column B but is lagged 10-minute to account for the time shift in occurrence of this rainfall and so on for the next 10-minute rainfall. The runoff due to the storm is, in accordance with the principle of superposition, the addition of the component hydrographs generated by the effective rainfall of each time-step (columns F, G, H and I)

(a) Identify Isochrones (b) Time-Area Curve

(c) Rainfall Hyetograph, Losses and Effective Rain

Catchment outlet

10 minute isochrone

20 minute isochrone

30 minute isochrone

A1 A2

A3

A4

40 minute isochrone


March 2009 4-53

(d) Component Runoff Hydrographs and Combined Runoff Hydrographs

Figure 4.22 Time-Area Method

Table 4.8 Derivation of Runoff Hydrograph Using the Time-Area Method

A B C D E F G H I J K

time (mins)

Incremental area (ha)

rainfall (mm)

Losses (mm)

Effective rain

(mm)

Runoff generated by each 10-minute effective rainfall (mm-

ha) combined runoff

10 18 6 8 (mm-ha) (m3/s)

0 0 0 0 0.0

10 2 15 5 10 20 0 20 0.3

20 5 20 2 18 50 36 0 86 1.4

30 6 8 2 6 60 90 12 0 162 2.7

40 4 10 2 8 40 108 30 16 194 3.2

50 0 72 36 40 148 2.5

60 Runoff generated by 1st 10-min effective

rain 0 24 48 72 1.2

70 Runoff generated by 2nd 10-min

effective rain 0

32 32 0.5

80 Runoff generated by 3rd 10-min

effective rain 0 0 0.0

90 Runoff generated by 4th 10-min

effective rain

Total 17 53 11 42 170 306 102 136 714


4-54 March 2009

Table 4.9 Methods of Estimating Loss Rates Recommended in MSMA Manual

Condition Loss Model Recommended Values

Impervious Areas

Initial Loss- Loss rate Initial Lost:1.5mm Lost rate:0mm/hr

Pervious Areas

Initial loss- Proportional loss or

Initial Lost: 10mm Proportional Loss” 20% rainfall

Initial Loss- Loss rate, or

Initial Loss: 10mm for all soil (i) sandy open structure soil (ii) Loam soil (iii) Clays, dense structured soil (iv) Clays subject to high

shrinkage and in a cracked state at start of rain

Loss rate 10-25 mm/hr 3-10 mm/hr 0.5-3 mm/hr 4-6 mm/hr

Horton Model Initial Infiltration Capacity f ° A. DRY soils (little or no vegetation) Sandy soil: 125 mm/hrLoam soil: 75 mm/hr Clays soil: 25mm/hr For dense vegetation, multiply values given in A by 2 B. MOIST soil Soil which have drained but not dried out: divide values from A by 3 Soil close to saturation: values close to saturated hydraulic conductivity Soil parties dried out: divides values from A by 1.5 – 2.5 Recommended value of k is 4/hr

Ultimate infiltration rate fc (mm/hr), for hydrologic soil Group (See Note) A 10-7.5 B 7.5-3.8 C 3.8-1.3 D 1.3-0

Note: Hydrological Soil Group corresponds to the classification given by the U.S. Soil Conservation Services. Well drained sandy soils are “A”; poorly drained clayey soil are “D”. The texture of the layer of least hydraulic conductivity in the soil profile should be considered. Caution should be used in applying values from the above table to sandy soils (GROUP A) Source: XP- SWMM Manual (WP-Software, 1995)

Unit Hydrograph (UH) Method The unit hydrograph (UH) method defines for a catchment, a t-duration unit hydrograph. The t-duration UH describes the runoff distribution due to a unit effective rainfall input over a duration t. Figure 4.35shows a 4-hour unit hydrograph due to a unit effective rainfall of 1 cm.

Figure 4.35 4-hour UH for 1 cm Effective Rain and Tabulated UH Ordinates

Time (hours)

Direct runoff (m3/s)

Time (hours)


0 0.0 10 12.0 1 0.6 11 8.0 2 2.0 12 5.0 3 8.0 13 3.0 4 15.0 14 2.0 5 18.0 15 1.5 6 19.0 16 1.0 7 18.6 17 0.8 8 17.5 18 0.5 9 15.0 19 0.2


March 2009 4-55

Application of UH to rainfall-runoff modelling involves convoluting the effective rainfall hyetograph with the unit hydrograph in accordance with the principle of proportionality and superposition (a process demonstrated in the previous section on time-area method) to compute component runoff hydrographs and adding the component hydrographs to derive the final direct runoff hydrograph(DRH) The following example illustrates the concept of using the UH Method for rainfall-runoff modelling. The 4-hour UH of a catchment is as given in Figure 4.35.The objective is to derive the resulting DRH given a 4-hourly interval effective rainfall hyetograph as shown in Table4.10

Table 4.10 4-hourly Effective Rainfall Hyetograph

Time (hrs) 0 4 8 12 16 20 Effective Rain, ER (cm) 0 1 4 3 1 2

Note that as the UH is a 4-hour UH, the effective rainfall hyetographs applied has to be presented in the form of a 4-hourly interval hyetographs. Table 4.11 shows the computations involved in deriving the DRH. The computation process is known as convolution and is very similar to the process involved in time-area method. Figure 5.5 shows the convolution process graphically.


4-56 March 2009

Table 4.11 Derivation of the DRH by Convolution Process

Time (hours

)


Effective Rain, ER

(cm)

Runoff generated by each 4-hour effective rain (ER) (m3/s)

Convoluted Hydrograph

(m3/s) 1 cm 4 cm 3 cm 1 cm 2 cm

0 0.0 0.0 0.0

1 0.6

1

0.6 0.6

2 2.0 2.0 2.0

3 8.0 8.0 8.0

4 15.0 15.0 0.0 15.0

5 18.0

4

18.0 2.4 20.4

6 19.0 19.0 8.0 27.0

7 18.6 18.6 32.0 50.6

8 17.5 17.5 60.0 0.0 77.5

9 15.0

3

15.0 72.0 1.8 88.8

10 12.0 12.0 76.0 6.0 94.0

11 8.0 8.0 74.4 24.0 106.4

12 5.0 5.0 70.0 45.0 0.0 120.0

13 3.0

1

3.0 60.0 54.0 0.6 117.6

14 2.0 2.0 48.0 57.0 2.0 109.0

15 1.5 1.5 32.0 55.8 8.0 97.3

16 1.0 1.0 20.0 52.5 15.0 0.0 88.5

17 0.8

2

0.8 12.0 45.0 18.0 1.2 77.0

18 0.5 0.5 8.0 36.0 19.0 4.0 67.5

19 0.2 0.2 6.0 24.0 18.6 16.0 64.8

20 0.0 0.0 4.0 15.0 17.5 30.0 66.5

21 3.2 9.0 15.0 36.0 63.2

22 2.0 6.0 12.0 38.0 58.0

23 0.8 4.5 8.0 37.2 50.5

24 0.0 3.0 5.0 35.0 43.0

25 2.4 3.0 30.0 35.4

26 1.5 2.0 24.0 27.5

27 0.6 1.5 16.0 18.1

28 0.0 1.0 10.0 11.0

29 0.8 6.0 6.8

30 0.5 4.0 4.5

31 0.2 3.0 3.2

32 0.0 2.0 2.0

33 1.6 1.6

34 1.0 1.0

35 0.4 0.4

36 0.0 0.0


March 2009 4-57

Figure 4.36 Convolution of UH of Figure 5.4 with Effective Rain (ER) Hyetograph of Table 4.9

0

2

4

6

8

10

12

14

16

18

200.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

0 4 8 12 16 20 24 28 32 36

Rai

nfal

l (cm

)

Run

off (

m3/

s)

Time (hours)

Rain 1cm ER

4 cm ER 3 cm ER

1 cm ER 2 cm ER

Convoluted Hydrograph


4-58 March 2009

4.6 ESTIMATION OF PMF PMF or probable maximum flood is the flood to be designed for in design of spillways for large dams or dams whose failure would result in heavy losses in properties and lives. The categorization of dams that requires PMF to be designed for varies. There are criteria set up by USBR, WMO and these are often be adopted in the absence of a local criteria. The PMF is basically the flood that results from a PMP and there are various ways of routing the PMP to yield the PMF. Since most dam catchments are undeveloped and rural in nature, many of PMFs derived for the dams in Malaysia uses HP11 to rout the PMP. To cater for large catchment areas the PMPs are often reduced with the application of areal reduction factors (ARFs). But with the availability of more sophisticated models, there are PMFs derived using more sophisticated rainfall-runoff models such as the RORB model, HEC-HMS. When such models are adopted, then the need arises for determination of temporal and areal distribution of the PMP. An example of PMF derivation is as follows. A 24-hour PMF is to be derived for Timah Tasoh Dam. Given is the 24-hour PMP derived as shown in the example in Section 2.5.8.2. This example illustrates the routing of the PMP using the triangular unit hydrograph method of HP11. The 24-hour catchment PMP is 540 mm. Rainfall losses can be estimated in several ways. DID’s HP11 computes effective rain Pe (mm) from rainfall P (mm) using the equation : Pe = P2 / ( P + 6 * 25.4 ) (4.33) = 5402 / ( 540 + 6 * 25.4 ) = 421 mm The Timah Tasoh Dam catchment is divided into three sub catchments:

• Catchment A of 17 km2 - the inundated reservoir area – which is considered impervious and all rain falling directly is converted to runoff

• Catchment B of 48 km2 - Sg Timah catchment upstream of the reservoir area • Catchment C of 126 km2 – Sg Tasoh catchment upstream of the reservoir area

Synthetic triangular unit hydrographs for duration D= 2 hours and for unit effective rainfall of 1 cm is to be derived for catchments B and C. Table 4.12 shows the computation of lag time Lg

Table 4.12 Computation of Lag Time Lg

Catchments B C River Sg timah Sg Tasoh main stream length L m 19 21 main stream length from outlet to catchment centroid Lc m 8 11

slope S 0.01029 0.00575 Catchment Group 2 3 Ct (based on catchment group) Ct 4 8

Computed lag time Lg hrs 8.27 21.20


March 2009 4-59

Figure 4.37 Triangular Unit Hydrograph

Table 4.13 shows the computation of the unit hydrograph parameters which is then used to develop the unit hydrograph ordinates in Table 4.14.

Table 4.13 Computation of Unit Hydrograph Parameters

Catchments B C Sg timah Sg Tasoh Area A Km2 48 126 Dp 0.89 0.75 slope U cm 1 1 Unit hydrograph duration D hrs 2 2

C hrs 9.27 22.20 Computed lag time Qp m3/s 12.70 11.73 Base of unit Hydrograph Tb hr 22 60 Time to peak Tp hr 8 12

Rainfall Excess

U

Q/2 Q/2

D

C

Tb

D /2 Lg

TrTp

qp


4-60 March 2009

Table 4.14 Ordinates of the Synthetic 2-hour UHs for Unit Rainfall of 1cm

Time Ordinates of UH

Sg timah Sg Tasoh hrs m3/s m3/s 0 0.00 0.00 2 3.18 1.96 4 6.35 3.91 6 9.53 5.87 8 12.70 7.82 10 10.89 9.78 12 9.07 11.73 14 7.26 11.24 16 5.44 10.75 18 3.63 10.26 20 1.81 9.78 22 0.00 9.29 24 8.80 26 8.31 28 7.82 30 7.33 32 6.84 34 6.35 36 5.87 38 5.38 40 4.89 42 4.40 44 3.91 46 3.42 48 2.93 50 2.44 52 1.96 54 1.47 56 0.98 58 0.49 60 0.00

The 24-hour PMP of 540mm needs to be distributed temporally and reference is made to the World Curve in Table 2.14. The World curve is used as a basis for temporal distribution of the PMP. Two sets of PMP hyetographs were derived. (see Table 4.15) For Catchment A rain falls directly onto the reservoir water body and hence no losses. The total hyetograph ordinates adds to 540mm. For catchments B and C the PMP hyetograph applied is the effective rainfall hyetograph of 42.1 mm. Note that since the UH is derived for a unit rainfall of 1 cm, the hyetograph units are expressed in cm.

0

2

4

6

8

10

12

14

0 20 40 60

Discharge

in m

3/s

Hours

Catchment B ‐ Sg Timah

Catchment C ‐Sg Tasoh


March 2009 4-61

Table 4.15 2- Hourly Effective Rainfall Hyetographs

Time (hrs)

Effective Rainfall Hyetographs (cm)

Catchment A

Catchment B and C

0 0.0 0 2 14.1 11 4 8.1 6.3 6 6.3 4.9 8 4.8 3.8 10 3.6 2.8 12 2.8 2.1 14 2.2 1.7 16 1.9 1.5 18 1.9 1.5 20 2.3 1.8 22 2.9 2.2 24 3.1 2.5

Total 54.0 42.1 Tables 4.16, 4.17, 4.18 shows the convolution of the PMP hyetographs with the respective unit hydrographs (UH) to derive the Direct Runoff Hydrographs (DRH) for the three catchments. Baseflow was added tpo the DRH of catchment B and C and for this purpose the baseflow rate of 0.055 m3/s/km2 recommended in HP11 was adopted. All three hydrographs from catchments A, B and C were combined to form the designed 24-hour PMF (see Table 4.19 and Figure 4.38)

Table 4.16 Derivation of DRH for Catchment A


4-62 March 2009

Table 4.17 Derivation of DRH for Catchment B


March 2009 4-63

Table 4.18 Derivation of DRH for Catchment C


4-64 March 2009

Table 4.19 Combining the Catchment hydrographs to form the PMF

Time (hrs)

Total Runoff Hydrographs (m3/s) For Catchments

PMF Hydrograph

(m3/s) A B C 0 0 2.64 6.93 9.57 2 332.76 2.64 6.93 342.33 4 191.16 37.62 28.49 257.27 6 148.68 92.524 62.288 303.492 8 113.28 163.057 105.737 382.074

10 84.96 245.578 156.538 487.076 12 73.16 282.171 212.885 568.216 14 68.44 293.919 273.262 635.621 16 66.08 286.687 310.086 662.853 18 54.28 265.181 334.443 653.904 20 51.92 234.54 349.755 636.215 22 44.84 199.062 359.409 603.311 24 44.84 162.161 366.474 573.475 26 0 145.659 373.28 518.939 28 0 133.089 375.906 508.995 30 0 120.441 374.868 495.309 32 0 103.696 370.142 473.838 34 0 79.569 361.023 440.592 36 0 59.239 346.509 405.748 38 0 42.007 326.005 368.012 40 0 27.484 305.457 332.941 42 0 15.697 284.899 300.596 44 0 7.165 264.333 271.498 46 0 2.64 243.732 246.372 48 0 2.64 223.124 225.764 50 0 2.64 202.512 205.152 52 0 2.64 181.898 184.538 54 0 2.64 161.394 164.034 56 0 2.64 140.846 143.486 58 0 2.64 120.288 122.928 60 0 2.64 99.722 102.362 62 0 2.64 79.121 81.761 64 0 2.64 63.903 66.543 66 0 2.64 51.768 54.408 68 0 2.64 42.032 44.672 70 0 2.64 34.158 36.798 72 0 2.64 27.659 30.299 74 0 2.64 22.193 24.833 76 0 2.64 17.563 20.203 78 0 2.64 13.643 16.283 80 0 2.64 10.458 13.098 82 0 2.64 8.155 10.795 84 0 2.64 6.93 9.57 86 0 2.64 6.93 9.57


March 2009 4-65

Figure 4.38 The 24-Hour PMF and the Component Catchment Hydrographs REFERENCE [1] HP 4. Magnitude and frequency of floods in Peninsular Malaysia - 1987. [2] HP 5. Rational method of flood estimation for rural catchment in Peninsular Malaysia - 1989. [3] HP 11 Design Flood Hydrograph Estimation for Rural Catchments in Peninsular Malaysia (1976). [4] HP 16. Flood Estimation for urban areas in Peninsular Malaysia - 1976. [5] Tan Hoe Tim, Manual of Department of Irrigation and Drainage Hydrology (revised and Updated), Department of Irrigation and Drainage Ministry of Agriculture,Malaysia,1988.

0

100

200

300

400

500

600

700

0 20 40 60 80

Discharge

in m

3/s

Hours

Sg Tasoh

Sg Timah

Reservoir

24‐hr PMF


4-66 March 2009



March 2009 4A-1

APPENDIX 4A MOVING BOAT METHOD BY KLEIN, ET. AL.


4A-2 March2009


March 2009 4A-3


4A-4 March2009


March 2009 4A-5


4A-6 March2009


March 2009 4A-7


4A-8 March2009


March 2009 4A-9


4A-10 March2009


CHAPTER 5 STATISTICAL HYDROLOGY

Chapter 5 STATISTICAL HYDROLOGY

March 2009 5-i

Table of Contents Table of Contents ................................................................................................................................5‐i

List of Tables ....................................................................................................................................... 5‐ii

List Figures .......................................................................................................................................... 5‐ii

5.1 HYDROLOGICAL DATA .......................................................................................................... 5‐1

5.2 MEAN, STANDARD DEVIATION, SKEWNESS AND KURTOSIS .............................................. 5‐4

5.3 FREQUENCY ANALYSES ........................................................................................................ 5‐6

5.3.1 Methodology ................................................................................................. 5-6

5.3.2 Parameter Estimation of Probability Distribution ................................................ 5-8

5.3.3 Regional Frequency Analysis .......................................................................... 5-12

5.4 EXAMPLE OF INTENSITY DURATION-FREQUENCY ANALYSES USING THE LOG-NORMAL DISTRIBUTION ................................................................................................................... 5‐18

REFERENCE ...................................................................................................................................... 5‐26


5-ii March 2009

List of Tables Table Description Page 5.1 Annual Rainfall Time Series (Sg Kerian Catchment) 5-3

5.2 Computation of Sample Statistics by MOM 5-10

5.3 Computation of L-Moments of a Data Series 5-12

5.4 Annual Maximum Peak Flows of Sg Rajang at Bakun 5-15

5.5 Estimation of Qt based on the EV1 Frequency Distribution 5-15

5.6 Probability Plotting Positions Using the Standard Plotting Position Method

by US Water Resources Council 5-17

List Figures Figure Description Page 5.1 Daily, Monthly and Yearly Rainfall Time Series of Sg Kerian Catchment (Derived from Data

Extracted from DID’s Hydrological Database) 5-2

5.2 Mean Monthly Rainfall of Sg Kerian Catchment 5-4

5.3 Mean Monthly Discharge (Sg Kerian at Selama) 5-4

5.4 Probability Distribution and Skewness 5-5

5.5 Kurtosis and Shapes of Distributions 5-6

5.6 Probability Distribution Graph Papers Normal Probability Graph Paper (top and Gumbel Probability Graph Paper (bottom) 5-8

5.7 Annual Peak Flows of Sg Rajang @ Bakun Plotted onto a EV1 Probability Paper 5-16


March 2009 5-1

5.0 STATISTICAL HYDROLOGY

5.1 HYDROLOGICAL DATA Statistics gives hydrologists a methodology to derive from observed data estimates of extreme hydrological values such as extreme floods, rainfall and low often required for planning and design of water resources projects. It also allows the hydrologist to understand uncertainty and confidence in hydrological observations and estimates. Hydrological time series data collected by DID comprises:

• Daily rainfall time series by daily read rain gauges • Daily read evaporation time series • Discrete water level series by daily read stick gauges (normally read 2 times a day) • Continuous rainfall time series by automatic recording rain gauges • Continuous water level time series by automatic recording water level recorders

Besides time-series data, DID also performs regular stream-flow gauging at river stations to develop and update stage-discharge curves. At selected river stations, suspended sediment samplings are also carried out to develop suspended sediment load – discharge curves. With these two rating curves applied to the water level time series, DID can derive:

• discharge time series and • suspended sediment time series

From the recorded time series users are able to perform various numerical computations to obtain:

• Daily time series from continuous time series e.g. Daily rainfall, evaporation, water level, discharge and suspended sediment discharge

• Monthly time series of rainfall, evaporation, water level, discharge and suspended sediment • Annual time series of rainfall, evaporation, water level, discharge and suspended sediment

load.

Daily, monthly and yearly time series of rainfall derived for the Sg Kerian Catchment upstream of the Kerian Barrage is as shown in Figure 5.1. Viewed as daily data, rainfall appears random in nature, some pattern of rainfall behaviour can be seen in the monthly time series and the annual rainfall time series shows appears as a jagged line wavering over a central value of 3364 mm. The central value is the mean value and the variation about the mean value is often measured by a statistic called the standard deviation. The standard deviation of the Annual Kerian Rainfall is 338 mm. The annual series is presented in Table 5.1 together with standard statistics often computed i.e. the mean, standard deviation, skewness and kurtosis.

The mean and the standard deviation are the most common statistics applied to observations, including hydrological observations. The mean is a measure of central tendency while the standard deviation is a measure of deviation from the mean.


5-2 March 2009

Figure 5.1 Daily, Monthly and Yearly Rainfall Time Series of Sg Kerian Catchment (Derived from Data Extracted from DID’s Hydrological Database)


March 2009 5-3

Table 5.1 Annual Rainfall Time Series (Sg Kerian Catchment)

Year Annual Rainfall (mm)

Year

Annual Rainfall (mm)

1947 4098 1974 2805 1948 3134 1975 3639 1949 4018 1976 2942 1950 3254 1977 2881 1951 3843 1978 2987 Mean 3364 1952 3604 1979 2863 Std Dev 338 1953 3760 1980 3664 Skew 0.21 1954 3070 1981 3078 Kurtosis -0.68 1955 3855 1982 3498 1956 3515 1983 3245 1957 3255 1984 3644 1958 3210 1985 3314 1959 3352 1986 3233 1960 3126 1987 3950 1961 3414 1988 3699 1962 3265 1989 3197 1963 3376 1990 3548 1964 3126 1991 3394 1965 3242 1992 2859 1966 3456 1993 3788 1967 3082 1994 3483 1968 2743 1995 2998 1969 3543 1996 3262 1970 3504 1997 2848 1971 3118 1998 3785 1972 3160 1999 3944 1973 3400 2000 3596

Average monthly series is also another dataset often derived and in the case of rainfall and discharge the monthly series gives an indication of seasonal variation in rainfall and flows respectively (see Figure 5.2 and 5.3).


5-4 March 2009

Figure 5.2 Mean Monthly Rainfall of Sg Kerian Catchment

Figure 5.3 Mean Monthly Discharge (Sg Kerian at Selama)

5.2 MEAN, STANDARD DEVIATION, SKEWNESS AND KURTOSIS

Mean and standard deviation equations are given by,

x =Average ∑ xi

n (5.1)

= Standard Deviation =∑ XI-X

2nI=1

n-1 (5.2)

Besides mean and standard deviation, the other statistics applied to hydrological observation is skewness and kurtosis.

Data is skewed when there are unbalanced distribution of high and low observations. The normal distribution is symmetrical about the mean and has zero skewness (see Figure 5.4). The shapes of the probability distributions that are positively skewed and negatively skewed are shown in Figure 5.4.If the larger observations are further away from the mean than the smaller observations then the data shows positive skew and vice versa.


March 2009 5-5

Figure 5.4 Probability Distribution and Skewness

The skew also called third moment about the mean is given by

n

n-1 n-2∑ Xi-X

s

3 (5.3)

Kurtosis is the fourth standardized moment and is given by

μ4

σ4 (5.4)

where μ4 is the fourth moment about the mean and σ is the standard deviation.

A distribution with high kurtosis (Leptokurtic) has a sharper peak and longer, fatter tails, while a low kurtosis distribution (platykurtic) has a more rounded peak and shorter thinner tails (see Figure 5.5)

normal distribution (zero Skew)

negative skew positive skew

http://en.wikipedia.org/wiki/Standardized_moment�

http://en.wikipedia.org/wiki/Moment_about_the_mean�

http://en.wikipedia.org/wiki/Standard_deviation�


5-6 March 2009

Figure 5.5 Kurtosis and Shapes of Distributions

5.3 FREQUENCY ANALYSES Frequency analysis of hydrological events is statistical analyses of extreme hydrological events in order to determine the frequency of occurrence of an extreme hydrological event (usually applied to maximum rainfall or discharge but it can be also applied to minimum rainfall or discharge) or to extrapolate observed extreme or discharge data to estimate extreme rainfall events of a designated Average Recurrence Interval (ARI). A key concept is extreme rainfall event and an example of an extreme rainfall event would be the maximum 3-hour rainfall. Note that a rainfall event always has a specified duration i.e. a 30 minute rainfall, a 1-hour rainfall or a 48 hour rainfall. Whilst frequency analysis is often carried out for maximum rainfall events, it is also carried out for minimum rainfall events such as the minimum monthly or minimum annual rainfall usually for drought events.

Another key concept is average recurrence interval (ARI) which is a measure of the frequency of occurrence of an extreme event. A 100-year ARI storm is a storm that occurs or recurs on the average once in 100 years. Likewise a 5-year ARI rainfall is a rainfall that recurs once in 5-years not necessarily at exactly every 5-year interval but if over a 50-year record the event is exceeded 10 times within the 50 years then the average recurrence interval is 5 years.

5.3.1 Methodology The steps involved in conducting frequency analysis of rainfall using the method of frequency factor are shown below:

• Select the rainfall station of interest. This should be the rainfall station nearest to the project area. The data quality of the station is also important and so is the length of records available (the longer the period of records the more reliable the analysis)

• Retrieve from each year of record, the maximum rainfall occurring over the specified storm duration. The result is a series of the annual maximum rainfalls.

• Compute the mean (Xm) and standard deviation (SD) of the annual maximum rainfall series.

(+) Leptokurtic

(0) Mesokurti (Normal)

(-) Platykurtic

General Forms of Kurtosis


March 2009 5-7

• Using the method of frequency factors, the rainfall (Xt) of t years ARI is given by:

Xt = Xm + Kt . SD (5.5)

Where: Kt is the frequency factor whose value depends on the probability distribution selected to fit the annual maximum rainfall series.

Another popular method of fitting a rainfall frequency distribution is by plotting the annual maximum rainfall series in a probability distribution graph paper. A probability graph paper is a graph paper where the scale is distorted such that data that fits the probability distribution plots as a straight line. This is possible for 2-parameter probability distributions. A three parameter probability distribution will not plot as a straight line. Examples of probability graph papers are shown in Figure 5.6.

The simplest and most common probability distribution is the normal probability distribution but it is usually found not suitable for extreme value hydrological data. Other probability distributions commonly used in hydrology are the Gumbel and Pearson Type III distributions. The log transforms of the normal and Pearson Type III distributions are also commonly used.


5-8 March 2009

Figure 5.6 Probability Distribution Graph Papers Normal Probability Graph Paper (top) and Gumbel Probability Graph Paper (bottom)

5.3.2 Parameter Estimation of Probability Distribution

There are various approaches towards estimation of parameters of probability distributions. The most commonly used method is the method of moments but increasingly there are preferences for other methods such as the method of L-moments.


March 2009 5-9

Method of Moments (MOM)

The MOM assumes that the moments of the probability density function (pdf) are equal to the corresponding moments of the sample data.

The first moment of a probability distribution about the origin μ is the centroid of the probability distribution and is given by

μ= x f x dx (5.6)

And this is also the mean of the probability distribution.

The moments of a probability distribution about the centroid or the central moments of the probability distribution are given by:

μr=E X-μr= x-μ

rf x dx (5.7)

Variance:σ2=μ2 (5.8)

Skewness = β1=μ3

μ23/2 (5.9)

Kurtosis = β2=μ4μ2

2 (5.10)

The first moment of the sample data about the origin is given by:

∑ (5.11)

And this is the sample mean. The moments of the sample data about the mean are given by:

mr=n-1 ∑ (Xi -X)r (5.12)

Sample moments for r>1 are not unbiased and unbiased estimators are given by:

s2=(n-1)-1

∑ (xi-x)2 (5.13)

m3*= n2

n-1 (n-2)m3 (5.14)

k4*= n2

n-2 n-3

n+1

n-1m4-3m2

2 (5.15)


5-10 March 2009

MOM is an established method and is easy to apply and hence in most hydrological studies MOM is applied. Table 5.2 shows an example of how moments of sampled data (Annual Maximum Discharge) are computed.

Table 5.2 Computation of Sample Statistics by MOM

Year Annual Maximum Discharge X

(X-Xm)2

(m3/s) 1981 59 42 1982 95 870 1983 67 2 1984 56 90 1985 64 2 1986 71 30 1987 69 12 1988 53 156 1989 41 600 1990 45 420 1991 65 0 1992 92 702 1993 44 462 1994 96 930

Total 917 4322

Number of samples n= 14

The mean = sum of moments about the origin/n = 917/14

= 65.5 m3/s The variance s2 = sum of squares of moments about the mean/(n-1) = 4322/13 = 332.4

Method of L-Moments (MLM)

L-moment is an alternative system of describing probability distribution proposed by Hosking (1990). L-moments according to Hosking (1990) “provide measures of location, dispersion, skewness, kurtosis, and other aspects of the shape of probability distributions or data samples -- but are computed from linear combinations of the ordered data values (hence the prefix L)”

The L-moment of X is a function of probability weighted moments (PWM). PWMs are computed from ranked observations Xj.


March 2009 5-11

The PWM of sampled observations is given by:

bi= n-1 ∑ x(j)nj=1+i

j-1 j-2 (j-i)

n-1 n-2 (n-i) (5.16)

Based on the PWM estimators the first 4 L-moments are defined as follows:

l1=b0, (5.17)

l2=2b1-b0, (5.18)

l3=6b2-6b1+b0, (5.19)

l4=20b3-30b2+12b1-b0 (5.20)

l1 is a measure of central tendency, l2 a measure of dispersion. The following are dimensionless L-moment ratios defined by Hosking (1990):

t= l2 l1⁄ (L-coefficient of variation, L-Cv) (5.21)

t3= l3 l2⁄ (L-skewness) (5.22)

t4= l4 l2⁄ (L-kurtosis) (5.23)

L-moments are linear combinations of observations and do not involve squares or cubic functions of observations. Therefore they are less sensitive to large values in observations and outliers.

For probability distribution with cumulative frequency function F(X), the PWMs are defined by:

βr= x F x rdF x , r=0,1,2 (5.24)

And correspondingly the L-moments are:

λ1=β0 (5.25)

λ2=2β1-β0 (5.26)

λ3=6β2-6β1+β0 (5.27)

λ4=20β3-30β2+12β1-β0 (5.28)

Computed L-moments of several theoretical probability distributions are as follows:

• Uniform (rectangular) distribution on (0,1):

λ1=12 , λ2=

16 , τ3=0, τ4=0 (5.29)

• Normal distribution with mean 0 and variance 1:

λ1=0, λ2=1

√π , τ3=0, τ4≈0.123 (5.30)

• Exponential distribution with mean 1:

λ1=1, λ2=12 , τ3=

13 , τ4=

16 (5.31)


5-12 March 2009

Table 5.3 Computation of L-Moments of a Data Series

Year Annual

Maximum Discharge

Rank j

Ranked Data Xj

i PWM bi

L-Moments Li

(m3/s) (m3/s) 0 65.50 1981 59 1 41 1 38.01 65.50 1982 95 2 44 2 27.32 10.52 1983 67 3 45 3 21.49 1.32 1984 56 4 53 4 17.77 0.88 1985 64 5 56 1986 71 6 59 L-Cv 0.16 1987 69 7 64 L-Skewness 0.13 1988 53 8 65 L-Kurtosis 0.08 1989 41 9 67 1990 45 10 69 1991 65 11 71 1992 92 12 92 1993 44 13 95 1994 96 14 96

5.3.3 Regional Frequency Analysis Regional frequency analyses is a method for extending frequency statistics to sites with little or no data. Dalrymple applied regional frequency analyses to derive average dimensionless flood frequency distributions applicable to all drainage basins within a homogeneous region. The average dimensionless flood frequency curve can only be defined if all the individual station curves have almost the same slope. Normally, homogeniety test is required before regional curves can be defined. Examples of regional frequency analyses can be seen in the following hydrological procedures (HPs) developed by DID:

• HP4: Magnitude and Frequency of Floods in Peninsular Malaysia (Revised and Updated 1987)

• HP12: Magnitude and Frequency of Low flows in Peninsular Malaysia (Revised and Updated 1985)

The assessment of regional homogeneity is carried out in regional frequency analysis and three tests of homogeneity are available:

• the Discordancy measure, D test and • the Heterogeniety measure, H test and • the goodness of fit measure ZDIST of fitted probability distribution

In the discordancy measure test, L-moment ratios: L-Cv, L-skewness and L-kurtosis of a site are computed for a group of homogeneous sites.

Di=1

3N ui-u

TK-1(ui-u) (5.32)

where ui = vector of L-moment ratios for station i,

K = covariance matrix of ui, u = mean of vector ui


March 2009 5-13

The station i is declared to be discordant, if Di is greater than the critical value of the discordancy statistic given in a tabular form by [Hosking and Wallis, 1993]

The heterogeneity measure H compares the inter-site variations in sample L-moments for sites within a region deemed homogeneous and is given by:

H= V-μvσv

(5.33)

where V = weighted standard deviation of L-coefficient of variation values, μV, σV = the mean and standard deviation of number of simulations of V.

The criteria for deciding heterogeneity of a region is as: if H < 1, region is acceptably homogeneous, if 1 ≤ H < 2, region is possibly heterogeneous, if H ≥ 2, region is definitely heterogeneous.

The goodness of fit measure, ZDIST, indicates how well the L-skewness and L-kurtosis of a fitted distribution matches the regional average L-skewness and L-kurtosis values of the observed data. The goodness-of-fit criterion for each distribution is defined as:

ZDIST=(τ4DIST-τ4

R+B4)/σ4 (5.34)

where t4

R = average value t4 obtained from the data of a given region, B4, σ4 = bias and standard deviation of t4, respectively, defined as:

B4=Nsim-1 ∑ (t4

(m)Nsimm=1 -t4

R) (5.35)

σ4= (Nsim-1)-1

∑ (t4m -t4

RNsimm=1 )

2-NsimB4

21/2

(5.36)

Where Nsim = number of simulated regional data sets generated using a Kappa distribution,

m = mth simulated region obtained using a Kappa distribution. The fit is deemed adequate if ZDIST is sufficiently close to zero, a reasonable criterion being (5.37) .1.64 ≥ ׀ZDIST׀ EXAMPLE OF FLOOD FREQUENCY ANALYSES. In flood frequency analyses the objective is to carryout statistical analysis to find a probability distribution that fits the observed flood data and thereafter extrapolating the probability distribution to estimate extreme ARI flood events such as the 100-year flood. To carry out statistical analysis there must be data and hence flood frequency analyses can only be carried out for catchments with flood data collected.


5-14 March 2009

Like rainfall, flood events varies from time to time but if data of flood magnitudes is collected over long periods of time one gets an idea of the central tendency (mean, average, etc.) and variability (the standard deviation) of the flood event.

If the objective is to find the 100-year ARI flood then the approach would be to extract for each year the maximum flood discharge (one value per year or the annual flood event). A year is usually taken as the calendar year for convenience in data extraction. But this need not be so in all cases. The idea is to make sure that the flood events considered are independent (i.e. clearly separate events). A case for deviating from the standard calendar year would be floods in Kelantan. The big floods in Kelantan occur during the North-East Monsoon Season and the season occurs in November, December and January with floods occurring anytime between these months. Should the big flood occur at the end of December and the heavy rainfall and flood persists until January then there may be a possibility that the selected peak flood discharge for both years comes from the same flood event. So statistically they are not independent events. So for Kelantan and some other east coast states of Peninsular Malaysia, the hydrological year need not be the calendar year and June to June would be a better choice of hydrological year. Table 5.4 presents the annual maximum peak discharge records of Sg Rajang @ Bakun. As in many stations records are often not complete. There are missing records for certain years. The mean annual flood peak discharge is 5,487 m3/s and the standard deviation of the annual flood peak discharge is 1,624 m3/s. Using the method of frequency factors and applying the EV1 distribution floods of various ARI (T years) can be computed.

QT = Qm + KT * S (5.38)

Where QT = the T-yr ARI peak flood discharge KT = the frequency factor corresponding to T-yr ARI KT for EV1, one of the probability distributions commonly used for flood frequency analysis is given by:

KT=- 6π 0.5772+ln ln T

T-1 (5.39)


March 2009 5-15

Table 5.4 Annual Maximum Peak Flows of Sg Rajang at Bakun

Year Qp (m3/s) Year Qp (m3/s) 1968 7670 1981 5925 1969 5838 1982 9574 1970 - 1983 6680 1971 6565 1984 5575 1972 4182 1985 6350 1973 6671 1986 7144 1974 5933 1987 6852 1975 - 1988 5334 1976 7601 1989 4075 1977 - 1990 4523 1978 - 1991 6538 1979 - 1992 9210 1980 - 1993 4364

1994 9620

Mean, Qm 6,487 m3/s Standard Deviation, S 1,624 m3/s

Based on the frequency factor equation above the KT and QT for various ARI are computed and presented in Table 5.5.

Table 5.5 Estimation of Qt based on the EV1 Frequency Distribution

ARI T

(years)

Frequency factor, KT

Estimated Flood, QT

(m3/s) 2 -0.164 6220 5 0.719 7655 10 1.304 8606 20 1.866 9517 50 2.592 10697 100 3.136 11581

Frequency analysis can also be presented graphically by plotting the observed annual floods onto probability distribution paper. In this example, the EV1 distribution was chosen and hence the plot is on EV1 paper. When plotted the goodness -of-fit (although there are statistical tests of goodness-of –fit that could be applied) of the proposed probability distribution can be viewed and be used as a check (see Figure 5.7) Table 5.6 shows how the data from Table 5.4 is ranked and assigned plotting positions (probability of exceedance). The plotting position formula adopted is the US Water Resources Council’s standard method. The plotting position of a particular annual flood peak value is its position along the probability or return period axis and is given by:


5-16 March 2009

T = (n + 1) / m (5.40)

Where T = the ARI (years)

n = number of annual flood peak data m = rank of the flood peak discharge sorted in descending order

There are other plotting position formulas such as those advocated by Blom and Gringorton but experience so far shows that the choice of plotting position does not significantly affect the results of analysis.

Figure 5.7 Annual Peak Flows of Sg Rajang @ Bakun Plotted onto a EV1 Probability Paper


March 2009 5-17

Table 5.6 Probability Plotting Positions Using the Standard Plotting Position Method by US Water

Resources Council

Q (m3/s)

rank m

ARI T =

(n+1)/m (years)

Exceedance Probability P = (1-1/T)

(%) 9620 1 22.0 95.5 9574 2 11.0 90.9 9210 3 7.3 86.4 7670 4 5.5 81.8 7601 5 4.4 77.3 7144 6 3.7 72.7 6852 7 3.1 68.2 6680 8 2.8 63.6 6671 9 2.4 59.1 6565 10 2.2 54.5 6538 11 2.0 50.0 6350 12 1.8 45.5 5933 13 1.7 40.9 5925 14 1.6 36.4 5838 15 1.5 31.8 5575 16 1.4 27.3 5334 17 1.3 22.7 4523 18 1.2 18.2 4364 19 1.2 13.6 4182 20 1.1 9.1 4075 21 1.0 4.5

So far DID has adopted EV1 in many flood studies. Likewise many consultant studies have adopted the EV1 distribution for flood studies. EV1 was adopted by the Natural Environment Research Council (NERC) of UK in their Flood Studies Report (1975). In the US the U.S. Water Resources Council recommended using the log-Pearson Type III distribution as the base distribution in all federal planning related to water and related land resources to promote a uniformity and consistency in planning. With the availability of powerful spreadsheets the frequency factor method is more popularly used compared to the graphical approach in flood frequency analysis. Frequency factor of the normal probability distribution corresponding to probability of exceedance p, is given by:

KT=w-2.515517 + 0.802853w + 0.010328w2

1 + 1.432788w +0.189269w 2+0.001308 w3 (5.41)

Where w = (ln(1/p2))0.5 (0 < p ≤ 0.5)


5-18 March 2009

If p > 0.5 then p = 1 – p and KT = -KT And the frequency factor of Log-Pearson Type III distribution is given by:

KT = z + (z2-1)k + 1/3(z3-6z)k2 -(z2-1)k3 + zk4 + 1/3 k5 (5.42)

where k =Cs/6

5.4 EXAMPLE OF INTENSITY DURATION-FREQUENCY ANALYSES USING THE LOG-NORMAL DISTRIBUTION IDF or Intensity-Duration-Frequency curves are often required in estimating design rainfalls subsequently used as input of rainfall-runoff models. The basic data for IDF analysis is maximum annual rainfall for various durations. Figure 5.7 shows the data extracted from DID’s rainfall records of a rainfall station in Pejabat JPS Pahang. Note that DID can retrieve such data from the hydrological database via its TIDEDA software package and in this case the TIDEDA routine used to extract the data is PMOVE. This example shows the fitting of a log-normal distribution to the data to derived extreme rainfall intensities of various ARIs (average recurrence intervals).

The maximum rainfall data (mm) is first converted to maximum rainfall intensities (mm/hour) by dividing the maximum annual rainfall by its duration (hours). The maximum annual rainfall intensity data is presented in Table 5.8. The LN (natural log) is applied to the intensity data to transform the original data to a LN-transformed data (see Table 5.9)

The mean and the standard deviation of the LN-transformed data is then computed and presented in Table 5.10. The statistics i.e. the means and the standard deviations of the LN-Transformed data were computed and the LN-Transformed values for various ARIs were computed using the frequency factor method.

