ce235-eh-lec-2

NUST Institute of Civil Engineering/Engr Sajjad Ahmad 1

Engineering Hydrology (CE- 235)

CHAPTER - 2

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PRECIPITATION

NUST Institute of Civil Engineering/Engr Sajjad Ahmad

PRECIPITATION - OUTLINE−Forms of precipitation−Factors influencing precipitation

formation−Measurement of precipitation−Computation of average rainfall over

a basin

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PRECIPITATIONFalling of any form of moisture from atmosphere to ground

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Water vapors

Atmospheric system

Evaporation Transpiration

Precipitation

FORMS OF PRECIPITATION~ Drizzle~ Rain~ Glaze~ Sleet~ Snow~ Snowflake~ Hail

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−Minute particles of water−Drop size < 0.5 mm−Intensity < 1.0 mm/hr−Do not produce runoff

DRIZZLE

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−Minimum drop size ≈ 0.5 mm−Maximum drop size ≈ 6.25 mm−Can produce runoff if

~Rainfall intensity greater than the rate of infiltration

RAIN

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~Ice coating on drizzle or rain drops~Ice coating forms when

~Drops comes in contact with cold surfaces on the ground

GLAZE

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~Drizzle or rain drops in frozen form~Drops freezes while falling through

air at subfreezing temperature

SLEET

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~Crystals of ice~Form due to sublimation

SNOW

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−Ice crystals fused together

SNOWFLAKE

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−Balls or lumps of ice−Diameter > 5 mm−Formed due to alternate freezing and

drying

HAIL

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−Mechanism of cooling of air−Water vapor condensation and

formation of droplets−Mechanism for growth of cloud

droplets−Mechanism for accumulation of

moisture

FACTORS INFLUENCING PRECIPITATION FORMATION

MECHANISM OF COOLING OF AIR

~The warm air near the surface rises carrying moisture/water vapors with it

~The air cools as it ascends due to lower temperature

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CODENSATION OF WATER VAPORS

~Water vapors condensed in presence of hygroscopic nuclei

~These nuclei comes from sea salt or combustion products of sulfurous and nitrous acids



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~Drops should be of sufficient size to over come the drag and uplift forces of air

~Droplet size can grow by two means~Coalescence of droplets through

collisions~Bergeron’s effect

GROWTH OF CLOUD DROPLETS

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~If temp is b/w 10 to 20 F then mixture of water droplets and ice crystals exists in atmosphere

~Due to higher saturation capacity around the droplets, some water evaporates

~While due to low saturation capacity around the ice crystals vapors condense and ice crystal size increases

BERGERON’S EFFECT

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~The quantity of water fall over basin is much more than the amount of moisture over the basin

~Therefore continuous supply of moisture is required

~This process is known as convergence

ACCUMULATION OF MOISTURE

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~It is the process of inflow of moisture from other areas to the basin

~It can be defined as~The net horizontal influx of air per unit

area

CONVERGENCE

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~Convectional precipitation~Orographic precipitation~Cyclonic precipitation~Turbulent ascent precipitation

PRECIPITATION CLASSIFICATION BASED ON

LIFTING MECHANISM

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CONVECTIONAL PRECIPITATION

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~Natural rising of warmer, lighter air in colder, denser surroundings

~In the hot day, the ground surface gets heated, causing the warmer air to lift up

~the colder air comes to take its place~The vertical air currents develop

tremendous velocities the vapors condensed and Convective precipitation occurs


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OROGRAPHIC PRECIPITATION

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~Orographic precipitation is caused by air masses which strike some natural topographic barriers like mountains, and cannot move forward

~Hence rise up, causing condensation and precipitation

~All the precipitation in Himalayan region is because of this nature

OROGRAPHIC PRECIPITATION

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~Cyclonic precipitation is caused by lifting of an air mass due to the pressure difference

~Cyclonic precipitation may be either frontal or non-frontal cyclonic precipitation

CYCLONIC PRECIPITATION

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~It results from the lifting of warm and moist air on one side of a frontal surface over colder, denser air on the other side

~A front may be warm front or cold front depending upon whether there is active or passive accent of warm air mass over cold air mass

FRONTAL PRECIPITATION

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COLD FRONT

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WARM FRONT

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STATIONARY FRONT

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~Amount of precipitation~Vertical depth of water on a level

surface~Expressed in inches, ft, cm etc

MEASURMENT OF RAINFALL

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~Intensity or rate of precipitation~Precipitation amount per unit time~Expressed in inch pr hour or cm per hr

etc


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~Primary input used to estimate~Stream flow~Ground water infiltration

