Tarbela Reservoir

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A presentation by

Muhammad Suleman Bahca


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The current study focuses the siltation problem in

Tarbela reservoir.

Siltation is caused by the gradual deposition of

the silt transported by the river upstream.

Tarbela reservoir is a major source of Pakistan in

terms of Electricity production and irrigation

supplies.

Tarbela dam was constructed in 1974 but now theSiltation has reached a stage where major

problems have occurred which have to be dealt

early.

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Introduction


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To study the current prevailing situation of

Siltation in Tarbela reservoir.

To study the annual deposition rate of

sedimentation in Tarbela reservoir.

To study various impacts of Siltation on the

reservoir.

To check out the possible remedial measures to

minimize the effects of Siltation in Tarbelareservoir.

Objectives

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Tarbela Reservoir

Tarbela is one of the worlds greatest water

resources development project built on the river

Indus

It is located about 100 km North West ofIslamabad

The construction work on Tarbela Dam was

started in 1968 and all the civil works were

completed in 1974. Salient features of the Project are given in Table

in the following slides.

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Characteristics Values

Location Distt. Haripur, River Indus

Completion year 1968

Length of the main dam 2744m

Reservoir length 97km

Reservoir area 260km2

Maximum depth 137m

Dam height 148m

Gross storage 14.34 BCM

Live storage 11.95 BCM

Dead storage 2.39 BCM

Mean Annual inflow 79 BCM


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Maximum gross capacity level 472m

Actual minimum operating level 396m

Minimum reservoir level in 2006 417

Capacity below min. reservoir level 1.55 x 109 m3

Hydro power capacity 3478MW

Electrical energy generation 14.8 GWh yr-1

Number of tunnels for electricity 3

Number of tunnels for irrigation 2

Tunnels diameter (constant) 13.7m

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To provide an 11.5 billion cubic metres (bcm) storage dam

on River Indus in order to partly replace the water of

eastern rivers ceded to India, and provide additional

supplies during the low flow period to facilitate further

development of irrigated agriculture.

To increase food production to achieve self sufficiency,

especially in wheat.

To generate cheap hydropower through staged development

of 2100 MW capacity.

Purpose of Tarbela Reservoir

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Siltation or sedimentation is a process of gradual deposition

or accumulation of silt in the water bodies i.e. Streams,

Reservoirs (Dams).

Indus River is one of the largest sediment producing rivers in

the world.

The main source of sediment is from the glacial landscape

and erosion from steep sided barren slopes.

The predicted rate of sediment inflow was 0.294 bcm per

year meaning that the dam would silt up to 90% capacity in 50

years

Siltation

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In practice, the actual sediment inflow rate has been

significantly lower than predicted, with an average rate of

0.106 bcm, i.e. 36% of the predicted inflow rate.

The useful life of the dam is now considered to be 85 years.

Average annual sediment deposit 0.134 x 109 m3

Sediment volume up to 2006 4.23 x 109 m3

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Siltation contn


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The drainage area of Indus at Tarbela Dam is 169,600km2out of which 10,400 km2lies immediately above TarbelaDam, falls in the active monsoon zone with annualprecipitation range of 800-1500 mm.

Additional 7700 km2 located further northward is alsoexposed to the receding effects ofmonsoon rains annuallyaveraging 600 to 1200 mm.

The bulk of Indus drainage area, 94% of total catchmentarea, lies outside the monsoon belt.

The northern areas generally receive scanty winter rains andprecipitation largely in the form of snow.

The moving glaciers crush rocks on their way and leavebehind a lot of sediments when they melt which is carriedby the river due to steep gradient.

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Sedimentation Process


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The velocity of the inflows containing the sedimentdecreases upon entering Tarbela reservoir, which reducesthe sediment carrying capacity of the river water.

The coarse sediment tends to deposit in the upper reaches

of the reservoir, while the finer particles travel downstreamtowards the dam and settle in the reservoir.

Young geological formations of erodible nature in the Induscatchment are also responsible for higher sediment yield.

Sediment carried by Indus is deposited in the reservoir at an

annual rate of about 200 million tons, corresponding toabout 98 % of the sediment inflow.

More than up to 80% of the sediment load is transported inthe reservoir during the months of July and August.