Y T = Ym + KT . S (5.43)

Where Yt is the LN-Transformed value corresponding to ARI of T- year KT the the normal frequency factor corresponding to ARI of T-year Ym the mean of the LN-Transformed data S the standard deviation of the LN-Transformed data

The normal distribution KT values for various ARI and the results of the LN-Transformed analyses are also presented in Table 5.10 The final step is to find the exponent of the LN-Transformed results to obtain the Log-Normal extreme values of the original data. (see Table 5.11) The LN-Transformed Values in Table 5.10 were plotted against the LN(duration in minutes) and best fit curves plotted and equations of best fit polynormial derived using Ms Excel’s trendline function. From the derived polynormial equations of figure 5.8,the IDF table can be derived and curves plotted (See Figure 5.9)


March 2009 5-19

Table 5.7 Annual Maximum Rainfalls for Various Storm Durations (DID rainfall Station in Pejabat JPS

Pahang)

YEAR Annual Maximum Rainfall(mm) for Various Storm Durations 10 min

15 min

30 min 1 hour 2 hour 3 hour 6 hour 12

hour 18 hour

24 hour

1971 13.90 20.80 29.40 51.90 60.50 71.90 114.70 157.00 197.70 236.30 1972 18.20 18.80 25.90 47.40 71.60 96.70 175.20 287.20 354.30 368.30 1973 19.90 25.60 38.70 64.40 81.90 82.80 83.30 83.40 83.40 83.40 1974 19.00 24.00 41.90 69.30 111.60 134.60 187.20 249.90 275.90 346.10 1975 18.80 26.90 34.60 50.50 97.80 106.30 144.00 159.00 198.00 204.00 1976 23.60 33.50 45.30 69.10 117.20 154.00 286.70 389.50 439.50 479.40 1977 23.10 28.90 46.10 66.10 90.10 100.60 118.50 122.00 195.70 208.00 1978 21.40 27.40 40.40 62.50 93.90 109.30 132.50 163.00 166.50 166.50 1979 25.10 28.90 37.00 54.00 81.60 86.70 90.00 139.90 149.00 152.30 1980 22.80 28.30 35.10 45.20 72.10 97.40 146.80 206.30 236.40 260.50 1981 11.70 17.50 26.70 34.20 49.10 50.00 50.50 50.50 50.50 50.50 1982 18.90 26.50 41.30 55.40 77.40 106.20 163.40 222.00 225.50 239.00 1983 18.70 25.20 37.00 49.50 51.00 51.00 51.50 51.50 53.30 71.00 1984 18.80 28.20 51.40 67.60 70.00 87.70 123.60 179.90 211.00 226.10 1985 20.50 20.50 34.20 63.30 79.00 79.50 116.80 134.30 135.50 140.50 1986 25.00 25.00 32.00 41.30 52.00 52.00 69.90 80.50 85.50 95.50 1987 19.10 28.60 51.90 63.90 76.00 76.00 97.30 144.70 167.30 172.40 1988 15.70 23.50 47.10 58.60 77.00 95.30 145.60 187.90 258.40 306.20 1989 15.70 19.10 29.20 49.40 65.50 77.30 106.30 146.70 186.40 201.00 1990 20.60 30.90 56.90 91.80 152.00 189.50 309.10 470.00 506.10 527.50 1991 7.50 10.50 18.80 34.20 59.90 73.80 118.90 195.30 231.80 274.80 1992 12.20 18.30 36.50 51.10 73.30 87.10 107.30 136.60 179.20 242.60 1993 11.20 16.80 26.60 51.50 73.60 102.70 174.20 259.00 273.50 278.50 1994 12.50 14.70 29.40 53.50 76.80 100.00 140.40 238.00 251.20 252.50 1995 16.40 17.30 27.40 48.80 69.10 84.80 105.50 138.20 163.10 198.20 1996 22.20 22.80 29.10 49.40 50.00 50.00 63.20 109.20 137.30 138.50 1997 32.10 34.30 41.00 54.30 106.50 108.00 110.50 110.50 110.50 110.50 1998 28.00 30.70 32.70 36.70 46.40 62.00 76.40 107.40 141.90 147.50 1999 23.20 25.00 46.00 59.00 74.50 101.00 153.00 194.50 235.00 247.50 2000 29.70 31.30 36.30 46.00 79.20 98.90 119.70 123.70 123.70 123.70 2001 23.60 26.00 40.50 53.50 69.00 88.50 98.90 120.80 132.00 138.00 2002 21.30 24.00 44.00 77.50 84.50 85.50 115.20 121.30 126.30 150.60 2003 20.00 25.00 36.00 49.50 69.00 82.00 105.50 140.50 175.50 182.00 2004 24.60 28.80 46.50 64.70 97.20 144.00 225.80 378.70 405.00 429.90 2005 24.30 31.00 37.90 51.70 62.50 66.50 99.20 118.70 156.10 204.90 2006 22.00 28.70 49.30 82.20 116.20 136.30 187.20 233.00 298.50 323.50 2007 27.40 37.90 60.10 106.40 117.50 117.60 154.00 189.50 235.00 268.00 2008 28.00 38.00 52.50 71.00 78.50 78.50 80.30 97.30 100.00 133.50 2009 21.50 30.60 49.20 62.50 62.90 76.20 109.20 199.30 230.20 237.20


5-20 March 2009

Table 5.8 Annual Maximum Rainfall Intensities (mm/hour)

YEAR Annual Maximum Rainfall Intensities (mm/hour) for Various Storm Durations

10 min 15 min 30 min 1 hour 2 hour

3 hour

6 hour

12 hour

18 hour

24 hour

1971 83.40 83.20 58.80 51.90 30.25 23.97 19.12 13.08 10.98 9.85 1972 109.20 75.20 51.80 47.40 35.80 32.23 29.20 23.93 19.68 15.35 1973 119.40 102.40 77.40 64.40 40.95 27.60 13.88 6.95 4.63 3.48 1974 114.00 96.00 83.80 69.30 55.80 44.87 31.20 20.83 15.33 14.42 1975 112.80 107.60 69.20 50.50 48.90 35.43 24.00 13.25 11.00 8.50 1976 141.60 134.00 90.60 69.10 58.60 51.33 47.78 32.46 24.42 19.98 1977 138.60 115.60 92.20 66.10 45.05 33.53 19.75 10.17 10.87 8.67 1978 128.40 109.60 80.80 62.50 46.95 36.43 22.08 13.58 9.25 6.94 1979 150.60 115.60 74.00 54.00 40.80 28.90 15.00 11.66 8.28 6.35 1980 136.80 113.20 70.20 45.20 36.05 32.47 24.47 17.19 13.13 10.85 1981 70.20 70.00 53.40 34.20 24.55 16.67 8.42 4.21 2.81 2.10 1982 113.40 106.00 82.60 55.40 38.70 35.40 27.23 18.50 12.53 9.96 1983 112.20 100.80 74.00 49.50 25.50 17.00 8.58 4.29 2.96 2.96 1984 112.80 112.80 102.80 67.60 35.00 29.23 20.60 14.99 11.72 9.42 1985 123.00 82.00 68.40 63.30 39.50 26.50 19.47 11.19 7.53 5.85 1986 150.00 100.00 64.00 41.30 26.00 17.33 11.65 6.71 4.75 3.98 1987 114.60 114.40 103.80 63.90 38.00 25.33 16.22 12.06 9.29 7.18 1988 94.20 94.00 94.20 58.60 38.50 31.77 24.27 15.66 14.36 12.76 1989 94.20 76.40 58.40 49.40 32.75 25.77 17.72 12.23 10.36 8.38 1990 123.60 123.60 113.80 91.80 76.00 63.17 51.52 39.17 28.12 21.98 1991 45.00 42.00 37.60 34.20 29.95 24.60 19.82 16.28 12.88 11.45 1992 73.20 73.20 73.00 51.10 36.65 29.03 17.88 11.38 9.96 10.11 1993 67.20 67.20 53.20 51.50 36.80 34.23 29.03 21.58 15.19 11.60 1994 75.00 58.80 58.80 53.50 38.40 33.33 23.40 19.83 13.96 10.52 1995 98.40 69.20 54.80 48.80 34.55 28.27 17.58 11.52 9.06 8.26 1996 133.20 91.20 58.20 49.40 25.00 16.67 10.53 9.10 7.63 5.77 1997 192.60 137.20 82.00 54.30 53.25 36.00 18.42 9.21 6.14 4.60 1998 168.00 122.80 65.40 36.70 23.20 20.67 12.73 8.95 7.88 6.15 1999 139.20 100.00 92.00 59.00 37.25 33.67 25.50 16.21 13.06 10.31 2000 178.20 125.20 72.60 46.00 39.60 32.97 19.95 10.31 6.87 5.15 2001 141.60 104.00 81.00 53.50 34.50 29.50 16.48 10.07 7.33 5.75 2002 127.80 96.00 88.00 77.50 42.25 28.50 19.20 10.11 7.02 6.28 2003 120.00 100.00 72.00 49.50 34.50 27.33 17.58 11.71 9.75 7.58 2004 147.60 115.20 93.00 64.70 48.60 48.00 37.63 31.56 22.50 17.91 2005 145.80 124.00 75.80 51.70 31.25 22.17 16.53 9.89 8.67 8.54 2006 132.00 114.80 98.60 82.20 58.10 45.43 31.20 19.42 16.58 13.48 2007 164.40 151.60 120.20 106.40 58.75 39.20 25.67 15.79 13.06 11.17 2008 168.00 152.00 105.00 71.00 39.25 26.17 13.38 8.11 5.56 5.56 2009 129.00 122.40 98.40 62.50 31.45 25.40 18.20 16.61 12.79 9.88


March 2009 5-21

Table 5.9 Ln-Transformed of Annual Maximum Rainfall Intensities

YEAR LN-Transform of Intensity Data for Various Storm Durations

10 min 15 min 30 min 1 hour 2 hour

3 hour

6 hour

12 hour

18 hour

24 hour

1971 4.42 4.42 4.07 3.95 3.41 3.18 2.95 2.57 2.40 2.29 1972 4.69 4.32 3.95 3.86 3.58 3.47 3.37 3.18 2.98 2.73 1973 4.78 4.63 4.35 4.17 3.71 3.32 2.63 1.94 1.53 1.25 1974 4.74 4.56 4.43 4.24 4.02 3.80 3.44 3.04 2.73 2.67 1975 4.73 4.68 4.24 3.92 3.89 3.57 3.18 2.58 2.40 2.14 1976 4.95 4.90 4.51 4.24 4.07 3.94 3.87 3.48 3.20 2.99 1977 4.93 4.75 4.52 4.19 3.81 3.51 2.98 2.32 2.39 2.16 1978 4.86 4.70 4.39 4.14 3.85 3.60 3.09 2.61 2.22 1.94 1979 5.01 4.75 4.30 3.99 3.71 3.36 2.71 2.46 2.11 1.85 1980 4.92 4.73 4.25 3.81 3.58 3.48 3.20 2.84 2.58 2.38 1981 4.25 4.25 3.98 3.53 3.20 2.81 2.13 1.44 1.03 0.74 1982 4.73 4.66 4.41 4.01 3.66 3.57 3.30 2.92 2.53 2.30 1983 4.72 4.61 4.30 3.90 3.24 2.83 2.15 1.46 1.09 1.08 1984 4.73 4.73 4.63 4.21 3.56 3.38 3.03 2.71 2.46 2.24 1985 4.81 4.41 4.23 4.15 3.68 3.28 2.97 2.42 2.02 1.77 1986 5.01 4.61 4.16 3.72 3.26 2.85 2.46 1.90 1.56 1.38 1987 4.74 4.74 4.64 4.16 3.64 3.23 2.79 2.49 2.23 1.97 1988 4.55 4.54 4.55 4.07 3.65 3.46 3.19 2.75 2.66 2.55 1989 4.55 4.34 4.07 3.90 3.49 3.25 2.87 2.50 2.34 2.13 1990 4.82 4.82 4.73 4.52 4.33 4.15 3.94 3.67 3.34 3.09 1991 3.81 3.74 3.63 3.53 3.40 3.20 2.99 2.79 2.56 2.44 1992 4.29 4.29 4.29 3.93 3.60 3.37 2.88 2.43 2.30 2.31 1993 4.21 4.21 3.97 3.94 3.61 3.53 3.37 3.07 2.72 2.45 1994 4.32 4.07 4.07 3.98 3.65 3.51 3.15 2.99 2.64 2.35 1995 4.59 4.24 4.00 3.89 3.54 3.34 2.87 2.44 2.20 2.11 1996 4.89 4.51 4.06 3.90 3.22 2.81 2.35 2.21 2.03 1.75 1997 5.26 4.92 4.41 3.99 3.97 3.58 2.91 2.22 1.81 1.53 1998 5.12 4.81 4.18 3.60 3.14 3.03 2.54 2.19 2.06 1.82 1999 4.94 4.61 4.52 4.08 3.62 3.52 3.24 2.79 2.57 2.33 2000 5.18 4.83 4.28 3.83 3.68 3.50 2.99 2.33 1.93 1.64 2001 4.95 4.64 4.39 3.98 3.54 3.38 2.80 2.31 1.99 1.75 2002 4.85 4.56 4.48 4.35 3.74 3.35 2.95 2.31 1.95 1.84 2003 4.79 4.61 4.28 3.90 3.54 3.31 2.87 2.46 2.28 2.03 2004 4.99 4.75 4.53 4.17 3.88 3.87 3.63 3.45 3.11 2.89 2005 4.98 4.82 4.33 3.95 3.44 3.10 2.81 2.29 2.16 2.14 2006 4.88 4.74 4.59 4.41 4.06 3.82 3.44 2.97 2.81 2.60 2007 5.10 5.02 4.79 4.67 4.07 3.67 3.25 2.76 2.57 2.41 2008 5.12 5.02 4.65 4.26 3.67 3.26 2.59 2.09 1.71 1.72 2009 4.86 4.81 4.59 4.14 3.45 3.23 2.90 2.81 2.55 2.29


5-22 March 2009

Table 5.10 Statistics of the LN-Transformed Data

YEAR Statistics of LN-Transformed Intensity Data for Various Storm Durations

10 min 15 min 30 min 1 hour 2 hour 3 hour 6 hour 12 hour

18 hour

24 hour

Mean 4.77 4.60 4.33 4.03 3.65 3.40 2.99 2.57 2.30 2.10

SD 0.30 0.27 0.25 0.24 0.27 0.30 0.40 0.48 0.51 0.51

Computed LN-Transformed Values for Storms of Various Durations

ARI (yrs)

Duration (mins) 10 15 30 60 120 180 360 720 1080 1440

LN(Duration) 2.30 2.71 3.40 4.09 4.79 5.19 5.89 6.58 6.98 7.27

2 4.77 4.60 4.33 4.03 3.65 3.40 2.99 2.57 2.30 2.10

5 5.02 4.82 4.54 4.23 3.87 3.65 3.33 2.98 2.73 2.53

10 5.16 4.94 4.65 4.34 3.99 3.78 3.51 3.19 2.95 2.75

20 5.26 5.04 4.74 4.43 4.08 3.89 3.65 3.36 3.13 2.94

50 5.39 5.15 4.84 4.53 4.19 4.01 3.81 3.56 3.34 3.14

100 5.47 5.22 4.91 4.59 4.27 4.10 3.93 3.69 3.48 3.28

ARI

Freq. Factors

2 0 5 0.845 10 1.285 20 1.645 50 2.054

100 2.335


March 2009 5-23

Table 5.11 Statistics of the LN-Transformed Data

ARI (yrs

Statistics of LN-Transformed Intensity Data for Various Storm Durations

10 min 15 min 30 min 1 hour 2 hour 3 hour 6 hour 12 hour

18 hour

24 hour

2 122.80 102.54 78.05 57.92 39.67 31.18 21.61 14.61 11.23 9.21

5 149.98 123.22 93.94 70.32 49.08 39.40 29.40 20.90 15.89 12.96

10 164.13 133.99 102.22 76.78 53.98 43.67 33.45 24.17 18.32 14.91

20 175.71 142.80 108.99 82.06 57.99 47.17 36.77 26.85 20.31 16.51

50 188.87 152.81 116.68 88.06 62.54 51.15 40.54 29.89 22.57 18.33

100 197.90 159.69 121.97 92.18 65.68 53.88 43.13 31.98 24.12 19.58


5-24 March 2009

ARI (years) Equation

100 y = ‐0.0109x3 + 0.1721x2 ‐ 1.2674x + 7.6176

50 y = ‐0.0092x3 + 0.1422x2 ‐ 1.118x + 7.3252

20 y = ‐0.0068x3 + 0.0988x2 ‐ 0.9007x + 6.8995

10 y = ‐0.0046x3 + 0.0605x2 ‐ 0.7093x + 6.5249

5 y = ‐0.002x3 + 0.0137x2 ‐ 0.4755x + 6.067

2 y = 0.003x3 ‐ 0.0761x2 ‐ 0.0264x + 5.1876

Figure 5.8 Plot of Log Transformed IDF Values (from Table 5.10)

With Polynormial Best Fit Curves and Equations

2.00

2.50

3.00

3.50

4.00

4.50

5.00

5.50

6.00

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

LN-T

rans

form

ed D

isch

arge

LN (storm duration)

2 yr ‐ ARI5 yr ‐ ARI10 yr ‐ ARI20 yr ‐ ARI

6.00

5.50

5.00

4.50

4.00

3.50

3.00

2.50

2.00

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

2yr-ARI

5yr-ARI 10-ARI

20yr-ARI


March 2009 5-25

Duration (mins)

Intensity (mm/hour) for Various ARI Storms

2yr 5 yrs 10 yrs 20 yrs 50yr 100yr

15 101.25 126.48 142.08 156.00 173.77 186.99

30 76.36 92.71 102.65 111.21 122.20 130.18

60 55.13 67.53 75.13 81.55 90.02 96.09

120 38.33 48.67 55.20 60.69 68.20 73.58

240 25.81 34.58 40.35 45.21 52.18 57.20

600 14.73 21.38 26.03 29.99 36.02 40.42

1200 9.43 14.46 18.13 21.25 26.25 29.91

1500 8.15 12.68 16.02 18.86 23.47 26.84

Figure 5.9 IDF curves Plotted Plotted on Log-log Graph Paper (Based on Equations in figure 5.8)

1

10

100

1000

10 100 1000 10000

Inte

nsity

, I (

mm

/hr)

Duration, t (min)

2 yrs5 yrs10 yrs20 yrs

1000

100

10

1

10 100 1000 10000

2 yrs

5 yrs

10 yrs

20 yrs


5-26 March 2009

REFERENCE [1] DID, Varieties of rainfall with area in peninsular Malaysia, water resources Publication No 17. [2] DID (2000). Urban Stormwater Management Manual For Malaysia. Water Resouces Publication,

No.1 Introduction to Manual. Dept. of Irrigation and Drainage, Ministry of Agriculture, Kuala Lumpur, Malaysia.

CHAPTER 6 LOW FLOWS, DROUGHT ANALYSIS AND MONITORING

Chapter 6 LOW FLOWS, DROUGHT ANALYSIS AND MONITORING

March 2009 6-i

Table of Contents Table of Contents………………………………………………………………………………………………………………….6-i

List of Tables ......................................................................................................................... 6-iii

List of Figures ........................................................................................................................ 6-iii

6.1 GENERAL ................................................................................................................... 6-1

6.2 DROUGHT TERMINOLOGY ............................................................................................ 6-2

6.2.1 Agricultural Drought ...................................................................................... 6-3

6.2.2 Hydrological Drought ..................................................................................... 6-3

6.2.3 Socioeconomic Drought ................................................................................. 6-3

6.3 PURPOSE OF LOW FLOW AND DROUGHT MONITORING ................................................ 6-3

6.4 REAL TIME DATA COLLECTION FOR DROUGHT .............................................................. 6-4

6.5 USE OF FLOW DURATION CURVES ................................................................................ 6-5

6.5.1 Introduction ................................................................................................. 6-5

6.5.2 Methodology ................................................................................................. 6-5

6.5.3 Procedure ..................................................................................................... 6-5

6.5.4 Interpretation of Results ................................................................................ 6-6

6.6 FREQUENCY ANALYSIS OF LOW FLOWS: LOW FLOW HYDROLOGY .................................. 6-7

6.6.1 Introduction ................................................................................................. 6-7

6.6.2 Criteria and Standard .................................................................................... 6-8

6.6.3 Low Flow Frequency Analysis: Less Than 1-Month Duration .............................. 6-9

6.6.4 Derivation of the Drought Flow Sequence ...................................................... 6-17

6.7 OVERVIEW OF DID HP 12 ON LOW FLOW ESTIMATION ............................................... 6-21

6.7.1 Introduction: Regionalization and Transposition ............................................. 6-21

6.7.2 HP 12: Magnitude and Frequency of Low Flows in Peninsular Malaysia (Revised and Updated) 1985 ....................................................................... 6-21

6.7.2.1 Introduction ............................................................................. 6-21

6.7.2.2 Notation .................................................................................. 6-22

6.8 HP 12 (NWRS, 2000) .................................................................................................. 6-26

6.9 DROUGHT INDEX (SPI AND PALMER) .......................................................................... 6-29

6.9.1 Standardized Precipitation Index (SPI) .......................................................... 6-30

6.9.2 The Palmer Drought Severity Index .............................................................. 6-31

6.9.3 Crop Moisture Index (CMI) ........................................................................... 6-31

6.9.4 Surface Water Supply Index (SWSI) ............................................................. 6-32


6-ii March 2009

6.10 WEB-BASED INFORM KEMARAU .................................................................................. 6-33

6.10.1 Introduction ............................................................................................... 6-33

6.10.2 Methodology ............................................................................................... 6-33

6.11 IMPACT OF CLIMATE CHANGE .................................................................................... 6-38

REFERENCE ......................................................................................................................... 6-41


March 2009 6-iii

List of Tables


6.1 Normal Probability 6-10

6.2 Normal Distribution Table 6-12

6.3 EV Type III parameters 6-23

6.4 Standard Precipitation Index Values 6-30

6.5 The Palmer Index 6-31


6.1 Flow Duration Curve Monthly Time Step Sg. Papar@ Kaiduan Streamflow Station. 6-7

6.2 Low Flow Frequency Analysis 1- And 7-Day Example 6-13

6.3 Example of Extreme Value DistributionExample of Low Flow Frequency Analysis Using EV 3 and log EV 3 6-16

6.4 General Extreme Value Distribution 6-17

6.5 Low Flow Frequency Analysis Drought Sequence Example 1 Of 2 6-19

6.6 Low Flow Frequency Analysis Drought Sequence Example 2 Of 2 6-20

6.7 RC Regions for Peninsular Malaysia (HP 12) 6-24

6.8 RE Regions for Peninsular Malaysia (HP 12) 6-25

6.9 NWRS Regionalized Low Flow Analysis: 1/2 6-28

6.10 NWRS Regionalized Low Flow Analysis: 2/2 6-29

6.11 WWW site for Infokemarau (JPS) 6-34

6.12 Real Time Rainfall and Water Level Monitoring 6-34

6.13 Drought Monitoring by River Flow 6-35

6.14 Drought Monitoring by Dam Levels 6-36

6.15 Isohyet Map of three-monthly Rainfall Distribution 6-37

6.16 Standard Precipitation Index (SPI) For Kuala Nerang 6-38

6.17 Global Temperature Anomaly 6-40


6-iv March 2009



March 2009 6-1

6 LOW FLOWS, DROUGHT ANALYSIS AND MONITORING

6.1 GENERAL Drought is a normal, recurrent feature of climate, although many consider it to be a rare and/or random event. It occurs in virtually all areas, whatever their normal climate may be, and the characteristics of a drought may be very different from one region to another.

Technically, drought is a “temporary” condition, even though it may last for long periods of time. Drought is by technical definition, a natural hazard. Unlike many other disasters which are sudden and/or spontaneous in terms of the time of occurrence, on the contrary droughts develop when there is an unfavorable meteorological condition that less than normal precipitation or streamflow over an extended period of time, usually for a continuation of several months or more. The decreased water input in the form of precipitation and subsequent runoff generation results in an inconvenient water shortage for water supply to both domestic and irrigation sector.

On the other hand, drought can also occur when the temperature is higher than normal for a longer period of time. As a result, more water is to be drawn off by evaporation and evapotranspiration alike. In agricultural or irrigation sector, there are also other probable causes of drought due to delays in the start of the rainy season or timing of rains in relation to principal rain-fed crop growth stages (e.g. rain at the “wrong” time). In addition, high winds and low relative humidity can also be contributing factors to the droughts as well.

Drought, in its nature, is not to be considered as a disaster, the disaster occurs as the demand of the people such as water supply can not be satisfactorily met. Human activities may have increased the impact of drought because of high demand of water which cannot be supplied sustainably when the natural supply decreases. Drought can have significant impacts on the economic and environmental impacts and personal hardships and inconvenience.

An example on the hardships was caused by the 1997-1998 drought in Klang valley due to the effect of the El Nino / Southern Oscillation (ENSO) phenomenon, when the rainfall patterns shift over different regions of the world, especially the eastern and western pacific ocean. During this period, it is in fact the entire South East Asia and Australia continent are extremely affected by one of the most severe ENSO episode of the century, due to the sea surface temperature rises in the east coastal region of South America, i.e. near the coast of Peru. In broad perspectives, the region experiences extreme low rainfall episode over a period of two years, which then led to severe drought conditions in the entire western pacific regions.

The 1997-1998 low rainfalls associated with the ENS0 event also affected some parts of Malaysia other than the first ever imposition of water rationing of the entire Klang Valley, such as Malacca, Penang, Kedah, Kelantan, the Borneo states of Sarawak and Sabah as well.

The extremely lower-rainfall-than-normal episode translates into low river flows at the respective water supply intakes in most parts of Malaysia. The raw water sources could not met the targeted demand at the intakes and water treatment plants. Thus this causes disruptions in the continuous supply of treated water to various demand centers. The shortage of water production and distribution resulted hardships and subsequently major economic and financial losses.

Additional woes to the situation were the frequent episodes of pollution (including illegal dumping of solid wastes) in the water bodies concurrently. Due to lesser flows prevailing in the rivers, the dilution effect is therefore insignificant to assimilate additional pollutant loads, the concentration of the pollutants in the river therefore increased to unacceptable levels.


6-2 March 2009

The state of Selangor was the hardest hit area during the 1997-1998 ENSO episode. Due to lower-than-expected low flows in almost all the intakes and reservoirs in the state of Selangor including Kuala Lumpur, the state authority therefore imposed a state-wide water rationing exercise to its 1.8 million residents (including Kuala Lumpur) in both Sg. Klang and Sg Langat Basins for durations from April to September 1998. Similarly, in Kelantan, though the ENSO episode did not result in domestic water supply, the dry conditions in March to May 1998 due to low river flow and level of Sg. Kelantan had caused irrigation water supply shortage in KADA. Sand bunds were constructed to raise the hydraulic head of Sg. Kelantan so that diversion of water to the existing pump houses along Sg. Kelantan could be made possible.

6.2 DROUGHT TERMINOLOGY The term “Drought” is rather difficult and subjective to define precisely, but conventional operational definitions may help to define the sequence of occurrence based on (1) onset, (2) severity, and (3) end of droughts. No single operational definition of drought works in all circumstances, and this is a big part of why policy makers, resource planners and others have more trouble recognizing and planning for drought than for other natural disasters. In fact, most drought planners now rely on mathematic indices to decide when to start implementing water conservation or measures in response to drought. As cited in University of Florida (1998), research by Donald A. Wilhite, director of the National Drought Mitigation Center, and Michael H. Glantz, of the National Center for Atmospheric Research, in the early 1980s uncovered more than 150 published definitions of drought. The definitions reflect differences in regions, needs and disciplinary approaches.

Wilhite and Glantz categorized their collection of definitions into four basic approaches to measuring drought: (1) meteorological, (2) hydrological, (3) agricultural and (4) socioeconomic. The first three approaches deal with ways to measure drought as a physical phenomenon. Last approach deals with drought in terms of its supply and demand scenarios by tracking the effects of water shortfall as it ripples through the socioeconomic systems.

Meteorological drought is usually measured by how far from normal the precipitation has been over some period of time. These definitions are usually region-specific, and presumably based on a thorough understanding of regional climates.

Examples of “meteorological droughts” from different countries at different times show why it is a poor idea to apply a definition of drought developed in one part of the world to another:

• United States (1942): less than one tenth inch of rainfall in 48 hours • Great Britain (1936): fifteen consecutive days with daily precipitation totals of less than one

hundredth of an inch • Libya (1964): when annual rainfall is less than 7 inches • India (1960): actual seasonal rainfall is deficient by more than twice the mean deviation • Bali (1964): a period of six days without rain. Under any circumstances, meteorological

measurements are the first indicators of drought.


March 2009 6-3

Operational definitions help people identify the beginning, end, and degree of severity of a drought. (An abbreviated description of operational definitions is also available.) To determine the beginning of drought, operational definitions specify the degree of departure from the average of precipitation or some other climatic variable over some time period. This is usually done by comparing the current situation to the historical average, often based on a 30-year period of record. The threshold identified as the beginning of a drought (e.g., 75% of average precipitation over a specified time period) is usually established somewhat arbitrarily, rather than on the basis of its precise relationship to specific impacts. An operational definition for agriculture might compare daily precipitation values to evapotranspiration rates to determine the rate of soil moisture depletion, then express these relationships in terms of drought effects on plant behavior (i.e., growth and yield) at various stages of crop development. A definition such as this one could be used in an operational assessment of drought severity and impacts by tracking meteorological variables, soil moisture, and crop conditions during the growing season, continually reevaluating the potential impact of these conditions on final yield. Operational definitions can also be used to analyze drought frequency, severity, and duration for a given historical period. Such definitions, however, require weather data on hourly, daily, monthly, or other time scales and, possibly, impact data (e.g., crop yield), depending on the nature of the definition being applied. Developing a climatology of drought for a region provides a greater understanding of its characteristics and the probability of recurrence at various levels of severity. Information of this type is extremely beneficial in the development of response and mitigation strategies and preparedness plans.

6.2.1 Agricultural Drought

Agricultural drought occurs when there isn't enough soil moisture to meet the needs of a particular crop at a particular time. Agricultural drought happens after meteorological drought but before hydrological drought. Agriculture is usually the first economic sector to be affected by drought.

6.2.2 Hydrological Drought Hydrological drought refers to deficiencies in surface and subsurface water supplies. It is measured as streamflow, and as lake, reservoir and ground water levels. There is a time lag between lack of rain and less water in streams, rivers, lakes and reservoirs, so hydrological measurements are not the earliest indicators of drought. When precipitation is reduced or deficient over an extended period of time, this shortage will be reflected in declining surface and subsurface water levels.

6.2.3 Socioeconomic Drought Socioeconomic drought is what happens when physical water shortage starts to affect people, individually and collectively. Or, in more abstract terms, most socioeconomic definitions of drought associate it with the supply and demand of an economic good.

6.3 PURPOSE OF LOW FLOW AND DROUGHT MONITORING Contribution by monitoring, prediction, early warning and mitigation of adverse impacts of extreme climatic events such as droughts on agricultural production and food security, water resources, energy, and health among other socio-economic sectors are important for Malaysia.

One of the effective strategies is to minimize negative impacts associated with the climate extremes are enhanced monitoring and timely availability of weather and climate information and prediction products, so that decision making processes can be swiftly implemented together with the availability of effective disaster preparedness policies.


6-4 March 2009

This enhanced applications of such products in order to reduce climate and weather related risks. It is principally to be used in the water resources management for raw water supply to various intakes for irrigation and water supply scheme in Malaysia.

6.4 REAL TIME DATA COLLECTION FOR DROUGHT

As mentioned earlier drought is a normal and randomly recurring feature of all climate regimes that is affecting most of the countries at one time or the other for various degrees of severity. It is characteristically differentiated itself from other natural hazards, such as floods, earthquakes, landslides, etc as it has a slow onset and also a lack of the universal definition, It is often difficult to define the boundaries of the affected, usually large area, its severity and the complex impacts. Therefore the mitigation interventions of drought are basically less obvious. It often occurs in several economic and social sectors and affects large geographic areas.

Real time monitoring program has been initiated and subsequently implemented by JPS in 2001. This on line and real time service of hydrometric data acquisition is needed for effective and efficient drought management in Malaysia. The main task of the monitoring and early warning systems is to provide timely information on the formation, development, persistence of drought to the users and decision makers. The system collects, analyzes and disseminates drought information.

The new measurement instruments based on the latest technology can conduct observations rather frequently, these time steps of measurements are shorter than it is necessary for drought monitoring. The ’creeping’ character of drought does not require monitoring in small time steps.

Based on timely and acquisition of crucial data for the drought prediction and monitoring systems, allows for the early detection of drought conditions. It also enables the planners and management to respond in a proactive rather than reactive manner.

Drought can be closely monitored by the application of various statistical techniques. This further requires the availability of long term series of historical hydrometric data and information as base reference by checking side-by-s9de on the deviation of the hydrometric variables from their long-term mean and other statistical parameters.

Due to the fact that the drought affected areas can be very different from one another , several operation drought definitions which had been mentioned before can be grouped in the following types: (1) meteorological, (2) hydrological, (3) agricultural and (4) socioeconomic.

As noted, there is no single index that can be used satisfactorily to evaluate the combined effects of meteorological, agricultural, or/and hydrological droughts, a variety of indices should therefore be used. Meteorological drought is expressed solely on the basis of dryness or precipitation deficiency. The other definitions are more concerned with the effects on water flows, agriculture or economy and society; reflecting the fact that impacts of drought are not limited on agricultural production, other sectors of economy like water management systems, transport, industry can be affected as well

A total of 167 streamflow and rainfall stations are available for such purpose in the Infokemarau network.

Application of information acquired other than for on line prediction of drought occurrence, they can be useful for hydrological assessment of low flow for individual streamflow stations and extended regionally using various regionalization approaches.


March 2009 6-5

6.5 USE OF FLOW DURATION CURVES

6.5.1 Introduction

The flow duration curve is a production of the analysis of flow discharge records that shows the percentage of time that flow in a river is probably to be equal or exceeded at a pre-determined magnitude of flow. To illustrate an example for clarity, it can show the percentage of time that the magnitude of a river flow can be expected to exceed a design flow of some specified value. On the other hand it is able to show the flow discharge of the river that occurs or is exceeded some percent of the time.

6.5.2 Methodology For preparation of a flow duration curve of a “point” or a gauging station is to gather all the available time series records, irrespectively of the time sequence. The basic time scale is normally mean daily or mean monthly discharge. Selection of basic time unit greatly affects the appearance and shape of the flow duration curve. For example, a steeper curve for the mean flow over a long period is used such as mean monthly flow. This is due to averaging effects of the short term peak flow with intervening smaller flow with a month. In other words, extreme values of short time step are in fact being averaged out as the time step get larger. In general, both monthly and daily time step flow duration curves are presented side by side and it is rarely to analyze and plot flow duration curves of longer than monthly time step, say annually. The reason is rather obvious, (1) the annually flow duration curve cannot capture the time scale variability of random variables. (2) there is a shortage length of records if a yearly or longer time step is used.

6.5.3 Procedure Step 1: Sort (rank) average daily discharges for period of record from the largest value to the smallest value, involving a total of n values.

Step 2: Assign each daily or monthly flow value a rank (M), starting with 1 for the largest daily or monthly flow value.

Step 3: Calculate Weibull exceedence probability (P) as follows:

p=100 x m

n+1 (6.1)

P = The probability that a given flow will be equaled or exceeded (% of time) M = The ranked position on the listing (dimension less) n = The number of events for period of record (dimension less)

Step 4: Plot the flow discharge versus the Weibull exceedence probability on a arithmetic paper

Adopted from http://water.oregonstate.edu/streamflow/analysis/flow/index.htm


6-6 March 2009

6.5.4 Interpretation of Results A flow duration curve plot in arithmetic scale shows the magnitude of flow or discharge vs. percent of time that a particular flow or discharge was equaled or exceeded. The area under the flow duration curve (with arithmetic scales) gives the average daily flow, and the median daily flow is the 50% value. It is also useful to plot the graph on a probability paper with or without the distortion of flow or discharge scale based on the extent of the extremity in low and high flow. A flow duration curve demonstrates the ability of river basin at the point of analysis to contribute to flows of various magnitudes. Information such as the relative number of time that flows past through the point or gauging station are likely to equal or exceed a specified value of interest is useful for river engineering design undertaking, establishment of the magnitude of environmental or riparian flow, water supply yield calculation, calculation of firm power in hydropower development, etc. The shape of a flow-duration curve in its upper and lower limits is important in evaluating the river flow and catchment hydrological characteristics and behaviors. The shape of the curve in the high-flow region indicates the type of flood regime the river basin is likely to have. A shape curve at the higher flow suggests infrequent and rare extremity in flows occurred within the river catchment. A very steep curve (high flows for short periods) would be expected for rainfall induced floods on small catchments. On the other hand, the shape of the low-flow region characterizes the ability of the basin to sustain low flows during extremely drought or dry seasons. If the upper catchment of a point gauging station is regulated by reservoir/dam storage, the flow duration curve will show a very flatter curve to reflect that moderate or constant flows are sustained throughout the year due to artificial regulation. However there are exceptions to the conventional interpretation of the flow duration curves, such as if there is large ground water aquifer adjacent to the effluent rivers which contributes to the base flow. An example of the flow duration curve plot is illustrated in Figure 6.1

March 2

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6-8 March 2009

In terms of the impacts of low flow occurrence in a river system, due to exceptionally low velocity prevails in the river, mostly in the stagnant water bodies, stenches and odors emitted by both dissolved and undissolved pollutants are repugnant and offensive. These are the major environmental problems faced by most riparian users along the river corridors in Malaysia. Both the point and nonpoint pollutants that are generated from human activities in the cities and townships mostly discharge directly with or without any forms of treatment processes. These mass of pollutant flow into adjacent streams and drainage networks en route to the main stems of the river system. Domestic and industrial wastewater effluents are being identified as the culprits responsible for water quality deterioration and degradation in the riverine system. Due to lacking of diluting capacity of the river especially during low flows, the impacts of pollution on the riverine system are considered serious with prolonged drought events. The magnitude of low flows that associated with specific severities and durations are of critical in assessing the impact of water quality and pollution in a water body. These could be quantified and derived at each point of interest within the river basin using results of “point” analysis of long-term hydrometric (mostly streamflow) records. These results serve as input for subsequent works and studies carried out on water quality assessment.

6.6.2 Criteria and Standard Frequency analysis is the most conventional tool used in Malaysia for low flow analysis. The minimum annual low flow series are extracted from streamflow gauging stations of interest. The records are then ranked and plotted for frequency analysis. Subsequently best fit curves are then drawn through their midst. The magnitudes of low flow of specific probabilities of occurrence are then inferred from the graphs or computations. The conventional low flow hydrological output of a selected N-year severity or probability of occurrence and specific durations is deemed adequate for the purpose of assessing the amount of raw water or continuous resources available at a specific point or outlet of a particular watershed. These magnitudes of flow of specific return periods and duration could also be equally applicable as “driver” for pollutant fate and transport mechanisms in the river system. For example, a 50-year severity or reliability represents the lowest flow/yield with an average recurrence interval (ARI) of 50 year on average is adopted in low flow analysis. It is also known as 98% reliability mainly in the water supply sector. The term, percentage reliability means the percent of the time the flow being greater or equal to. Other return periods are of equal importance such as 5- and 20-years for irrigation water supply. Two (2) types of quantitative/magnitudes of low flows using frequency analysis are typically assessed for low flow. They are as follows (a) Flow duration analysis and 1:N-year D-day (less than 30-day, from 1- to 30-day), (b) 1:N-year D-month drought sequence that represents the lowest drought flow series of D month

or more duration (normally 36 to 48 consecutive month duration is adequate to characterize low flow in a river system).


March 2009 6-9

6.6.3. Low Flow Frequency Analysis: Less Than 1-Month Duration (a) Log Normal 2 P and 3 P In Malaysia, assessment of low flow regime of streamflow station is best carried out using frequency or probability distributions, such as Log-Normal distributions, of 2P or 3P parameters and Weibull (Extreme Value type III) distribution. They have been used in several major hydrological and water resources studies in Malaysia (SMHB, 1992, 1994; SMHB/RHB/ZAABA, 2000).

1:N-Year D day (1-, 7-, 14-, and 30-Day) Low Flow Frequency Analysis

Tasks of Frequency analyses on n-day low flows are carried out for point streamflow stations in a river basin. The Normal and Log-Normal of 2P and 3P distribution are commonly used technique for frequency analysis. Initially, the D-day low flows are tabulated from the daily flow records. The selection of the n-day annual series is based on lowest moving n-day sum of the daily flow records. The D-day data flow data are then duly tabulated and ranked in ascending order and plotted against the Weibull normal unbiased plotting position or other plotting positions that might be suitable for specific types of distribution as shown below.

