Graduate School of Water Resources

Design of Diversion Dam in Tarbela Dam Pakistan

Designed by: Muhammad ShoaibStudent # : 2015730558

Introduction

Contents 1.Site Selection 2.Site Study 3.Dam Type selection 4.Embankment 5.Geology and foundation conditions 6.Reservoir investigations7.Test fills8.Study of causes of dam failure9.Flood Hydrograph10.Basic Hydrologic and Meteorological11.Flood Hydrology Reports12.Engineering Design13.Penstock14.Construction

The project is located at a narrow spot in the Indus River valley at Tarbela in Haripur.

The main dam wall, built of earth and rock fill, stretches 2,743 meters (8,999 ft.) from the island to river right, standing 148 meters (486 ft.) high.

The spillways, located on the auxiliary dams, in turn consist of two parts. The main spillway has a discharge capacity of 18,406 cubic meters per second (650,000 cu ft. /s) and the auxiliary spillway, 24,070 cubic meters per second (850,000 cu ft. /s).Now a new diversion dam project is going to start on Tarbela dam.

Introduction

(Tarbela Dam Pakistan)

1.Site Selection

Site Selection continues…..

(Site Selection Front view)

(Site Selection Back view)

(Site Selection top view)

Reasons for selecting this site

If from the selected point the river flows forward, it will flow through the valley of steep mountains and will flow again in the Indus River.

Advantages : There is no need to make tunnel to divert the flow during

construction Disadvantage/Environmental Issues: A bridge will need to be constructed to replace the road

to let the river flow in Indus River.

2.Site Study First we will conduct detailed geological and subsurface explorations, which characterize the foundation, abutments, potential borrow areas and type of Dam selection.

2.1 Geology of the Tarbela area: The phyllite unit forms the base of the Kingriali Formation

nearly everywhere in the Tarbella area. This unit was called the "basal conglomerate member" by Ali (1962, p. 34). It is a gray- and brown-weathering phyllitic sequence of shale and siltstone.

Pebbles and cobbles in the conglomerate consist mainly of Tanawal quartzite but also include phyllite, shale, and vein quartz.

The dolomite unit of the Tarbela area consists of dark-weathering interlayered brown and gray microcrystalline dolomite.

In the Sherwan syncline, distinct layers of undolomitized gray limestone are present within the dolomite.

3.Dam Type selection

Site conditions lead to selection of an earth-fill dam rather than a concrete dam (or roller-compacted concrete dam) because it includes a wide stream valley, lack of firm rock abutments, considerable depths of soil overlying bedrock, poor quality bedrock from a structural point of view, availability of sufficient quantities of suitable soils.

The geology of the dam also supports the construction of earthfill dam as Shale, Limestone, Siltstone and dolomite are all soft minerals which when compacted alongside the soil can produce a strong embankment.

Furthermore, the earthfill dams are the most common type of dam, principally because their construction involves the use of materials from required excavations and the use of locally available natural materials requiring a minimum of processing.

Moreover, the foundation and topographical requirements for earthfill dams are less stringent than those for other types (Earthfill Embankment Dam)

3.1 Technical requirements

The dam, foundation, and abutments must be stable under all static and dynamic loading conditions.

Seepage through the foundation, abutments, and embankment must be controlled and collected to ensure safe operation

The freeboard must be sufficient to prevent overtopping by waves and include an allowance for settlement of the foundation and embankment.

The spillway and outlet capacity must be sufficient to prevent over-topping of the embankment by the reservoir.

3.2 Administrative requirements Environmental responsibility. Operation and maintenance manual. Monitoring and surveillance plan. Adequate instrumentation to monitor performance. Documentation of all the design, construction, and

operational records. Emergency Action Plan: Identification, notification, and

response sub plan. Schedule for periodic inspections..

(Dam Foundation)

4.Embankment

Many different trial sections for the zoning of an embankment should be prepared to study utilization of fill materials; the influence of variations in types, quantities, or sequences of availability of various fill materials; and the relative merits of various sections and the influence of foundation condition.

4.1 Other Considerations

Other design considerations include the influence of climate, which governs the length of the construction season and affects decisions on the type of fill material to be used.

5.Geology and foundation conditions

The foundation is the valley floor and terraces on which the embankment and appurtenant structures rest Gravel foundations, if well compacted, are suitable for earthfill dams.

Because gravel foundations are frequently subjected to water percolation at high rates, special precautions will be taken to provide adequate seepage control or effective water cutoffs or seals. The liquefaction potential of gravel foundations will be investigated.

5.1 Comprehensive field investigations and/or laboratory testing will be required where conditions such as those listed below are found in the foundation: Deposits that may liquefy under earthquake shock or

other stresses. Weak or sensitive clays. Dispersive soils. Varved clays. Organic soils.

(Weak or sensitive clay)

5.2 Subsurface investigation for foundations should develop the following data

Subsurface profiles showing rock and soil materials and geological formations, including presence of faults, buried channels, and weak layers or zones. The RQD is useful in the assessment of the engineering qualities of bedrock.