~Accurate measurement is necessary


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~Types of rain gauges used are~Non recording rain gauge~Recording rain gauge

~Recording rain gauges also gives the intensity of rain fall


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~ The most common is the non recording gauge called a Standard Rain Gauge. The SRG is 200mm diameter cylinder of 600mm height. It has a funnel on top and a plastic measuring tube (1/10th of diameter of cylinder) in the middle

~ Due to smaller diameter 2.5mm rainfall fills 25mm of tube

~ Hence gives more accuracy

NON RECORDING RAIN GAUGE

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~Schematic diagram of symon’s rain gauge

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~Float type~Weighing type~Tipping bucket type

RECORDING TYPE RAIN GAUGE

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Float

Rotating drum and graph paper

FLOAT TYPE

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~ Standard instrument used to quantify rainfall~ Float within collecting bucket rises with level~ Vertical movement marked by pen and shows rainfall

depth on a paper chart~ The chart is rotated by a spring-driven or electric

clock at speeds of 1 revolution in 6, 9, 12, 24, or 192 hours

~ The rain gauge chart is a record of the accumulated of rainfall for the selected time interval

FLOAT TYPE

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FLOAT TYPE

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WEIGHING TYPE~ It consists of a storage bin, which is

weighed to record the mass~ It weighs rain or snow which falls into a

bucket, set on a platform with a spring or lever balance

~ The increasing weight of the bucket and its contents are recorded on a chart

~ The record shows accumulation of precipitation

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NUST Institute of Civil Engineering/Engr Sajjad AhmadTIPPING BUCKET TYPE

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~ Two containers on balance beam form a “tipping bucket”

~ Rain fills one container until its threshold weight reached

~ Bucket then tips over, emptying collected water into total container and continues to collect rainfall in other container

~ Magnet generates electric pulse which is recorded

TIPPING BUCKET TYPE

45 NUST Institute of Civil Engineering/Engr Sajjad Ahmad

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→Mistakes in reading the scale of gauge

→Dents in collector rim may change its receiving area

→Instrumental error in gauge or in their recording or measuring arrangements

→Some rainwater may get lost due to splash from the collector

SOURCES OF ERROR

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→Some initial rainwater may get lost in moistening gauge funnel and inside surfaces

→Blowing wind may tilt the rains from vertical which thus brings less rain catch in the gauge

→Vertical upward air currents may impact upward acceleration to precipitation thus brings less rain catch in gauge

SOURCES OF ERROR

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→Gauge inclined 10° from vertical will approx cater 1.5% less rainfall than it should

→No rainfall recorded during tipping of bucket

→Tipping of bucket may be affected due to rusting or accumulation of dust on pivot

SOURCES OF ERROR

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oUnavoidable Errors– Equipment failure– Observer error

o Avoidable Errors (related to site) inclination - perpendicular to ground Obstructions height Wind

REMEDIAL MEASURES

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Minimum distance from obstruction

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−A rain gauge recorded 125mm of precipitation. It was found later that the gauge was inclined at an angle of 20 degree to the vertical. Find the actual precipitation.

Example 1

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Rainfall is recorded automatically, therefore no need of attendant

Recording rain gauge also gives intensity of rainfall at any time, while non-recording gauge gives only total rainfall for time interval

Recording rain gauges can be installed far off places

Possibility of human error is obviated

ADVANTAGES OF RECORDING RAIN GAUGES

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o Costly in comparison to non-recording gauges

o Error in recording rainfall due to fault in electrical or mechanical mechanism

DISADVANTAGES OF RECORDING RAIN GAUGES

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−Point data analysis−Estimation of missing precipitation

record−Simple arithmetic mean method−Normal ratio method

−Consistency of precipitation data or double mass curve analysis

→Estimation of average precipitation over a basin

Analysis of precipitation data

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−Interpolation station ≈ gauging station whose data is missing

−Index station ≈ surrounding stations around interpolation station whose data is used to calculate the missing rainfall data

Arithmetic Mean Method

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−If Px is the missing precipitation then

−Here n ≈ number of nearby stations

−And Pi is the precipitation at ith station


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−Lets suppose there are three index stations then the precipitation at interpolation station will be given as


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−If Px is the missing precipitation then

−Here n ≈ number of nearby stations

−And Pi is the precipitation at ith station

−Nx and Ni are the normal annual precipitation value for station x and ith respectively

Normal Ratio Method

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~Arithmetic average method can be used iff following satisfies

~Nx and N1 , N2 and N3 are the normal annual precipitation value for station x and 1,2 and 3rd station respectively

Applicability of above methods

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~If the condition not satisfies then only normal ratio method can be used

Applicability of above methods

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