T= MN+1

(6.2)

1T= Weibull polotting position of an annual n – day flow, year

M= Length of n – day records year

N= Rangking of the annual flow in the n – day series, ND

For consistency in calculations, the 1:N-year and other return periods low flow value was determined analytically using the Normal, LN2P or LN3P distribution as appropriate instead of ‘eye’ fitting.

Normal Distribution

The normal distribution is adopted to fit a best fit curve through the scattering low flow plotting on a specially designed probability paper series data (a straight line if a normal probability assumption is made). The Frequency factor technique was based on Chow et al. (1988) where standard moment estimator is used to inferred statistical parameters. Other parameter estimation techniques, such as maximum likelihood (ML), method of least square (MLS), probable weighted moment (PWM), L-moment, etc. could also be used as appropriate. .

Qn year= Q+Kn year x σ (6.3)

Sample Statistics: Mean, Standard deviation and Skewness

a) Mean: Q= 1N

∑ (Q)iNi=1 (6.4)

b).Standard Deviation: ∑ (Q)i-(Q i)

2Ni-1

N-1

1/2

(6.5)

c) Skewness: G= N

N-1 (N-2)

∑ log Q - log(Q)Ni=1

σ3

3

(6.6)


6-10 March 2009

Frequency factors associated with each return period or probability of non-occurrence could be interpolated from the normal probability table (see Table 6.1) or estimated numerically by integrating the Gaussian Probability Density Function (PDF), whichever is convenient. For brevity, commonly used frequency factors are tabulated below. Frequency Factors as Derived from Gaussian Normal Distribution

Table 6.1 Normal Probability

Return Period Frequency Factor K

100 year 2.326 50 year 2.054 30 year 1.881 10 year 1.282 1.5 year 0.840 2.0 year 0.000

The plot of percentage of probability of non exceedance and flow on a distorted normalized and arithmetic scale paper show a best fit line traverse through the randomly sampled flow annual series.

(b) Log Normal Distribution 2 Parameter

The LN2P distribution assumed that the logarithms of the discharges were themselves normally distributed Most of the hydrologic parameters are log-normally distributed. In essence, the randomly sample variables could be transformed into either natural or base 10 logarithm. However the latter is the most commonly and frequently used technique. The logarithmic values of the random variables, in this case, flows a formed and the statistics of 1st, 2nd, and 3rd moments are calculated accordingly as follows

re trans

log Qh year= Qlog19 + Kn year x σ log 10 (6.7)

Qn year =10log Qn year (6.8)

Sample Statistics: M andard eviation, Skewness ean, St d

Log Mean: log (Q)i = 1 N

∑ log(Q)iNi=1 (6.9)

Log Standard Deviation:σ log Q = ∑ log (Q)i- log (Q)i

2Ni=1

N-1

1/2

(6.10)

Log Skewness: Glog Q N

N-1 N-2

∑ log Q ‐ log Q3N

i=1σlog Q 3

(6.11)

The plot of percentage of probability of flow non exceedance and the magnitude of flow on a probability and transformed log scale paper shows as in normal distribution, a straight line fitting through the scattering annual flow series.


March 2009 6-11

(c) Log Normal Distribution 3 Parameter

As in the LN2P, The LN3P distribution assumed that the logarithms of the discharges were themselves normally distributed. If the distribution was non-normal, a 3-parameters or LN3P distribution was used instead. This was accomplished by transforming the variable i.e. n-day average low flow either adding or subtracting a fitting parameter, X’ such that the skewness of the logarithm of the series was more or less approaching zero.

By transforming the D-day low flow data, it was found out that a better fit could be obtained, in most cases. In the final step, each data on the annual flow series was transformed into a logarithmic value. Upon taking the logarithm of each value the mean, standard deviation, and skewness of the transformed variable were obtained from,

log Q+x' i= 1N

∑ log Q+x'Ni=1 (6.12)

σlog Q+X =∑ log Q+X' -log(Q+X')i

2Ni=1

N-1

1/2

(6.13)

G= N

N-1 N-2

∑ log Q+X' - log (Q+X)3N

i=1σlog Q+X' 3

(6.14)

The 1:50 year and other years (return period) run-of-river yield using frequency factor techniques as attributed to Chow (Chow et al, 1988) was then calculated as follows:

log Q+X' 50 = log Q+X' i +Kσlog Q+X' (6.15)

Q50 =10log (Q+X')-X (6.16)

Q50=1:50 year n-day low flow, m3/s

log Q= Logarithmic value of annual series of n – day low flow m3/s

σlo

X'= 3rd parameter, dimensionless

g Q+X = Logarithmic standard deviation of annual series of flow n – day flow, m3/s

N= Number of annual series of n- day low flow, dimensionless

G= Skewness of annual series of n-day low flow, m3/s

K= Frequency factor for normal probability distribution with the following values, ND

It was generally assumed that the abstraction upstream of the streamflow station was negligible. If not, any known abstraction upstream can be added to the calculated low flow yields accordingly. Figure 6.2 shows an example of LN2P frequency distribution curves of a streamflow stations in the state of Perak


6-12 March 2009

Table 6.2 Normal Distribution Table


March 2009 6-13

Figure 6.2 Low Flow Frequency Analysis 1- And 7-Day Example


6-14 March 2009

(d) Weibull Distribution (Extreme Value Type III)

The Weibull distribution, is a well known probability distribution and also known as a variant of the General Extreme Value (GEV) family, or Extreme Value Type III distribution. It is generally used for the low flow frequency analysis of point streamflow or gauging station records. The distribution is normally consists of two parameters to represent the probability density function (pdf):

f X = βλ

xλ

β-1e -X

λ

β

:x>0 (6.17)

denotes the characteristic smallest value of the distribution and it is also called a location parameter. On the other hand β is a measure of dispersion or a shape factor. The corresponding cumulative density function (cdf) is the integral form of the pdf as follows:

f x =1-e -xλ

β

:x>0 (6.18)

Even though the Weibull distribution was originally developed to address the problems in material science engineering discipline, it is now being widely used in many other areas, such as in low frequency rainfall analysis, probability of occurrence of earthquake and other natural disasters, low flow water quality frequency analysis, to mention a few. When β =1, this distribution reduces itself to the Exponential model, and when β =2, it mimics the Rayleigh distribution which is mainly used in telecommunications. In addition, it resembles the Normal distribution when β =3.5: The expected value and varianc ( irst a d second m ments) are estimated from the samples statistics as e f n o

Mean: E X =ε+ λ-ε Γ 1+ 1β (6.19)

Variance: Var X = λ-ε2Γ 1+ 2

β-Γ2 1+ 1

β (6.20)

By taking the ratio between the first and second moment of the population, then the coefficient of variation is only depend on the shape factor.

Vx2=

Γ 1+2β

-Γ2 1+1β

-1 (6.21)

The locat n factor io λ is then estimated using the first moment equation.

E x =μ=λΓ 1+ 1β (6.22)

λ= μ

Γ 1+1β

(6.23)

(e) Modified Weibull Distribution

Some transformation to the original hydrological data helps to better fit by Weibull distribution. The modification to the data that is normally associated with hydrologic practices of both low and high flow frequency analysis is the logarithmic transformation. This results in better fit of the hydrological annual series as shown in Figure 6.3.


March 2009 6-15

(f) Shifted Weibull Distribution The shifted Weibull distribution is introduced to fit the lower bound of the distribution using a pre-determined parameter,ε. This is in essence introducing another variable to the distribution. As such third moment of the sample is neeed. This is represented by the skewness statistic of the sample in tandem with the other two, previously mentioned first and second moments. The probability density function (pdf) and cumulative density function (cdf) are modified accordingly by including the third parameter as a correction factor, ε.

f x = β

λ-ε

x-ε

λ-ε

β-1e

-x-ελ-ε

β

:x>0 (6.24)

f x =1-e-x-ελ-ε

β

:x>0 (6.25)

The moments are i en a follows: g v s

Mean : E x =ε+ λ-ε Γ 1+ 1β (6.26)

Variance: Var x = λ-ε2Γ 1+ 2

β-Γ2 1+ 1

β (6.27)

Skewness: γx=Γ 1+3

β -3Γ 1+2β Γ 1+1

β +2Γ3 1+1β

Γ 1+2β -Γ2 1+1

β

3/2 (6.28)


6-16 March 2009

1

1000

Figure 6.3 Example of Extreme Value DistributionExample of Low Flow Frequency Analysis Using EV

3 and log EV 3 (g) General Extreme Value Distribution Weibull distribution is also commonly known as Extreme Value type III distribution within the family of General Extreme Value (GEV). The shape parameter, defines the category of the GEV that are known as

• Gumbel Distribution or EV type I if the shape factor, =1

• Frechet Tippet Distribution or EV type II if the shape ctor, <0 fa

• Weibull Distribution or EV type III if the shape factor >0

10

100

1000

0

0

red te

Q m

3/s

-6.00 -4.00 -2.00 0.00 2.0

uced varia

o dbserve EV 3 log EV 3

10000

1000

100

Q m

3/s

10

1

-6.00 -4.00 -2.00 0.00 2.00

Reduced variate

observed EV 3 Log EV 3


March 2009 6-17

Sketch below shows the general shape of the GEV distribution

Figure 6.4 General Extreme Value Distribution

6.6.4 Derivation of the Drought Flow Sequence Derivation of a 1:N-year and D-month drought sequence using frequency analysis is an industrial standard in water supply sector in Malaysia. The purpose of the drought sequence analysis is to derive a 1N-year D-month synthetic drought or low flow hydrograph using long-term monthly historical records of a streamflow station. The derived drought sequences are then used to represent inflows in the water resources studies. It is mostly used for estimating the reliable yield of a reservoir system. The rationale to adopt D-month duration drought sequence is fairly straightforward. The step-by-step procedure of drought sequence analysis for 48-month is presented as follows. Probability analysis using LN2P/LN3P is undertaken for independent low flow sequence of up to 12-month duration period, alternatively a double logarithmic plot (power equation) could also be used if the data contains significantly higher skewness. The lowest sum of flow volume of a given duration as obtained from the historical records are tabulated and ranked in ascending order. Their respective plotting positions are computed using acceptable plotting position. The 1- to 12-months duration values (or annual series) were then obtained for each record year. For duration shorter than 12 months, lowest moving average sums were selected for drought sequence purposes. To illustrate, the lowest 1-month duration flow can be selected 12 times from a 12-month record. However for duration more than 1-month, the available selection was 11 moving average sum of the flow volume. For instance, for a 11-month duration, only 2 moving average sums are available. It should be reiterated that the lowest moving average flow volume for 1 to 12-months duration are selected at all time. In short, for the first 1 to 12 month duration, the lowest 1- to 12- month total flow volume is selected based on the criteria of lowest moving average value.


6-18 March 2009

For a longer duration, such as more than 12 months, the selection becomes limited as the duration of the series increases. The sum of flow volume is no longer independent in a sense that, as the series would continue or “spill-over” to next year. In this case, a fix-start event option was devised to overcome this shortcoming of the “dependency” of the drought sequence of more than 12-month duration (Twort et al., 2000). The stating months for the derivation of the drought sequence was based on the lowest moving average sum of the mean monthly flow and of a specific duration. For example, the starting month of the 13-month duration is determined by selecting the lowest 13-month of the historical average flow. The calculations were then repeated for other duration up to 48 months. As mentioned above, 1:N-year cumulative drought flow volume is calculated analytically the same way in the derivation of 12-month independent drought sequence previously discussed.

The annual series are then plotted on a logarithmic-normal probability paper using Weibull plotting position or any suitable plotting position. For some shorter records, a double logarithmic paper sometimes is used in replacement. The reason of logarithmic transformation is due to most of the time, by transforming the annual series, a best-fit line was then normally drawn through the individual annual series. In most cases, a straight line can be plotted through the data set. Though sometimes it might be necessary to attempt a nonlinear curve fitting to the data series as deemed appropriate. This has to be done because of the skewness in the annual series.

Generally, shorter annual series records tend to deviate from conventional normal distribution assumption. However it could not be said confidently the same as if the absence of skewness might be the result of longer annual series. Even if there is skewness in the annual series, it is most likely due to existence of a small numbers of “outliers” in their midst. By visual observation, sometimes these “outliers” could be excluded from computation without much reservation. Other a rigorous test for “outliers” could be carried out prior to purging any extreme values from the annual series.

For consistency, the 1:D-year drought for any duration would be calculated analytically instead by “eye-fitting”. This is accomplished by firstly transforming the annual series to logarithmic values. The first (Mean), second (Standard Deviation) and third (skewness) moments of the logarithmic series are then calculated. The steps involved are the same as above.

Once the 1:N-year D-month drought volume is calculated, then it is followed by the desegregation of the estimated flow volume into a 1 to D month incremental flow volume. Then the D-month series are rearranged to follow the most prevailing flow pattern, in most cases, the average observed monthly pattern of the streamflow station is adopted.

Other patterns could also be used such as choosing the lowest observed historical records. Several scenarios could be easily carried out with different temporal low pattern. Figure 6.5 shows an example of 1 to 36 month duration drought sequence curves of Sg. Sarawak @ Kg Git streamflow station in Sarawak.


March 2009 6-19

Figure 6.5 Low Flow Frequency Analysis Drought Sequence Example 1 of 2


6-20 March 2009

Figure 6.6 Low Flow Frequency Analysis Drought Sequence Example 2 of 2


March 2009 6-21

6.7 OVERVIEW OF DID HP 12 ON LOW FLOW ESTIMATION

6.7.1 Introduction: Regionalization and Transposition

Regionalization and transposition approach in the low flow hydrological assessment is another important step once the magnitude of low flow associated with specific N-year return period or probability of occurrence are established from the routine frequency analysis of point streamflow gauging station.

The regionalized approach is a commonly adopted methodology of taking into consideration that the “point” streamflow station may not be able to represent the spatial variability or “areal wide” of the entire watersheds or hydrological region. Therefore a group of streamflow stations in the region and vicinity could better performed the intended low flow estimation. The reason of areal wide estimate is required is due to the fact that it is rarely streamflow stations are conveniently located in the potential intake sites or vicinity as they are costly to build and maintained. As a matter of fact, most often than not, only few streamflow stations are available compared to the vast numbers of rainfall station in a given hydrological region or watershed.

Regionalization and transposition approach provides flow estimates and prediction to ungaged points of interest in a river basin using the results of the point gauging streamflow station. This is in essence similar to catchment wide rainfall estimates using several point raingauge records in a catchment. Techniques of regionalization of low flows are narrated in the following procedures (Tucci et al, 1995).

Fit a frequency distribution to point streamflow gauging station and relate the distribution parameters to observed hydrological and physical basin characteristics, such as catchment area, average catchment slope, mean annual rainfall volume, etc. Prior to this, normally the low flows of N-year severity are transformed to non-dimensional form by normalizing with the annual average flow (AAF).

Procedures of low (drought) and high (flood) flow regionalization approaches were carried out by Department of Irrigation and Drainage (DID) in the middle of 1980’s. The hydrological procedure of HP 4 (Ong, 1987) and HP 12 (Toong, 1985) are being widely used in Malaysia for low flow assessing using regionalization techniques. Other variants for low flow estimates of ungaged sites are being made or evolved (i.e. NWRS SMHB/RB/JPZ, 2000).

6.7.2 HP 12: Magnitude and Frequency of Low Flows in Peninsular Malaysia (Revised and Updated) 1985

6.7.2.1 Introduction

The purpose of this Hydrological Procedure (HP 12) is to estimate the design low flow (mainly for run-of-river scheme) of ungauged catchments in Peninsular Malaysia. This procedure has been in use since its inception in the late 1980’s for preliminary screening and planning of water resources projects in Malaysia. Especially the low flow yield of various duration and return period of probability of non exceedance occurrence is required for mostly potential run-of-river water supply scheme.

The procedure is based on a regionalization approach by collating pertinent hydrological as well as topographic/geographical variables together. The hydrological element of the procedure is the results of observed individual annual series of streamflow stations in Peninsular Malaysia. Two variables are of important in determining the low flow yield (a) the catchment area, and (b) the mean annual rainfall of the catchment.


6-22 March 2009

6.7.2.2. Notation This hydrological procedure is derived based on the following criteria.

a) The catchment land use has not changed significantly over the period of record. b) The low flows have not been systematically regulated or affected by the extraction, storage or

diversion of water upstream. c) There are eight or more years of stream flow record available. d) The catchments are predominantly rural. e) The catchment areas are greater than 20 km2. f) The maximum return period is 50 years. It is assumed that this procedure can yield reasonable results if the above criteria are met. Additional prerequisite of applying this hydrological procedure is to ascertain that the catchment must not have significant storage (swamps or lakes). Caution must be exercised on catchment areas less than 20 km2, and also for catchments located in areas where the density of river stations used for deriving the regional curves is sparse.

General Approach

The input parameters required are catchment area, mean annual rainfall (MAR), and the desired return period (T). This procedure only caters for low flow estimation up to 50-year return period.

The mean annual minimum flow (MAM) for a given region is calculated by a multiple non-linear (2 variables) equation as shown in Table below based on the catchment area and the MAR.

Mean Annual Minimum Flow (MAM) equations

RE1:1.097x10-8A1.092MAR1.663 (6.29)

RE2:1.675x10-10A0.920MAR2.387 (6.30)

RE3:1.675x10-16A1.197MAR3.856 (6.31)

= area km2 A

MAR = Mean annual rainfall, mm/year obtained from 1:1,000,000 Peninsular Malaysia MAR map

After selecting the appropriate geographical region, (from RC 1 to RC 4; see Figure 6.5), the

dimensionless quotient, Qd,T

MAMis calculated by equations in Table below. The low flow for 1-, 4-, 7-, 30-

day duration are calculated by multiplying the appropriate quotient to the MAM value, i.e.

Qd,t=Kx MAM (6.32)

The table below represents the Extreme Value (EV) Type III fit parameters for the 1-, 4-, 7-, and 30-day duration of four (4) hydrological regions (see Figure 6.6). The probability distribution is described as follows:

y=-ln -ln 1- 1

T (6.33)

1-e-ky (6.34) Qd,T

MAM=u+ α

k


March 2009 6-23

Where K = Dimensionless quotient,dimensionless

T = Return period, year

Y = Reduced variate, dimensionless

u = EV parameter, dimensionless

α = EV parameter, dimensionless

k = EV parameter, dimensionless

Table 6.3 EV Type III parameters

κ Region μ α RC 1 1.093 -0.437 0.583

1.176 -0.462 0.577 1.241 -0.480 0.594 1.619 -0.614 0.632

RC 2 1.121 -0.497 0.492 1.222 -0.514 0.465 1.310 -0.536 0.458 1.726 -0.676 0.474

RC 3 1.114 -0.482 0.540 1.212 -0.525 0.553 1.265 -0.535 0.543 1.749 -0.718 0.519

RC 4 1.154 -0.609 0.490 1.206 -0.609 0.490 1.261 -0.669 0.497 1.632 -0.876 0.506


6-24 March 2009

RC1

RC2

RC1 RC3

RC1

RC4

Figure 6.7 RC Regions for Peninsular Malaysia (HP 12)


March 2009 6-25

RE1

RE2

RE3

Figure 6.8 RE Regions for Peninsular Malaysia (HP 12)


6-26 March 2009

6.8 HP 12 (NWRS, 2000)

The regionalized approach is a commonly adopted methodology of taking into consideration that the “point” streamflow station may not be able to represent the spatial variability or “areal wide” of the entire watersheds or hydrological region. Therefore a group of streamflow stations in the region and vicinity could better performed the intended low flow estimation. The reason of areal wide estimate is required is due to the fact that it is rarely streamflow stations are conveniently located in the potential intake sites or vicinity as they are costly to build and maintained. As a matter of fact, most often than not, only few streamflow stations are available compared to the vast numbers of rainfall station in a given hydrological region or watershed.

As a result, the regionalized approach takes into account both the spatial variability in topographic and hydrological variations such as catchment areas and the magnitude of rainfall (normally mean annual average rainfall [MAR], annual actual evapotranspiration [AE]) of the hydrological regions to estimate the low flow yields of specific return periods and duration.

NWRS (2000) adopts a slightly different approach vis-à-vis HP 12 (Toong, 1985). Taking into the advantage of availability of additional records of low flow annual series for various streamflow stations in the region (i.e. additional 10 to 20 years of records available), efforts to recalculate the “point” low flow yields at respective streamflow stations were made. Subsequently as in the HP 12 (Toong, 1985), a regionalized approach to estimate reliable yields of specific return periods at sites of interest in the project areas.

The regionalized approach combines the results of frequency analysis of individual streamflow stations in the hydrological homogeneous regions to derive meaningful relationships of low flow yields of specific duration (normally, 1-, 7-, 14-, and 30-day) and return periods with the geographical (catchment area) and hydrological (mean annual rainfall and actual evapotranspiration rates) factors.

The mean annual minimum flow (MAM) is first calculated for a specific node (i.e. potential intakes) of interest in the Larut Matang and Selama District which fall into Regions LF2 and LF3. The MAM is estimated as follows:

MAM=eaAb MAR-AEc (6.35)

LF REGION2:MAM=e5.2669A0.689 MAR-AE-1.087

(6.36)

LF REGION3:MAM=e-6.582A0.889 MAR-AE0.467

(6.37)

MAM = Mean annual minimum flow (m3/s)

A = Catchment area (Km2)

MAR = Mean annual rainfall (mm/year)

AE = Actual evapotranspiration (mm/year)

The MAM equation is a simple multivariate (two variables) regression where geographical parameters, i.e. catchment area (A) draining the potential intakes and hydrological parameters i.e. mean annual rainfall (MAR) and actual annual evapo-transpiration rates (AE) of the basin are the independent variables.

For a 1-, 7- and 30-day low flow, the reliable yields are then estimated using relationships between the return periods and MAM


March 2009 6-27

LF REGION 3:MAM=e-6.582A0.889 MAR-AE0.467

(6.38) Q7,T

MAM=aRD3+bRD2+cRD+d (6.39)

LF REGION 2

1-day: Q1,T

MAM =-0.0193(RD)3+0.1818(RD)2-0.6607(RD)+1.0985 (6.40)

7-day: Q7,T

MAM=-0.0212(RD)3+0.1956(RD)2-0.7118(RD)+1.2633 (6.41)

30-day: Q30,T

MAM=-0.0231(RD)3+0.2240(RD)2-0.8883(RD)+1.8383 (6.42)

LF REGION 3

1-day: Q1,T

MAM=-0.0081(RD)3+0.0757(RD)2-0.3236(RD)+0.9158 (6.43)

7-day: Q7,T

MAM=-0.0098(RD)3+0.0862(RD)2-0.3398(RD)+0.9810 (6.44)

30-day: Q30,T

MAM=-0.0118(RD)3+0.1072(RD)2-0.4311(RD)+1.2861 (6.45)

MAM= Mean annual minimum flow (m3/s)

Qn,T= flow discharge of n day duration and T return period (m3/s)

RD=-ln ln T

T-1reduce variate (nd)

T= Return period (year)

Different coefficients, a, b, c, and d are used for different durations. For information, these coefficients are the results of regression analysis using computed yields of various return periods (and transformed to reduced variates accordingly).

The differences between the JPS (Toong, 1985) and NWRS (2000) methodology are summarized as follows:

a) Way of mean annual minimum (MAM) flow estimation: An extra variable, the annual actual evapotranspiration rate (AE) is taken into consideration (in conjunction with MAR and catchment area) in the HP 12 NWRS (2000), where as the MAM calculation in the HP 12 (Toong, 1985) is only based on MAR and catchment area.

b) Way of regression or curve fitting techniques used in their respective computation: A

multiple regression of low flow quotient (ratio of flow for specific return period and duration over the mean annual minimum low flow[MAM]) is used in lieu of relatively mathematically rigorous and theoretically correct version of Gumbel or type I of Extreme value (EV) frequency distribution in HP 12 (Toong, 1985).

c) Way of grouping the hydrological homogeneous regions: seven (7) groupings of the

hydrological homogeneous regions are made in the HP 12 (NWRS, 2000). It is assumed that the MAM and subsequent low flow yield calculations are of the same region. In other words it is akin to combining three (3) RE and four (4) RC regions of HP 12 (Toong, 1986) into one


6-28 March 2009

single homogeneous zone. RE regions are for estimation of MAM whereas RC zones are for estimation of flow quotient in HP 12 (Toong, 1985).

d) Way of increasing the hydrological/flow gauged records. In the HP 12 (Toong, 1985), only

some twenty years of records were available for analysis and subsequent preparation and delineation of RE and RC regional maps. In the NWRS (2000), another ten (10) years or more gauged records are available. This undoubtedly enhances the reliability of the calculation of low flow yields.

As a result of the different techniques in computation, the MAM and LF regions in the NWRS (2000) are basically redrawn and expectedly different from the RE and RC regions in the HP 12 (Toong, 1985). Figure 6.9 shows the LF zoning for Peninsula Malaysia.

REGIONAREA 32 km2 Data LF3MAR 4000 mm a -6.582AE 1200 mm b 0.899

c 0.467

MAM 1.272 m3/sT 1-day 7-day 30-day2 1.026 1.103 1.4525 0.729 0.804 1.06910 0.609 0.688 0.92120 0.522 0.605 0.81725 0.496 0.579 0.78630 0.475 0.558 0.76050 0.413 0.490 0.681100 0.306 0.366 0.538

T 1-day 7-day 30-day2 89 95 1255 63 69 9210 53 59 8020 45 52 7125 43 50 6830 41 48 6650 36 42 59100 26 32 46

Q D,T

HP12 - LOW FLOW ANALYSIS, NATIONAL WATER RESOURCES STUDY 2000

Q D,T

Figure 6.9 NWRS Regionalized Low Flow Analysis: 1/2


March 2009 6-29

LF3

LF5

LF6

LF7

LF1LF2

LF4

Figure 6.10 NWRS Regionalized Low Flow Analysis: 2/2

6.9 DROUGHT INDEX (SPI AND PALMER) Drought index is used to provide fore-warning on the imminent on set of the drought episode based on measureable or quantitative hydrological parameters such as rainfall and soil moisture content, to an extent streamflow. The science and terminology on drought occurrence and analysis are fairly well established. The content of this subchapter is mainly derived in full or in part from http://drought.unl.edu/whatis/indices.htm#spi. There are many indices available for drought prediction and assessment. Only three popular indices are introduced and outlined in this subchapter. Surface Water Supply Index (SWSI) which is based on the streamflow records is briefly presented as it is not suitable for Malaysia climates.


6-30 March 2009

Standard Precipitation Index (SPI) which is based primarily on precipitation records alone is the index adopted in the Malaysia Infokemarau network project. As it is only based on single hydrometric variable, it is easy to extend to drought forecasts. One of its advantages is soil moisture and other hydrological conditions are not required. On the other hand, Palmer Drought Severity Index (PDSI); which is based on the water balance equation, is a rather complex technique for use. The prerequisite is the soil moisture condition of the region to be monitored must be continuously monitored. This therefore, adds additional cost to the effort.

6.9.1 Standardized Precipitation Index (SPI)

The Standardized Precipitation Index (SPI) is an index based on the probability of precipitation for any time scale. Many drought planners and decision makers appreciate the versatility of this particular index. The SPI can be computed for different time scales as mentioned earlier. It can also provide early warning of onset of drought and help to assess drought severity. One of its advantages is less complex than the other indices. This temporal flexibility allows the SPI to be useful in both short-term agricultural as well as long-term hydrological applications. The SPI was developed by T.B. McKee, N.J. Doesken, and J. Kleist and in 1993. The data and climate division boundaries are from the National Climatic Data Center (NCDC). The Western Regional Climate Center (WRCC) uses these data to calculate SPI values for each climate division. The information is then reclassified and mapped at the National Drought Mitigation Center (NDMC) using a Geographic Information System (GIS). The maps are then based on preliminary precipitation data, and the data’s source and methods used in incorporating the data into a final product must be considered carefully when analyzing these maps. Table below shows the SPI values that are classified from extreme wet to extreme dry with corresponding values ranging from +2.0 to 02.0 or less.

Table 6.4 Standard Precipitation Index Values

SPI Values 2.0+ Extremely wet

1.5 to 1.99 Very wet 1.0 to 1.49 Moderately wet -.99 to .99 Near normal

-1.0 to -1.49 Moderately dry -1.5 to -1.99 Severely dry -2 and less Extremely dry

This understanding shows a deficit of precipitation has different impacts on groundwater, reservoir storage, soil moisture, snowpack, and streamflow. This has then led McKee, Doesken, and Kleist to develop the Standardized Precipitation Index (SPI) in 1993. The SPI was designed to quantify the precipitation deficit for multiple time scales. These time scales reflect the impact of drought on the availability of the different water resources. Soil moisture conditions respond to precipitation anomalies on a relatively short scale. Groundwater, streamflow, and reservoir storage reflect the longer-term precipitation anomalies. For these reasons, McKee et al. (1993) originally calculated the SPI for 3–, 6–,12–, 24–, and 48–month time scales. The SPI calculation for any location is based on the long-term precipitation record for a desired period. This long-term record is fitted to a probability distribution, which is then transformed into a normal distribution so that the mean SPI for the location and desired period is zero.


March 2009 6-31

In terms of interpretation of the result, positive SPI values indicate greater than median precipitation, and on the other hand, negative values indicate less than median precipitation. Because the SPI is normalized, wetter and drier climates can be represented in the same way, and wet periods can also be monitored using the SPI. 6.9.2 The Palmer Drought Severity Index

The Palmer is a soil moisture algorithm calibrated for relatively homogeneous regions. It has been adopted by many U.S. government agencies and states rely on the Palmer to trigger drought relief programs.

It is the first comprehensive drought index developed in the United States. Palmer values may lag emerging droughts by several months; less well suited for mountainous land or areas of frequent climatic extremes; complex that has an unspecified, built-in time scale that can be misleading.

The Palmer Index was developed by Wayne Palmer in the 1960s and uses temperature and rainfall information in a formula to determine dryness. It has become the semi-official drought index.

The Palmer Index is most effective in determining long term drought—a matter of several months—and is not as good with short-term forecasts (a matter of weeks). It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought.

Table 6. 5 The Palmer Index

Palmer Classifications 4.0 or more Extremely wet 3.0 to 3.99 Very wet 2.0 to 2.99 Moderately wet 1.0 to 1.99 Slightly wet 0.5 to 0.99 Incipient wet spell o.49 to -0.49 Near normal -0.5 to -0.99 Incipient dry spell -1.0 to -1.99 Mild drought -2.0 to -2.99 Moderate drought -3.0 to -3.99 Severe drought -4.0 or less Extreme drought

The Palmer Index can also reflect excess rain using a corresponding level reflected by plus figures; i.e., 0 is normal, plus 2 is moderate rainfall, etc.

The advantage of the Palmer Index is that it is standardized to local climate, so it can be applied to any part of the country to demonstrate relative drought or rainfall conditions. The negative is that it is not as good for short term forecasts, and is not particularly useful in calculating supplies of water locked up in snow, so it works best east of the Continental Divide.

6.9.3 Crop Moisture Index (CMI)

The Crop Moisture Index (CMI) is also a formula derived from the original Palmer Index and was also developed by Wayne Palmer subsequent to his development of the Palmer Drought Index. The CMI reflects moisture supply in the short term across major crop-producing regions and is not intended to assess long-term droughts.


6-32 March 2009

The CMI responds more rapidly than the Palmer Index and can change considerably from week to week, so it is more effective in calculating short-term abnormal dryness or wetness affecting agriculture. CMI is designed to indicate normal conditions at the beginning and end of the growing season; it uses the same levels as the Palmer Drought Index. It differs from the Palmer Index in that the formula places less weight on the data from previous weeks and more weight on the recent week. The Crop Moisture Index (CMI) uses a meteorological approach to monitor week-to-week crop conditions. It was developed by Palmer (1968) from procedures within the calculation of the PDSI. Whereas the PDSI monitors long-term meteorological wet and dry spells, the CMI was designed to evaluate short-term moisture conditions across major crop-producing regions. It is based on the mean temperature and total precipitation for each week within a climate division, as well as the CMI value from the previous week. The CMI responds rapidly to changing conditions, and it is weighted by location and time so that maps, which commonly display the weekly CMI across the United States, can be used to compare moisture conditions at different locations. Weekly maps of the CMI are available as part of the USDA/JAWF Weekly Weather and Crop Bulletin.

6.9.4 Surface Water Supply Index (SWSI)

The SWSI is designed to complement the Palmer Index, where mountain snowpack is a key element of water supply; calculated by river basin, based on snowpack, streamflow, precipitation, and reservoir storage. It represents water supply conditions unique to each basin. However changing a data collection station or water management requires that new algorithms be calculated, and the index is unique to each basin, which limits inter-basin comparisons. The Surface Water Supply Index (SWSI) was developed by Shafer and Dezman (1982) to complement the Palmer Index for moisture conditions The Palmer Index is basically a soil moisture algorithm calibrated for relatively homogeneous regions, but it is not designed for large topographic variations across a region and it does not account for snow accumulation and subsequent runoff. Shafer and Dezman designed the SWSI to be an indicator of surface water conditions and described the index as “mountain water dependent”, in which mountain snowpack is a major component. The objective of the SWSI was to incorporate both hydrological and climatological features into a single index value resembling the Palmer Index for each major river basin in the state of Colorado (Shafer and Dezman 1982). These values would be standardized to allow comparisons between basins. Four inputs are required within the SWSI: snowpack, streamflow, precipitation, and reservoir storage. Because it is dependent on the season, the SWSI is computed with only snowpack, precipitation, and reservoir storage in the winter. During the summer months, streamflow replaces snowpack as a component within the SWSI equation. The procedure to determine the SWSI for a particular basin is as follows: monthly data are collected and summed for all the precipitation stations, reservoirs, and snowpack/streamflow measuring stations over the basin. Each summed component is normalized using a frequency analysis gathered from a long-term data set. The probability of non-exceedence—the probability that subsequent sums of that component will not be greater than the current sum—is determined for each component based on the frequency analysis. This allows comparisons of the probabilities to be made between the components. Each component has a weight assigned to it depending on its typical contribution to the surface water within that basin, and these weighted components are summed to determine a SWSI value representing the entire basin. Like the Palmer Index, the SWSI is centered on zero and has a range between -4.2 and +4.2.


March 2009 6-33

The SWSI has been used, along with the Palmer Index, to trigger the activation and deactivation of the Colorado Drought Plan. One of its advantages is that it is simple to calculate and gives a representative measurement of surface water supplies across the state. It has been modified and applied in other western states as well. These states include Oregon, Montana, Idaho, and Utah. Monthly SWSI maps for Montana are available from the Montana Natural Resource Information System (http://nris.state.mt.us/wis/SWSInteractive/). Several characteristics of the SWSI limit its application. Because the SWSI calculation is unique to each basin or region, it is difficult to compare SWSI values between basins or regions (Doesken et al., 1991). Within a particular basin or region, discontinuing any station means that new stations need to be added to the system and new frequency distributions need to be determined for that component. Additional changes in the water management within a basin, such as flow diversions or new reservoirs, mean that the entire SWSI algorithm for that basin needs to be redeveloped to account for changes in the weight of each component. Thus, it is difficult to maintain a homogeneous time series of the index (Heddinghaus and Sabol, 1991). Extreme events also cause a problem if the events are beyond the historical time series, and the index will need to be reevaluated to include these events within the frequency

6.10 WEB-BASED INFORM KEMARAU

6.10.1 Introduction

Drought monitoring program was initiated since early 2001 (see Figures 6.12 to 6.13) with the aims and objectives to

• disseminate information, on-line on the water resources Status • provide early warning on potential drought • advice the general public on the drought status

6.10.2 Methodology

Closed relationship exists between water resources status and current rainfall amount received. In this Infokemarau program rainfall data of 41 selected stations are used in the analysis to reflect the water resources status of Peninsular Malaysia.

Real time hydrometric information such as streamflow and rainfall for about 167 stations could be obtained on line via www.infokemarau.water.gov.my (see Figure 6.8). Figures 6.9 to 6.11 show the web pages for real time data acquisition for river and dam streamflow stations respectively.

The percentage of deviation from the long term mean (LTM) value of 3 monthly moving rainfall totals is used as indicator of the water catchments condition. A negative deviation from the LTM value indicates that the particular region is experiencing a dryer than normal condition and vice versa.

In general if a certain catchment is receiving rainfall (3 monthly rainfall total) less than 60% of the LTM for a consecutive period of 3 months and more. Then, it may be expected that the amount of run-off or water resources of that catchment to be adversely affected, Figure 6.15 shows an isohyetal map for entire peninsula Malaysia. An example of Standard Precipitation Index (SPI) for Kuala Nerang is shown in Figure 6.16.

This monthly monitoring program provides early warning on the impending water shortages of a particular region. Thus, follow up analysis and measures can be activated by the relevant agencies to mitigate the water stress problem.

Reference: www.infokemarau.water.gov.my

http://infokemarau.water.gov.my/Selected%20Station


6-34 March 2009

Figure 6.11 WWW site for Infokemarau (JPS)

Figure 6.12 Real Time Rainfall and Water Level Monitoring


March 2009 6-35

More than 150 hydrometric stations, i.e. streamflow and rainfall stations are included in this on line program.

Figure 6.13 Drought Monitoring by River Flow Source: www.infokemarau.water.gov.my


6-36 March 2009

Figure 6.14 Drought Monitoring by Dam Levels Source: www.infokemarau.water.gov.my


March 2009 6-37

Figure 6.15 Isohyet Map of three-monthly Rainfall Distribution

Source: www.infokemarau.water.gov.my


6-38 March 2009

SPI VALUES

2.0+ - extremely wet 1.5 to 1.99 - very wet 1.0 to 1.49 -moderately wet

-0.99 to 0.99 -near normal -1.0 to -1.49 -moderately dry -1.5 to -1.99 -severely dry -2 and less -extremely dry

Figure 6.16 Standard Precipitation Index (SPI) For Kuala Nerang

Source: www.infokemarau.water.gov.my

6.11 IMPACT OF CLIMATE CHANGE

Global warming is the raising of average measured air temperature near the earth surface and oceans since the mid-20th century (see Figure 6.17), and it is projected to continue and increase above long-term average in the near future.

This alarming state of global warming is due primarily to the increase in green house gases, i.e. carbon dioxide (CO2), methane (CH4), NOx, CFC and others gases in the atmosphere. Increasing of CO2 and other trace elements in atmosphere will alter energy balance of climate system, and cause global warming in the future. It has received much attention in recent year (IPCC, 2008).

The presence of these high concentration green house gases in the troposphere, hinder the emission of long-wave radiation processes from the earth to the atmosphere. The heat is therefore trapped within the layer of green house gases (known as green house glasses) and reradiate back to the earth. By doing so, this radiation hear balance raises the air temperature on earth in general.


March 2009 6-39

The impacts of global warming on water resource in particular are not quantitatively studied in Malaysia (NWRS 2000) but in general consensus, the impact could be experienced in various dimensions, such as it is believed that the impending rising in sea level at un precedent rate due to accelerated melting of icebergs and glaciers in both north and south poles. Thus causes erosion and recession of the coastal lines. It also affects the hydrologic cycles, etc. The impacts of global warming suggest extremities in both climatologic hydrologic events, such as recent more frequent occurrences and exceptionally high severity of tropical storms and monsoon events accompanied with fiery wind gusts.

Potential direct impacts of global warming to water resources are systematically summarized as follows (Pittock, 2003):

a. Reduce or increase inflows to water/hydrologic natural and artificial storages b. Reduce streamflow/discharge in major river basins/catchments c. Reduce or increase in water availability for rain fed agriculture and irrigation d. Reduce recharge of groundwater due to less infiltration from surface waters e. Increase in severity of both droughts and floods f. Increase salinity of surface and ground waters. g. Increase inundation of coastal freshwater wetlands and lowlands, rivers, and estuaries h. Change in weather pattern from the past i. Increase sediment and nutrients in streams

Some of the indirect impacts or consequences are as follows:

a. Threatened water supplies: Possible shortage of water supply to meet the increasing demand by cities and towns, agricultural, industrial, environmental flows. This is primarily due to low flows in the rivers or other water bodies. Relocation of water supply intakes further upstream of the saline-freshwater interface due to saltwater intrusion by sea level rising.

b. Increased risk of euthrophication in water bodies: algal blooms or enrichments and impairment to the water quality in general due to ineffective dilution. Algal blooms degrade the water quality by excessive growth of oxygen demanding organisms. When algal die off, the biomass exerts oxygen demand for their biodegradation processes and in turn reduce the oxygen concentration of the water bodies. Without oxygen, massive aquatic fauna depth as a result.

c. Probable changes in ecological water requirements: Possible alternations/changes to ecosystems. Displacement, reduction or loss of vulnerable ecosystems or species might occur due to lower water availability.

d. Increase pressure on water related storages and infrastructure: With the extremity in fluctuation in river water flows, some existing water related infrastructures such as water supply schemes, flood mitigation projects, coastal protection, etc might not be able sustain the intended design standards. As a result, inadequate raw water source might interrupt the smooth operation of water supply schemes. Frequent flooding events and overtopping of levees and embankments might occur in flood defense projects and thus increasing the risk of flood damages. Coastal erosion and sedimentation might be recurring and frequent episodes.

e. Increased competition for water: Global warming and climate change increase the competition amongst nations for precious water commodity in many countries and many regions within a country. It is especially vulnerable for competition amongst the countries that are sharing common riparian boundaries. Water resources scarcity spurs competition in dam building for storage in time of need.