5.2.1 Fault

To the researchers knowledge, there has as yet been no historic case of an operating dam being displaced by a fault during an earthquake, although there have been some "near misses."

This good record of worldwide performance is particularly remarkable in view of the fact that many if not most dams are located in river canyons whose courses are controlled by preferential erosion along underlying faults and joints.

Not surprisingly, almost all foundations for large dams display some faults, however minor, and the geologic and seismologic challenge is to determine whether such faults are likely to rupture during the life of the structure (i.e., are they "active"?) and, if so, with what displacements, with what geometries, with what magnitudes, and with what likelihoods

(Real time example of fault)

6. Reservoir investigations

The sides and bottom of a reservoir should be investigated to determine if the reservoir will hold water and if the side slopes will remain stable during reservoir filling, subsequent drawdowns, and when subjected to earthquake shocks

7.Test fills

In the design of earth and rock-fill dams, the construction of test embankments can often be of considerable value and in some cases is absolutely necessary.

Factors involved in the design of earth and rockfill dams include the most effective type of compaction equipment, lift thickness, number of passes, and placement water contents; the maximum particle size allowable; the amount of degradation or segregation during handling and compaction; and physical properties such as compacted density, permeability, grain-size distribution, and shear strength of proposed embankment materials.

8.Study of causes of dam failure An understanding of the causes of failure is a critical element in the design and construction

process for new dams and for the evaluation of existing dams. The primary cause of failure of embankment dams in the is overtopping as a result of

inadequate spillway capacity. The next most frequent cause is seepage and piping. Seepage through the foundation and

abutments is a greater problem than through the dam. Therefore, instrumentation in the abutments and foundation as well as observation and

surveillance is the best method of detection. Other causes are slides (in the foundation and/or the embankment and abutments) and

leakage from the outlet works conduit8.1 Other factors that increase the likelihood of internal erosion and backward erosion piping incidents developing at a given site include:

Conduits constructed across abruptly changing foundation

Circular conduits constructed without concrete bedding

Conduits with an excessive number of joints

Excavations made to replace unsuitable foundation

Conduits with compressible foundations Conduits located in closure sections in

embankment dams

9.Flood Hydrograph

Design-flood hydrographs or parts thereof (peak or volume) are required for sizing the hydraulic features of a variety of water control and conveyance structures.

In the case of dams and their appurtenant features, flood hydrographs are required for the sizing of spillways and attendant surcharge storage spaces

9.1 PMF Hydrograph

The PMF (probable maximum flood) hydrograph represents the maximum runoff condition resulting from the most severe combination of hydrologic and meteorological conditions considered reasonably possible for the drainage basin under study.

The PMF is used by design and construction organizations as a basis for design in those cases where the failure of the dam from overtopping would cause loss of life or widespread property damage downstream.

10. Basic Hydrologic and Meteorological Data-compilation and analysis of hydrologic and meteorological data accumulated during and

after severe flood events is necessary for every flood hydrology study

10.1 Hydrologic Data

10.1.1 Recorded Stream flow Data These data are collected primarily by the (Pakistan Geological Survey) at continuous recording

stream flow gauging stations. Generally, these publications present the stream flow in terms of the average daily flow for each day for the period the stream gauge has been in operation

10.1.2 Peak Discharge Data Because the cost of installing, operating, maintaining, compiling, and publishing the

data is high, there are relatively few continuous-recording stream gauges, considering the number of rivers and streams in the Pakistan

10.2 Meteorological Data Systematic acquisition of precipitation data is accomplished primarily through the efforts of the

NWS (National Weather Service). The NWS maintains a network of “first order” weather stations. Each station in this network collects continuous precipitation, temperature, wind, and relative humidity data.

11.Flood Hydrology Reports

Envelope curves Reservoir routing criteria Antecedent flood Frequency analysis Probable maximum flood Snowmelt Loss rates Unit hydrograph Storm study Basin description General Summary of study results Authority

12.Engineering Design12.1Dam Capacity

Dam capacity = [Reservoir Length x Reservoir Width (at the dam) x Depth of the Water (maximum)] / 3

In our case Reservoir Length = 900 meter Reservoir Length = 257 meter feet Dam height = 15.20 meter Total Dam capacity = 900*257*15.20 = 3,515,760 cubic meter.

Average pressure = γ * h/2 = 9.81 * 15.20/2 = 74.556 Length along which pressure acts = L = h/sin θ = 15.20/ sin 60 = 49.86 m Force = pA = 257 * 74.556 * 49.86= 955362.07 KN Center of pressure = h/3 from bottom = 15.20/3 = 5.06 m

12.2 Force & Center of Pressure

Where: A is the catchment area in hectares (ha) R is the average annual rainfall in millimeters (mm) Y is the runoff as a percentage of annual rainfall A= 23.13 Ha R=750 mm (average per year) Y= 7.5

% Therefore runoff = 23.13*23.13*750*7.5 = 3,009,357 Liters.Note: Indus basin has never experienced a rainfall of more than 800 mm/year.