6-40 March 2009

Figure 6.17 Global Temperature Anomaly Sources: NOAA www site


March 2009 6-41

REFERENCE [1] University of Florida, The Disaster Handbook, Institute of Food and Agricultural Sciences,Cooperative Extension Center, National Edition, Gainsville, Florida,1998.

[2] Chow, VT, DR Maidment, L, Applied Hydrology, McGraw Hill, NY, NY, Mays.1988 [3] Ong, CY, Magnitude and Frequency of Flood Flows in Peninsular Malaysia, HP No: 4, Jabatan Pengairan dan Saliran, Kementerian Pertanian, Malaysia,1987. [4] Institute of Hydrology (IH), HYRROM User Manual, Institute of Hydrology, UK, 1988. [5] SMHB/RB/JPZ, National Water Resources Study, 2000-2050. Final Report, EPU, Government of Malaysia, 2000 [6] Toong AT, Magnitude and Frequency of Low Flows in Peninsular Malaysia. HP No: 12, Jabatan Pengairan dan Saliran, Kementerian Pertanian Malaysia ,1985. [7] Tucci, C, A. Sliveira, F Albuquerque. Flow regionalization in the upper Paraquay basin. Brazil. Hydrological sicnece, 40 (4), 485-497. 1995. [8] Twort, A.C., F.M. Law, F.W. Crowley, and D.D. Ratnayaka, Water Supply, Forth Edition, Edward Arnold Ltd, London, UK, 1994.


6-42 March 2009


CHAPTER 7 RIVER SEDIMENTATION

Chapter 7 RIVER SEDIMENTATION

March 2009 7-i

Table of Contents

Table of Contents ................................................................................................................... 7-i


7.1 INTRODUCTION ........................................................................................................ 7-1

7.2 TYPE OF SEDIMENT LOAD .......................................................................................... 7-1

7.2 MEASUREMENT OF RIVER SUSPENDED SEDIMENT ....................................................... 7-2

7.2.1 Type of Sediment Sampling in River ................................................................ 7-2

7.2.2 Depth Integrating Samplers ............................................................................ 7-2

7.2.3 Point Integrating Samplers .............................................................................. 7-4

7.3 MEASUREMENT OF BED LOAD .................................................................................... 7-5

7.3.1 Bedload Samplers .......................................................................................... 7-5

7.3.2 Bed Material Samplers .................................................................................... 7-6

7.4 MECHANICS OF SEDIMENT TRANSPORT ...................................................................... 7-7

7.5 SEDIMENT TRANSPORT MODELS ................................................................................ 7-8


7-ii March 2009

List of Figures


7.1 US DH – 48 (Wading Type) 7-3

7.2 US DH 59 (handle type) 7-3

7.3 US DH 49 (Suspension type) 7-4

7.4 USP P - 61 –A 7-5

7.5 Helley Smith Bed Load Sampler 7-6

7.6 Bed Material Sediment Samplers US BM 53 (Piston Type) 7-7

7.7 Sediment Rating Curves for catchment type I and II (NK/SMHB, 1999) 7-11


March 2009 7-1

7 RIVER SEDIMENTATION

7.1 INTRODUCTION

Basic understandings and knowledge of the quantitative amount, both spatial and temporal distribution of sediment transport and movement in a river system is vital important to the development and management of river flow. These data are often being used to judge the health of watershed and river system and the success or failure of human induced activities designed to mitigate adverse impacts of sediment on rivers and its associated riparian habitats.

7.2 TYPE OF SEDIMENT LOAD

The technical classification on the type of sediment transport is mostly based on the mode of transport in the river reach. There are three types of transportation mechanisms that rely on the hydraulic parameters of the river channel, such as flow rates, cross sectional areas, hydraulic radius, etc. Another type of the sediment transportation class is entirely independent to the hydraulic properties of the river channel, known as Wash Load. The mechanism is highly hinged on the amount of supply of sediment available from the upstream.

For an initiation of incipient motion to facilitate the sediment transport, the bed shear stress exerted by the river flow must exceed a specific critical shear stress of the bed as explained in terms of Newtonian mechanics. The bed shear stress is estimated based on the bed or energy slope of the river reach, hydraulic radius of the cross section, and the specific weight of the flow. Only if this critical bed shear stress is exceeded, the way in which the sediment is transported depends on the characteristics of the sediment interaction with river water. As always the flow itself carries some suspended particles as it flows downstream. Else the sediment will be setting down due to the action of gravity. This settling velocity is the minimum velocity a flow must have in order to transport, rather than deposit, sediments, and (for a dilute suspension) is described by Stoke’s law.

The best description of this type of sediment is the derivation of flow-sediment rating curve, where the amount of sediment influx at any specific boundary is solely dependent on the flow rates.

(a) Suspended Load (particulate form)

The sediment composition consists of both organic and inorganic particulate matter that is suspended in and carried by moving water. Sediment particles that are mechanically transported by the sediment suspension due to turbulence within a stream or river suspended load is always associated with sandy particles. Due to their lightweight and relatively smaller in size, the turbulence generated by the water movement could literally throw them up in suspension.

Sediment occurred as suspended load if the velocity in the river channel is greater than the settling velocity of the sediment particle. There is always possibility of non uniform sediments with a wide spectrum of different particle sizes in the flow, some of these particles will have sufficiently large diameters that they settle on the river or stream bed, but still move downstream. This mode of transport is termed as bed load and is discussed in details later.

(b) Dissolved Load

All organic and inorganic material carried in solution by moving water. This is a better known as solute transport phenomena.

http://en.wikipedia.org/wiki/Settling


7-2 March 2009

(c) Bed load Coarse materials such as a larger size sand, gravel, stones, and boulders that are not transported by the turbulence created by the water flow (primarily velocity field) are settled down by the force of gravity to the bottom of the river channel. At this juncture, they move along mainly by the “saltation” or bouncing around or near the bottom of the channel (saltation: jumping up into the flow, being transported a short distance then settling again). On the other hand, these relatively coarse materials move primarily by skipping, rolling, and sliding along the channel bed.

7.2 MEASUREMENT OF RIVER SUSPENDED SEDIMENT

Sediment measurement involves sampling the water-sediment mixture (sand, silt, and gravel in a river) to determine the

(a) mean suspended sediment concentration, (b) particle size distribution, (c) specific gravity, (d) temperature of the water sediment mixture, and (e) other physical and chemical properties of the transported sand and gravel particles in the rivers. Suspended sediment concentration in a natural river system varies spatially and temporally from the water surface to the bottom of the river bed and both longitudinally and laterally along and across the river cross section. Most of the time, concentration generally increases from a minimum at the water surface to a maximum at or near the bed of the river.

7.2.1 Type of Sediment Sampling in River

7.2.2 Depth Integrating Samplers

Depth integrating samplers are basically used to continuously obtain and extract a sample specimen of sediment-water mixture as they are lowered from the water surface to the bottom of the river bed and returned at a constant rate of travel to the river water surface again. Both ascending and descending speeds however, need not be the same, but the rate of travel must be constant in each direction. As the sediment-water sample is collected, air in the container is compressed so that the pressure balances the hydrostatic pressure at the air exhaust and the inflow velocity is approximately equal to the stream velocity. The analysis of the sediment in the sediment-mixture bottle is expressed as weight of the sediment in a unit volume of water.


March 2009 7-3

(a) US DH - 48 (Wading type)

It is a light weight sampler for collection of suspended sediment samples where a wading road sampler suspension is used in the sampling operation. The intake nozzle is orientated into the current and held in a horizontal position while the sample is lowered at a uniform rate from the water surface to the bottom of the stream instantly reserved and then raised to the water surface at a uniform rate.

Figure 7.1 US DH – 48 (Wading Type) Reference: www.hydromatinstruments.com

(b) US DH 59 (Handle Type)

It is a medium weight suspended sediment samplers for hand line type. This sampler is equipped with a tail vane assemble to orient the intake nozzle of the sampler into the approaching flow as the sampler enters the water the sample container is sealed against a gasket in the head cavity of the casting by pressure applied to the base of the bottle by a hand operated spring tensioned pull rod assembly at the tail of the sampler.

Figure 7.2 US DH 59 (handle type) Reference: www.hydromatinstruments.com

(c) US DH 49 (Suspension type)

It is a sampler for suspension by gauging winch to take suspended sediment samplers in the streams not greater then 7.4 meter in depth. The head of the samplers is hinged to permit access to the sample container. Tail fins are provided to orient the instrument into the stream flow. The head of the sampler is drilled and tapped to receive the 6.3 mm or 4.8 mm or 3.2 mm intake nozzle which projects into the current for collecting the sample.


7-4 March 2009

Figure 7.3 US DH 49 (Suspension type) Sources: www.hydromatinstruments.com

(d) US DH 74 (Reel Type)

It is a 30 kg/CM2 sampler for suspension by cable Reel and Crane to take suspended sediment samples in streams up to 5.5 meter in depth, the body of sampler is cast bronze or cast aluminium.

(e) US DH 75 (Wading Rod Type)

It is a 29.5 kg/CM2 sampler for suspension by cable reel and crane to take larger suspended sediment or water quality samples originally designed to take samples in streams that are below freezing it has mainly been used as a water quality sampler in its epoxy coated trace metal version. The Sampler has a cast bronze streamlined body which is completely open in the front to hold a 3 liter plastic container.

7.2.3 Point Integrating Samplers Point integrating samplers on the other hand, are equipped with an electrically controlled rotary valve which opens and closes the sampler on command. They are designed to take a sample at any point in a stream over a short period of time interval. With the control valve fixed in the open position, these samplers are also used to obtain depth integrated samples. One-way depth integrated samples may be obtained by opening the valve with the sampler at the water surface and lowering it to the streambed at a constant speed. This permits sampling to greater depths. Suspended-sediment samplers should be used only with the specified nozzle to give a truly representative sample. All US-series samplers are designed to sample isokinetically which means that water is entering the nozzle at the same speed as the water would be traveling if the sampler wasn't there. The US DH-2TM is a bag-type suspended-sediment/water-quality sampler, capable of being used as a hand-line sampler. The sampler is designed to meet the requirements of the U.S. Geological Survey (USGS) for a "clean" suspended-sediment sampler. The sampler will collect at least 1 liter of sample isokinectally to 10 m


March 2009 7-5

(a) US P - 72 It is electrically operated sampler for collection of suspended sediment samplers at any point beneath the surface of a stream, or for taking a sample continuously over a range of depth. The sampler is made of cast aluminum it is electrically actuated valve mechanism to start and stop the sampling process is located in the sampler head.

(b) USP P - 61 -A

It is electrically operated sampler for collection of suspended sediment samplers at any point beneath the surface of a stream, or for taking a sample continuously over a range of depth. The sampler is made of cast bronze, the sampler is streamlined and equipped with tail fins to orient it in the stream.

Figure 7.4 USP P - 61 -A Sources: www.hydromatinstruments.com

(c) USP - 63

This is a electrically operated suspended sediment sampler for collecting of suspended sediment sampler at any point beneath the depths and high velocities, the US P-63 A- 91 kg sediment samplers, the P-63 is made of cast bronze, 1m inches long and has the capacity for a quart sized round milk bottle.

(d) USP - 50

It is electrically operated sampler for collection of suspended sediment samplers at any point beneath the surface of a stream, or for taking a sample continuously over a range of depth. In extremely deep rivers of high velocity, the USP-50, A-136 KG, sediment samplers, the USP - 50 made cast bronze.

7.3 MEASUREMENT OF BED LOAD

7.3.1 Bedload Samplers

Suspended sediment samplers only sample the sediment water mixture to a point about slightly over 100 mm above the river bed. In contrast to this category of sampler for bed load where it is used to sample sand, silt, gravel, or rock debris transported by stream on or immediately above its bed. The bed load is the sediment that moves in the stream at velocities less than the surrounding flow by sliding, rolling, or bounding on or very near the streambed. The size of particles moving as bed load is identical with samples of bed material in the movable part of the stream bed.


7-6 March 2009

In wide sand-bed streams with shallow flow depths and high sand concentrations, more sediment may be transported in the unsampled zone than in the sampled zone. As flow depth increases, the proportion of sediment in the unsampled zone becomes smaller, often accounting for only a small fraction of the total sediment load. The bed load portion of sediment discharge is primarily sampled using Helley Smith sampler.

Figure 7.5 Helley Smith Bed Load Sampler Source: www.stream.fs.fed.us

7.3.2 Bed Material Samplers Bed material samplers are designed to collect samples from the bed of a stream, or from lake or reservoir deposits. These samplers take an undisturbed consolidated bed sample and are therefore designed to prevent sample washout or disturbance during retrieval. This is an important aspect of bed material sample integrity. Several types of bed material load samplers are introduced below.

A) Bed Material Sediment Samplers US BM 54 Type

This is a sampler used to collect samples from the bed of a stream or reservoir of any depth, the material to be sampled may be firm, soft, plastic or granular. Particles in size up to one inch or larger may be picked up in the sampler bucket the sampler which weights 100 pounds, the streamlined body os made of cast steel and it is equipped with tail fins. when, the sampler is supported by a steel cable the bucket may be cocked, that is set in the open position for taking a bed sample when tension on the cable is released by resting the sampler on the stream beb or other support the bucket snaps shut, taking a sample.

B) Bed Material Sediment Samplers US BM 60 (Bucket Type)

This is a sampler used to collect samples from the bed of a stream, lake or reservoir, this is a hand line sampler about 56 CM approximate and 40 pounds. the sampler mechanism consists of a scoop or bucket driven by a crosscurved constant torque motor type spring that rotates the bucket from front to back, the scoop when activated by release of tension on the hanger rod can penetrate into the bed about 430 mm and can hold approximately 175 cc of material


March 2009 7-7

C) Bed Material Sediment Samplers US BM 53 (Piston Type) This is a sampler used to collect sample the bed of wadable streams. The instrument is 1.2 m in total length and usually is made of corrosion resistant materials. The collecting end of the sampler is a stainless steel thin walled cylinder 2 inches in diameter and 8 inches long with a tight fitting brass piston.

The piston is held in position by a rod which passes through the handle to the opposite end. The piston created a partial vacuum above the material being sampled and here by compensates in a reverse direction for some of the frictional resistance required to push the sampler into the bed

Figure 7.6 Bed Material Sediment Samplers US BM 53 (Piston Type)

7.4 MECHANICS OF SEDIMENT TRANSPORT

The overall balance in momentum between sediment transport and sediment being deposited due to gravity on the channel bed can be quantitatively described by the Exner equation. This equation on the exchange of sediment in mobilization and incorporated in the channel is vitally important in that exchanges in river water depth and the slope will change the bed shear stresses, and thus causing local areas of erosion or deposition.

The Exner Equation describes the conservation of both suspended load and bed load sediment in a riverine system (Yang, 1996). In its most commonly-used form, it is a mass conservation equation between the sediment in the bed and the sediment in transport.

http://en.wikipedia.org/wiki/Exner_equation

http://en.wikipedia.org/wiki/Conservation_of_mass

http://en.wikipedia.org/wiki/Sediment

http://en.wikipedia.org/wiki/Sediment


7-8 March 2009

The alternative name for Exner equation is sediment routing or continuity equation.

∂(CvA)∂t

+ 1-p ∂(As)∂t

+ ∂Qs∂x

+ Clql=0 (7.1)

Cv = suspended load concentration

A = area of flow

P = porosity of bed

As = area of bed

Qs = suspended & bed load discharge Cl = lateral concentration ql = lateral flow

7.5 SEDIMENT TRANSPORT MODELS Similar to water quality model, the utility of sediment transport modelling exercise is far reaching other than to understand the underlying mechanism of sediment movement and behaviour in a riverine system. The sediment transport model has been used to routinely describe the impact of engineering project on the riverine environment, especially in the aggradation (deposition) and degradation (scouring) of the river bed. Other specific application of sediment transportation is to study the behaviour of sediment current influx into a reservoir system. The model could be generally classified as steady (time invariant) and unsteady (time varying) in terms of solutions for Exner equation. Water flow equations are normally coupled with the sediment routing equation in modelling the behaviour of sediment in a riverine system. The velocity fields and water depths or elevations are generally solved numerically a priori from the full or partial form of St. Venant equations of flow continuity and conservation of momentum. The full St Venant equations are described below in both flow continuity and momentum terms: Flow Continuity equation

∂A∂t

+ ∂Q∂x

+ p ∂As

∂t+ q1=0 (7.2)

Momentum equation

∂(ρQ)∂t

+ ∂∂xρ Q2

A+ gρA ∂y

∂x+ ρgAS

f=0 (7.3)

A full analytical solutions for both hydrodynamic (flow and depth) and sediment routing are normally not available except for a special circumstance where boundary and initial conditions are simple and straightforward. Even recourse made to solve the system of equation by numerical means are not a simple task, therefore, for simplicity, various modification of the flow routing exercises could be made as deemed appropriate. With the cancellation of some terms in both continuity and momentum equations, a simpler version of flow routing procedure could be obtained, such as approximations using diffusion and kinematic equations for velocity field and depth computation.


March 2009 7-9

On the other hand, solutions of the sediment routing Exner equation requires a full understanding of the sediment transport mechanisms with regard to the hydraulic parameters in the river channel. The sediment flux (in terms of mass per unit width per unit time) that serves as input to the Exner solution is required a priori. This subset of sedimentation engineering has been well studied throughout the years either in the laboratory flumes or limited field studies. Various empirical sediment transport formulas with the purpose for estimating sediment influx (mass (per unit time) has been proposed from time to time since the earlier 1900’s. These formulas are mainly of empirical nature and based mostly on the calibration of limited sediment and hydraulic data collected in flume or field studies. Some of these formulas that are best suited for one form of river system might not be feasible for others. One of the obvious examples is the formulas that are derived based on mostly gravel mountainous river might not be applicable in low land alluvial bed rivers. A further classification of these formulas is the estimation of either (a) bed load, (b) suspended load transport or (c) both, which is termed as total load. Another important feature of these formulas is the treatment of the sediment state variables. Some formulas proposed the adopted of an index sediment size for estimating the sediment flux while the others take into account of the non uniformity of the sediment available in the river bed. Examples of these formulas throughout the century, from the beginning of earlier 1900’s, are Schoklitsch A, (1917). Duboys (1942) Meyer-Peter and Muller (1948), Einstein-Brown and variants (1950), Laursen (1958), Colby (1964), Toffaletti (1969), Englund and Hansen (1972), Acker and White (1973; revised 1990), Yang Unit Stream Power and variants (1973, 1979, 1984, 1996), Van Rijn (1984), Parker (1990), Laursen-Madden (1993), Karim and Kennedy (1990), Karim (1998), Wu et al (2000), Wilcox (2001), Yang and Lim (2003), and many others. These formulas are generally can be presented in a quantitative functional form of

Qs=f(V, D, S, B,de,ρs,Gsf, ds,ib, ρ, T) (7.4)

V= average flow velocity D = effective depth of flow S = energy slope B = effective width of flow de = effective particle sizes of sediment mixture ρs = density of sediment Gsf = grain shape factor ds = geometric mean of particle diameter in each size class ib = percentage/fraction of particles ith size found in the bed ρ = density of flow water T = water temperature These sediment and hydraulic parameters or variables could be combined and rearranged in dimensionless forms through a dimensional analysis for ease of derivation of sediment transport equations. Such undertakings using Buckingham Pi Theorem are numerous; Yang (1996) and Karim (1998) are representative of this form of analysis.

As mentioned earlier, most of these formulas are developed by empiricism mainly in flume studies in a laboratory setting. Extrapolations of these formulas outside of their calibration range are generally being prudently cautioned by the respective developers or originators of the formulas.


7-10 March 2009

Perhaps with the absence of flume and field studies in Malaysia context, it might not be wise that these formulas are being adopted from time to time in engineering design undertakings without the crucial and importance step of calibration and not to mention, a subsequent important and integral part of modelling exercise, verification and validation. Several studies in Malaysia addressed the issues of sedimentation impacts on river engineering. In the feasibility study for interstate water transfer project, sedimentation rating curves as a function of unit flow discharge per unit catchment area was developed albeit with limited suspended load concentration database of selected stations in the states of Pahang, Selangor and around Kuala Lumpur (NK and SMHB 1999).


March 2009 7-11

1 Sg leper @ Jam Gelugor 2 Sg. Bernam @ SKC 3 Sg. Jelai @ Kuala Medang 4 Sg. Lipis @ Benta 5 Sg. Jelai @ Jeram Bungor 6 Sg. Bentong @ Kuala Marong 7 Sg. Selangor @ Rasa 8 Sg. Selangor @ Rantau Panjang 9 Sg. Langat @ Dengkil 10 Sg. Langat @ Kajang 11 Sg. Gombak @ Jalan Tun Razak 12 Sf. Semenyih @ Rinching

10

100

1000

10000

0.001 0.01 0.1 1

Discharge/Unit Area (m3/s/km2)

Sus

pend

ed S

edim

ent (

mg/

l)

1 2 34 5 6

7 8 910 11 12

Figure 7.7 Sediment Rating Curves for catchment type I and II (NK/SMHB, 1999)


7-12 March 2009


CHAPTER 8 RIVER WATER QUALITY

Chapter 8 RIVER WATER QUALITY

March 2009 8-i

Table of Contents Table of Contents ................................................................................................................... 8-i

List of Tables ........................................................................................................................ 8-ii


8.1 STREAM ECOLOGY ..................................................................................................... 8-1

8.2 PURPOSE OF SAMPLING ............................................................................................. 8-3

8.3 MEASUREMENT OF RIVER WATER QUALITY ................................................................ 8-3

8.4 RIVER WATER QUALITY MANAGEMENT ..................................................................... 8-11

8.4.1 POLLUTANT SOURCES ......................................................................................... 8‐12

8.4.2 WATER QUALITY INDICES AND STANDARDS ....................................................... 8‐13

8.4.3 Discharge Standard (IWK, 2008) ......................................................................... 8‐14

8.5 WATER QUALITY PARAMETER .................................................................................. 8-15

8.6 WATER QUALITY MODELLING .................................................................................. 8-16

8.6.1 Classification of Water Quality Model ................................................................ 8‐18

8.6.2 Process Description ............................................................................................. 8‐20

8.6.2.1 Advection ..................................................................................... 8-20

8.6.2.2 Diffusion ..................................................................................... 8-20

8.7 MATHEMATICAL MODELS FOR RIVER WATER MANAGEMENT:

SELECTION AND ADOPTION ..................................................................................... 8-20

8.7.1 Calibration and Validation ................................................................................... 8‐21

8.8 ENVIRONMENTAL FLOW ........................................................................................... 8-22

8.8.1 Environmental Flow: A Definition ....................................................................... 8‐22

8.8.2 Environmental Flow in Malaysia ......................................................................... 8‐23

8.8.3 Minimum Average Monthly Flow ....................................................................... 8‐24

8.8.4 1:50‐year 7‐day duration Minimum Flow (7Q50) ............................................... 8‐24

8.8.5 98% and 99% Probability of Exceedance ............................................................ 8‐25

8.8.6 A Fraction of (Annual Average Flow) Volume ..................................................... 8‐25

8.8.7 Other Environmental Flow Techniques ............................................................... 8‐27

8.8.8 Comprehensive Review by www.eflownet.org ................................................... 8‐27

8.8.9 ENFRAIM: Environmental Flow Requirements; an Aid for

Integrated Management ..................................................................................... 8‐29

8.8.10 Application in Malaysia ......................................................................... 8‐30

8.8.11 Global Warming and Water Resources ................................................. 8‐30

REFERENCES ...................................................................................................................... 8-32

APPENDIX 8A: LIST OF GLOSSARY ....................................................................................... 8A-1


8-ii March 2009

List of Tables


8.1 The classification of River Water Quality Based on Their Beneficial Uses 8-13

8.2 The present range of WQI 8-14

8.3 Discharge Effluent Standard A and B 8-15

8.4 The water quality parameters 8-16

8.5 The criterion on classification of the water quality model. 8-18

8.6 Tennant Method: In Stream Flow Requirement 8-26

8.7 Categorizations of international environmental flow assessments 8-28

8.8 Durations and major advantages and disadvantages of environmental flow assessment methodologies 8-29

List of Figures


8.1 A typical freshwater stream food web 8-1

8.2 2.5 L Van Dorn Acrylic Water Bottle 8-4

8.3 Integrated depth water sampler. 8-5

8.4 BOD samplers. 8-5

8.5 COD sampler 8-6

8.6 A handheld DO meter 8-6

8.7 Multiparameter sondes 8-7

8.8 The flow diagram of calibration processes. 8-21


March 2009 8-1

8 RIVER WATER QUALITY

8.1 STREAM ECOLOGY Streams like springs and rivers are also known as lotic habitat. A lotic habitat and its ecology is primarily characterized by unidirectional flow, constant state of physical change and a high diversity of microhabitats, where the fauna and flora has adapted themselves to the flow conditions. The ecology of lotic habitat is governed by abiotic and biotic factors, the former being flow, light, temperature, chemistry and substrate. Flow is the most influential abiotic component in determining a stream ecosystem as it is responsible for producing riffles, pools and gliders via erosion and deposition. Light is the main source of energy in a lotic environment and is required for primary production which provides the foundation for the trophic web (Figure 8.1) in an aquatic system. Aquatic plants and algae including periphyton (filamentous and tufted algae which clings on rocks) need light for photosynthesis and other animals depend on them as part of the food chain. The varying degree of light available in the water column as well as the stream surroundings also help dictate the community structure of both fauna and flora. Likewise differential temperatures resulting from varying light intensities in a stream affects the biological population in a stream ecosystem. Deeper waters tend to host fish with preference for colder temperature while shallower exposed streams are dominated by species with tolerance for higher temperatures. Stream water temperature usually varies diurnally and for some localities seasonally e.g. lower temperatures are normally associated with wet season and vice versa.

Figure 8.1 A typical freshwater stream food web Source: http://lakewhatcom.wsu.edu/display


8-2 March 2009

The chemistry of a stream greatly depends on the inputs from the geology of its watershed, or catchment area, but can also be affected by precipitation and pollutants from anthropogenic sources (Allan 1995; Cushing and Allan 2001). Due to a high rate of mixing small streams are usually less variant than larger lotic systems as the latter exhibit less nutrients, dissolved ions and pH as distance increases from the river’s source. (Giller and Malmqvist 1998). Physico-chemical parameters such as dissolved oxygen, pH, temperature, depth, conductivity, salinity, turbidity, total suspended solids, nutrients such as nitrate and phosphate all play a key role in determining the overall makeup of the ecosystem. Amongs them dissolved oxygen is likely the most necessary component of lotic systems, as survival of all aerobic organisms depends on it. Oxygen enters the water mostly via diffusion at the water-air interface and its solubility in water decreases as water temperature increases. Rapid, turbulent streams tend to have more oxygen content as more of the water’s surface area is exposed to the air (Giller and Malmqvist 1998). Ecosystems highly populated with aquatic algae and plants produce high levels of oxygen during the day as a byproduct of photosysnthesis but the level drops significantly during the night when primary producers switch to respiration. Large amount of detritus, por circulation and high aquatic animal activity can also cause low levels of oxygen in lotic systems (Cushing and Allan 2001).

Stream substrate type is normally a product of local geological material present in the catchment that is eroded, transported, sorted, and deposited by the current. Inorganic substrates are usually classified by size on the Wentworth scale consisting of boulders, pebbles, gravel, sand and silt (Giller and Malmqvist 1998). As a rule of thumb, the further away the stream from its source the finer the substrate type. The stream bed can also consist of organic substrate such as fine particles, submerged wood, dead leaves and plants. Stream bed morphology is also dynamic as it is very much affected by the flow especially during flooding events.

Normal organisms found in a stream biota or biological community are bacteria, tiny plants or phytoplankton, zooplankton, fishes, insects and other invertebrates including those living within the substrate (also known as benthos). Zooplanktons are tiny animals including fish larvae (ichthyoplankton) which predate on smaller planktons. All these organisms interact with each other and play a key role in driving the trophic (or energy) relationship in a stream ecosystem. Bacteria are found everywhere in a stream and they play a large role in recycling energy in the system. Phytoplankton and other aquatic plants tie up food or carbon via photosynthesis which in turn is passed on to zooplankton and other grazers including fish when they get eaten. The zooplankton then is fed upon by juvenile fishes which in turn are preyed upon by larger fishes. Aquatic plants also offer shelter and food source to other organisms thus forming microhabitats (Brown, 1987). Insects can make up to 90% of a lotic system invertebrate population and are found in various microhabitats or niches in the system. They feed on plants, decaying matter and other smaller insects and pass on energy when they get eaten by a fish or any organism on higher trophic level. Benthic invertebrate such as polychaetes (worms), mollusks and gastropods are common denizens of the stream bed and banks and are also fodder for higher positioned animals within the trophic web. Fishes are usually near the top of the food chain in a stream ecosystem and usually consist of planktivores, herbivores/detritivores, omnivores and carnivores. They are flexible in their feeding role and their diet may vary according to food availability and their respective development stage (young of different species usually require different prey sizes).

The trophic web in a stream ecosystem is dependent on two carbon or energy sources, autochthonous and allocthonous. The former refers to energy sources derived from within the ecosystem itself e.g. primary producers and also decaying organisms while the latter alludes to organic based inputs e.g. leaves, detritus, dead organisms etc. brought into the lotic system by the stream flow, wind, erosion, run-offs or ground water. Tropical streams are normally characterized by high numbers of detritivores as the ecosystem is mainly dependent on allocthonous inputs brought on by currents upstream. Therefore maintaining a good riparian cover along a stream goes a long way in preserving its ecosystem.

http://en.wikipedia.org/wiki/Wentworth_scale


March 2009 8-3

At present stream and river ecosystems in Malaysia are facing numerous threats from various sources, almost all of them caused by humans. These threats include pollution, flow modification and invasive species. Pollution can be from point source and/or non-point source. Point source of pollution refers to a single identifiable localized source such as untreated discharge from palm oil refineries, aquaculture and domestic sewage while non-point source implies pollution coming from various and diffused sources such as agricultural and urban run-offs which can contain large amounts of pesticides, herbicides, unused fertilizer and other chemicals. Human activities such as logging, dredging, reclamation, construction of dams etc. can directly or indirectly impact the flow and stream morphology, causing an imbalance in the ecosystem. Level of biodiversity may be reduced when species which are very sensitive to physicochemical changes are displaced by those more tolerant to the same alterations. The building of dams also cause fragmenting of rivers where both anadromous (upriver bound) and catadromous (ocean-bound) fish species find themselves cut off from their regular migratory routes for spawning. Local fishes like ‘kelah’ migrate up the rapids to spawn in shallow waters whereas others like freshwater eels migrate to the open sea to breed before their young return to the same river and stream their parents came from.

The introduction of invasive or non-native species of fish (Malaysian context: peacock bass, arapaima, pacu and arowana from South America, ‘flower horn’ from local fish ornamentalists etc), snails (in Malaysia padi farmers face economic threat from the ‘siput gondang emas’) etc. can cause untold damages to not just the ecosystem but also the local economy. Introduced species are generally known to be more competitive and aggressive than their native counterparts, therefore ultimately displacing the locals in their respective niches. Once established, these species can be difficult to control or eradicate, particularly because of the connectivity of lotic systems

8.2 PURPOSE OF SAMPLING Water quality defines the physical, chemical, and biological properties of water against a specific set of standards for it intended purposes, i.e. water supply, stormwater discharge, irrigation, recreation etc. Water quality standards are typically tailored for different types of water bodies, such as lakes, rivers, estuaries, seas, etc for intended utilization. The primary objectives of water quality management are to measure and manage the properties of water to suitable levels for the purpose of potable water supply, safety of human contact, aesthetics, and for the health of ecosystems. Hydrometry such as streamflow gaging, current velocity measurement, etc is used to quantify the flow rates in the water body. Likewise, chemical and electromechanical techniques are used to determine the quantitative amount of both physical and chemical parameters of the water body.

The water on earth is finite. Amongst these, sea water accounts for about 97% of total water resources in the hydrosphere and the other meagre 3% comprising, surface waters in lakes and rivers, icebergs, glaciers, and groundwater. Needless to say, the vast salt water resources could not be fully utilized without additional cost in treatment. The need for water quality management is pressing in Malaysia. Although blessed with more than 2500 mm/year of rainfall on average throughout Malaysia, water quality impairment impedes full utilization of precious resources for beneficial uses. With increased threats to streams and rivers as described in the previous section, it has become an imperative mission for most major streams and river basins to be monitored on a long term basis.

8.3 MEASUREMENT OF RIVER WATER QUALITY Sampling Methods & In-situ Measurements There are two main types of water quality parameters, chemical and physical. The third type involves biological parameters which can be used in assessing not just the water quality but is also useful in indicating the overall health of the ecosystem.


8-4 March 2009

Depending on the objective, the Water Quality Index (WQI) adopted by the DOE in 1990 (refer Appendix) covers chemical parameters such as pH, dissolved oxygen (DO), chemical oxygen demand (COD), biological oxygen demand (BOD), ammonium (NH4) while physical parameters include total suspended solids (TSS). Other parameters commonly measured in a routine water quality study are alkalinity, total hardness, conductivity, temperature, oxidation-reduction potential (ORP), chlorophyll a (which reflects the density of algae in the water column), turbidity, nutrients such as nitrates, phosphates and silicates and faecal coliforms. In some cases heavy metals and oil and grease are also quantified. Sampling methods can vary for different parameters sought; however most form of water sampling take place from either a boat or bridge as protocols require that samples be retrieved from mid-stream at mid depth. Water sampling using this approach is usually accomplished by lowering into the water column a Van Dorn PVC or acrylic body 2.5 L water bottle (see figure 8.2) triggered to close by manual operation or by mechanically by a messenger (a streamlined weight sent down the line which is attached to the sampler). A list of glossary is attached in Appendix 8A for further explanation and definition of the parameters and their associated terminology.

Figure 8.2 2.5 L Van Dorn Acrylic Water Bottle

Stream water sampling can be conducted using either an integrated water sampler (Figure 8.3) where samples can be retrieved from top to bottom in the middle of the channel or from side to side at mid depth, or a grab sample using one of the Van Dorn water bottle samplers as described above. When using the Van Dorn water sampler samples must be taken at various points of equal distance across the stream. If only one sample can be collected then it should be done in the middle of the channel and at mid depth. Due to possible loss of volatile compounds sampling should be avoided where the water is turbulent. Instead sampling should be done beneath the surface in relatively calm waters with the opening of the sampler facing away from the surface and against the flow in order to avoid collecting surface scum unless oil and grease is the constituent of interest. Other parameters such as BOD and COD require special samplers (Figures 8.4 & 8.5) while for heavy


March 2009 8-5

metals the insides of the Van Dorn PVC bottles should be coated with Teflon or a special Teflon FEP bottle is used.

Figure 8.3 Integrated depth water sampler.

Source: www.rickly.com

Figure 8.4 BOD samplers.


8-6 March 2009

Figure 8.5 COD sampler

With the current technology advancement many of these parameters described above can now be measured in-situ. Handheld instruments (Figure 8.6) and multiparameter sondes (Figure 8.7) are capable of measuring on site parameters such as temperature, pH, DO, conductivity, salinity, turbidity, chlorophyll a, depth, ORP and ions such as ammonium, nitrate and chloride. These instruments when properly calibrated can yield very good accuracy and are relatively easy to maintain.

Figure 8.6 A handheld DO meter Source: www.ysi.com


March 2009 8-7

Figure 8.7 Multiparameter sondes Source: www.eurekaenvironmental.com

The quality of data collected and the subsequent results from analyses are much dependent on how the sampling was performed and the performance and reliability of the instruments used. Regardless of the type of methods and instruments used the following information (CRC, 2009) are useful in ensuring data integrity:

a) Precision

Precision is a measure of how reproducible the data collected is between samples. It determines the consistency of repeated samples that are tested. Precision measurements are obtained by taking duplicate samples each sampling day for each parameter recorded. The samples shall be taken at the same time and the same place to ensure the precision of the measurement. The relative percent difference will show how precise the data is for the parameters sampled. Precision is also known as repeatability.

b) Accuracy

This is a measure of confidence that the data collected in the field and in the laboratory reflect the true value of a given parameter. Each instrument used to obtain the water quality parameters will have various ranges of expected values. For example, when calibrating the pH meter, a known pH buffer solution of 7.0 will be sampled using the pH probe. If the value of the pH measured shows a reading of 8.1, the difference between the average pH value is off, or biased, by 1.1 unit or having 86.4% accuracy. The laboratory will determine its level of accuracy for the fecal coliform bacteria samples, and the turbidity samples. Accuracy, therefore, is simply a quantification of the difference between the measured value and the true value.

c) Representativeness

Representativeness is a measure of the extent to which the measurements obtained (water quality parameters) actually depict the true environmental condition being evaluated. For example, a sample taken near a manure spill will not be indicative of the entire stream. Samples must be taken at approximately the same location in the stream each sampling day.


8-8 March 2009

d) Completeness The completeness of data quality controls relies on how many samples need to be taken to be able to use the information that is collected. (For example, should the required parameters at each of the fifteen stations plus a duplicate sample at each station be taken the completeness factor will have been met. However, should only 10 stations be sampled out of 15, then the percent completeness would be approximately 67 %.) Percent completeness is the number of planned measurements judged valid divided by the total number of measurements taken multiplied by 100.

e) Comparability

The data gathered should be preferably by the same team or individual and over a minimum period of one year. Both wet and dry sampling conditions should be monitored. Comparability can only be measured by data gathered on the same stream or on a similar stream with similar conditions. If the data is gathered over a period of two years, the data may be compared on an annual basis.

Sampling for Physical, Chemical and Biological Analyses & Sample Preservation

For temperature, pH, DO, conductivity, salinity, turbidity and ORP.

As previously described the sampling for and measuring of physical, chemical and biological parameters is usually a combined method involving mechanical aid and digital instruments. In-situ measurements of temperature, pH, DO, conductivity, salinity (automatically calculated from conductivity and temperature values), turbidity and ORP save time and can be accomplished using multiple handheld instruments (pH meter, conductivity meter, turbidity meter, D.O. meter etc.) with single or multiple sensors e.g. YSI, Hach, Orion etc. or a multi-parameter sonde (a much bigger cylindrical probe which can host up to 14 sensors or more) from YSI, HydroLab, Eureka Environmental etc. Both type of instruments need to undergo calibration (if necessary) prior to deployment. Usually the instrument is lowered down throughout the water column and real-time readings are displayed via a handheld unit or notebook PC and the data manually recorded into a field book or field forms. For long term monitoring purposes more advanced models can capture and log the data onto their internal memory and subsequently retrieved by downloading to a notebook PC.

For total suspended solids, ammoniacal nitrogen, nitrate and other nutrients

Mechanical sampling with the aid of a 2.5 L Van Dorn sampler is normally conducted for parameters such as total suspended solids, ammoniacal nitrogen, nitrate and other nutrients. Samples are collected from mid stream and at mid depth and kept on ice in a cooler for further lab analyses. For Dissolved Metals For metals samples are collected as single grabs in a 500ml Teflon FEP bottle (EPA Method 1669) using the stainless steel metals sampler or by hand. Alternatively a teflon coated PVC 2.5 L Van Dorn bottle can also be utilized. Care must be used at all times when collecting and processing metals samples to avoid contaminating the inside of the sample bottle or cap with debris or ambient air. Also, samples need to be preserved with acid and placed in ice in a cooler as soon as possible after collection. The holding time prior to analysis for all metals, except mercury, is six months. The holding time for mercury is 28 days.


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For Faecal Coliform Standard procedures involve using a bacteria sampler with an empty faecal coliform glass bottle inside it to retrieve samples from the stream. Great care has to be exercise at every step of the sampling exercise to avoid any possible contamination of samples. After removing the stopper the sampler is lowered down just to the water surface to have its bottom rinsed. Similar to metal sampling, the sampler is submerged completely at about 0.5 m depth before retrieval. The inner glass bottle is then carefully removed, tagged and placed on ice in a cooler.

For biological oxygen demand (BOD)

Water sample is retrieved and transferred with great care to a BOD sample bottle without introducing any air bubbles. The bottle is then capped and stored on ice in a cooler. All BOD samples must be analyzed within 48 hours.

For chemical oxygen demand (COD)

Similar to BOD sampling method water sample is retrieved and transferred with great care to a COD sample bottle containing sulfuric acid without the introduction of air bubbles. The bottle is then capped and stored on ice in a cooler. All COD samples must be analyzed within four days.

For chlorophyll a

Samples retrieved from the Van Dorn water sampler are quickly filtered and preserved on ice in a cooler before subsequent extraction by acetone and measurement by a fluorometer or spectrophotometer in the lab. Some handheld instruments and multiparameter sonde can also measure chlorophyll a in situ but these are just approximated readings and lab extraction is still necessary to obtain the true concentration of chlorophyll a. If proper procedure is adhered to a good correlation curve between in vivo (live) chlorophyll a and extracted concentrations can be obtained. This curve is useful and applicable when regular or routine samplings are required from the same locality. A new curve is however required to account for seasonal changes in the algal assemblage or for a new sampling site i.e. stream or river as the probability of a different algal composition is very high. In other words different algal communities give different chlorophyll a readings, depending on geographical location and the time of the year.

For step-by-step procedure on sampling procedures of all the parameters described avoe please refer to the American Public Health Association (APHA) Standard Methods for Examination of Water & Wastewater (21st ed.). Flow measurements Often water quality is measured without the flow of the stream or river quantified at the time of sampling. Without flow values, the sampling results and in-situ measurements would not be as meaningful as quantitative information is required for water quality modeling etc. Flow can be measured on site during sampling with simple methods using float and stopwatch or with a current velocity meter. For more details on various gauging method please refer to Chapter 4.

Trend in Water Quality Monitoring Using Bio-Indicators and Bio-Assay

Most if not all aquatic communities whether micro- or macroscopic, may provide information on the quality of its environment. The community normally used in evaluating lotic systems condition are large, readily visible invertebrate animals colonizing the substrata of all rivers. These animals are collectively referred to as macroinvertebrates of which the main constituents are young aquatic stages of insects.


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Within this community each species tend to exhibit varying degree of sensitivity and tolerance to pollution. Some species are, for example, are very intolerant to high levels of silt i.e. turbidity thus will not be found where the concentration of suspended solids is high. A characteristic feature of polluted environments is a reduction in overall species assemblage and an increase in the density of tolerant species. Benthic macroinvertebrates generally inhabit their respective microhabitats in parts of a stream throughout their life cycle and tend to remain localized. Therefore they are continually exposed to any changes that occur in the environment. The composition of a macroinvertebrate community at any point in a stream or river then reflects the average water quality at that particular point. Hence the common objective in bio-monitoring projects is to detect stream and river degradation and the extent of it due to forest and agricultural practices, urbanization, or other controllable sources of impact.

In the United States and European countries macroinvertebrates have long been utilized as a bio-indicator in assessing river quality. Majority of this indicator are bottom dwellers such as crustaceans while others like fishes depending on their degree of availability can also make good candidates for monitoring and assessment purposes. The common criteria in selecting the type of organisms suitable for long term monitoring are:

a) The animal must relatively abundant and be easily captured or sampled, so size and density are important;

b) The animal must have a good range of distribution and in varying forms of lotic conditions for comparative investigations; and

Preferably, the biological characteristics e.g. morphological, physiological and behavioural adaptations and the immense variation in life history patterns and reproductive traits of the animal must already be well documented (to save additional baseline studies).

In a typical study design for biomonitoring of stream and riverine system the following guideline (Plotnikoff & Wiseman, 2001) would assist the lead researcher in reaching their objectives:

i) Choose representative riffle-habitat (broken surface water) sampling of benthic macroinvertebrates, physical habitat, and water quality to describe biological community condition as a result of natural and human-induced disturbance. Normally, samples are collected from riffles to characterize the benthic macroinvertebrate community unless degradation is suspected in pool habitat (slow moving or eddying water). To distinguish natural versus human influence, data must be collected at reference sites and at degraded sites over a period of time to address spatial and temporal variability.

ii) Reference sites are intended to represent relatively unimpacted or least impacted conditions.

Minimally disturbed conditions reflect sites that have experienced very little historical activity that alters stream integrity. Least disturbed sites have been degraded historically, but exhibit some level of recovery. Reference sites are used to describe biological variability due to natural disturbances (e.g. precipitation, drought). Degraded sites are surveyed to describe a continuum of human influence on natural stream communities. Identification of what a degraded macroinvertebrate community is and the factor(s) that caused the resulting condition defines severity of impact

iii). This gradient of biological condition is used to determine the levels of human-induced

disturbance that are excessive in a waterbody. iv) Besides high-quality reference conditions, sites with high levels of physical and chemical

modifications should be surveyed to obtain a data set that represents a gradient of biological conditions as a response to the existing stream condition.


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v) Choose one or two species of macroinvertebrates which life history and wide distribution is well

known using the set of criteria aforementioned. vi) Additional information on stream site characterizations should be gathered: canopy cover, stream

bed substrate, flow, turbidity, water temperature, pH, and dissolved oxygen. Detecting degradation through evaluation of invertebrate communities requires establishment of a description for reference condition. This is the focal point for developing analytical tools commonly used to evaluate stream condition and "biological integrity".

Complementing biomonitoring studies are bioassay work which in the environmental context refers to a broad-range survey of toxicity, and a toxicity identification evaluation is conducted to determine what the relevant toxicants are. Bioassays are typically conducted to measure the effects of a substance on a living organism. Bioassays may be qualitative or quantitative. Qualitative bioassays are used for assessing the physical effects of a substance that may not be quantified, such as abnormal development in fish larva, or heavily biased sex ratio in fish population. In some countries the law requires some industrial dischargers and sewage treatment to conduct bioassays. These procedures, also known as whole effluent toxicity tests, include acute toxicity tests as well as chronic test methods. The methods involve exposing living aquatic organisms to samples of wastewater. All said, biological monitoring provides useful information that can also serve as an early warning system. 8.4 RIVER WATER QUALITY MANAGEMENT Water quality defines the physical, chemical, and biological properties of water against a specific set of standards for it intended purposes, i.e. water supply, stormwater discharge, irrigation, recreation etc. Water quality standards are typically tailored for different types of water bodies, such as lakes, rivers, estuaries, seas, etc for intended utilization. The primary objectives of water quality management are to manage and measure the properties of water that suitable for the purpose of water supply, safety of human contact, aesthetics, and for the health of ecosystems. The hydrometry such as, streamflow gaging, direct velocity measurement, etc are used to quantify the flow rates in the water body. Likewise, chemical and physical techniques are used to determine the quantitative amount of both physical and chemical parameters of the water body. The water on earth is finite. Amongst these, sea water accounts for about 97% of total water resources in the hydrosphere and the other meagre 3% comprising, surface waters in lakes and rivers, icebergs, glaciers, and groundwater. Needless to say, the vast salt water resources could not be fully utilized without additional cost in treatment (because they are too salty to drink).

The need for water quality management is pressing in Malaysia. Although blessed with more than 2500 mm/year of rainfall on average throughout Malaysia, water quality impairment impedes full utilization of precious resources for beneficial uses.

Another generalized perception is that the water quality is merely a simple property that indicative whether the water body is polluted or not by both qualitative and quantitative forms. However, water quality issue is a far more complicated subject matter, in part of because water itself is a complex medium implicitly tied to the ecosystems on earth. Industrial and domestic pollution is merely one of the major culprits responsible for water pollution, as well as runoff from overland and built up areas, urban stormwater runoff and discharge of untreated sewage.

The needs for water quality management are vital to aesthetics, socio-economical growth, and subsequently environmental sustainability in Malaysia. This applies to the good quality water bodies such as lakes and rivers that meandering through cities and towns.

http://en.wikipedia.org/wiki/Toxicity

http://en.wikipedia.org/wiki/Toxicant

http://en.wiktionary.org/wiki/organism


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The degradation of water quality disrupts the environmental well-beings and harmonies of the water bodies and their surrounding landscapes. Ability to predict the changes to water quality is imperative to management of riverine resources. “Water quality management" is the management processes and techniques with the objective of planning and implementing the protection of a water body, such as rivers, lakes, and seas, for various beneficial uses. The objective of the water quality management is two folds; (a) to provide a healthy and clean water body, and (b) to effectively management the finite resources in the water body. Provision of adequate wastewater (domestic/industrial/agricultural) collection, treatment, and disposal for domestic and industrial wastes is a prerequisite in the water quality management. In addition for activities that might create water quality problems, and regulating and enforcing programs to accomplish the planning goals and laws and regulations dealing with water pollution control.

DID is in charge of river water management in Malaysia. A special case of water quality issue worthy of attention is the water quality downstream of reservoir or dam prevailing mostly in the upper reaches of the river basin. Examples are dams/reservoirs under the jurisdiction of the DID. These dams/reservoirs are mainly for irrigation as well as domestic water supply. The impacts of reservoirs/dams in the upper reach of a river basin are far reaching both in terms of temporality and spatiality. The impacts and repercussion on water quality downstream of the water retenting structures can be far reaching over time. Impacts such as relatively clear water outflows from the reservoir could erode and scour the river conveyance channels further downstream. Sometimes, releases from the dam/reservoir bottom will degrade the water quality downstream with its anoxic contents and could affect the fauna and flora and riparian community downstream.

8.4.1 POLLUTANT SOURCES Water pollution is the consequence of a body of water being adversely affected due to the addition of large amounts of materials to the water. These materials by nature will reduce the aesthetic values of water and bring impairment if not disastrous consequences to the quality of water in general. In addition, when the water body is unfit for its beneficial use, water is then considered polluted. Two types of water pollutants exist; (a) point source and (b) nonpoint source. Point sources of pollution occur when harmful substances are emitted directly into a body of water. This connotes to “end of pipe” discharges from any known or readily identifiable points along the water body. An example of the point sources pollutions are attributed to effluents discharged by the sewage treatment plants, industrial/manufacturing and household septic tank effluents directly into surface drain, and effluents from yards and farms, to mention a few. Typical point source pollutants to river water body are originated from industrial and domestic sources. These include a variety of organic and inorganic, dissolved or solid form. The most common pollutants are nutrients (nitrogen and phosphorus, inclusive of micronutrients), dissolved or undissolved forms of metal ions, sediments, etc. Whereas the non-point sources pollution which are mainly “diffused” and not attributed to a single traceable source such as in agricultural activities and cities, where rainfall induced surface runoffs could not be traced to a single end-of-pipe outlet. An example of this type of water pollution is when fertilizer from a field is carried into a stream by rain, stormwater discharges into river that carry sediments and other pollutant washoff along their paths. Nonpoint pollutants could also be attributed to areal airborne contribution of nutrients, i.e. particulate nitrogen and phosphorus in the forms of wet and dry deposition.


March 2009 8-13

Nonpoint source also delivers pollutants indirectly through adverse environmental changes. Examples such as a sudden pulse of pollutant being mobilized and eventually flushes into nearby water body and en route to river network. Discharges or effluents from domestic wastewater treatment facilities such as industrial and domestic treated effluents are also considered as input into the receiving river water body. Incidents of untreated effluents could cause fishkills in river due to their undesirable raw content of pollutant and moreover, also affect the operation of raw water intakes downstream of the discharge point. Incidents of water intake shut down were reported on several occasions.

8.4.2 WATER QUALITY INDICES AND STANDARDS

River water quality standard in Malaysia is based on Phase I and II studies of water quality criteria and standards for Malaysia. In the Phase I of the studies, recommendation was made on the classification of river water quality based on their beneficial uses. Six (6) classifications were designated as follows (Table 8.1):

Table 8.1 The classification of River Water Quality Based on Their Beneficial Uses

Classification Use

Class I

Conservation of the natural environment Water supply I-practically no treatment necessary (except by disinfection or boiling only) Fishery I- very sensitive aquatic species

Class IIA Water supply II- conventional treatment required Fishery II- sensitive aquatic species

Class IIB Recreational use with body contact

Class III

Water supply III- extensive treatment required Fishery III- common species of economic value and tolerant species Livestock drinking

Class IV Irrigation

Class V Water unsuitable for any of above uses

In the Phase II study, another criterion was developed on the need of quantitative assessment of the river water quality as the phase I study was merely a fuzzy qualitative index on the health of river system in Malaysia. The phase II study proposed and subsequently adopted a numerically based Water Quality Index (WQI) in the Environmental Quality Report 1990. The WQI relates a cohort of measured water quality parameters to a common scale and combined them into a single indicator index. The water quality parameters used in the WQI formulation include pH, Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Suspended Solid (SS) and Ammoniacal Nitrogen (AN). The present range of WQI ave been grouped into five (5) classes as shown in table 8.2.


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Table 8. 2 The present range of WQI

Parameter Unit Classes

I II III IV V

Ammoniacal Nitrogen mg/l < 0.1 0.1 - 0.3 0.3 - 0.9 0.9 - 2.7 > 2.7

Biochemical Oxygen Demand mg/l < 1 1 - 3 3 - 6 6 – 12 > 12

Chemical Oxygen Demand mg/l < 10 10 - 25 25 - 50 50 – 100 > 100

Dissolved Oksigen mg/l > 7 5 - 7 3 - 5 1 – 3 < 1

pH mg/l > 7.0 6.0 - 7.0 5.0 - 6.0 < 5.0 > 5.0

Total Suspended Solids mg/l < 25 25 - 50 50 - 150 150 – 300 > 300

Water Quality Index > 92. 7 76.5 - 92.7 51.9 - 76.5 31.0 - 51.9 < 31.0

8.4.3 Discharge Standard (IWK, 2008) Domestic sewage treatment is designed with the objective to produce an effluent low in solids and organic matters in both dissolved and solid forms. Standards have been established for the quality of effluent discharged from treatment plants to receiving waters. Discharge effluent standards by sewage treatment processes are imposed either classified as Standard A or B depends on the location of the point sources. Effluent that is discharged upstream of a water supply intake should meet Standard A, while effluent that is discharged downstream has to meet Standard B. These standards are set by the Environmental Quality Act 1974. If the discharge is upstream of a raw water intake, Standard A is relevant. On the other hand, if the discharge is downstream of an intake, Standard B is imposed. Table 8.3 below shows both Standard A and B.


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Table 8.3 Discharge Effluent Standard A and B

Parameter Unit Standards

A B

Temperature C 40 40

pH Value unit less 6.0-9.0 5.5-9.0

BOD5 at 20C mg/l 20 50

COD mg/l 50 100

Suspended Solids mg/l 50 100

Mercury mg/l 0.005 0.05

Cadmium mg/l 0.01 0.02

Chromium, Hexavalent mg/l 0.05 0.05

Arsenic mg/l 0.05 0.10

Cyanide mg/l 0.05 0.10

Lead mg/l 0.10 0.5

Chromium, Trivalent mg/l 0.20 1.0

Copper mg/l 0.20 1.0

Manganese mg/l 0.20 1.0

Nickel mg/l 0.20 1.0

Tin mg/l 0.20 1.0

Zinc mg/l 1.0 1.0

Boron mg/l 1.0 4.0

Iron (Fe) mg/l 1.0 5.0

Phenol mg/l 0.001 1.0

Free Chlorine mg/l 1.0 2.0

Sulphide mg/l 0.50 0.5

Oil and Grease mg/l Not Detectable 10.0

(Source: www.iwk.com.my) 8.5 WATER QUALITY PARAMETER The river water quality for selected river basins is monitored by a private concessionaire, ASMA under agreement with the Malaysian Department of Environment (DOE). Other than these six (6) parameters used for the computation of the Water Quality Index (WQI; i.e. Dissolved oxygen, Biochemical Oxygen Demand, Chemical Oxygen Demand, Ammoniacal Nitrogen, Suspended solids, and pH), other parameters such as heavy metal ions, microorganisms, organic chemicals presented in the river water might also relevant.


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The water quality parameters are important in assessing the state of the well being of the water bodies. They are categorically regrouped into three (3) subgroups; (a) Physical, (b) Chemical (c) Biological Parameters respectively, as shown in Table 8.4 below.

Table 8.4 The water quality parameters

Physical Parameters Chemical Parameters Biological Parameters

Streamflow/hydrometry

Temperature

Specific Conductance

pH

Turbidity

Dissolved Oxygen

Hardness

Alkalinity

Sediment/Solid

Total Suspended Solids

Total Dissolved Solids

Nutrients

BOD/COD

Phosphate

Orthophosphate

Total Organic Carbon

Dissolved Organic Carbon

Ammonia

Nitrate

Nitrites

Sulfate

Chloride

Fluoride

Metal Ions

Heavy Metals (Ca, Mg, Na, Fe, Ba, Cd, Cr, Pb, Mn, Zn)

Total Coliform Bacteria

Fecal Coliform Bacteria

E. Coli Bacteria

Virus

8.6 WATER QUALITY MODELLING The prediction of the ecological as well as environmental impacts of waste and pollutant disposal as a result of associated land use changes and modification is now appeared to be more than a fundamental requirement for river engineering personnel. Investigating ways and means of linking land use, pollutant loading and disposal, water quality and ecosystem impacts together. The use of computer-based water quality models is widely accepted for such purpose. These models could be a merely simple "black box type" mass balance models to be used as planning and screening tools to commercially available dynamic and complex water quality models primarily used for strategic planning purposes Water quality modelling techniques have evolved as an accepted tool to support the surface water management. Modelling techniques are used to carry out a systematic and methodical analysis, aiming at understanding the cause and effect relationships and assessing the impact of changes in ambient water quality due to various possible scenarios, such as changes in land use and loadings, etc. Water-quality modelling is the linkage between the sources of pollution and the in stream water quality processes of a given water body. In summary, a model is not perceived to be more than a representation of the physical, chemical, and biological water-quality processes and mechanisms that occur in a water body.


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Water quality models are tools for simulating the movement of precipitation and pollutants from the ground surface through pipe and channel networks, storage treatment units and finally to receiving waters. Both single-event and continuous simulation may be performed on catchments having storm sewers and natural drainage, for prediction of flows, stages and pollutant concentrations.

Each water quality model has its own unique purpose and simulation characteristics and data requirement. A thoroughly review of each individual model therefore should be undertaken prior adoption. Water-quality modelling can be resource-intensive. Different levels of complexity may be used depending on the level of confidence required in a given situation. Learning curve could be steep and time and resources should be invested as appropriate in order to perform a successful modelling endeavour.

Water quality models can be classified according to its special features, such as model complexity, type of water body, and the extensive of water quality parameters to be modelled. The more complex the model is, the more difficult and expensive will be the successful and meaningful execution of the modelling exercise.

In addition, observed/measured records of water parameters to be modelled must be available sufficiently a priori for model calibration and validation purposes. These two steps are vital to the credibility of modelling endeavour. Without them, the modelling exercise is simply reduced to “guessing game” and no substantial understanding on the water quality process could be discerned. Factors to be considered in the modelling exercises are:

a. On the number and type of water quality indicators or parameters to be modelled: Generally speaking, the more the parameters are to be included in the model, the more complex the model will be. Furthermore, some parameters are more complicated than the other to be modelled and studied, especially if they are interrelated to one another and subject to physical aspects of the environments.

b. On the level of spatiality: The complexity of the modelling exercise increases with the

increase in number of sources/input that have to considered. This leads to the additional demand in the data collection effort such that calibration and verification could be carried out amicably.

c. On the level of temporality: Long- term simulation undertaking requires fairly long-term

commitment in data collection and computational tasks. A long-term simulation provides static averages are easily to be accomplished than a relatively short term dynamic variation in water quality.

d. On the type of water body: The physical size of the river, such as length, width, and depth could influent the mechanisms of pollutant transport in the modelling exercise. Representation of the water quality parameters in the transport processes in both longitudinal and traverse directions varies for a wide river channel vis-à-vis a lake or a narrow water body. The summary is shown in Table 8.5.


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Table 8.5 The criterion on classification of the water quality model.

Criterion Comment

Single-plant or regional focus

Simpler models can usually be used for single-plant “marginal” effects. More complex models are needed for regional analyses

Static or dynamic Static (constant) or time-varying outputs

Stochastic or deterministic Stochastic models present outputs as probability distributions; deterministic models are point estimates

Type of receiving water (river, lake or estuary)

Small lakes and rivers are usually easier to model. Large lakes, estuaries, and large river systems are more complex

Water Quality parameters

Dissolved oxygen Usually decreases as discharges increases. Used as a water quality indicator in most water quality models.

Biochemical oxygen demand (BOD)

A measure of oxygen-reducing potential for waterborne discharges. Used in most water quality models.

Temperature Often increased by discharges, especially from electric power plants. Relatively easy to model.

Ammonia nitrogen Reduces dissolved oxygen concentrations and adds nitrate to water. Can be predicted by most water quality models.

Algal concentration Increases with pollution, especially nitrates and phosphates. Predicted by moderately complex models.

Coliform bacteria An indicator of contamination from sewerage and animal waste

Nitrates A nutrient for algal growth and a health hazard at very high concentrations in drinking water. Predicted by moderately complex models.

Phosphates Nutrient for algal growth. Predicted by moderately complex models

Toxic organic compounds A wide variety of organic (carbon-based) compounds can affect aquatic life and may be directly hazardous to humans. Usually very difficult to model.

Heavy metals Substance containing lead, mercury, cadmium, and other metals can cause both ecological and human health problems. Difficult to model in detail.

Adopted from Pollution Prevention and Abatement Handbook, World Bank Group, 1998

8.6.1 Classification of Water Quality Model The first known water quality model approach and attempt to river management application and planning begins with a simple parameter, i.e. dissolved oxygen (DO) balance sag curve in Ohio river, USA in the 1920’s by Streeter and Phelps (1925). At that time, the dissolved oxygen concentration in water body appears to be the most significant criteria in sanitation as the discharges of impurity and organics from domestic as well as industrial wastewater has had major impact on the receiving water course. Bacteria, which are present in abundance in wastewater consume oxygen in waters to breakdown organic matter.


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A reduction of dissolved oxygen is therefore served as an important indicator for the state of water quality and ecological health in the river course. However, rapid running water and self purification of river waters was also taken place. Both water flows and agitation due to turbulence nature of running waters and wind induction on surface aeration somewhat inducing oxygen from the air into the water body by apparently the diffusion mechanism. This particular mechanism is known as reaeration. Oxygen in the air is resupplied to the water body via both natural and mechanical aeration processes. It was fundamental and practical for river water to purify itself naturally or with the aid of minimum engineering approaches other than the natural course of running water. Dobbins and O’Connor (1958) carried out an empirical study in the late 1950’s to relate the reaeration rate of a natural river reach with measured mean flow velocity and hydraulic parameter, i.e. average depth of the river cross section. Subsequently other researchers followed the lead and derived various forms of reaeration coefficients using the same methodology as Dobbins and O’Connor (1958); (Churchill et al, 1962; Owens et al. 1964; Langbein and Durum, 1967). Other variant is Tsivoglou and Neal (1976) where only the average stream velocity and stream slope are used. Wind induced reaeation on surface water was also been carried out by Chu and Jirka (2003). Ramakar et al (2003) presented a variant form of Dobbins-O’Connor type reaeration formula based on least square (curve fitting) approach of the hydraulic parameters such as stream/channel velocity, bed slope, flow depth, cross-sectional area and measured dissolved oxygen concentration. Reaeration attributed to moving waters by momentum and water surface wind blow by no mean the only oxygen diffusion mechanisms. Other factors such as biological processes might be played a role in reintroducing oxygen into waters as well. This is primarily caused by autotrophic primary producer organisms that produce cell biomass by photosynthesis.

The DO model of Streeter and Phelps (1925) evolved to the next plateau after three decades by taking into account of other instream reaeration processes such as photosynthesis, respiration, and sediment oxygen demand (SOD) in the oxygen balance computation (O’Connor, 1960) in the 1960’s. These advances marks the foundation of modern water quality modelling of natural waters, i.e. river, lake, estuary, etc.

Ultimately, it should be reiterated that the classification of the water quality model depends on the purpose and objective of the water quality modelling exercise. The models could range from simple to complex based on the number of parameters and state variables to be simulated in the modelling exercises and on both spatial and temporal scales of the river reach to be modelled.

Before performing the water quality modelling assignment, hydrologic and hydraulics submodelling to determine the discharges and the water surface elevation should be carried out a priori. The submodels can be divided into stationary and non-stationary modes. Stationary in the connotation means the flow or discharge to be constant during the simulation time period. On the other hand, in the non-stationary mode, the discharge or flow varies with time.

In general, water quality model can be broadly defined as (a) lumped or distributed-parameter; (b) deterministic and physically based, or stochastic types.

Lump Parameter type of water quality model can be considered as deterministic or stochastic in its formulation. It treats the entire water body as a single entity or “compartment” in its representation of the river reach. The mechanism of transport and chemical as well as biological processes takes place only within the compartment. Mass balance equations are formulated accordingly based mainly on the law of mass conservation and continuity. The utility of lump parametered water quality model is limited and could perhaps address only a very simple process and confine to a fairly simple and straightforward undertaking. Mathematically speaking, ordinary differential equation (ODE) can be used to represent adequate the processes of pollutant fates, transport, and transformation in simple analytical or numerical solution. A short river reach and a lake are representative of lump


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parameter model. The derivative of the pollutant with time depends on the mass conservation of the pollutant entering and leaving the compartments and in addition, subject to in-stream or in-lake chemical as well as biological reaction processes. Rate of change of pollutant = inflow – outflow ± reaction

d(VC)

dt= QinCin-QoutCout±∑ kVC (8.1)

Distributed parameter type of water quality model on the other hand, represents relative sophistication in representing the fate, transport, and transformation of water quality parameters in a water body in space. Generally the processes could be mathematically represented by the various genres of partial differential equations (PDEs) with reaction components as appropriate. Solutions to this type of mathematically function depend on the appropriate boundary and initial conditions that reflect the physical and hydraulics reasoning of the river basin model. The 3-dimensional PDE for a single chemical variable is as follows:

(1) rate of change = (2)advection + (3)dispersion ±(4)reaction:

∂C

∂t= - Vx

∂C

∂x + Vy

∂C

∂y+ Vz

∂C

∂z+ ∂

∂xDx

∂C

∂x+ ∂

∂yDy

∂C

∂y+ ∂

∂z Dz

∂C

∂z+∑ reaction (8.2)

8.6.2 Process Description The fate and transport processes of pollutants in a water body hinges on physical, chemical, and/or biological processes and their associated transformation. Physical processes to describe the transport of pollutants are (1) advection and (2) diffusion in both longitudinal and traverse directions along the river reach.

8.6.2.1 Advection

Advection is the physical process where the waterborne variables i.e. pollutant is transported by the motion of water particle or current velocity in a river reach. It describes the state of pollutant transport downstream of a reference point at the mean flow velocity of the river reach. Actual velocity though varies temporarily and spatially. For a narrow river channel, one dimension model representation is suffixed to describe the fate and transport of pollutant due to the influence of channel velocity. Most of the time, an average representative velocity is chosen for this purpose.

8.6.2.2 Diffusion

As a result of the action of velocity movement, mixing process takes place. This mixing is caused by both the turbulent and molecular diffusion and by dispersion. Turbulent diffusion represents the pollutant mixing due to temporal deviations or fluctuations of the flow velocit from the local mean velocity value. Dispersion on the other hand, describes mixing due to spatial velocity fluctuation along the river.

8.7 MATHEMATICAL MODELS FOR RIVER WATER MANAGEMENT: SELECTION AND ADOPTION Selection of the model to be adopted in a water quality modelling study is nonetheless based on several factors such as the individual and the unique objective of the water quality management and planning, resources availabilities and expertise, availability of observed and measured water quality parameters, etc. In other words, model selection can be reasonably made based on the management objective and familiarity and affinity of the users to the specific models.

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8-22 March 2009

8.8 ENVIRONMENTAL FLOW Scarcity in water resources has been well recognized as an en emerging global issue that deserved utmost importance and attention amongst the water resources planners and professionals. Growing awareness on the increased water demand by human activities does not only reduce the amount of water available for future industrial and agricultural development but also appears to have significant impacts on aquatic (fauna and flora) ecosystems and the inhabitable species. Increasing water consumption and demand by the domestic, industrial as well as irrigation sectors, and other environmental unfriendly human activities such as deforestation by opening of large tract of land mass for agricultural cultivation expansion, wetland/peat swamp draining and reclamation, commission and construction of reservoirs, and other water diversion structures significantly alter the natural hydro-geomorphic processes and regimes. Hence these impacts result in a series of water related environmental problems such as increasingly frequent freshwater shortage, declining groundwater tables, dangerous levels of pollution, serious habitat degradation and disastrous and extreme event flooding over the past decades in many parts of the world. Reaching a consensus on satisfying the needs of both the aquatic environment and other beneficial uses for expanded and intensive human activities is an emerging as well as a critical issue in many countries for sustainability in economic development and at the same with minimum disturbance and detriment to the environment as a whole. Competition for water resources utilization in world’s river basins is destined to be intense as human population and associated water demands attributed to higher living standard. With the needs of reaching harmony with the environment in mind, the ecological well being of the riverine community must not be in any way compromised. The water demand to satisfy these riparian communities is termed as environmental or compensation flow. The raison d’etre of putting forward the concept of environmental flow is to set up a threshold value while increasing the demand for human activities but without scarifying the riverine natural function. Protection must therefore be considered at an acceptable level. In essence, compensation or environmental flow could be considered at the continuous flow that keeping the river at an acceptable natural functional level. It must be borne in mind that drastic reducing or altering the flow regime in a river would seriously pose long-term detrimental consequence to the riparian community as a whole. 8.8.1 Environmental Flow: A Definition Environmental flow or riparian release is defined as the prior release that is required to be made by statue downstream of any water resources abstraction or storage projects i.e. intakes and reservoirs. It is also rarely termed as river maintenance flow in some studies (RB/SSP 2003) purportedly necessary quantity of flow to sustain healthy riverine environments. Another often used term is instream flow. As the name implied, it is the flow or water/hydraulic depth that has to be maintained after significant diversion of water resources from a water body. The term is defined as following by Washington State Department of Ecology. The term "instream flow" is used to identify a specific stream flow (typically measured in cubic feet per second, or cfs) at a specific location for a defined time, and typically following seasonal variations. Instream flows are usually defined as the stream flows needed to protect and preserve instream resources and values, such as fish, wildlife and recreation. Instream flows are most often described and established in a formal legal document, typically an adopted state rule. Instream flow is the amount of water needed in a stream to adequately provide for downstream uses occurring within the stream channel. Instream uses may include some or all of the following: aquatic habitat, recreation, wetlands maintenance, navigation, hydropower, riparian vegetation, and water quality.


March 2009 8-23

The purpose of these mandated releases by law is to provide adequate living conditions and spaces for fauna and flora and/or other riparian communities downstream. Else detrimental long term ecological consequences will be forthcoming in both short- and long-term. Definition advocated by Global Environmental Flows Networks (2006) is as follows: Environmental flow is the amount of water needed in a water course to maintain healthy, natural ecosystems. The term is used in the context of rivers which have been dammed, with most or all of the flow trapped by the dam — the failure to provide an environmental flow can have serious ecological consequences. The terms, environmental flow, instream flow, maintenance flow, and compensation flow are used interchangeably in this report.

8.8.2 Environmental Flow in Malaysia

For most of the water resource development projects in Malaysia, there are always some riparian stakeholders/users, water extractors for irrigation and WTP operators downstream, which divert waters in the rivers for their respective utilization. Therefore it is of utmost importance at this juncture to differentiate between compensation flows during average, wet and drought years which are in essence seasonally varied according to their own unique requirement. In the first two categories, the issue of compensation flow does not seem to be an issue as abundant water would be available downstream during these periods of higher and average flows in the rivers. This however, does not hold true during extreme drought periods, when water levels are comparatively lower than average. Only during these periods of time and depending also on the duration of low flow regime, the riparian communities and extractors/stakeholders downstream of the water resources projects may not receive their fair share or allocation of precious water resources. Any detrimental non-action will have environmental as well as ecological impacts to the downstream communities. The general issues of the compensation flows are discussed and appropriate recommendations or decisions could then be made on the adoption of the amount of the compensation releases and especially a prolonged drought period where competitive demand for water by various riparian users are both critical and crucial. The practice of compensation or environmental flow release is in fact an integral part of the water management operation requirement. For the case of reservoir design and operation, it is an important parameter for determining the optimum yields and their corresponding live storage capacities of the reservoirs. On other words, this essentially means that to a certain extent, the available reliable yield depends on the amount of water that is needed to release a priori to the riparian users downstream before it can be supplied to the WTP. On the contrary, generous and less optimal allocation of compensation flow reduces the beneficial uses and vice versa. This is an attempt to summarize the general concepts, rationales and findings of the past studies carried out in Malaysia with respect to the issues of compensation or environmental flows. The experiences are mainly drawn from the water supply industry with regards to the issue of compensation flow. In the past experience, the issue of compensation flows has been addressed in two major water resources studies (Pahang and Johor Water Resources Studies, PWRS; 1992, JWRS; 1994). These studies’ findings were used as a basis to establish the magnitude of compensation flows for river system in the state of Pahang and Johor.


8-24 March 2009

This guideline on water releases from dam or reservoir is being used to date in most of the water supply projects in Malaysia (SMHB, 2003). It must be cautioned that these practices of compensation releases are mainly of mechanistic nature or based solely on quantum of flows availability assessed by hydrological study output. Therefore, the practices do not take into account the actual of amount of flows that are needed to be releases for sustainability of aquatic flora and fauna communities in the riverine environments. As a matter of fact, the type of study to meet the requirement of fauna and flora communities in their life cycles, to the best of knowledge is not yet being carried out systematically in Malaysia. In this transition period, only techniques derived from hydrological assessment are being considered. Four (4) techniques that have been practiced solely based on quantitative hydrological assessment of flow are described briefly below. They are namely, (a) Observed minimum average (monthly or daily) flow, (b) 1:50-year 7-day low flow obtained from a low flow frequency analysis, (c) A percentage of the Annual Average Flow (AAF). (d) Flow duration curve is included for comparison purpose. The last technique, 99% and 98% probability of flow exceedance as interpolated from flow duration curves have been practiced from time to time in Malaysia although JICA (1982) in its National Water Resources Study for Malaysia has advocated some higher percentage, i.e. 90 to 80% to be adopted as compensation or environmental flow. Each individual method with its merits and demerits is briefly discussed in a more systematic manner. 8.8.3 Minimum Average Monthly Flow The minimum monthly average flow volume is the simplest technique among other. The minimum flow is basically selected from the long-term historical monthly flow records of the gauged streamflow stations and appropriately transposed to the sites of interest using appropriate transposition factors. This is in fact a hydrological index number based solely on observed records with some uncertainties. Notwithstanding the simplicity of this technique in deriving the magnitude of low flow, however, it should be cautioned that the lowest observed record of streamflow rate might not be a better and reasonable indicator as operational difficulties are always encountered during field gauging exercises and instrumentation errors are prevailing if they are not properly maintained and calibrated. The lowest flow might therefore be recorded erroneously due to variety of reasons, such as instrument malfunction, one-time freak event, etc. Hence the observed lowest flow records should somehow be interpreted with due diligence and prudence before they can be adopted for estimating compensation flow. 8.8.4 1:50-year 7-day duration Minimum Flow (7Q50) The 1:50-year 7-day (7Q50) low flow can be conveniently extracted from the result of probability/frequency analysis of minimum low flow annual/partial series of gauged streamflow stations of significant years of records and appropriately transposed to the site of interest. The frequency analysis is normally carried out using both LN2P and LN3P distributions as deemed fit. Sometimes, Log-EV type I (EV1, Gumbel) and Log-Pearson type III (LP3) distributions are also considered amongst the popular choice. This is however depends mostly on the preference of the individual hydrologists. The minimum record length of annual series is preferably more than 30 years for extrapolation to 50-year return period or 2% chance of occurrence.


March 2009 8-25

Most of the time, the results using various distributions in frequency analysis do not vary significantly to warrant unnecessary concerns. The low flow frequency analysis is part of the routine hydrological estimation tasks commonly undertaken in waster resources assessment. The magnitude of flows for various durations and return periods or specific probabilities of occurrence are readily available. The origin of 7Q50 is perhaps based on USA technique of 7Q10 approach in determining the low flow regime in the river for waste assimilation purpose. 7Q10 is used by EPA (2008) on the premise that 7Q10 design flow was similar to the biologically-based 4B3 design flow and recommended the use of either design flow for water quality standards and toxic waste load allocation studies relating to chronic effects on aquatic life. 7Q10 is the average seven-day consecutive low flow with a ten year return frequency or probability of occurrence. In layman’s term, as depicted in the lexicon of frequency analysis this is the lowest stream flow for seven consecutive days that would be expected to occur once in ten years in average. The same magnitude of flow however could occur consecutively. It is used in the USA in setting point load discharge limits from municipal wastewater treatment plants (WWTP) in National Pollutant Discharge Elimination System (NPDES) water quality permits. This permit will only be granted if the proposed amount of pollutants that will be discharged into a river will not significantly impair its beneficial uses, such as extraction for drinking or swimming. When the river discharge is below the 7Q10 level, under the stipulated NPDES permit requirement, the WWTP operators are then restricted from discharging pollutants that would cause concentrations in the receiving water to exceed their permit limits, even at very low (i.e. 7Q10) streamflow levels. Although such a low streamflow value, roughly equivalent to a ten-year drought, is appropriately used in the context of limiting pollution discharges, the 7Q10 flow statistic is sometimes inappropriately claimed to represent an adequate streamflow for maintaining a healthy aquatic ecosystem, when in fact much higher streamflow levels are required. Due to the fact that 7Q10 is somewhat higher in magnitude in some studies, in some practices, only a fraction of 7Q10 is assumed as environmental flow.

8.8.5 98% and 99% Probability of Exceedance

Interpolation from the results of flow duration analysis was introduced earlier in the Pahang and Johor water resources studies (1992, 1994) as an additional technique in order to examine the sensitivity of the water resources model simulation with respect to the magnitude of compensation/environmental flows released downstream of the proposed dams and intakes. The criterion is originally based on the recommendation in the National Water Resources Study (JICA, 1982) in which 90% and 95% exceedance of the flow duration curves are generally adopted in the planning and design of the proposed water resource development projects in Malaysia. However, subsequent works in hydrological and water resources assessment by SMHB (2002, 2004), opined that the values proposed by JICA (1982) are considered too stringent for conditions in Malaysia. This is due to the fact that the natural fluctuations between highest and lowest flows are too extreme high. The lower flow at flow duration of lesser strict such as 99% and 98% are adopted instead. These values are obtained from the daily or monthly flow duration curves analysis of gauged streamflow stations and appropriately transposed to the site of interest.

8.8.6 A Fraction of (Annual Average Flow) Volume

The origin of this technique being adopted in water resources studies in Malaysia unfortunately is not known at the time of writing this report.


8-26 March 2009

It was first appeared in the Pahang Water Resources Study (PWRS; 1992) study where conclusions was made on the fact that a 10% non-exceedance flow as equivalent to about 10 to 25% of the AAF after analyzing the flow duration curves of the selective streamflow stations in the state of Pahang. This technique is almost similar to Tennant or Montana method (1976) adopted in estimating the instream flow requirement in the rivers for fish habitats in the Midwest region of the USA. In 1976 Tennant introduced a method for determining instream flow requirements for fish, known as the Montana method or more commonly the Tennant method. The method uses a percentage of average annual flow (AAF) to determine fish habitat quality. From field data collected of 58 cross sections from 11 streams in Montana, Nebraska, and Wyoming, Tennant concluded that 10% of AAF is the minimum for short term fish survival, 30% of AAF is considered to be able to sustain fair survival conditions, and 60% of AAF is excellent to outstanding habitat. These quantities are employed internationally, regardless of physical and hydrologic setting, due to the simplicity of using only the average annual hydrograph. The Tennant method was developed for eleven (11) streams in the state of Montana, Wyoming, and Nebraska. It is also known as “Montana” method in its less recognized and alternate name outside of the USA. It is used to find satisfactory level of discharges for fish passing. A flow of 30% of the annual average flow was found to maintain satisfactory widths, depths ad velocities. The study was mostly carried out by field observation. In the temperate climate where it was originated, Tennant technique can be branched to both winter and summer month requirement. The criteria and condition where Tennant method based on is rather site specific, it is therefore rather difficult to transfer this technique to sites. Furthermore, it does not account for daily, seasonally, or annual flow variations (see table 8.6 below).

Table 8.6 Tennant Method: In Stream Flow Requirement

Description of flow Recommended base flow regime

Oct –March April-September

Flushing or maximum 200%

Optimum range 60 to 100%

Outstanding 40% 60%

Excellent 30% 50%

Good 20% 40%

Fair or degrading 10% 30%

Poor or minimum 10% 10%

Severe degradation 0 to 10%

Problems with Tennant methods are again of empirical nature. The method could not be used outside of its calibration ranges, i.e. such as applications to hydrologically and geomorphological inhomogeneous basins. This is to say that other than, the validity in theory on ability to quantify the magnitude of flow of a river basin accurately, the quantum of environmental releases that are suitable for fishes or other fauna and aquatic communities are therefore uncertainty. This is best undertaken on specific locality basis, such as, field studies may be needed to determine the quantum of flow in selected river basins in the state of Sarawak.


March 2009 8-27

Another shortcoming of this technique is its inability to provide environmental release to account for daily, seasonal, or annually flow variations, since the methodology is solely based on the average annual flow. Sometimes, comparison with the average 10- and 30-day natural flows maybe advisable to determine whether the flows are available naturally during the low flow periods or durations. Checking with monthly or 30-day average low flow seems to be a reasonable approach. Several variants of Tennant method evolved throughout the years. The modification is based on the premise of different water requirements for geographical and physiological factor of fish species in other regions of the USA or perhaps in other parts of the world. The improved Tennant method purportedly computed eco-environmental water requirement for reach by taking other riverine factors into consideration. This is basically Tennant method in essence is only a hydrological based or commonly known as indexed technique. Recognizing the shortcomings of Tennant method, other variables, apart from the hydrological magnitude of the flow, such as toe-depth, river channel hydraulic parameters are also taken into account in the development of instream or compensation water requirement models not only for fishes but for other aquatic fauna and flora habitats as well. These models are such as Range of Variability Approach (RVA), PHAMSIM, IFIM, amongst others. 8.8.7 Other Environmental Flow Techniques The issues of environmental flows and their quantification have spurred many interests amongst the team members and professionals in the water resources development sectors internationally. Protection of the salmon fisheries and habitats and further extension to other threatened aquatic species is the raison d’etre for the proliferation of various environmental flow assessment techniques around 1960’s. The techniques range from a simple hydrological or hydraulic index techniques to some sophistications by taking into account the ecological and environmental factors into account. These models are being practiced throughout the world. Comprehensive review of environmental flow models is described in the following subsection, which is an excerpt in toto from www.eflownet.org. 8.8.8 Comprehensive Review by www.eflownet.org In the most recent review of international environmental flows assessments, Tharme (2003) recorded 207 different methods within 44 countries. Several different categorizations of these methods exist, three of which are shown in table 8.7 below.