12.3 Catchment runoff Catchment runoff = 100 *A*R*Y liters

12.4Volume of Embankment V = D/6[A1+4M+A2] Where M is the area of the cross-section midway

between A1 and A2. Height = 15.20 Meter. Bottom Length is 2/H. Bottom Length = 10.13 Meter D = 2.5 meter Dam Width = 257 Meter Bottom Length = 10.13-2.5=7.63A1 = 17.007*

257= 4370.79 meter square A2=A1 M= 2.5*257=642.5 meter square V = 4370.79/6[4370.79+4(642.5) +4370.9] Volume of Embankment = 8,240,250 Cubic

Meter

12.5 Power of Dam Power The electric power in kilowatts (one kilowatt equals 1,000 watts).Height of Dam The distance the water falls measured in feet.River Flow The amount of water flowing in the river measured in cubic feet per second.Efficiency How well the turbine and generator convert the power of falling water into electric

power. While for newer, well operated plants this might be as high as 90% (0.90). 11.8 Converts units of feet and seconds into kilowatts.

Power = (Height of Dam) x (River Flow) x (Efficiency) / 11.8Power = (49.992 feet) x (70962 cubic feet per second) x (0.80) / 11.8 = 258,002 kilowatts

To get an idea what 258,002 kilowatts means, let's see how much electric energy we can make in a year.Since electric energy is normally measured in kilowatt-hours, we multiply the power from our dam by the number of hours in a year.Electric Energy = (258,002 kilowatts) x (24 hours per day) x (365 days per year) = 2,260,097,520 kilowatt hours.The average annual residential energy use in the Pakistan is about 1,500 kilowatt-hours for each person. So we can figure out how many people our dam could serve by dividing the annual energy production by 1,500.People Served = 2,260,097,520 kilowatts-hours / 1,500 kilowatt-hours per person) = 1,506,731.68people.

13.Penstock Metal pipes will be used in the construction of conduits. Steel is a strong alloy of iron and carbon that contains

lower carbon content than cast iron (lower than 2 percent).

The amount of carbon determines the steel’s hardenability.

Advantages of using Steel pipes Welded joints provide water tightness High compressive and tensile strength Flexible and deformable under stress High modulus of elasticity to resist buckling loads Various types of joints possible Flanges provide a rigid connection to gates and

valve Has the ability to be easily used as a redundant

system

Disadvantages of Steel pipes High material costs Requires a concrete encasement

for significant and high hazard embankment

Requires special linings at reservoirs

The proper selection of linings and coatings and any associated maintenance are required to prevent corrosion.

14.Construction14.1 Construction of road to access the site

14.2 Leveling and excavation of the damsite The site need to be leveled and the required

ditching should be done to make the site ready for the embankment construction and ultimately dam construction

14.3 Clearing The area to be covered by the embankment* should be pegged out prior to commencement

of any works. The embankment and the area to be excavated should be cleared and grubbed. Topsoil should be heaped in areas outside of the area to be covered by the embankment and

all trees, scrub and roots removed. Topsoil should be placed in layers not exceeding 200 mm and planted with grass if it is to be

left for a considerable time (more than 6 months).

14.4 Foundation construction 14.4.1 Grouting Grout holes for the cut-off curtain are drilled to a depth where the grout curtain will effectively

seal off the seepage of water beneath the proposed data

14.5 Installing penstocks Then the next step is the installing of Penstock.

14.6 Embankment compaction All fill material for the embankment should be

placed in layers (or lifts) no greater than 150mm thick.

The largest size particle should not be greater than 1/3rd the height of the lift, that is, 50mm.

Each layer should be thoroughly compacted before the next layer is place

The compaction effort achieved should be on average 98% Standard Maximum Dry Density

The minimum compaction effort should be 95% Standard MDD

The material forming the embankment should be placed with sufficient moisture to ensure proper compaction

Before each additional 150mm lift is added to the embankment, the preceding lift should be scarified to ensure that the two lifts are properly joined

A wheeled scraper or truck should be used for placing the clay on the dam site

(Installing the Penstock)

(Compaction of Embankment)

14.7 Settlement of the embankment Settlement of soil banks is common and an allowance must be made for settlement of the dam

embankment. The embankment may settle to a level where it is overtopped by water and failure will result. Or overtime settlement may result in the height of the embankment becoming lower than the

spillway.14.8 Vegetation Topsoil should be spread over the exposed

surfaces of the embankment to a depth of at least 150mm and sown with pasture grass to establish a good cover as soon as possible.

Never allow any vegetation larger than pasture grass to become established on or near the embankment.

Tree roots, especially eucalyptus tree roots can cause the core to crack resulting in the failure of the dam.

As a rule of thumb, trees and shrubs should be kept to a minimum distance of 1½ times the height of the tree away from the embankment of the dam.

This especially applies to eucalypts. (Vegetation on Embankment)

14.9 SpillwayThe purpose of the spillway is to pass flood flows without overtopping the dam wall. Particular attention must be paid to providing adequate width and depth (or freeboard) of the spillway as per the specifications given in the dam permit.  The following guidelines apply to spillways: The absolute minimum width of a spillway is three meters. Minimum spillway dimensions are given on the permit.

Thank you for your time and attention!

Congratulations Dam Construction completed!