8-28 March 2009

Table 8.7 Categorizations of international environmental flow assessments

Organisation Categorization of methods Sub-category Example

IUCN (Dyson et al.

2003)

Methods Look-up tables Hydrological (e.g. Q95 Index) Ecological (e.g. Tennant Method)

Desk-top analyses Hydrological (e.g. Richter Method) Hydraulic (e.g. Wetted Perimeter Method)

Ecological

Functional analyses BBM, Expert Panel Assessment Method, Benchmarking Methodology

Habitat modeling PHABSIM

Approaches Expert Team Approach, Stakeholder Approach (expert and non-expert)

Frameworks IFIM, DRIFT

World Bank (Brown & King,

2003)

Prescriptive approaches

Hydrological Index Methods

Tennant Method

Hydraulic Rating Methods

Wetted Perimeter Method

Expert Panels

Holistic Approaches BBM

Interactive approaches

IFIM DRIFT

IWMI (Tarme, 2003)

Hydrological index methods Tennant Method

Hydraulic rating methods Wetted Perimeter Method

Habitat simulation methodologies IFIM

Holistic methodologies BBM DRIFT

Expert Panel Benchmarking Methodology

The following review is based on Tharme (2003), Dyson et al. (2003), Brown & King (2003) and Acreman & Dunbar (2004), but follows the categorization of Tharme (2003). The table 8.8 below shows durations of assessment needed and major advantages and disadvantages of environmental flow assessment methodologies.


March 2009 8-29

Table 8.8 Durations and major advantages and disadvantages of environmental flow assessment

methodologies

Duration of assessment (months)

Major advantages Major disadvantages

Hydrological Index ½ Low cost, rapid to use Not site-specific, ecological

links assumed

Hydraulic rating 2-4 Low cost, site specific Ecological links assumed

Habitat simulation 6-18 Ecological links included

Extensive data collection and use of experts, high

cost

Holistic 12-36 Covers most aspects Requires very large

scientific expertise, very high cost, not operational

8.8.9 ENFRAIM: Environmental Flow Requirements; an Aid for Integrated Management In 2001, the ENFRAIM project began with a review of existing environmental flow requirement methodologies. The main conclusion arising from this review is that there is a vast amount of methods being developed throughout the world geared towards setting Environmental Flow Requirements, either for specific rivers or specific regions. Particularly in North America, many Environmental Flow Assessments are developed with respect to the conservation of fish habitats, whereas in South Africa, for example, a holistic approach is most frequently used. However, it often occurs that not all of a river’s functions are taken into account (functions which include flood mitigation, recession agriculture, drinking water, local fisheries, delta formation and stabilisation). ENFRAIM is a decision making tool for water resources managers for determining the quantum of environmental flow requirement in a river basin. It is a new generation of model framework with clarity and transparency in operational management. The flow assessment is though a much more complex procedure, especially if both riverine and coastal downstream requirements are to be included in the process. It was a Dutch funded project to develop the concept of Environmental Flow Requirements into an adequate (i.e. effective and efficient) planning tool for integrated river and coastal management. International collaboration were also been carried out with case studies and field applications in Vietnam, Thailand, and Bangladesh. Attention was focused on the way in which one should deal with these functions when setting environmental flow requirements. Specific local issues such a climate, geomorphology, functions sustained by the river and requirements of different parts of the complete river basin ecosystem should be taken into account accordingly.


8-30 March 2009

8.8.10 Application in Malaysia Most of the environmental or compensation flow requirements for Malaysia are based on hydrological index techniques where a fraction of river flows are released downstream of a dam or diversion structure. These four (4) techniques are explained in brief in previous subsections. Admittedly this is most simple mechanistic approach to date on the quantification of environmental flow requirement. Without consideration of the requirement of fauna and flora communities in the river system makes the hydrological index techniques seems simplistic. In theory, the environmental flow requirement should be tailored to the need of the aquatic species. The need for water to growth hinges on the growing and life cycle of the communities, to be exact. Without taking this factor into consideration, it seems flaws in the allocation of environmental flow requirement. Due to primarily the limitation in knowledge and understanding of fish population and their behavior and life cycle in a river system. It makes the estimation on the quantum of environmental flows or releases a difficult task laden with uncertainty. Attempt to link the environmental flow requirement to the complexity and sustainability of fauna and flora habitats in a river system was plausible but unfortunately appropriate and suitable references and undertakings are not available in Malaysia, to the best of the knowledge. However this does not preclude attempt to tailor the environmental flow requirement to the instream water quality parameters. An example of such attempt was a master water resource study for Sg Selangor Basin utilized criteria other than hydrologic index technique in estimating the environmental flow requirement (RB/SSP, 2003). One of these techniques was the flow requirement at each tributary to meet the DOE type II or III instream water quality requirement, i.e. BOD in a river water body. Mappings were carried out a priori for each tributary, Landuse and intent of present and future utilization were amongst other criteria were been taken into consideration in tandem. To meet the DOE type II or III requirement, in essence, adequate water must be released from a dam, i.e. Sg. Selangor, in the headwater zone of the basin so that dilution of the BOD could be taken place. Other water quality parameters can also be used to determine the environmental flow requirement. The study did not attempt to estimate the flow requirement based on the fauna and flora needs. This is essentially the dilemma faced by the water resources planners in Malaysia. At best perhaps a compromise could be made. This will therefore enable interim solutions using hydrological index techniques could be used. Further refinement on this issue could be pursued once adequate information and knowledge on the fauna and flora life cycle could be confidently established. 8.8.11 Global Warming and Water Resources Global warming is the raising of average measured air temperature near the earth surface and oceans since the mid-20th century and it is projected to continue and increase above long-term average in the near future. This alarming state of global warming is due primarily to the increase in green house gases, i.e. carbon dioxide (CO2), methane (CH4), NOx, CFC and others in the atmosphere. Increasing of CO2 and other trace airs in atmosphere will alter energy balance of climate system, and cause global warming in the future. It has received much attention in recent year (IPCC, 2008). The presence of these high concentration green house gases in the troposphere, hinder the emission of long-wave radiation processes from the earth to the atmosphere. The heat is therefore trapped within the layer of green house gases (known as green house glasses) and reradiate back to the earth. By doing so, this radiation hear balance raises the air temperature on earth in general.


March 2009 8-31

The impacts of global warming on water resource in particular are not quantitatively studied in Malaysia (NWRS 2000) but in general consensus, the impact could be experienced in various dimensions, such as it is believed that the impending rising in sea level at un precedent rate due to accelerated melting of icebergs and glaciers in both north and south poles. Thus causes erosion and recession of the coastal lines. It also affects the hydrologic cycles, etc.

The impacts of global warming suggest extremities in both climatologic hydrologic events, such as recent more frequent occurrences and exceptionally high severity of tropical storms and monsoon events accompanied with fiery wind gusts.

Potential direct impacts of global warming to water resources are systematically summarized as follows (Pittock, 2003):

a. Reduce or increase inflows to water/hydrologic natural and artificial storages b. Reduce streamflow/discharge in major river basins/catchments c. Reduce or increase in water availability for rain fed agriculture and irrigation d. Reduce recharge of groundwater due to less infiltration from surface waters e. Increase in severity of both droughts and floods f. Increase salinity of surface and ground waters. g. Increase inundation of coastal freshwater wetlands and lowlands, rivers, and estuaries h. Change in weather pattern from the past i. Increase sediment and nutrients in streams

Some of the indirect impacts or consequences are as follows:

a. Threatened water supplies: Possible shortage of water supply to meet the increasing demand by cities and towns, agricultural, industrial, environmental flows. This is primarily due to low flows in the rivers or other water bodies. Relocation of water supply intakes further upstream of the saline-freshwater interface due to saltwater intrusion by sea level rising.

b. Increased risk of euthrophication in water bodies: algal blooms or enrichments and

impairment to the water quality in general due to ineffective dilution. Algal blooms degrade the water quality by excessive growth of oxygen demanding organisms. When algal die off, the biomass exerts oxygen demand for their biodegradation processes and in turn reduce the oxygen concentration of the water bodies. Without oxygen, massive aquatic fauna depth as a result.

c. Probable changes in ecological water requirements: Possible alternations/changes to

ecosystems. Displacement, reduction or loss of vulnerable ecosystems or species might occur due to lower water availability.

d. Increase pressure on water related storages and infrastructure: With the extremity in

fluctuation in river water flows, some existing water related infrastructures such as water supply schemes, flood mitigation projects, coastal protection, etc might not be able sustain the intended design standards. As a result, inadequate raw water source might interrupt the smooth operation of water supply schemes. Frequent flooding events and overtopping of levees and embankments might occur in flood defense projects and thus increasing the risk of flood damages. Coastal erosion and sedimentation might be recurring and frequent episodes.


8-32 March 2009

e. Increased competition for water: Global warming and climate change increase the

competition amongst nations for precious water commodity in many countries and many regions within a country. It is especially vulnerable for competition amongst the countries that are sharing common riparian boundaries. Water resources scarcity spurs competition in dam building for storage in time of need.

REFERENCES [1] Allan, J.D. 1995. Stream Ecology: structure and function of running waters. Chapman and Hall, London. Pp. 388 [2] Brown, A.L. 1987. Freshwater Ecology. Heinimann Educational Books, London. Pp. 163

[3] Carl von Ossietzky Universität Oldenburg, FB Biologie (ICBM), Postfach 2503, D-26111 Oldenburg, Germany. e-mail: [email protected]

[4] Cushing, C.E. and J.D. Allan. 2001. Streams: their ecology and life. Academic Press, San Diego. Pp. 366.

[5] Chehalis River Council, Lewis Conservation District, Washington State Department of Ecology. 2009. Water Quality Monitoring - A How to Guide

[6] Giller, S. and B. Malmqvist. 1998. The Biology of Streams and Rivers. Oxford University Press, Oxford. Pp. 296.

[7] Plotnikoff R.W. and Wiseman C. 2001. Benthic Macroinvertebrate Biological Monitoring Protocols for Rivers and Streams. Washington State Department of Ecology. http://www.ecy.wa.gov/biblio/0103028.html

[8] Rinderhagen M.., Ritterhoff J. and Zauke G.P. Crustaceans as Bioindicators.

http://www.ecy.wa.gov/biblio/0103028.html


March 2009 8A-1

APPENDIX 8A: LIST OF GLOSSARY

Alkalinity Alkalinity is not a pollutant. It is a total measure of the substances in water that

have "acid-neutralizing" ability. AT is a measure of the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. Alkalinity is closely related to the acid neutralizing capacity (ANC) of a solution and ANC is often incorrectly used to refer to alkalinity. Alkalinity is equal to the stoichiometric sum of the bases in solution. In the natural environment carbonate alkalinity tends to make up most of the total alkalinity due to the common occurrence and dissolution of carbonate rocks and presence of carbon dioxide in the atmosphere.

BOD Biochemical Oxygen Demand or Biological Oxygen Demand. Its is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water.

Chloride Chlorine is a greenish-yellow gas that dissolves easily in water. It has a pungent, noxious odor that some people can smell at concentrations above 0.3 parts per million. Because chlorine is an excellent disinfectant, it is commonly added to most drinking water supplies in the US. In parts of the world where chlorine is not added to drinking water, thousands of people die each day from waterborne diseases like typhoid and cholera. Chlorine is also used as a disinfectant in wastewater treatment plants and swimming pools. It is widely used as a bleaching agent in textile factories and paper mills, and it’s an important ingredient in many laundry bleaches. Free chlorine (chlorine gas dissolved in water) is toxic to fish and aquatic organisms, even in very small amounts.

Chlorophyll-a Chlorophyll-a is the green pigment found in plants and algae. Plants and algae use this pigment to trap the energy from the sun so they can grow. Chlorophyll a is the most common of the six types, present in every plant that performs photosynthesis. The reason that there are so many pigments is that each absorbs light more efficiently in a different part of the spectrum. Chlorophyll a absorbs well at a wavelength of about 400-450 nm and at 650-700 nm. Chlorophyll a is measured in micrograms per liter (µg/l) units. Micrograms per liter is micrograms of chlorophyll a per liter of water. In estuaries, chlorophyll a measurements can range from 1 µ g/L to higher than 20 µ g/L. Scientists measure chlorophyll a in the lab by separating the chlorophyll a from the algae in the water.

COD Chemical Oxygen Demand. Its is a test, commonly used to indirectly measure the amount of organic compounds in water. Most applications of COD determine the amount of organic pollutants found in surface water (e.g. lakes and rivers), making COD a useful measure of water quality


8A-2 March 2009

Conductivity Conductivity is a measurement of the ability of an aqueous solution to carry an

electrical current. An ion is an atom of an element that has gained or lost an electron which will create a negative or positive state. For example, sodium chloride (table salt) consists of sodium ions (Na+) and chloride ions (Cl-) held together in a crystal. There are several factors that determine the degree to which water will carry an electrical current. These include the concentration or number of ions, mobility of the ion, oxidation state (valence) and temperature of the water.

DO Dissolved Oxygen. Its refer to the amount of gaseous oxygen (O2) dissolved in an aqueous solution.

Faecal coliform Fecal coliforms (sometimes faecal coliforms) are facultatively-anaerobic, rod-shaped, gram-negative, non-sporulating bacteria. They are capable of growth in the presence of bile salts or similar surface agents, oxidase negative, and produce acid and gas from lactose within 48 hours at 44 ± 0.5ºC. Fecal coliforms include the genera that originate in feces; Escherichia as well as genera that are not of fecal origin; Enterobacter, Klebsiella, and Citrobacter. The assay is intended to be an indicator of fecal contamination, or more specifically E. coli which is an indicator microorganism for other pathogens that may be present in feces.

NH4 Ammonium. It is the ionized form of ammonia, which is occurs when the water is acidic.

Nitrates In inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms (NO3

−). In organic chemistry the esters of nitric acid and various alcohols are called nitrates. Nitrate reactions [NO3-] in fresh water can cause oxygen depletion. Thus, aquatic organisms depending on the supply of oxygen in the stream will die. The major routes of entry of nitrogen into bodies of water are municipal and industrial wastewater, septic tanks, feed lot discharges, animal wastes (including birds and fish) and discharges from car exhausts. Bacteria in water quickly convert nitrites [NO2-] to nitrates [NO3-].

ORP Oxidation Reduction Potential also known as redox potential. It is the tendency of a chemical species to acquire electrons and thereby be reduced. Each species has its own intrinsic reduction potential; the more positive the potential, the greater the species' affinity for electrons and tendency to be reduced.

pH Potential of Hydrogen. It is a measure of the acidity or basicity of a solution.

Phosphate The element phosphorus is necessary for plant and animal growth. Nearly all fertilizers contain phosphates (chemical compounds containing the element, phosphorous). When it rains, varying amounts of phosphates wash from farm soils into nearby waterways. Phosphates stimulate the growth of plankton and water plants that provide food for fish. This may increase the fish population and improve the waterway’s quality of life. If too much phosphate is present, algae and water weeds grow wildly, choke the waterway, and use up large amounts of oxygen. Many fish and aquatic organisms may die.


March 2009 8A-3

Salinity Salinity is a measure of the mass of dissolved salts (ionic constituents) in a given

mass of solution and usually expressed as parts per thousand (ppt). Ions commonly found in water include calcium, magnesium, potassium and sodium cations and bicarbonate, carbonate, chloride, nitrate, and sulfate anions.

Silicates Silicates are those compounds which have a silicon-oxygen anion chemically combined with such metals as aluminum, calcium, magnesium, iron, potassium, sodium and others to form silicate salts. Most silicate salts, with the exception of sodium silicate, are only slightly soluble in water and are widely distributed in nature. Minerals such as asbestos, mica, talc, lava, etc., contain silicates.

Temperature In physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature. Temperature is one of the principal parameters of thermodynamics. For water quality study, to having its own toxic effect, temperature affects the solubility and, in turn, the toxicity of many other parameters. Generally the solubility of solids increases with increasing temperature, while gases tend to be more soluble in cold water. Temperature is a factor in determining allowable limits for other parameters such as ammonia.

Total hardness In water quality, the total hardness is due to the presence of multivalent metal ions which come from minerals dissolved in the water. Hardness is based on the ability of these ions to react with soap to form a precipitate or soap scum. In fresh water the primary ions are calcium and magnesium; however iron and manganese may also contribute. Carbonate hardness is equal to alkalinity but a non-carbonate fraction may include nitrates and chlorides.

TSS Total Suspended Solids. It is one of parameter use for water quality measurement. Usually refers to the identical measurement of the dry-weight of particles trapped by a filter, typically of a specified pore size


8A-4 March 2009


CHAPTER 9 FLOOD FORECASTING AND WARNING SERVICES

Chapter 9 FLOOD FORECASTING AND WARNING SERVICES

March 2009 9-i

Table of Contents


9.1 DEFINITION OF FLOOD .......................................................................................... 9-1

9.2 TYPES OF FLOODS ................................................................................................. 9-1

9.3 FLOOD PREPAREDNESS .......................................................................................... 9-2

9.4 ROLE OF DID IN FLOOD FORECASTING AND WARNING ............................................ 9-4

9.5 TELEMETRY SYSTEMS AND CHOICE OF TELEMETRY SYSTEMS .................................. 9-5

9.6 FLOOD FORECASTING TECHNIQUES ....................................................................... 9-6

9.7 ACCURACY AND TIMELINESS OF FORECAST ............................................................ 9-6

9.8 FLOOD WARNING SYSTEMS .................................................................................... 9-7

9.9 DISSEMINATION OF FORECAST AND WARNING ..................................................... 9-11

REFERENCE ......................................................................................................................... 9-13


9-ii March 2009

List of Figures


9.1 Form for Reporting Current River Level Status (Form C) ..................................... 9-3

9.2 Form P for Reporting Flood Events ................................................................... 9-4

9.3 The river basins .............................................................................................. 9-7

9.4 Locations of Telemetric Flood Monitoring Stations .............................................. 9-8

9.5 An Example of A Flood Warning Board .............................................................. 9-9

9.6 Locations of Flood Warning Siren Stations ....................................................... 9-10

9.7 Processes and Information Flow of DID’s Flood Warning Service ....................... 9-11

9.8 Processes and Information Flow in DID’s Flood Forecasting Service ................... 9-12


March 2009 9-1

9 FLOOD FORECASTING AND WARNING SERVICE

9.1 DEFINITION OF FLOOD An area is said to be flooded if stormwater runoff cause a significant rise in water level above the ground level inflicting damage to properties or crops or disrupting the normal activities in the area. The source of stormwater runoff can be from a catchment upstream or it could be from an adjacent river overspilling its banks or bunds. Not all floods are caused directly by stormwater. It could be caused by a dam-break event or from releases from pond or dam storage. Flood is of concern if it causes substantial property damage or it causes significant disruption to normal activities. Therefore a swampy area or an undeveloped open space experiencing a 0.3 m of rise in water level above the ground may not be viewed as flooding by some and that’s the reason some floods in undeveloped areas are not recorded as flood prone areas. On the other hand the same 0.3 m inundation along a highway would cause havoc to traffic and be given lots of media attention. Damage to properties will occur if flood rises above the floor level of properties and damage increases drastically as flood level rises. Beyond 1.0m flood becomes potential danger to lives. Swift flowing flood waters of 0.6m can carry away most vehicles due to buoyancy of the vehicle in water and momentum of flowing water. Agricultural areas are more tolerant to flooding than urban areas. Most crops especially tree crops (rubber, fruits) can withstand shallow inundation for periods of 48 hours. Oil palms are more resistant to flooding and can stand longer periods of inundation and the loss is often attributed to loss of harvest when access id cut off during a flood. 9.2 TYPES OF FLOODS There are two main forms of flooding, flash flood and river flood. Flash floods usually occur in urban areas. Flooding is usually caused by short, intense localized thunderstorms, the type of storm usually experienced in the evening. Runoff rate and volume from the relatively impermeable urban area is high and time of concentration short and in many urban areas with flash flood problems, the existing drains were designed for previous catchment condition which would be a catchment less densely urbanized with more open spaces, for instance the Klang and Batu Pahat urban areas. Besides flash flood, there is also river flood. When flow in a river exceeds its conveyance capacity, the water in the river rises above its bank level and overspills into adjacent low-lying areas causing river floods. River floods are commonly experienced in Kelantan and Pahang. The monsoon rains that occur in Kelantan and Pahang during the months of November and December are widespread, heavy and prolonged. Continuous raining of three days are common. The stormwater runoff from the vast catchment converges to the main river and when flow exceeds the capacity of the river, it overspills and cause extensive flooding due to the sheer volume of runoff coming from a large catchment. In Malaysia, flash floods are common occurrences and may occur from time to time in flood prone areas throughout the year. In terms of flood extent, flash floods affect smaller areas but because of its tendency to occur in densely urbanized areas, the value of property damaged is high and disruption to traffic and businesses substantial.


9-2 March 2009

However, river floods especially the river floods of Kelantan and Pahang the flood extent is large can extend over 1000 square kilometres. Although the value of property and density of population is lower, the flood damage inflicted can also be high because the area affected is large. The annual average flood damage of the river basins Sg Kelantan and Sg Pahang amounts to RM 93 million and RM 76 million respectively. 9.3 FLOOD PREPAREDNESS Floods cause damage to properties and also endanger lives and preparedness of the community and government agencies to handle an emergency flood situation is important to minimize losses in the event of a flood. Preparing the public for emergency flood response is outside the scope of hydrology although in an emergency flood situation, DID vehicles, boats and personnel are sometimes mobilized to assist in evacuation and related activities. To DID, in particular to the Hydrology Division of DID, flood preparedness would mean preparedness to provide flood warning services and flood forecasting services. For the purpose of flood forecasting, DID has set up a central Flood Forecasting Centre (FFC) at DID Ampang. This centre will be manned 24-hours by teams working on shifts should there be any indication of impending severe floods in the country. There is also an annual meeting of state hydrological officers before the onset of the monsoon months to prepare DID for flood forecasting for rivers in the east coast states of peninsular Malaysia. Flood forecasting relies on real-time rainfall and water level data obtained from DID’s network of telemetric stations. In preparation for floods during the monsoon season, the performance of the telemetric stations will be checked and repairs carried out and forecasting models tested and parameters adjusted if necessary. DID’s standard operating procedures (SOP) in flood disaster management are documented and they are as follows:

Before the flood season • Prepare flood operation check list for district, state and federal DID offices • Ensure that all rivers, main drains, dams, pumps, river and coastal bunds and drainage

facilities under JPS are in good condition. • Ensure that all telemetric systems, river gauges, flood warning stations, communication

equipments and vehicles are in good condition. During the flood season • Monitor flood levels at flood warning stations • Submit flood level status report (see Form C in Figure 9.1) to the respective district and state

DID Flood Operations Centre. • Prepare and submit flood reports (see Form P in Figure 9.2) • Carry out flood forecasting for selected rivers (currently seven rivers are provided with flood

forecasting services) and submits forecasts to the respective district and state DID Flood Operations Centre.

• Provide logistic support (vehicles and boats) for flood disaster rescue operations. • Provide technical input and advice in assessment of flood disaster and flood mitigation.


March 2009 9-3

After Occurrence of Flood • Record flood and related hydrological data • Determine source and causes of flood and recommend mitigation measures • Assess damage to DID infrastructures • Implement rehabilitation works to DID infrastructures

Figure 9.1 Form for Reporting Current River Level Status (Form C)


9-4 March 2009

Figure 9.2 Form P for Reporting Flood Events

9.4 ROLE OF DID IN FLOOD FORECASTING AND WARNING Flash floods occurs intermittently throughout the year and because flash flood usually occurs suddenly and time available for response is short, DID has installed flood warning sirens along identified flood prone areas to provide flood warnings to residents. These sirens trigger when flood level exceeds specified threshold levels. DID also monitors the flood status throughout the country via its network of flood warning stations. The flood warning stations are equipped with water level sensors so that river flood levels can be monitored. To assist in assessment of severity of flood, DID has designated threshold flood levels at all the flood warning stations. The threshold flood levels are: ALERT LEVEL, WARNING LEVEL and DANGER LEVEL colour coded in green, orange and red respectively. ALERT LEVEL is taken as the level whereby, DID officers are alerted of an impending flood and hence officers on standby for flood warning duties are alerted and they have to start monitoring the flood status. WARNING LEVEL is a level whereby the flood situation has deteriorated further and flood is about touching DANGER LEVEL DANGER LEVEL would be the level where lives and properties are now in danger. The flood has reached the level beyond which property gets inundated in water.


March 2009 9-5

Setting these threshold levels depends on the flood behaviour in the area which will vary from one location to another. DANGER LEVEL is the level where substantial property damage starts occurring and as a rough guide the DANGER LEVEL can be taken as the mean annual flood level. The ALERT LEVEL can be taken as the 10 percentile of the annual flood level. (The ALERT LEVEL can be taken as the maximum of the mean annual water level) The WARNING LEVEL is the level exactly between ALERT LEVEL and DANGER LEVEL. Not all these flood warning stations are telemetric stations and readings are sometimes manually read. For large rivers like Sg Kelantan and Sg Pahang, the distance of the upstream catchment to the flood affected areas (eg. Pasir Mas and Kota Bahru in the case of Sg Kelantan) is long and therefore the flood experiences longer lag time. This provides sufficient lead time for forecasting to be effective. Lead time is the time between forecast of a flood and the actual occurrence of the flood and a longer lead time allows the recipient of the forecast to respond. The Hydrology division of DID provides flood forecasting services to these two rivers during the north-east monsoon months of November and December. With the implementation of sophisticated flood mitigation projects such as the SMART flood diversion tunnel project in Kuala Lumpur, DID is now involved in flood forecasting which will provide the basis for operation of projects during a flood. The SMART tunnel operation requires flood forecasting as the tunnel fulfils dual role of vehicular traffic tunnel and flood diversion tunnel and to activate the flood diversion sufficient lead time must be available for the traffic within the tunnel to be cleared. As projects become more sophisticated, flood simulation and forecasting will become an important component in the operation of the projects. It need not be limited to the SMART tunnel project. There may be other projects where operation can be further fine tuned with the incorporation of flood simulation models in decision making. To date, flood forecasting services are provided to 7 river systems namely Sg Kelantan, Sg Golok, Sg Besut, Sg Pahang, Sg Perak, Sg Muda and Sg Batu Pahat. Flood warning service is provided to 39 river and the rivers are equipped with telemetric systems for real-time monitoring of flood levels. 9.5 TELEMETRY SYSTEMS AND CHOICE OF TELEMETRY SYSTEMS

For flood forecasting to be effective, sufficient lead time must be available. Lead time of a flood forecast is the time between the detection or forecast of a flood event and the time of actual occurrence of the event. In fact for large rivers in the world such as the Ganges, the Mississippi and the Rhine, the flood has to travel over large distances therefore can be detected days before it reaches the target area. In a sense flood forecasting for these rivers are easier. Simple stage correlation methods would probably yield quite realistic forecast of floods which can be issued with sufficient lead time for the flood forecast to be meaningful in terms of getting people to evacuate. But, a flash flood such as those experienced in Kuala Lumpur the flood arrives less than 2 hours after the rain and therefore to provide sufficient lead time for the forecast to be meaningful, DID is also looking into the possibility of forecasting rainfall quantitatively (quantitative precipitation forecast or QPF).


9-6 March 2009

Therefore the flood forecasting systems under DID are all equipped with telemetric rainfall and water level stations so that data are collected in real-time. DID’s hydrological stations are equipped with sensors, raingauge for collecting rainfall data and water level sensors for collecting water level data. These data are traditionally read by field technicians when they visit the stations. Data used to be recorded on paper charts but with modern electronics these data are now recorded digitally in memory cards. But readings are not available immediately despite the upgrade to electronic storage systems. For flood forecasting data must be available in real-time and to achieve that sensors are now incorporated with remote terminal unit (RTU). The RTU is essentially a telecommunications unit which gets the data read by the sensor and transmit the data via telemetry to a central data collection station which is usually at the state DID office. Telemetry stands for remote measurement and using telecommunications to transmit the measured data to a master station where the data picked up could be used for flood forecasting for early response. 9.6 FLOOD FORECASTING TECHNIQUES Various flood forecasting techniques have been adopted by DID and they depend on the nature of the river, the flood (flash flood or river flood, long lag time or short lag time, tidal influence or no tidal influence), availability of data and supporting systems such as hydrological instruments and SCADA . The flood forecasting techniques adopted include:

• simple stage correlation method applied to forecast Kota Bahru Flood based on Kuala Krai Flood level.

• black box type of model e.g. the multiple linear regression model which was at one time used for Sg Kelang and the linear transfer function model currently used for Sg Pahang.

• conceptual rainfall runoff models such as the Tank Model currently used for Sg Kelantan • The unit hydrograph based models which is also a form of conceptual rainfall-runoff model

such as the Flash Flood Model which was also at one time applied to Sg Kelang.

9.7 ACCURACY AND TIMELINESS OF FORECAST Obviously for any forecast to be effective it must be accurate and in the case of flood it must also be timely. In terms of accuracy we have to be realistic. Rainfall collected by the network of telemetric rain gauges can never truly record the actual areal rainfall whose spatial variability can be quite high. The spatial variability of rainfall is higher for localized convectional rain compared to the monsoonal rainfall. The real catchment process is quite complex and we are merely trying our best to represent the rainfall runoff process by viewing the process in a simple manner so that the process can be described by equations. In flood forecasting the lead time of forecast is important. The lead time of a flood forecast is the time between the prediction of the occurrence of the flood and the actual occurrence of the flood. At various DID flood warning stations three flood levels have been defined i.e. the Alert Level (Green), the Warning Level(Orange) and the Danger Level(Red). If Danger Level is the threshold level whereby flood occurs then the period between the time when forecast is made and the time the flood level rises to the Danger Level is the lead time. Lead time varies with the catchment characteristics especially the length of the catchment and also with the nature of rainfall. Lead time also depends on the technique of flood forecasting. To give an idea of lead time in DID ‘s flood forecasting service, the lead time of a flood forecast of Sg Kelantan at Kuala Krai is about 12 hours while the lead time of a flood forecast of Sg Kelang at Jambatan Sulaiman is about 1.5 hours.


March 2009 9-7

Obviously the longer the lead time the more useful the forecast as it allows time for action to be taken to save lives and properties. To improve lead time, information on rainfall should be obtained in real-time and this is made possible through the establishment of telemetric stations and SCADA systems. In the case of the SMART Tunnel project where sufficient lead time is critical for its operation, DID is contemplating applying quantitative precipitation forecast (QPF) to further increase the lead time despite the uncertainty over the reliability of QPF. 9.8 FLOOD WARNING SYSTEMS The flood warning systems varies from sophisticated flood forecasting operated by DID, to community self help system such as the flood warning board system to simple flood siren system. Whilst ideally flood forecasting should be a component of a flood warning system, sometimes flood forecasting is not possible due to reasons such as:

• Insufficient lead time. To achieve a reasonable lead time of forecast, real time monitoring of rainfall and water level.

• A suitable flood forecasting model has not been developed yet • Lack of manpower, hardware and software to process the data and expertise to operate the

flood forecasting model.

Flood warning in many areas is achieved using other methods such as Flood Warning Boards (FWB) or flood sirens. Telemetric Flood Monitoring Stations

As of 2008, there are thirty nine (39) river basins are equipped with telemetric flood monitoring stations where the flood levels and rainfalls are monitored in real-time. The river basins are as follows:

Figure 9.3 The River Basins


9-8 March 2009

The breakdown of telemetric stations is: 42 telemetric water level stations, 127 telemetric rainfall stations and 193 combined water level and rainfall stations. The locations of the telemetric stations are as shown in Figure 9.4.

Figure 9.4 Locations of Telemetric Flood Monitoring Stations

Flood Warning Board

DID flood warning board (FWB) systems were established in various river systems. Three rivers where FWBs have been established are Sg Kelantan and Sg Pahang. The obvious similarity of both rivers is they are large rivers with long lag time. Another prerequisite for FWB system is that there must be a reliable stage correlation between flood levels upstream and flood levels downstream at target FWB sites.

In the FWB system, a reference station upstream is identified e.g. Kuala Krai in Sg Kelantan. In analyses of past records it was found that the flood levels at various towns downstream at Pasir Mas and Kota Bahru areas is strongly correlated with the flood levels at the upstream reference station, Kuala Krai. The travel time of the flood is also known.


March 2009 9-9

Real-time data of the flood level at Kuala Krai (which is equipped with a telemetric water level station) is obtain at regular intervals (1-hourly to 6-hourly). When the level at Kuala Krai reaches alert level, the flood levels are broadcasted via the local radio stations so that residents in flood prone areas downstream are constantly informed of the Kuala Krai Levels. Though analyses of past flood records DID has established a reasonably reliable stage correlation of flood levels at Kuala Krai with flood levels at various target flood prone areas and are able to mark on the FWB the various flood levels that would be experienced at the FWB site corresponding to various Kuala Krai flood levels. At the flood prone areas, the FWB are located where the affected people congregates, i.e. near to a local coffee shop. The community affected by the flood is able to determine the expected flood level in their area by noting the flood level at the reference station Kuala Krai and the time of travel of flood is also displayed in the FWB as a guide to residents in planning their evacuation. After the 1983 flood in the east coast states of Peninsular Malaysia there was a review report prepared. The report concluded that FWBs are practical tools for delivering warnings direct to the public and some suggestions to improve the FWBs were made. A typical flood warning board is shown in Figure 9.5. The FWB system continues to be implemented and to date about 138 FWBs have been established.

Figure 9.5 An Example of A Flood Warning Board


9-10 March 2009

Flood Siren A flood siren is located in flood prone areas where due to short forecast lead time or for some other reasons, flood forecasting and FWB systems are not applicable. An example would be an area affected by flash floods. Flash floods are floods which occur suddenly (short lag time) and flood siren is definitely one of the options in providing flood warning. The operation of a flood siren is simple. If the flood level rises to a set threshold flood level then the siren is triggered, thereby warning residents nearby of an impending flood and flood can occur anytime even at night when those asleep can be alerted by the siren. There are variations to the simple flood siren where several threshold levels can be defined and different siren tone assigned to each threshold level. About 300 flood sirens have been set up and they are located in locations prone to flash flooding (see Figure 9.6)

Figure 9.6 Locations of Flood Warning Siren Stations


March 2009 9-11

9.9 DISSEMINATION OF FORECAST AND WARNING For the flood forecast to be useful, it must be disseminated to the intended stakeholders. The stakeholders need not be directly to the affected people and can be the authorities in charge of flood evacuation. The FWB and flood siren system disseminate warnings directly to the affected residents. In DID’s flood forecasting service, the forecast or prognosis of flood forecast is disseminated to Bilik Gerakan, Bahagian Keselamatan Negara and Pusat Kawalan Malaysia (Polis). The flow chart of processes and the various agencies involved in DID’s flood warning service is presented in Figure 9.7. For River Basins with Flood Forecasting Service the processes is slightly different (See Figure 9.8)

Figure 9.7 Processes and Information Flow of DID’s Flood Warning Service


9-12 March 2009

Figure 9.8 Processes and Information Flow in DID’s Flood Forecasting Service


March 2009 9-13

REFERENCE [1] DID “Review of the Flood Warning Board System”, Department of Drainage and Irrigation, Malaysia, 1984.


9-14 March 2009


CHAPTER 10 CATCHMENT MODELLING

Chapter 10 CATCHMENT MODELLING

March 2009 10-i

Table of Contents

Table of Contents .................................................................................................................. 10-i


10.1 OVERVIEW OF MODELLING OF RAINFALL, RUNOFF AND STORM DRAINAGE ................. 10-1

10.2 EVENT BASED AND CONTINUOUS SIMULATION OF RAINFALL-RUNOFF ........................ 10-3

10.3 TANK MODEL ........................................................................................................... 10-4

10.4 TIME SERIES MODELLING ......................................................................................... 10-7

REFERENCES ...................................................................................................................... 10-12


10-ii March 2009

List of Figures


10.1 A Simple Catchment Model 10-1

10.2 Configuration of a Distributed Catchment Model 10-2

10.3 A Basic Tank Model Configured for Sg Kelantan 10-4

10.4 The Tank Configurations for a Sub-basin and fora Channel Reach for the Modified Tank Model 10-5

10.5 Sub-Basins and Channel Reaches of the Modified Tank Modelfor Sg Kelantan and Tabulation of Model Parameters 10-5

10.6 Tank model Simulation Results for the 1992 and 1994 Floods 10-6

10.7 Unit Hydrograph (UH) as a Function that Transform Effective Rainfall to a Direct Runoff Hydrograph (DRH) 10‐8

10.8 A Typical Three-Layer Feed Forward ANN 10-9

10.9 Logsig Transfer Function 10-10


March 2009 10-1

10 CATCHMENT MODELLING

10.1 OVERVIEW OF MODELLING OF RAINFALL, RUNOFF AND STORM DRAINAGE In reality, hydrological processes are complex and attempts have been made to represent the processes in a simplified manner and these simplified representations of a catchment are models of the catchment. In many typical catchment models, the catchment is represented as storage and rainfall falling on the catchment goes into this storage. Water is lost from this storage via evaporation, infiltration and outflows. In a typical application of catchment models, the parameter of interest is usually the outflow or discharge from the catchment. Simple equations are used to compute outflow from this storage. Such models are also called rainfall-runoff models. See Figure 10.1.

R

Q

E

R

Q

I I

E

Model representation of hydrological processes

Catchment and hydrological processes

R: Rainfall E : Evaporation

I: Infiltration Q : Discharge

Figure 10.1 A Simple Catchment Model

With powerful computing tools made possible by computers, a more complex catchment model can be configured. The additional model features that are usually adopted are:

• Further differentiation of storage into: surface storage and groundwater storage. Outflow from surface storage comes out fast whilst outflow from groundwater storage comes out slowly. Rainfall enters surface storage first and water gets into groundwater storage via infiltration. Some models even differentiate groundwater into primary and secondary groundwater storages.

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March 2009 10-3

• SWMM: The EPA Storm Water Management Model (SWMM) is a rainfall-runoff simulation

model used for simulation of runoff quantity and quality from primarily urban areas. The runoff component of SWMM operates on a collection of subcatchment areas that receive precipitation and generate runoff and pollutant loads. The routing portion of SWMM transports this runoff through a system of pipes, channels, storage/treatment devices, pumps, and regulators. SWMM tracks the quantity and quality of runoff generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period comprised of multiple time steps. SWMM was first developed in 1971, and has since undergone several major upgrades since then. The latest version of SWMM 5 was produced by the Water Supply and Water Resources Division of the U.S. Environmental Protection Agency's National Risk Management Research Laboratory in a joint development effort with CDM, Inc. SWMM is also a freeware but there are third party developers packaging SWMM with pre and post processors. The software is used by some consulting firms in stormwater drainage design in Malaysia.

• Hec-1/Hec-HMS: Hec-1, developed by the Hydrologic Engineering Centre of the US Army Corps of Engineers, is a well known and popularly used software for hydrological modelling.. The model is designed to simulate the surface runoff response of a river basin to precipitation by representing the basin as an interconnected system of sub-basin and channel components. Hec-1 is rewritten to run under MS Windows and is now known as Hec-HMS. Hec-HMS is also a freeware and features an impressive user interface and features for very versatile model configuration and file management. Being a freeware with quite impressive and versatile functions, the software commands a large user base with active online user forums. As expected many consultants are using this software in catchment studies. Currently Hec-HMS also has functions to use radar rainfall data and can be linked to GIS software

10.2 EVENT BASED AND CONTINUOUS SIMULATION OF RAINFALL-RUNOFF

Many models used in catchment studies in particular the flood models are event based model. In event based model of flood flow, the groundwater contribution to flow is not significant as the surface runoff far exceeds the groundwater flow. Therefore simple rainfall loss models are often adopted simplifying the rainfall runoff modelling procedure. Loss models such as initial loss followed by constant continuous loss model or runoff coefficient loss model are often applied. There is sometimes a need to implement flow simulation or flow forecasting covering longer time durations covering both flood flows and extended periods of dry weather flow. When flow simulation is extended to the dry weather periods groundwater contribution becomes significant. Evaporation and evapotranspiration becomes significant and would have to be factored in the rainfall-runoff simulation process. Examples of such models are the flood forecasting models of Sg Kelantan and Sg Pahang. For such models continuous soil moisture accounting is applied. In continuous soil moisture accounting, the model keeps track of groundwater storages whereby infiltration from surface storage recharges groundwater and groundwater storage is depleted via both groundwater outflow and evapotranspiration. The Sg Kelantan flood forecasting model uses Sugawara’s Tank Model which has different levels of tanks representing surface and groundwater storages for soil moisture accounting. The Hec-HMS model although applied in many cases as event-based model do have a continuous soil-moisture accounting module which can be selected if required. The obvious reason for not using continuous soil moisture accounting is its complexity. Another area of simulation where soil moisture accounting plays an important role is the long term daily flow simulation carried out for water resources studies.


10-4 March 2009

The Thornwaite and Mather Water Balance Model described in DID’s Water Resources Publication No 6 (WRP6) is an example of a daily continuous soil moisture accounting model. The lumped catchment model of Blackie and Eeles adopted in the HYRROM software of UK’s Institute of Hydrology is another example of a daily runoff simulation model using continuous soil moisture accounting. 10.3 TANK MODEL Modelling for flood forecasting would be considered the most demanding rainfall-runoff modelling effort. The reliability of the model in simulating the catchment runoff can be compared with observed data immediately. One of the models adopted for flood forecasting is Sugawara’s Tank Model. The model was used for real-time forecasting of floods at Guillemard Bridge, Sg Kelantan. Sugawara’s Tank Model can be configured in many ways and one of the model configurations is as shown in Figure 10.3 below. The Sg Kelantan Tank Model comprises 3 tanks. One for the surface flow, a second tank for the interflow and outflows from both tanks is routed through a third tank to simulate river routing process. Rainfall R enters the first tank. First tank storage must rise above H1 before any outflow occurs (conceptually H1 is the interception storage). Water is lost from the first tank via evaporation EV and infiltration INFIL1, storage higher than H1 will result in surface flow Q2 and Q1 if rain so heavy raises the storage to above H2.

Figure 10.3 A Basic Tank Model Configured for Sg Kelantan

by Sugawara (1981)

The second tank receives water from INFIL1 and the second tank storage must be in excess of H3 before interflow Q3 occurs. INFIL2 represents deep percolation. A constant base flow is included. Flows Q1, Q2, Q3 and baseflow feeds into the third tank and the routed flow represents the simulated flow at the catchment outlet and in the case of the Kelantan Flood Forecasting model, the outlet is at Guillemard Bridge.


March 2009 10-5

The parameters initially configured for Sg Kelantan is presented in Figure 10.3 above. The original Tank model configured for Sg Kelantan is a lumped model. Towards the end of 90s, the Kelantan Tank Model was modified. The Tank configuration is as shown in Figure 10.4. This configuration is for a sub-basin and a channel reach. Availability of more powerful and cheaper computers allows DID to configure Sg Kelantan model as distributed model. The Sg Kelantan basin is divided into four sub-basins, B1 to B4 and the sub-basins are linked by channel reaches C1 to C3. A schematic of the Modified Tank Model for Sg Kelantan and the model parameters are presented in Figures 10.5 and 10.6.

Figure 10.4 The Tank Configurations for a Sub-basin and fora Channel Reach for the Modified Tank

Model

Figure 10.5 Sub-Basins and Channel Reaches of the Modified Tank Modelfor Sg Kelantan and Tabulation of Model Parameters


10-6 March 2009

02468

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Figure 10.6 Tank model Simulation Results for the 1992 and 1994 Floods


March 2009 10-7

10.4 TIME SERIES MODELLING

Whilst hydrological models described thus far are models which attempt to simulate the physical hydrological processes such as the various storages and flows in the hydrological cycle, there is another way to generate runoff data based purely on statistics of historical data. Such analyses are also called time-series analyses. A hydrological time series is a set of hydrological observations such as rainfall, evaporation and river discharges that are arranged chronologically. Ideally, long series of data can be derived from actual observations but this is usually not the case and studies are sometimes carried out using synthetic time series in particular rainfall and streamflow series. Synthetic flow series are generally adopted in many studies for reservoir sizing, for determining the reliability of water supply and for reservoir operation studies. Natural hydrological processes are either stochastic or combination of deterministic and stochastic processes. Inflow and rainfall processes often exhibits marked seasonal variability, superposed with random deviations from the seasonal variation. Therefore, in a typical time-series analysis, the historical time series data is taken and attempt is made to break down the data into the deterministic component and the stochastic component. The deterministic component of a monthly flow for instance would be the seasonal cycle of monthly flows. Such models do not place emphasis of the physical processes. The catchment is considered a system that transform the system input (rainfall) to yield system output (runoff) and there is no necessity to understand the physical processes that transform rainfall to runoff and therefore such models are also known as “black box” models. The transformation function (the mathematical or statistical function that transforms the rainfall input to runoff output) is determined using mathematical and statistical calibration. Examples of such models are:

o Simple and multiple regression, o transfer functions, o neural networks and o stochastic models

• Regression Model The most simple relationship between two variables (e.g. yearly runoff, Y versus yearly rainfall, X) is a linear regression equation of the form:

Y = a + b X (10.1) where a, b are equation constants or regression coefficients. Multiple regression is a straightforward extension of simple linear regression. It is used for correlating one variable with many variables, for example yearly runoff, Y as a function of yearly rainfall, X1 and yearly evaporation, X2.

Y = a + b X1 + c X2 (10.2)

where a, b and c are equation constants or regression coefficients. Regression equations that have higher degree polynomials.

Y = a + b X13+ c X2 (10.3)


10-8 March 2009

The regression coefficients are usually chosen to minimize the sum of squares of error e

Minimize Σ e2 (10.4)

Where e = difference between observed Y, Yi and predicted values of Y, Yi’ Regression methods are conceptually simple and have been applied to infill data series. Although attempts have been made to improve the reliability and accuracy of hydrological data collection, there are still data gaps (periods of missing data) in DID’s hydrological records. Regression methods are often be used to infill the missing records, the missing monthly and yearly rainfall or discharge data.

• Transfer or Transformation Functions Transfer or transformation function seeks to convert rainfall series to runoff series. In many applications, it is the runoff data that is of main interest. Runoff data is more difficult to collect and more difficult to extrapolate to the point of interest. In DID, rainfall records extends to the 1940s while streamflow records began during the 1960s. In terms of numbers, there are more rainfall stations than streamflow stations. One way of generating runoff data from rainfall data (apart from the other conceptual models such as the Tank Model and Lumped Catchment Model described in previous sections) is the unit hydrograph method. The unit hydrograph method is an application of the convolution integral procedure and is an example of a black-box transfer or transformation function (see Figure 10.7 Details of unit hydrograph method is described in section 5.2.3).

Transfer or

transformation function

Effective rainfall

hyetograph

Direct runoff

Figure 10.7 Unit Hydrograph (UH) as a Function that Transform Effective Rainfall to a Direct Runoff

Hydrograph (DRH)

Unit

hydrograph


March 2009 10-9

• Artificial Neural Network (ANN)

Artificial Neural Networks (ANNs) are basically computing systems similar to biological neural networks. They are characterized by three components:

• Nodes • Weights (connection strength) • An activation (transfer) function

In an ANN, there are layers and at each layer there are nodes. A commonly used ANN is the three-layer feed-forward ANN. A typical three-layer feed-forward ANN, consists of a layer of input nodes, a single layer of hidden nodes, and a layer of output nodes, as shown in Figure 10.8. In the figure, i, j, k denote nodes inner layer, hidden layer and output layer, respectively. w is the weight of the nodes. Subscripts specify the connections between the nodes. For example, wij is the weight between nodes i and j. The term "feed-forward" means that a node connection only exists from a node in the input layer to other nodes in the hidden layer or from a node in the hidden layer to nodes in the output layer; and the nodes within a layer are not interconnected to each other. i

j

k

wij

wjk

Input Layer Hidden Layer Output Layer Figure 10.8 A Typical Three-Layer Feed Forward ANN

Each node in the input layer receives an input variable and passes it to the nodes in the hidden layer. In addition, a bias node, which is also a weight with a fixed input, 1.0, is usually added to the input layer and to the hidden layer. The nodes in the hidden layer and in the output layer are nonlinear nodes meaning the weights multiplied by inputs.

Activation function determines the response of a node to the total input it receives. The most commonly used sigmoid function given as,

y=f x =1

1+ exp (-x)=logsig(x)

(10.5)

Sigmoid functions are used to bound the outputs of the weighted sum of all the incoming inputs x. Whatever the output of x becomes, the result will be limited to [0, 1] interval by sigmoid function in a nonlinear manner. Since, it is easy to take derivative of sigmoid function; it is more popular than any other functions. A log sig transfer functions is given Figure 10.9.


10-10 March 2009

1

y

x‐1

0

Figure 10. 9 Logsig Transfer Function

The process of fitting the network to the experimental data is called training. It consists of adjusting the weight associated with each connection (synapse) between neurons. Training and testing concept is similar to the idea of calibration, an integral part of most hydraulic modelling studies. The available data set is generally grouped into two parts, one for training and the other for testing.

The purpose of training is to determine the set of connection weights that cause the ANN to estimate outputs within the given tolerance limits to target values. The data set reserved for training is used for this purpose. This grouping of the complete data to be employed for training should contain sufficient patterns so that the network can learn the underlying relationship between input and output variables adequately. That is why the training part generally consists of most of the data available. In the literature, there is no specific rule while grouping total data into training and test divisions. It became standard for some years to train artificial neural networks by a method called Backpropagation. Backpropagation models, in a feedforward architecture, contain three components. They are an input layer, an output layer and at least one hidden layer. In backpropagation algorithm there are two main steps. The first step is a forward pass, which is also called as activation phase. In that step, inputs are processed to reach the output layer through the network. After the error is computed, a second step starts backward through the network, which is also called as error backpropagation. During the training phase, an error value, usually mean square error (MSE) is calculated between the desired output and the actual output. The MSE is then propagated backwards to the input layer and the connection weights between the layers are readjusted. After the weights have been adjusted and the hidden layer nodes have generated an output result, the error value is again re-determined. If the error has not reached, which is usually defined by a particular iteration number, the error will then again be propagated backwards to the input layer. This procedure continues until the model has finally reached to the predetermined tolerance limit. The weights in backpropagation algorithm are adjusted according to the direction in which the performance function, in this study MSE, decreases rapidly. Although the function decreases most rapidly along the steepest descent direction (negative of the gradient), it may not produce the fastest convergence. A search is performed along conjugate directions, which produces generally faster convergence than steepest descent directions


March 2009 10-11

The number of input, output, and hidden layer nodes depend upon the problem being studied. If the number of nodes in the hidden layer is small, the network may not have sufficient degrees of freedom to learn the process correctly. If the number is too high, the training will take a long time and the network may sometimes overfit the data. • Stochastic Models Stochastic models are used in hydrology for time series data generation often for water resources simulation studies. The performance of a proposed water resources system, often involving a combination of direct river extraction, dam storage and perhaps diversions from another catchment is assessed by simulating the flows and storages in the proposed system using long term hydrological time series. However, in many cases, the observed records are short. In Malaysia, many areas do have about 20 years of streamflow records available. Based on statistics of available time series records which are often too short or fragmented to be useful, stochastic models are applied to generate longer hydrological time series with statistics that preserved the statistics of the available time series. There are a number of commonly used stochastic models such as the autoregressive (AR) and the Periodic Autoregressive (PAR) models The objective of stochastic models is to generate hydrological time series, which preserve the statistical properties of the original data at more than one time level (typically annual and seasonal). For instance, generated monthly inflows must reproduce the basic statistics (e.g., mean, standard deviation and skewness coefficient) of observed monthly flow data. Further, they should represent adequately the statistics of the annual historical series. The autoregressive model assumes that there is persistence in successive hydrological values e.g. rainy days occurs in a stretch or dry weather persists for many days. Therefore a simple approach to predicting river flow is to correlate it with flows 1 day before (lagged by 1 day). An example of a simple first order AR model for predicting flow xt is as follows:

xt = xm + ρk(xt-k – xm) + e (10.6)

Where xm = mean x: xt-k = the k time step lag time Q: ρk = the lag k serial correlation coefficient and e = a random error term

The serial correlation coefficient is given by:

ρεt,εt-1=

∑ εt,εt-1

∑ ε12 ∑ ε

t-12 (10.7)

Where εt = (xt – xm) and εt-k = (xt-k – xm)

where k = 0, 1, 2, … is the time lag. The zero lag coefficient r0 is equal to one, and higher lag coefficients generally damp towards small values with increasing lag. The autocorrelation coefficients can be plotted versus lag in a plot known as a correlogram. Besides flows and rainfall, the AR model is also used to model residual errors in flood forecasts. For example, adjustments to flood forecasts made using the DID’s Tank Model for Sg Kelantan is adjusted using the AR Model of the residual errors.


10-12 March 2009

The Thomas-Fiering model has been widely used for data generation and forecasting of hydrologic variables. Thomas and Fiering’s (1962) early model and its periodic autoregressive (PAR) and moving average (MA) extensions generate monthly or seasonal flow directly. An example of a Thomas-Fiering Model for predicting inflow qt to the reservoirs is as follows:

qt=μt+ρt-1,tσtσt-1

(qt-1-μt-1)+ξtσt 1-ρt-1,t2 (10.8)

where ρ, μ and σ are the estimated lag-one autocorrelation, mean and standard deviation associated with the inflows to the reservoirs. REFERENCES [1] Curtis, D.C. and Burnash, R.J.C., “Inadvertent rain gauge inconsistencies and their effect on hydrologic analysis”, 1996 California-Nevada ALERT Users Group Conference, Ventura, CA, May 15-17, 1996.

CHAPTER 11 SAFETY CONSIDERATIONS

Chapter 11 SAFETY CONSIDERATIONS

March 2009 11-i

Table of Contents

Table of Contents ................................................................................................................. 11-i

List of Figures ...................................................................................................................... 11-ii

11.1 OCCUPATIONAL SAFETY AND HEALTH ACT (OSHA) REQUIREMENTS 1994 ................... 11-1

11.2 GENERAL PRACTICES .......................................................................................... 11-3

11.2.1 Training for New Employees ......................................................................... 11-3

11.2.2 Communication ............................................................................................ 11-3

11.2.3 Preparing for the Field .................................................................................. 11-4

11.2.4 Transportation ............................................................................................. 11-4

11.2.5 In the Field .................................................................................................. 11-6

11.2.6 Equipment ................................................................................................. 11-10

11.2.7 Maintenance and Storage ........................................................................... 11-10

11.2.8 Safety Inspection ....................................................................................... 11-11

11.3 SPECIFIC PROCEDURES AND SAFETY ISSUES ........................................................... 11-11

11.3.1 Cableway Measurements ............................................................................ 11-11

11.3.2 Bridge Measurement .................................................................................. 11-12

11.3.3 Wading ..................................................................................................... 11-13

11.3.4 Moving boat .............................................................................................. 11-14

11.3.5 Helicopters ................................................................................................ 11-15

11.3.6 Safety Procedures When Working In Confined Space and Stilling Well ............ 11-19

11.3.7 Safety Procedures When Working At Height ................................................. 11-21

REFERENCE ....................................................................................................................... 11-22


11-ii March 2009

List of Figures

Figure Description Page 11.1 Using orange pylons for warning approaching motorists. 11-6

11.2 Examples of Poisonous and Thorny plants 11-9

11.3 Landing Area Dimensions 11-16

11.4 Approach helicopter between 10 and 2 o’clock zone 11-17

11.5 Approach and leave by the down slope side for rotor clearance 11-18

11.6 Example of the stilling well 11-19


March 2009 11-1

11.0 SAFETY CONSIDERATIONS

11.1 OCCUPATIONAL SAFETY AND HEALTH ACT (OSHA) REQUIREMENTS 1994

In certain works, the DID employee are required to undertake field works which may be hazardous such as making flood measurements. It is important that the safety aspects of the work procedure be implemented and adhered with, to avoid or minimize accidents from happening. OSHA (The Occupational Safety And Health Act) have been formulated to:-

• assist in the coaching and stimulation of the operating units’ safety and health objectives

• design, develop and implement the safety and health programs for the safety officer.

• manage and ensure all environment and OSHA legal requirement is in compliance.

The Department of Occupational Safety and Health (DOSH), under the Ministry of Human Resources, has been assigned the responsibility of administrating and enforcing legislation related to occupational safety and health (OSH) to ensure that safety, health and welfare of people at work as well as others are protected from hazards resulting from occupational activities.

The department carries out enforcement activities on industries governed by three legislations which are Occupational Safety and Health Act (OSHA) 1994, Factories and Machinery Act 1967; and Petroleum Act (Safety Measure) 1984.

To achieve a good record for occupational safety and health management at the workplace, Safety and health program audit is used as a tool for benchmarking a firm's safety and health efforts against accepted standards which are outlined in the MS 1722 : Part 1 : 2005 OSH MS ( Occupational Safety and Health Management Systems – Requirements ). This standard was developed by Department of Standards Malaysia and with other agencies collaboration. It provides a means of measuring both documentation and implementation of the safety and health program.

The Occupational Safety and Health Act (OSHA) 1994 provide the legislative framework to promote, stimulate and encourage high standards of safety and health at work. The aim is to promote safety and health awareness, and establish effective safety organisation and performance through self-regulation schemes designed to suit the particular industry or organisation. The long-term goal of the Act is to create a healthy and safe working culture among all Malaysian employees and employers. The Department of Occupational Safety and Health (DOSH) is responsible for enforcing compliance with OSHA 1994. DOSH also enforces compliance with the Factories and Machinery Act 1967.

OSHA 1994 defines the general duties of employers, employees, the self-employed, designers, manufacturers, importers and suppliers of plant or substances. Although these duties are of a general character, they carry a wide ranging set of responsibilities. The Act provides a comprehensive and integrated system of law to deal with the safety and health of virtually all people at work and the protection of the public where they may be affected by the activities of people at work.

The general duties of employers, employees, the self-employed, designers, manufacturers, importers and suppliers of plant or substances are clearly defined under OSHA 1994. Employers must safeguard so far as is practicable, the health, safety and welfare of the people who work for them. This applies in particular to the provision and maintenance of a safe plant and system of work.

Arrangements must also be made to ensure safety and health in the use, handling, storage and transport of plant and substances. Under OSHA 1994, 'plant' includes any machinery, equipment,


11-2 March 2009

appliance, tool and component, whilst 'substance' means any natural or artificial substance whether in solid, liquid, gas, vapor or combination thereof, form.

Risks to health from the use, storage or transportation of substances must be minimised. To meet these aims, all practicable precautions must be taken in the proper use and handling of any substance likely to cause a risk to health. It is the duty of employers to provide the necessary information, instruction, training and supervision in safe practices, including information on the legal requirements. Employers need to consider the specific training needs of their organisations with particular reference to processes with special hazards.

An employer employing 40 or more persons must establish a safety and health committee at the workplace. The committee's main function is to keep under review the measures taken to ensure the safety and health of persons at the workplace and investigate any related matters arising. An employer must notify the nearest occupational safety and health office of any accident, dangerous occurrence, occupational poisoning or disease which has occurred or is likely to occur at the workplace.

Some operation, installation, maintenance and dismantling of equipment and process need competent persons. Thus, during the installation of machinery and equipment such as cranes, lifts and local exhaust ventilation systems, competent persons are compulsory to ensure safe erection, whilst a boilerman and a steam engineer are required to operate high risk equipment such as boilers. Processes that use hazardous chemicals require competent persons to conduct the air quality and personal monitoring, and a safety and health officer and an occupational health doctor are required to ensure the proper surveillance of the workplace.

There are seven regulations under OSHA 1994 that enforced by DOSH. They are:

a. Employers' Safety and Health General Policy Statements (Exception) Regulations, 1995 b. Control of Industrial Major Accident Hazards Regulations, 1996

c. Classification, Packaging and Labelling of Hazardous Chemicals Regulations, 1997

d. Safety and Health Committee Regulations, 1996

e. Safety and Health Officer Regulations, 1997

f. Use and Standards of Exposure of Chemicals Hazardous to Health Regulations, 2000

g. Notification of Accident, Dangerous Occurrence, Occupational Poisoning and Occupational Disease Regulations, 2004

Contravention of some of the requirements can lead to prosecution in court. A person who fails to comply with an improvement or prohibition notice that is served on him is liable to prosecution, with a maximum fine of RM 50,000 or imprisonment for a term not exceeding 5 years, or both.

The objective of the Factories and Machinery Act (FMA) 1967, on the other hand, is to provide for the control of factories on matters relating to the safety, health and welfare of persons, and the registration and inspection of machinery. Some high risk machinery such as boilers, unfired pressure vessels, passenger lifts and other lifting equipment such as mobile cranes, tower cranes, passenger hoists, overhead traveling cranes and gondolas, must be certified and inspected by DOSH. All factories and general machinery must be registered with DOSH before they can be installed and operated in Malaysia.


March 2009 11-3

11.2 GENERAL PRACTICES This section discusses on policies, procedures, and safety issues that are common to most of the field activities or assignments. It includes training and responsibilities of supervisors, maintaining adequate communication, field preparations, transportation, field situations, operation and maintenance of tools, and safety inspections. 11.2.1 Training for New Employees Training includes a combination of formal classroom training, reading assignments, discussions with the supervisor, and on-the-job training. It will also include initial training and refresher courses on defensive driving, first-aid and cardiopulmonary resuscitation (CPR), boating and water safety, and swimming, if employee is a non swimmer. The employee's immediate supervisor has the primary responsibility of ensuring that adequate training is provided. Training begins with discussions between the supervisor and the employee on the mission of the Division, activities assigned to individual offices, and what is expected of the employee. The supervisor needs to assess the employee's capabilities and maturity, determine his or her previous safety record, determine the type of training and previous field experience, and discuss a specific training plan for the individual. The employee also will need to accurately state his or her previous experience and training, ask questions about the nature of potential assignments, and be confident that the instructions and training will be adequate for the employee to begin field activities. 11.2.2 Communication Communication is a two-way affair: it includes instructions and questions and answers if clarification is needed, it includes letting people know where you are going and why and checking to make sure of safe returns, and it includes contacting others to explain the plans and receive permission to enter on or use the property of others. Making assignments and conveying instructions for accomplishing assigned tasks is the responsibility of the supervisor; making sure the assignments and instructions are understood is the responsibility of both the supervisor and the employee and this constitutes an effective two-way communication. In addition to specific instructions, the supervisor needs to explain the purpose of the assignment and how it relates to the overall mission. Employees must be made aware of potential risks, how to avoid them, and how to get help if an accident does occur. Supervisors need to know where employees are going and when they will return to provide information to searchers and rescuers in case the employees do not return from an assignment. Special communication equipments, such as two-way radios or cellular telephones, are necessary if assignments are in remote locations. Remember to think, communicate, and consider the following procedures to ensure your safety:

• Prepare an itinerary and discuss it with your supervisor, including a list of field sites, a route-of-travel log, motel reservations, and a schedule for "check in" telephone calls to your home or office.

• If going to a remote area, use mobile or portable radios or a cellular telephone if coverage is adequate.

• Always have fully charged batteries and an extra set of batteries.


11-4 March 2009

• Whether using telephones or radios, keep to your pre-arranged check in schedule and

provide update information or adjustments to your itinerary. Another important aspect of communication is conveying plans for hydrologic data-collection activities to co-operators and landowners. Many field activities are conducted on land or water that is in private ownership or under the jurisdiction of local, state, or federal agencies. Before starting an assignment, contact the appropriate agencies or individuals. Discuss plans, and receive permission. Give them a schedule of planned activities and keep them posted on progress. Time spent talking with a public official or landowner may help avoid problems or gain their cooperation for activities in the future. We can do our job only if we have access to collect the needed data. 11.2.3 Preparing for the Field Adequate preparations before beginning a field assignment will help ensure that you complete assignments, avoid accidents, and return safely. Prepare yourself, plan specific activities and tasks, and check both your personal protection gear and tools and instruments needed for the job. Adequate planning for field assignments includes: • Discussing the assignment thoroughly with your supervisor to ensure that you understand

what is expected, why it needs to be done, and how it should be accomplished.

• Know where the nearest emergency medical facilities are located; make plans on how to contact these facilities if you are alone and severely injured (cellular phone).

• Reviewing maps, property descriptions, and notes made by yourself and others on previous

visits to ensure that you are aware of site conditions and potential hazards that may exist. • Reviewing technical manuals, memoranda, previous field inspection notes, and safety

procedures. • Preparing a schedule for completing individual tasks and making a list of required

instruments, tools, and supplies. • Contacting landowners, public officials and relevant authority such as Department of Forestry

and Military to inform them of your plans and obtain permission for access. • Checking weather forecasts. • Wear appropriate attire and protective safety gears for the assignment. A list of required tools, instruments, and supplies should be made when planning your trip, and these items should be checked to ensure they are in good operating condition before you start your trip. Bring along a first aid kit, flashlight, compass, personal floatation device (PFD), matches, cellular phone, insect repellant, suntan lotion, drinking water, machete (parang), and a triangular-shaped reflector device. 11.2.4 Transportation The common means of transportation for hydrologists and hydrologic technicians involved in field activities are passenger cars, vans, light trucks, and small boats. The first step in safe vehicle operation is an inspection of a vehicle before beginning a trip.


March 2009 11-5

This inspection will include • Checking for adequate fuel supply, engine oil, and coolant • Testing of headlights, tail lights, and turn signals.

• Checking all belts and hoses.

• Checking windshield washers and wipers.

• Testing brakes and checking tires.

• Checking to be sure that all equipment is properly stored and secured. Any piece of equipment or tool could become a flying projectile during a sudden stop or accident.

• Checking and adjusting rear-view mirrors.

• Ensuring vehicle is equipped with adequate safety and emergency equipment.

Another step required for a safe trip in your vehicle is a pre-planned schedule and route to save time and avoid known road hazards. Plan your trip to allow for a mix of driving and rest or other activity. For long trips, plan on rotating drivers. Also, plan your route to avoid potentially dangerous situations, such as having to:

• Obey the speed limit and traffic signs.

• Always drive defensively.

• Wear safe seat belts.

• Do not ford a stream or flooded road section without first checking for deep holes or washouts.

• Test brakes after fording a stream or any deep water; they probably will be wet and not as responsive as normal.

• Be aware of weather conditions. A dry road or track going into a field site may become wet and impassable coming out after changes in weather conditions.

• Be especially cautious on logging and mining roads. Logging trucks have the right-of-way on a logging road.

• Be especially aware of wild animals and open-range livestock while driving at night.

• Drive only on existing, well-established back roads or trails. Avoid driving on agricultural fields, open pasture or other unsafe area.

• Always attempt to park your vehicle completely off the road surface and shoulder. If this is not possible, use orange pylons, traffic cones, reflectors emergency flashers, etc. to warn approaching motorists as shown in fig. 11.1.


11-6 March 2009

Figure 11.1 Using orange pylons for warning approaching motorists.

11.2.5 In the Field

In the field, you will be subjected to many natural and manmade dangers. These include varying weather conditions, rough terrain, animals, and manmade structures such as flood control and drainage structures, overhead power lines, cableways, stream gage houses and stilling-wells hazards.

a) Thunderstorms

Thunderstorms are a serious danger while working in the field. Lightning is the storm's worst killer. While working in the field, keep an eye on the weather and notice whether towering cumulus clouds that mark the location of thunderstorms are approaching your work area. The safest place to be during a thunderstorm is in the field vehicle with doors and windows closed. General guidelines for avoiding dangerous conditions during thunderstorms are

• Stop making wading, bridge, or cableway measurements and seek shelter in a structure or your vehicle

• Do not use the telephone or work on electrical lines or steel structures, such as bridges or cableways, because a lightning strike some distance away could affect you

• Avoid isolated trees; seek shelter in dense stands or clumps of young trees

• Sit on your feet in a crouched position or sit on some insulating material, such as wood, rubberized material, or plastic sheet.

• Be aware of flash floods, avoid stream crossings, and move vehicle and equipment to higher ground.

b) Terrain

Working in and around streams and rivers will subject you to many conditions in which the local terrain may cause slips and falls that could result in serious injury to you and your coworkers. You can’t avoid all potential dangers, but you can minimize risk of accidents by considering the followings:

• Wear shoes or boots that provide good arch and ankle support instead of low-cut sneakers


March 2009 11-7

• Inspect the area before beginning work and locate gopher, muskrat, or other holes; isolated

rocks and boulders; fallen logs; loose and slippery rocks; and other obstacles

• Select a level work area and remove debris that could cause you to trip and fall. It may be necessary to cut tall vegetation to be sure the site is free of obstacles. Carry a machete in a sheath to cut vegetation instead of plowing through dense vegetation or taking another route over treacherous terrain to avoid dense vegetation

• If you must travel some distance from your vehicle, avoid taking shortcuts across treacherous terrain; consider the time lost by an accident than by going the longer and safer way around.

• Avoid steep slopes with loose rocks and boulders that could come loose and tumble down the slope.

• Select sites for wading discharge measurements very carefully; keep in mind both the hydraulic characteristics required and safe conditions for accessing and wading the stream.

• Traverse streams carefully and use a wading rod or stout stick to probe the bottom in advance. If you find deep holes or a highly irregular bottom, look for another section.

• Be careful while walking on rocks and boulders in streams; they are usually very slippery. Consider wearing some type of sole gripper to give you additional traction.

• Wear a life jacket when working in and around streams, rivers, and lakes.

c) Animals

There are numerous animals that may represent a risk. Snakes and insects are probably the most common that you will face, but other reptiles and domestic and wild animals can cause serious injury, illnesses, and fatalities. The most effective defense against snakes is to avoid being bitten. The following precautions should be taken to minimize the risk of snakes while working in the field:

• Familiarize yourself with the description and habits of all poisonous snakes indigenous to the work area.

• Wear protective clothing, including boots and knee-high or full-length leggings in prime snake terrain.

• Be observant and look before you step or reach for something.

• Use existing trails and use a walking stick to clear vegetation ahead of you. Don't step over logs without looking on the other side first.

• Don't climb among rocks where you have to reach above your level of sight for a handhold.

• Don't pick up rocks or other objects that might conceal a snake. Turn the object toward you with a stick or shovel. This could shield you from being bitten.


11-8 March 2009

Insects generally are a nuisance while working in the field, but they can be dangerous depending on the type of insect and your reaction to their sting or bite. Stinging insects, which include honey bees, killer bees, wasps, yellow jackets, hornets, and ants, are painful and can be dangerous to individuals that are allergic to the venom. The yellow jacket and hornet are the most dangerous because they are aggressive and can inflict multiple stings. You may be allergic to venom or you may develop an allergy with each new attack.

Reactions can range from fever, light-headedness, hives, and painful swelling to a sudden drop in blood pressure and breathing difficulties. Biting insects, such as mosquitoes, chiggers, ticks, and various flies, are generally of less immediate hazard than stinging insects, but they may be carriers of disease.

Common precautions against insect bites and stings are:

• Wear clothing that makes access to the skin difficult.

• Avoid tramping through heavy vegetation if another choice is available.

• Be observant of wasp nests, hornet hives, and ant hills.

• Use insect repellants on exposed skin and at openings in your clothing. Spraying pants cuffs and socks is a good preventative for chigger bites.

• In the case of ticks, inspect clothing and exposed skin periodically during the day, and disrobe completely and inspect your skin at the end of the day.

• Obtain immunization in advance if you are allergic to stings or will be working in an area with infestations of disease carrying insects.

• Carry appropriate medication for allergic reactions and inform your coworkers about how to administer it.

• Watch out for characteristic dense webs of spiders.

• Avoid reaching into dark places where you can't see and be careful picking up rocks and clothing that have been lying on the ground.

Other animals may be a dangerous depending on whether you surprise them or represent a threat to their young, food, or territory. Most wild animals will be frightened away at sight, but the more domestic they are and the more familiar they are with humans, the less likely they will run from you. Because of this, dogs probably represent the greatest threat while in the field.

Significant animal threats also come from wild boars, buffaloes, bull and some domestic livestock. These animals may actually chase people, so don’t challenge them. Expect animals to defend their territory. The following guidelines are recommended to avoid animal attacks while in the field.

• Avoid surprising animals by making noise while traversing a trail or open country.

• Choose open terrain with good visibility.

• Make a wide detour around any animals with young, or over a fresh kill.

• Avoid walking in pastures or fields with domestic bulls.


March 2009 11-9

• Avoid any animals acting abnormally. Many smaller mammals, including the fox, monkeys,

and squirrel may bite and transmit rabies

• Carry a walking stick to fend off attacks from domestic dogs.

• Watch out for rodent nests. New rodent nests must be noted for future removal.

d) Poisonous And Thorny Plants

The most common problem with poisonous plants is the allergic reaction that individuals have to their sap such as Rengas tree, Poison Ivy, Sesudu (Euphorbia Lactea) and wild yam. The sap can cause an allergic skin reaction of varying intensity depending on the amount of contact and the degree of susceptibility of the individual. The sap can be transferred directly by brushing against or handling the plants and indirectly from tools or clothing and from smoke of burning plants.

The jelatang plant (oxicodendron radicans - syn. Rhus toxicodendron, Rhus radicans) must be avoided because a slight brush with its ordinary looking leaves will give you an extraordinary itch and burning sensation for the next few days.

Thorny plants such as the touch me not and the rattan plant have sharp needle like thorns that could inflict pain if being pricked. Some example are shown in figure 11.2

Rengas tree jelatang plant

Poison Ivy Poison Ivy

Figure 11.2 Examples of Poisonous and Thorny plants


11-10 March 2009

Avoid allergic reactions to poisonous plants by:

• Learning to identify poisonous plants and avoiding contact

• Wear gloves and protective clothing when contacting plants cannot be avoided.

• Remove contaminated clothing as soon as possible and wash immediately to avoid contact by other individuals.

• Wash affected skin with abundant soap and water.

11.2.6 Equipment

Each hydrologist and hydrologic technician usually is equipped with and expected to handle a wide variety of tools and instruments. This equipment represents a potential risk to the user if it is improperly maintained and mishandled.

A wide variety of instruments in our assigned field tasks ranging from small hand-held levels to sophisticated electromagnetic data loggers and data storage devices are used by hydrological field personnel. While the use of these instruments usually do not represent a risk, they are expensive and irreplaceable in some cases, and they should be secured and protected from damage or theft. Instruments and other sensitive equipment should be stored and transported in protective cases to avoid damage. Levels, pH meters, and other instruments should not be routinely stored in vehicles—they should be stored in an assigned and locked storage area in the office or warehouse. Storage of instruments in cabinets in vehicles provides additional protection from being damaged and also keeps them out of sight of would-be thieves. Locking your vehicle at all times will provide security for the valuable equipment assigned to you.

List of safety equipment, walkie talkie, life jacket, safety harness, proper signboard and cones, swimming lessons.

11.2.7 Maintenance and Storage

One of the activities that is least adhered to because of other pressing tasks is the proper maintenance and storage of equipment and instruments. Many times personnel are rushed when returning from a field trip and do not take time to clean, service, and return equipment to the appropriate storage area. Often, work is postponed until the next day, but many times it never gets done, and then it may become someone else's problem. This can lead to hard feelings between co-workers, safety risks, and inefficiency. Some actions that may eliminate some of these problems are:

• Prepare an itinerary that includes time for cleaning and resupplying vehicle and cleaning and servicing equipment after the field trip.

• Schedule time during the trip to clean and service equipment after use each day.

• Note any equipment problems on field notes or personal diary as a reminder to get the equipment repaired upon return from the field.

• Inform supervisor or the person responsible for field equipment of the problems or personally insure that the equipment is repaired. At the very least, attach a note to the equipment indicating the problems encountered while using the equipment.


March 2009 11-11

• Maintain a log for each piece of equipment which includes instructions and a schedule for

servicing.

• Provide an assigned storage place for each of the equipment. Return them to the storage area rather than leaving it in vehicles, loading dock, hallways, or office space.

11.2.8 Safety Inspection

The inspection of all operations, equipment and facilities is a continuous part of each employee's responsibility. The identification of hazards require the routine review of facilities, equipment, and operations by every employee as part of the daily work routine. To maintain a safe work environment, a formal safety inspection of all facilities, equipment, and operations must be made each year to identify potential hazardous or substandard conditions. The inspections must be made by qualified personnel who are knowledgeable of the appropriate safety standards and procedures.

Any condition identified as not meeting established standards or creating a risk to the safety or health of the employee, or other employees, or to the public must be reported and brought to the attention of the responsible supervisor for corrective action.

11.3 SPECIFIC PROCEDURES AND SAFETY ISSUES

This section of the guide discusses specific procedures and safety considerations for most of the routine field assignments undertaken by hydrologists and hydrologic technicians. It includes discussions on safety guidelines that need to be observed when undertaking surface-water, ground-water, and water-quality activities.

11.3.1 Cableway Measurements Cableways have been used for many decades by DID in making discharge measurements especially during peak flood events. Cableways provide a track for suspending a travelling trolley which holds the current-meter suspended from it. The gauge person/hydrographer operates the cableway measurements from the banks. The trolley is moved from one point to another on the cableway by means of cable-pulley system. The following safety procedures are recommended to be followed when making cableway measurement:

• On reaching the station, check for any poisonous snakes, insect nest and other harmful creatures that may shelter in the station.

• Review field folder and note any special conditions or procedures to be used at the site.

• Before starting, undertake a close inspection of supporting frames, all cables, cable connectors, and all bolts for damages and possible failures, at both banks if possible.

• Inspect all areas of the travelling trolley for weak or missing parts; also check operation and condition of the braking system.

• After completing cableway inspection, proceed with setting up necessary measuring equipment on the trolley. Check that all cables are on the pulley wheel groves and not derailed.

• Avoid personnel working under cableway platform when assembling measuring equipment on the trolley.


11-12 March 2009

• The trolley when not in use, must be locked to the cable support by a bar hook, without

danger of it getting loose. • Wear life jacket and work gloves. Carry extra sounding reel, insect repellent, and necessary

tools for repairing and measuring equipment.

• Keep your hands off the cable when the trolley is moving to prevent possible injury.

• If the river is used by boats, some warning device must be used to alert the boat operators that there is a cable in the water ahead.

11.3.2 Bridge Measurement

Bridges are often used for making discharge measurements of streams that cannot be waded. Equipment needed in making bridge measurements differs from that used in wading measurements in that a portable metal crane is often used to mount a reel and suspend the meter, sounding weights, and cable over the bridge. Power equipment, which may be mounted on vehicles, is used for large rivers. Some bridges are not adaptable for cranes, and bridge boards must be used. On some foot bridges a special rod or handline is used.

Bridges are inherently dangerous because of vehicular traffic. The following safety procedures are recommended when making discharge measurements from a bridge:

• Review field folders to determine any hazards that are noted and maximum depths and velocities that have been observed.

• Know how to use the equipment. Make a dry run with new equipment or unfamiliar equipment at the office with someone who knows how it operates.

• Check the operation of the equipment before leaving the office to make sure that cranes, meters, reels, and motors are in good operating condition. Perform a visual inspection of batteries used with power cranes. Replace if unusual wear or cracks in the casing are observed.

• Follow the procedures outlined in the Traffic Control Plan (TCP) for each bridge site for placing traffic control devices, and keep a copy in the field folder. The plan must meet Federal standards as a minimum, or State or local standards, whichever prevails.

• Park the vehicle on the shoulder and use colored, revolving beams and emergency flashers on vehicle to warn oncoming traffic, as stated in the TCP.

• Set "caution" signs and plastic cones around work area and assign a person, when necessary, to watch for traffic and debris in the river and shout warnings as appropriate.

• Wear an orange-colored life jacket and work gloves.

• Using a reel and crane, either hand operated or power, can be dangerous because of the possibility of getting fingers caught under the cable or having the cable break and fly wildly. If at any time you lose your grip on the hand crank, make no attempt to grab the handle. Let it go! The flying handle can severely bruise an arm or even break a bone.

• When measuring at night, use adequate lights, especially if drift is running.


March 2009 11-13

• Keep a sharp look-out for drift when measuring. Have a pair of heavy duty wire cutters

handy to cut loose if drift is snagged.

• Work from upstream side of the bridges if at all possible, so that debris can be spotted moving downstream.

• Provide some device to alert boat operators that a cable is in the water.

• When working from a bridge that has hazardous power lines, provide a permanent warning sign on some part of the bridge directly above or below the hazard to alert the field person of the danger.

11.3.3 Wading

Discharge measurements using current meters are best made by wading. Wading measurements have a distinct advantage over measurements made from bridges, boats, or cableways in that it is usually possible to select the best available cross sections for the measurement. Constant awareness of wading dangers and weather conditions needs to be maintained to avoid accidents and potential injury.

Listed below are some safety guidelines that need to be observed: • Review the field folder to determine the best section for making wading measurements. Also

determine if any potential risks are noted and the maximum velocity (1m/s) and depths (1m) that may be encountered.

• Determine whether the river stage is rising or falling. Beware of rapid rises in river stage when wading and anticipate and allow for changes in flow conditions at the end of the measurement. It is a good idea to select an object (rock, stump, mark along bank, etc.) that is just above water surface and keep watching it to determine if the river stage is rising or falling.

• Always probe the stream bed ahead with a rod when moving from bank to bank. Keep your feet spread apart and alignment of legs parallel to the flow for better stability.

• If the velocity becomes too great for safe wading do not turn around, because when the greater area of the front or back of the body is exposed to the current, you may be swept downstream. Back out carefully, bracing yourself with the wading rod.

• Don't try to break the station discharge record for the maximum wading measurement.

• Wear a life jacket when wading and conducting discharge measurements. Tie the tagline securely so that you may pull yourself out, if necessary.

• Beware of sand channels where potholes, quicksand, and scour can be hazardous.

• Beware of slick, steep banks, and swampy areas.

• Watch for debris drifting.

• At controlled or regulated streams, consult recorder or instruction for pattern of regulation. Contact dam, reservoir or gate operators before entering stream.


11-14 March 2009

11.3.4 Moving boat Measurements made from boats require special equipment not used for other types of measurements. Generally, a cross-piece reaching across the boat is clamped to the sides of the boat and a boom attached to the center of the cross-piece extends out over the bow. The cross-piece is equipped with a guide sheave and clamp arrangement at each end, to attach the boat to the tag line and make it possible to slide the boat along the tag line from one station to the next. Power-operated equipment, which may be mounted on boats, is used for large rivers. The following safety procedures are necessary to prevent accidents or damage to equipment:

• Review field folders to determine potential risks and maximum depths and velocities.

• Select the proper boat and motor for the particular job and maintain them in good, workable condition.

• Follow all safety precautions during trailering, towing, and launching boat.

• After arriving at boat measuring site, locate launching area for the boat. Check this area for snakes and clear any bush.

• Unload boat from top of truck or trailer. Remember to use proper lifting techniques.

• At least two people on board and one on bank

• Boat trailer must have lights rather than reflector

• Operators of boats must be trained in cardiopulmonary resuscitation (CPR) and have completed an approved boating skills and seamanship course.

• Assemble all equipment associated with the boat measurement. Carry spare paddles, horn, cutting pliers, bailing devices, personal floating devices (PFD), and water bucket.

• Stretch a tagline (with white or red flagging attached) across the river and secure it to a tree or stake with a cable grip. Wear a life jacket, work gloves, and be observant for boat traffic. Provide an advanced warning to the boaters such as a compressed air horn, buoys, or flashing lights. Warning devices should be positioned 400 m upstream and downstream of tagline. Remember, the only practical way to avoid tagline accidents is to engage a tagline release person and provide them with equipment to release the tagline quickly

• Avoid or take special precautions in the vicinity of canal siphons, pumping intakes, bridge piers, docks, locks, and dams.

• The essence of boating safety is keeping out of trouble rather than getting out of trouble after you get into it. The operator of the boat is responsible for knowing all equipment requirements and safety procedures for the craft.

• Some general boating safety guidelines are:

• Distribute weight evenly when loading your boat.

• Know emergency procedures and distress signals.


March 2009 11-15

• In addition to the equipment required by law, carry a first aid kit, flashlight, distress flares,

paddle or oars, extra shear pins, bucket, extra anchor and plenty of anchor line, mooring lines, a good tool kit, compass, reserve fuel and extra spark plugs, emergency water and food, and a transistor radio capable of receiving on the marine band.

• Call for weather report for the area you will be in prior to each trip. Beware of weather, currents, and tide conditions.

• Know and obey state and federal rules.

• Leave a float plan.

• Follow all safety precautions when fueling the craft. Portable fuel tanks should be lifted out of the boat and placed on the dock to be fueled. Don't forget to secure the tank and wipe up and wash down any spillage.

• Pay attention to your boat's handling characteristics and know its capabilities for all types of weather conditions.

• Keep clear of fixed objects and watch out for overhead power lines.

• Keep the boat in good condition.

11.3.5 Helicopters

Access to some of the remote stations, inaccessible by roads, requires the use of helicopters to retrieve the recorded hydrological data, service and maintain these stations. In boarding the craft, do abide all the safety regulations. Inform the pilot, exactly where the station location are sited and obtain prior information on safe landing site for the craft and any foreseeable dangerous obstacles such as electrical cables, marshes, sloping ground terrain etc. that may deprive the pilot from making a safe landing.

In boarding the helicopters, safety care must be taken because it is an expensive and dangerous machine that can kill if safety is not adhered properly. Danger exists because people do not understand the potential hazards that are inherent on or near the helicopter.

This safety alert is to give you a basic understanding of where the potential dangers exist, and how to work around helicopters safely and effectively. Knowing the proper protocols and safety precautions prior to boarding a helicopter is required. Above all else the number one rule is to obey the pilot at all times and to stay alert in and around the operational area of a helicopter. If in doubt, ask your pilot. The pilot will provide clear and concise instructions to ensure a safe flight.

Planning the Flight

Inform and discuss with the pilot on the followings:

• The flight mission and the flight plan.

• Coordinate the mission/purpose with any other passengers.

• Suitability of landing zones in the area.

• Obstacles in the area of the landing zone (type, height, location, etc.)


11-16 March 2009

• Type of terrain in the landing zone (sand, rocks, trees, swampy, bushy, etc.). • Slope of the landing zone (much over ten degrees is cause for concern). Helicopter Landing Area At a minimum, the touchdown zone (please refer figure 11.3) should be as follows:

• During daylight hours – 30m X 30m square

• At night – 40m X 40m square

• The surface should be flat and firm, and free from debris that might blow up into the rotor

system.

Daylight Hours30m X 30m

Night Time Hours

40m X 40m

Figure 11.3 Landing Area Dimensions

As A Passenger You Will Be Expected To Know:

• What is expected of you on the flight

• How to embark and disembark

• In-flight and ground procedures

• Location and use of safety and survival equipment

• Emergency procedures

On The Ground Preparation and Conduct: • Conduct/Request a tail-gate safety briefing. Every helicopter is different.

• Discuss and predetermine the seating arrangements prior to boarding. Make sure the primary

observers have seats optimal for viewing


March 2009 11-17

• Do not smoke within 30 meter of a helicopter and 150 meter of a fuel truck.

• Stay well to the side of the helipad when the helicopter is arriving or departing

• Secure your clothing, equipment, and headgear against rotor winds.

• Protect your eyes against blown dust and particles.

• Secure adequate hearing protection.

• Keep the helipad clear.

• Always stay clear of the helicopter's main and tail rotors.

• Always obtain eye contact with the pilot when attempting to approach. Pointing first to yourself

then to the helicopter indicates you want to approach the craft. Don’t approach until you get the

ok nod from the pilot.

• A good rule of thumb is to approach from the pilot's 10 O'clock to 2 O'clock position.

• Crowds should be kept 30 meter from the helicopter at all times.

• Assure that all personal equipment is secure (i.e. no hats, umbrella and loose sheet cover that

can be blown away or up into the rotor system).

Helicopter Approach Zones

Figure 11.4 Approach helicopter between 10 and 2 o’clock zone

• If you can, wait until the rotors stop turning before boarding or embarking from the craft.

• Never touch a helicopter without the specific permission of a crew member.

• Approach and leave by the down slope side - for rotor clearance.


11-18 March 2009

Figure 11.5 Approach and leave by the down slope side for rotor clearance

• Never throw items toward or out of helicopters.

• Load cargo carefully and secure it against movement.

• Ensure baggage compartment doors are properly closed and latched.

• Secure seatbelts (and shoulder straps, if provided) while in flight.

• Remain in your seat unless given permission to move.

• Do not distract the pilot during takeoff, maneuvering or landing.

• Read instructions on the operation of doors, emergency exits, and the location of the ELT

(emergency locator transmitter) and emergency equipment.

• The pilot has the final say in any situation involving the safety of the crew, passengers, aircraft

or any aspect involving the helicopter's operation DURING AN EMERGENCY.

• Check that any loose gear in the cabin is secured.

• Wear helmet and approved floatation device if provided.

After An Emergency Landing

• Wait for instructions to exit, or until rotor stops turning.

• Assist others to evacuate well clear of the aircraft.

• Remove first aid kit and other emergency equipment after no threat of fire.

• Administer first aid if required.

• Remove ELT, read instructions and activate.

• Set up camp to be as comfortable as possible.

• Make the site as conspicuous as possible from the air.

• Stay near the aircraft - don't wander away from the site.


March 2009 11-19

When Flying Over Water

• Listen carefully to the pilot's over water pre-flight briefing. • Wear a lifejacket and/or immersion suit.

• Know seatbelt fastening, tightening, releasing procedures.

• Know the location and operation of doors and emergency exits.

• Know the location and operation of the ELT.

• During an emergency

• obey the pilot's ditching instructions. • assume brace position when advised by the pilot. • Wait for instructions to exit, or until rotor stops turning.

• After a ditching

• Establish a reference position. • Release seat belt. • Inflate lifejacket and life raft when clear of helicopter.

11.3.6 Safety Procedures When Working In Confined Space and Stilling Well

Figure 11.6 Example of the stilling well

The following sections provide safety procedures for working in confined space such as stilling well.

Potential Hazards

There are many hazards connected with entering Confined space such as stilling well. Some of the most common hazards are:

• Adverse Atmosphere The confined space may contain flammable or poisonous gases or the atmosphere may be deficient in oxygen. Forced ventilation may be necessary.


11-20 March 2009

• Deteriorated Rungs

Stilling well steps rung may be corroded and not strong enough to support a man. It may be difficult to inspect the rungs because of poor lighting. Use portable ladder when in doubt.

• Unwanted Creatures and Insects

Presence of dangerous creatures and insects like snakes, monitor lizard, scorpions and wasp.

• Falling Objects

Items placed near the well opening may fall and injure a worker in the well.

• Sharp Edges

Sharp edges of items in or near a stilling well may cause cuts or bruises.

Planning

Advance planning should include arrangements for test equipment, tools, ventilating equipment, protective clothing, portable ladders and safety harness. Time spent in the confined space should be kept to a minimum. Before workers enter a stilling well, tests should be made for explosive atmosphere, presence of hydrogen sulfide, and oxygen deficiency. Combustible or toxic vapors may be heavier than air, so the tests on the atmosphere must be run at least 3/4 of the way down the well. Whenever adverse atmosphere is encountered, forced ventilation must be used to create safe conditions. After the ventilating equipment has been operated for a few minutes, the atmosphere in the well should be retested before anyone enters the well. When explosive conditions are encountered, the ventilating blower should be placed upwind to prevent igniting any gas that is emerging from the opening. When a gasoline engine blower is used, it must be located so that exhaust fumes cannot enter the well. If testing equipment is not available, the stilling well should be assumed to contain an unsafe atmosphere and forced ventilation must be provided. It should never be assumed that a stilling well is safe just because there is no odor or the well has been entered previously. Use torchlight if necessary to check on any unwanted creatures that may lurk in the well.

Entering Stilling well

Persons who are entering stilling well If there is any doubt about the soundness of the stilling well steps, use a portable ladder instead. A person should never enter unless he is wearing personal safety equipment, including a safety harness and a hard hat. A person should be stationed at the surface continuously while someone is working inside a stilling well. This is to provide any emergency assistance in case the person in the well is injured. Falling Objects All loose items should be kept away from the well opening. This applies to hand tools as well as stones, gravel and other objects.


March 2009 11-21

Other Precautions Other precautions which should be taken when entering a stilling well are:

• Wear a hard hat.

• Wear coveralls or removable outer garment that can be readily removed when the work is completed.

• Wear boots or safety shoes.

• Wear rubberized or waterproof gloves.

• Wear a safety harness with a stout rope attached.

• Do not smoke.

• Avoid touching yourself above the collar until you have cleaned your hands. 11.3.7 Safety Procedures When Working At Height

When working at height (ex. Inspection and servicing antennae), be ready with the proper climbing equipment. Ladders and scaffolding should be securely placed such that it does not slip or tilt and fall. Use certified and approved safety harness and helmet. In areas where exposure to electrical and ultra high frequency and microwave radio wave radiation are present, extra care and precaution should be taken. In these environment, anti static gloves, and proper insulated boots must be worn. When working at top, safety harness must be secured at independent, safe and strong support that could withstand the human weight if a fall is to occur. Only personnel knowledgeable in the installation and maintenance of antenna supporting structures, antenna systems and transmission lines will perform the work operations.


11-22 March 2009

REFERENCE

[1] R. Imai, Health & Safety Alert – US Wildlife Operations, Department of Fish and Game (DFG) and the Office of Spill Prevention and Response (OSPR), 2004.

[2] D.K. Yobbi, T.H. Yorke and R.T. MYCYK, A Guide to Safe Field Operation, U.S. Geological Survey Open-File Report, Tallahassee, Florida 1996, 95-777.

CHAPTETR 12 EMERGING TECHNOLOGIES IN HYDROLOGICAL OBSERVATIONS AND INSTRUMENTATION

Chapter 12 EMERGING TECHNOLOGIES IN HYDROLOGICAL OBSERVATIONS AND INSTRUMENTATION

March 2009 12-i

Table of Contents

Table of Contents ................................................................................................................ 12-i

List of Table ............................................................................................................... 12-ii

List of Figure ............................................................................................................... 12-ii

12.1 REMOTE SENSING FOR DISCHARGE MEASUREMENT ................................................... 12-1

12.2 DISCHARGE MEASUREMENT USING ADCP .................................................................. 12-1

12.3 VERTICAL ADCP ....................................................................................................... 12-1

12.4 HORIZONTAL ADCP .................................................................................................. 12-2

12.7 RAINFALL MEASUREMENT USING RADAR ................................................................... 12-9

12.7.1 Principle of Radar Rain Gauge ................................................................... 12-10

12.7.2 Radar Display ................................................................................................... 12‐10

12.7.3 Interpreting Rainfall From Radar Raingauge ................................................... 12‐11

12.7.4 Errors and Problems ......................................................................................... 12‐12

12.7.5 Weather Radar in Malaysia .............................................................................. 12‐14

12.7.6 Types of Radar Systems .................................................................................. 12‐15

12.7.7 Quantitative Precipitation Forecast .................................................................. 12‐16

REFERENCES ............................................................................................................. 12-18


12-ii March 2009

List of Table


12.1 Reflectivity and Rainfall Rate 12-12

12.2 Types of Weather Radars under the Malaysian Meteorological Service 12-15

12.3 Comparison of X-, C-, and S- Band Radar Systems 12-16

List of Figure Figure Description Page

12.1 Horizontal ADCP with 3-cell measurement across river (plan view) 12-2

12.2 Areas in water column marked ‘X’ actually covered by ADCP during discharge measurement. 12-3

12.3 Unmeasured top and bottom by ADCP. 12-4

12.4 Measured and unmeasured areas ADCP. 12-4

12.5 ADCP vs. single current meters for current profiling 12-5

12.6 Moving boat method using ADCP showing multiple velocity profiles acquired as boat transects across. 12-5

12.7 Top two pictures show event loggers while the bottom two are multi-channel data loggers. 12-6

12.8 Department’s SCADA and telemetry flood forecasting and early 12-9

12.9 Radar Scanning of Rainfall 12-9

12.10 Radar Detects Objects in the Atmostphere 12-10

12.11 2 PPI Radar Displays from Kota Bahru Radar Station 12-10

12.12 Radar Uses Elevation Angle, Azimuth Angle and Distance D to Locate Position of Target 12-11

12.13(a) Error in Radar Detection of Rain 12-12

12.13(b) Error in Radar Detection in Rain 12‐13

12.14 Weather Radar Stations in Malaysia 12-14


March 2009 12-1

12 EMERGING TECHNOLOGIES IN HYDROLOGICAL OBSERVATIONS AND INSTRUMENTATION

12.1 REMOTE SENSING FOR DISCHARGE MEASUREMENT

The United States Geological Survey and the University of Washington collaborated on a series of initial experiments on the Lewis, Toutle, and Cowlitz Rivers during September 2000 and a detailed experiment on the Cowlitz River during May 2001 to determine the feasibility of using helicopter-mounted radar to measure river discharge. Surface velocities were measured using a pulsed Doppler radar, and river depth was measured using ground-penetrating radar. Both radars were mounted on a helicopter and flown over the rivers in a series of approximately 1-minute passes at heights of 2–15 m. Surface velocities were converted to mean velocities, and horizontal registration of both velocity and depth measurements enabled the calculation of river discharge. The magnitude of the uncertainty in velocity and depth indicate that the method error is in the range of 5 percent. The results of this experiment indicate that helicopter-mounted radar can make the rapid, accurate discharge measurements that are needed in remote locations and during regional floods.

12.2 DISCHARGE MEASUREMENT USING ADCP

Acoustic Doppler Current Profilers or ADCPs are currently part of the new generation of velocity-measuring instruments used from the deep ocean to estuaries and even shallow streams. It can also measure flow rates in open channels using the velocity-area method. Recent breakthrough in broadband acoustic technology, both firmware and software has allowed the industry to create and develop many variations of ADCP, allowing discharge measurements to be made from numerous perspectives.

Being more advanced than an acoustic current meter, ADCPs, depending on the frequency employed are able to measure the average speed and direction of the flow in the water column in bins or cell sizes as small as 5 cm. An ADCP can be deployed from a look-up or look-down position (also known as vertical ADCP) or even side-looking (horizontal ADCP).

12.3 VERTICAL ADCP

Conventional ADCPs measure velocities in a look-up or down position, usually from a stationary location. Some models such as the StreamPro (www.rdinstruments.com), Qliner (www.nortek-as.com) and RiverCat (www.sontek.com) are mounted on small remote-controlled crafts or tethered rafts or catamarans and pulled from one discrete gauging point to another across a bridge. Water depth is also measured by the ADCP viz bottom tracking while positioning is done by a survey-grade GPS. State-of-the-art hydrometry software allows the user to input measurement parameters such as depth-cell size, maximum profiling depth, averaging interval etc. for automated total discharge output immediately at the end of each run.. Other more advanced models such as the Rio Grande (www.rdinstruments.com), River Surveyor (www.sontek.com), and the FQ ADCP series (www.linkquest.com) can be deployed from a moving boat and give out real-time discharge rate via a PC as the vessel traverses the channel.


12-2 March 2009

12.4 HORIZONTAL ADCP Horizontal ADCPs or H-ADCP are a take off from their vertical counterpart with the former deployed on a side-looking approach across a channel. By employing three beams (Fig 12.1) which measure velocities at three different depths and locations across the channel, an average velocity is computed and multiplied with the cross-sectional area (previously surveyed) of the channel to yield the discharge rate. When regularly calibrated with the rating curve, the velocity index (VI) can also be depended to provide the discharge rate, Q on a real-time basis. The assumption is that as the water level rises and flow increases, the respective average velocities at these three depth cells shall also elevate accordingly. By continuing to change the positions of the three depth cells all across the river during different stage levels over a period of time, a comprehensive VI for low to high flow events is derived. This VI can also be used as part of a real-time monitoring and telemetry system in providing early warning to flood-prone areas.

Models available include the Channel Master (www.rdinstruments.com), EasyQ (www.nortekusa.com) and Argonaut SL (www.sontek.com).

Cell 1

Cell 2

Cell 3

Blanking

Cell size

Figure 12.1 Horizontal ADCP with 3-cell measurement across river (plan view)

The drawback of deploying an ADCP, both vertical and horizontal for velocity and flow measurement is the blanking distance or the blind spot where the mass of water flowing in the space closest to the sensor head and also to the riverbed is immeasurable. The size of the blanking distance from the sensor head commensurate with the ADCP’s frequency with a typical 1000 kHz ADCP having a blind spot of approximately 30 cm while the blanking distant from the river bottom would be 0.05 d. Therefore a look-down ADCP will not be able to measure the velocity between the water surface to its submerged sensor head plus the blanking distance. For example an ADCP which has a blanking distance of 30 cm and is submerged another 30 cm below the water surface will not yield any velocity data for the first 60 cm. Thus users will have to extrapolate the measured velocity profile to the water surface and the riverbed using the following equation:

Vy/Va = (y/a)1/n (12.1)


March 2009 12-3

In addition when used in a moving boat approach to gauging, the ADCP will not be able to measure velocities at the edge of both banks. Figs. 12.2 – 12.4 show the measurable area for discharge by the ADCP and also the immeasurable areas where discharge will have to be extrapolated. Nevertheless, the advantages of employing ADCPs for flow measurements greatly outweigh those of a conventional current meter. The ADCP measures the average velocity for many depth cells which would incidentally require as many single current meters for the same job (Figure 12.5) In addition conventional current meter only measures the velocity at one fixed point in the water column. This method has proven to be very advantageous when used in very wide rivers and during flooding events where gauging with cableways etc. were found to be futile. Furthermore, the time taken to complete a single discharge measurement using an ADCP with moving boat is normally one third of that consumed using a conventional cableway. Figure 12.6 shows a moving boat method using ADCP for real-time gauging. The real-time discharge, Q is calculated by the ADCP software at the end of each transect run.

Figure 12.2 Areas in water column marked ‘X’ actually covered by ADCP during discharge

measurement.

t 0 t mt k+1

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12-4 March 2009

Figure 12.3 Unmeasured top and bottom by ADCP.

Figure 12.4 Measured and unmeasured areas ADCP.

Vertical Shore estimate

Triangular Shore estimate

Top estimate

Bottom estimate


March 2009 12-5

123456789

1011

Depth cells Current Velocity Vector

DepthCell

Averages Vector WithinComplete Depth Cell Area

Measures Current Only at aLocalized Point in the Area

ADCP Moored Line ofsStandard Cueent Meter

Moored Line of

St d d C t M t

Figure 12.5 ADCP vs. single current meters for current profiling

Depth

0.2

0.2

0.1

0.3

0.3

0.3

0.2

0.3 0.4

0.4 0.5

0.4 0.5

0.40.3

0.3

0.2

0.3

0.5 0.6 0.7

0.6 0.6

0.60.5

0.50.4

0.40.3

0.30.2

0.8 0.8

0.8 0.8

0.70.7

0.50.54

0.40.4

0.30.3

0.2

1.0 1.0

1.0

0.9

1.0

0.9

0.70.7

0.50.5

0.40.4

0.20.2 Q ENSEMBLE = 28.2 m3/s

Σ Q = 84.9 m3/s

Q ENSEMBLE = 28.2 m3/sQ ENSEMBLE = 28.2 m3/s

Σ Q = 84.9 m3/sΣ Q = 84.9 m3/s

Distance = 33 m

Figure 12.6 Moving boat method using ADCP showing multiple velocity profiles acquired as boat

transects across.


12-6 March 2009

Data Logger

A data logger is an electronic device that records data over time and is usually integrated to a built in instrument or sensor or connected to single or multiple external instruments and sensors. Thus they are also called single channel or multiple channel data loggers. Another type of data logger works on pulsed voltage inputs and thus known as an event logger. The event logger is normaly useed to record the number of tips in a rainfall tipping bucket, hence recording the amount of rain. Most of the commercially available event loggers are generally very small, consume very little power (battery powered), equipped with a microprocessor, internal memory for data storage. Most of these event loggers interface with a notebook PC or PDA and utilize software to program the logger, view and analyze the collected data while bigger data loggers have a local interface device (keypad, LCD) and can be used as a stand-alone device (Figure 12.7).

Figure 12.7 Top two pictures show event loggers while the bottom two are multi-channel data

loggers.

Data loggers vary between general purpose types for a range of measurement applications to very specific devices for measuring in one environment only. The higher end data loggers usually have multiple channels and are programmable while the more affordable models are just static, single channel machines with only a limited number of changeable parameters. The reliability and flexibility of the new generation of data loggers plus their ease of maintenance have made many chart recorders obsolete in many applications.

Datalogger Ver

Mini LogHobo

Hyrologger RF14


March 2009 12-7

One of the primary benefits of using data loggers is the ability to automatically collect data on a 24-hour basis at a pre-programmed interval. Upon activation, data loggers are typically deployed and left unattended to measure and record information for the duration of the monitoring period. This allows for a comprehensive, long term trends of the environmental conditions being monitored such as weather, water level, rainfall etc. The newest of data loggers can serve web pages, allowing numerous people to monitor a system remotely.

Standardisation of protocols and data formats has always been an issue but is now being resolved in the growing industry and XML is increasingly being adopted for data exchange. The development of the Semantic Web is anticipated to accelerate this trend. Several protocols have been standardised including a smart protocol, SDI-12 exists that allows some instrumentation to be connected to a variety of data loggers. The use of this standard has not gained much acceptance outside the environmental industry in the U.S. SDI-12 also supports multi drop instruments. Other datalogging companies support the MODBUS communication standard, which is more commonly utilized and accepted in Malaysia.. Another multi drop protocol which is now starting to become more widely used is based upon Canbus (ISO 11898). Some data loggers though utilize a flexible scripting environment to adapt themselves to various non-standard protocols.

All data loggers have stand-alone memory that is used to store acquired data. Sometimes this memory is very large to accommodate many days, or even months, of unattended recording. This memory may be battery-backed static random access memory, flash memory or EEPROM. Given the extended recording times of data loggers, they typically feature a time- and date-stamping mechanism to ensure that each recorded data value is associated with a date and time of acquisition. As such, data loggers typically carry built-in real-time clocks whose inherent drift can be an important consideration when choosing between data loggers.

The unattended and remote nature of many data logger applications implies the need in some applications to operate from a DC power source, such as a battery. Solar power may be used to supplement these power sources. These constraints have generally led the data logger industry to ensure that the devices they market are extremely power efficient relative to computers. This unattended nature also dictates that data loggers must be extremely reliable.

For ease of data retrieval and also in some cases where constant or real-time feedback is critical, many data loggers are equipped to serve as remote terminal units or RTUs and are easily integrated into a telemery system. The role of RTUs in a telemetry system is further discussed in the following section.

Data Communication Systems

In many scenarios, quick and timely receipt of data or warning has made a difference between life and death. In mitigating natural catastrophes e.g. tsunami, earthquake, hurricanes, flood etc. the installation of telemetry systems to relay early warning has helped save many lives.

The key components in many telemetry systems are the RTU (Remote Terminal Unit), SCADA system (Supervisory Control and Data Acquisition) and communication protocol and physical communication network. The RTU is normally a reasonably advanced data logger responsible for acquiring the data from the field measuring equipment and sensors. It prepares (and in some cases interprets) the data from the equipment and formats this data according to a communication protocol, ready for transmission on a physical communication medium. RTUs may acquire their information through electrical signals connected to the RTU or from other intelligent devices via a serial data connection. RTUs may also perform local control functions. The communication protocol is the language used in the transmitting and receiving of data messages on the physical network.


12-8 March 2009

A protocol identifies the sender, the recipient, the meaning of the data in the message including verification information to ensure the complete message arrives and that it is error free. Both the transmitter and receiver of the data message must use the same protocol or “handshake” in order that both understand the data message. The communication network provides the physical means for the transfer of information (message data) from an RTU to a SCADA system, from an RTU to another RTU, and in some architecture between multiple SCADA systems. There is a large number of communication network technologies used with telemetry. Choice of communication network is critical to the operation of a telemetry system and can be a costly aspect of a telemetry system.

A SCADA system is made up of one or more computers, providing an interface to the physical communication network (and hence to the RTUs), and an operator interface to the data derived from RTUs. This data is interpreted, stored for subsequent retrieval, analysed and transferred to other computer systems. A SCADA system often provides a control interface for sending data to RTUs. This can be by way of operator commands, pre-programmed commands reactive to data from RTUs, or just information received from other computer systems. The physical communication component can be in the form as simple as a UHF or VHF radio or a fixed telephone line (PSTN) to the state-of-the art satellite communication package e.g. Immarsat-C, Aces. Many hydrological stations worldwide rely on GSM and GPRS technology to relay data to their respective headquarters and/or operation room while for very remotely located sites satellite communication is used instead. For instance the Department uses GSM for data telemetry both in rural and urban areas where coverage is available and GPRS for updating their ‘Infobanjir’ website from the master station. UHF and VHF are used in remote areas and data is transmitted via series of repeater/relay stations and finally to the master station. Other states use both Streamyx and PSTN to update the same website which provides for early flood warning. Immarsat-C is used to send data from the remote and challenging stations located in Sabah. IPVPN or leased line provides intranet connection between state departments and headquarters and also with their research arm in Ampang. The IPVPN is also used to connect the master station from G. Ledang to the state department office in Johor Bahru. At present most if not all State offices and the Department headquarters and research arm in Ampang have access to the internet via wireless system or Wi-Fi. Figure 12.8 depicts a typical Department SCADA and telemetry flood forecasting and early warning network system for the whole country.

Continuous power supply is always a challenge when installing a telemetry system as the consumption is significantly higher than the level required to run the field devices and RTUs e.g. datalogger. Therefore, due consideration should be given towards choosing the right type of power supply module for the telemetry. For very remote and inaccessible areas (except by helicopters) where site visits can be costly, a solar power panel is usually integrated into the system. Care must be taken though to ensure that the panel is large enough to recharge the DC battery even on a cloudy day. Routine maintenance activities include checking the battery power level, cleaning the battery terminals of oxidants and the solar panel from debris, dirt and animal droppings.

Anytime radio communication is used in a telemetry system there is a high probability of the system being struck by lightning. Therefore adequate steps have to be taken to ensure that the design of the telemetry and SCADA systems incorporate highly reliable lightning protection design and materials.

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12-10 March 2009

12.7.1 Principle of Radar Rain Gauge A radar emits electromagnetic pulse. This pulse hits a target (raindrops) and is scattered (see Figure 12.10). Some energy is reflected towards antenna as echo and the strength and position of echo is utilized to determine the intensity and location of rainfall.

Figure 12.10 Radar Detects Objects in the Atmostphere Figure 12.10 Radar Detects Objects in the Atmostphere Including Rain Droplets from Radio Signals Reflected Including Rain Droplets from Radio Signals Reflected

12.7.2 Radar Display 12.7.2 Radar Display

A typical radar display is the plan position indicator (PPI) as shown in Figure 12.11. Water droplets within the range of the rotating scanner will detect echoes reflected from raindrops and the strength of the echo is used as a measure of rainfall intensity. Echoes are also emitted by buildings, hills within the range of the scan.

A typical radar display is the plan position indicator (PPI) as shown in Figure 12.11. Water droplets within the range of the rotating scanner will detect echoes reflected from raindrops and the strength of the echo is used as a measure of rainfall intensity. Echoes are also emitted by buildings, hills within the range of the scan.

8/12/2003 ‐ 6:30 p.m 8/12/2003 ‐ 8:30 p.m

Figure 12.11 2 PPI Radar Displays from Kota Bahru Radar Station Figure 12.11 2 PPI Radar Displays from Kota Bahru Radar Station during the 2003 Flood in Kelantan (source: MMD) during the 2003 Flood in Kelantan (source: MMD)

The PPI displays radar data in a map like format with the radar at the centre. The distance to target is given by range marks/range rings. The direction to target is shown by the position of echo relative to the radar.

The PPI displays radar data in a map like format with the radar at the centre. The distance to target is given by range marks/range rings. The direction to target is shown by the position of echo relative to the radar.


March 2009 12-11

12.7.3 Interpreting Rainfall From Radar Raingauge Radar uses three basic parameters to detect the target location as shown in Figure 12.12

Figure 12.12 Radar Uses Elevation Angle, Azimuth Angle and Distance D to Locate Position of Target

Number of drops per unit volume and size of drops is correlated to the reflectivity of the echo. Rainfall intensity (mm/hr) is related to the reflectivity (dBZ) of the echo and the relationship is known as the Z-R relationship

• The general form of this relationship is z = a Rb (12.2)

where z = radar reflectivity in mm6/m3 R = rainfall rate in mm/hr

• Malaysian meteorological radar department uses standard Marshall-Palmer relationship for estimating rainfall rates

z =200 R 1.6 (12.2)

A simple guideline on the interpretation of rainfall rate (mm/h) with respect to reflectivity (dBZ) is given in Table 2.1

azimuth angle

Plan View

N

elevation angle

ground

Distance to target, D (determined by time lag between transmitting and receiving of radio signals


12-12 March 2009

Table 12.1 Reflectivity and Rainfall Rate

Reflectivity (dBZ)

Rain rate (mm/h) Comments

10 ~0.2 Significant but mostly non-precipitating clouds

20 ~1 Drizzle, very light rain 30 ~3 Light rain 40 ~10 Moderate rain, showers

50 ~50 Heavy rain, thundershowers, some hail possible

60 ~200 Extremely heavy rain, severe thunderstorm, hail likely

12.7.4 Errors and Problems The various errors and problems associated with measurement of rainfall using radar are as illustrated in Figures 12.13(a) to 12.13(b).

Radar beam above cloud and misses rain beneath the cloud

Radar beam blocked by terrain (hills,building)

Figure 12.13(a) Error in Radar Detection of Rain


March 2009 12-13

Radar beam lose power due to absorption by rain from nearer storm and underestimates the rain of storm further away

Radar beam bents due to refraction cause by variation in air density. Beam will return ground echoes

Figure 12.13(b) Error in Radar Detection of Rain


12-14 March 2009

12.7.5 Weather Radar in Malaysia The Malaysian Meteorological Service maintains weather radar stations in Malaysia, 7 in Peninsular and 4 in Sabah/Sarawak (see Figure 12.14)

Peninsular Malaysia East Malaysia

Kluang Kuching

Subang Bintulu

Butterwoth Kota Kinabalu

Alor Setar Sandakan

Kuantan

Kota Bharu

KLIA*

* KLIA-Kuala Lumpur International Airport, Sepang

Figure 12.14 Weather Radar Stations in Malaysia


March 2009 12-15

Table 12.2 Types of Weather Radars under the Malaysian Meteorological Service

Location No. Band Wavelength (cm)

Frequency (MHz)

Peninsular Malaysia 7 S 10 cm 2707-2900

East Malaysia 4 C 5.5cm 5500-5700

All 7 radar stations in peninsular Malaysia use S-band while the other 4 in East Malaysia use C-band. All radar stations maintained by Malaysian Meteorological Department and all of them are supplied by Enterprise Electronic Corporation (EEC) except for KLIA station.

Radar for KLIA station is a Doppler S-band by Mitsubishi radar division. This radar uses the SIGMET display system from U.S.A. 12.7.6 Types of Radar Systems The important aspects in selecting a radar system are the operating frequency and the transmitter type. Weather radars are commonly designated with X, C and S band with each one of them having different frequency range. In terms of operation, the occurrence of heavy rainfall event in Malaysia make the X band with shorter wavelength unsuitable since it is prone to severe attenuation (radar beam losing too much power). The use of longer wavelength is necessary to overcome the problem of severe radar beam attenuation but it increase the hardware cost. S-band is most suitable in overcoming the disadvantage of severe attenuation by X-band radar and to a lesser degree attenuation suffered by C band radar. S-band radars have sufficiently long wavelength to keep attenuation due to tropical rainfall in Malaysia to minimal levels. Apart from this, X-band radars are normally used outside tropical regions to resolve fine structure in precipitation systems and for short range measurements. For hydrological applications, the type of weather radar to select should be considered. There are two basic types of weather radar which is conventional pulse radar for rainfall intensity measurement and Doppler radar capable of wind measurement. In terms of data that can be derived, the Doppler radar provide much more advanced measurement of precipitation characteristics compared to conventional pulsed radar by measuring the radial velocity of precipitation targets. But nowadays most weather radar have incorporated hardware that can support Doppler measurement. In measuring precipitation intensity, the conventional pulsed radar system is pretty much reliable. But to consider advanced radar technology nowadays, it is advisable to incorporate the Doppler system since the incremental cost is not large and Doppler system can provide a host of useful data that can support quantitative precipitation forecast (QPF).

Transmitter plays an important role in radar performance whereby it determines the capability of radar to transmit stable radar signal. Generally there are two types of transmitter in use in weather radar systems which are klystron or magnetron. Klystron generates very stable frequency radar waves compared to magnetron. Note that the stable frequency is preferred in precipitation motion measurement.


12-16 March 2009

Table 12.3 Comparison of X-, C-, and S- Band Radar Systems

X-Band C-Band S-Band

Frequency (GHz) 8-12 4-8 2-4

Wavelength (cm) 2.5-4 4-8 8-15

Useful Range (diameter) 50 km ≈ 200km ≈ 300km

Attenuation (for 100mm/hour rainfall),dB/km

2.5 0.2 0.03

Application

Short range local warning radar (used in cloud physics/marine navigation)

Medium-range warning radar (used in temperate country

Networked long range warning radar (used in tropical country in cyclone area)

12.7.7 Quantitative Precipitation Forecast Quantitative precipitation forecast (QPF) methods using radar data benefit from the ability of weather radar to provide rainfall data with high spatial and temporal resolution. Areas of application are thunderstorm warnings for aviation as well as precipitation forecasts for hydrological models to prevent damage to human life and property.

To make the best use of QPF methods for providing input to hydrological models the technical aspects of radar measurements have to be reviewed. Difficulties encountered when using weather radar operationally for hydrological application are:

• radar system may be obsolete and therefore not eligible for automatic operation – continuous measurements not possible

• the quantitative estimate of precipitation may lack the needed precision because of technical

limits of the radar method, e.g. poor visibility in regions of interest

A quality control and correction is therefore necessary using radar data in QPF methods. Nonetheless, we should not forget the advantages of radar for hydrological applications: providing 1) automatic estimates of data with high spatial and temporal resolution for operational application, which are quickly accessible, 2) continuous data that reflect the structure of precipitation, measuring the variation of rainfall pattern in time and space, and 3) a detailed history of rainfall over the radar range.

To forecast the motion of radar echoes, their speed and direction of displacement has to be determined. Many radar-based QPF methods use tracking algorithms based on pattern recognition, either determining the echo motion applying cross correlation or tracking particular features of the radar echo. Problems with tracking algorithms are mainly caused by small-scale variations of the radar echo pattern, e.g. caused by orographical impacts. A further approach to advert radar echoes is using winds from the steering level, which can be provided by numerical weather prediction model outputs or Doppler radar measurements.

Cross correlation is used to determine the overall motion of radar echoes. Similar patterns of radar echoes are detected by comparing tracking areas in consecutive scans. The best fit between the tracking areas is found by optimizing the correlation coefficient.


March 2009 12-17

The distance between the tracking area and the time lag of the scans determine the displacement vector. There have been various extensions to the basic idea such as adding a geometric algorithm to provide a possibility to detect merging and splitting cells.

The most common approach to feature tracking is the mass centroid method, deriving the displacement vector between consecutive radar scans from the distance of the mass centres of two corresponding radar echoes. The centres are assumed to be representative for individual convective cells or storms. Thus, the mass centroid method provides detailed information about cell tracks and characteristics.

Current operational QPF systems using radar tracking algorithms are mainly based on three-dimensional data sets and partly aim to combine both approaches. The TITAN algorithm (Thunderstorm Identification, Tracking, Analysis and Nowcasting – in operational use at the Malaysian Meteorological Service) uses a mass centroid method to identify storms considered as “three-dimensional entities”.

The main limiting factor to intensity forecasts are initiation and dissipation of precipitation pattern as well as local effects like orography. A common approach to forecast intensity and echo size trends are weighted or unweighted fits of past developments of radar echo. Researchers have included the individual development of cells into a QPF technique to forecast the motion of precipitation areas by applying a nonlinear extrapolation to the intensity and echo size forecast (e.g. increase echo size for 15 minutes then decrease). Linear extrapolation achieved the best results. 30 minutes are supposedly the most appropriate time interval for fitting the extrapolation scheme on the basis that a trend has to be established for a developing cell. The TITAN algorithm applies a weighted linear fit of storm history for the storm size forecast. Statistical and case study-oriented comparisons of QPF schemes demonstrated during the first World Weather Research Programme Forecast Demonstration Project (FDP), held in Sydney, Australia during 2000, served to confirm many of earlier reported findings regarding QPF algorithm design and performance. With a few notable exceptions, QPF algorithms based upon the linear extrapolation of observed motion were generally superior to more sophisticated, nonlinear QPF methods. Centroid trackers (TITAN) were most reliable in convective scenarios and are therefore highly recommended for use in tropical regions. During widespread, stratiform rain events, the pattern matching extrapolators were superior to centroid trackers and wind advection techniques. There is some limited case study and statistical evidence from FDP to support the use of more sophisticated, nonlinear QPF algorithms. Wilson et al. demonstrate the advantages of combining linear extrapolation with algorithms designed to predict convective initiation, growth, and decay in a QPF scheme called Auto-nowcaster. Ebert et al. show that the application of a nonlinear scheme (called Spectral Prognosis) designed to smooth precipitation features at a rate consistent with observed temporal persistence tends to produce a nowcast that is superior to linear extrapolation in terms of root mean square error. QPF is relatively new technology and despite claims by many agencies and suppliers that QPF is successfully adopted, we cannot find quantitative evidence as to the degree of improvement or the accuracy of QPF. Most statements on the reliability of QPFs are qualitative in nature. In all fairness, it is noted that there are difficulties in trying to quantify the effectiveness of QPF such as:

• QPF can be of short range and longer range. Short range forecasts, typically up to 3hrs, are

also called nowcasts. They are generally more accurate than longer range forecasts.


12-18 March 2009

• The nature of precipitation system and its motion. The mesoscale convective system is large

scale phenomena and has a long lifespan making it easier to track and less susceptible to local variations. Single cell thunderstorms cover small areas and have short lifespan making it difficult to track and forecast.

In any case radar hardware nowadays normally comes with Doppler capability which can provide data in support of QPF and the additional costs of Doppler enabled radar is marginally higher compared to radar without Doppler capability. REFERENCES [1] Klein, G.S., Yufit, G.A. & Shkurko, V.K., A new moving boat method for the measurement of discharge in large rivers, 1993.

[2] McCuen, R.H., Hydrologic Design and Analysis; Prentice Hall, New Jersey, 1998, 814 pages.

[3] Riggs, H.C., “A simplified slope-area method for estimation flood discharges in natural channels”, Journal Research US Geological Survey, 4(3), 1976.