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S.Proess contin


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The average composition of the deposits determined from

samples collected from Tarbela reservoir is

28% of sand

55% of silt

and

17% of clay

Composition of the Deposits

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The predicted rate of sediment inflow was 0.294 billion m3annually.

but the actual sediment inflow rate has been significantlylower with an average rate of 0.106 billion m3 which is 36%

of the predicted rate. one aspect of the sediment deposition which was however

not expected so early is the rapid advancement of thesediment delta front which is 10.6 km from the dam in2006.

This endangers all the low level outlets including the powerstation which could be blocked by the liquefaction of thesediment under the severe earthquake.

The minimum reservoir drawdown level was raised frominitial 396 m to 417 m until 2006.

Current status

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The capacity of reservoir is decreasing due to this heavy

sediment load.

The remaining storage capacity calculated from

hydrographic survey 2005 is given in the table below

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ReservoirCapacity

Initial(1974) BCM

Year 2005BCM

Reduction %

Gross

Storage

14.344 10.295 28.23

Live Storage 11.948 8.695 27.22

Dead Storage 2.395 1.598 33.30


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A loss oflive storage, which is causing gradual reduction inthe regulated yield of reservoir. This in turn would result inreduction in water availability for the agriculture indownstream mainly Punjab and Sindh.

The physical effect of sediment, which includes the risk ofclogging of low level tunnel outlet particularly in a seismicactivity, the erosive action of sediment-laden water onoutlet concrete structures and Power turbines will result inexorbitant maintenance costs.

Due to low storage available in the reservoir, chances offlood situation also increase.

Sedimentation can also affect the aquatic fauna, mainlyfisheries.

Problems Encountered due to Siltationof Tarbela Reservoir

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The most desirable method for controlling reservoir

sedimentation is probably by reducing soil loss and erosion

from the reservoir watershed.

Can be achieved by

reducing sediment inflow through soil conservation,

watershed management,

Gully control,

vegetative screens building check dams.

Problem of having difficult to implement

Preserving Reservoir Capacity

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Poor or improper land use and management are

the major contributing factors to reservoir

sedimentation.

The need for watershed management to reducesediment input into the reservoirs is suggested

widely.

Measures like strip cropping, forestation, crop

rotation, terracing, and gully control cansignificantly reduce the soil loss.

Watershed Management

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Building debris dams or check dams (or trap dams)

as another way of reducing sediment inflow into

the reservoirs

It can control the flow of coarse sediments intothe reservoir.

These deposits can be flushed or bypassed via

diversion tunnels or pipes downstream of the

main reservoir dam during high flows, orperiodically cleaned during low flows when the

deposits are exposed.

Check Dams

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Reduction of sediment influx either by watershed

management or by construction of check dams in the upper

catchment is impractical as about 90% of total runoff isdominated by snow / glacier melt.

Nothing can be done at this attitude on the steep

mountains.

Most of the catchment area is out of the monsoon zone.

Water shed Management is being implemented by the NWFPForest Department up to Besham and it has very little

effect.

Diamer Basha Dam shall have some positive impact as it

would enhance its life.

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Density currents are caused by the difference in

density of the relatively cleaner reservoir water

and the sediment-laden incoming stream flow.

Density differences can be generated either bythe high concentrations of suspended sediment

particles in the incoming stream flow, or by the

temperature differences between the stream flow

and the reservoir waters.

Venting Density Currents

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Frequent flushing of sediments is done during the

high-inflow season

The purpose of regulating the flows during the

flood season is to release as much sediment aspossible from the reservoir.

This takes advantage of the silt-carrying capacity

of floodwaters in the reservoir and in the

downstream reaches of the river. This operation greatly reduces sediment

entrapment in the reservoir.

Flood Flushing

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Drawdown flushing is different from venting density

currents and flood flushing.

It is implemented to evacuate the sediments that

have already been deposited in the reservoir. Drawdown (or hydraulic) flushing involves the

release of water from a reservoir through a low-

level outlet, while reducing the pool to an

allowable minimum level.

Depends on the reservoir topography, outlet

capacity and elevation, characteristics of the

incoming sediment material, operation of the

reservoir, and duration of the flushing operation.

Drawdown Flushing

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In the case of Tarbela reservoir Evacuation of 200 million

tons of yearly sediments by flushing through four low level

high capacity outlets from the left bank has been proposed

by the consultants.

This option would comprise four 12 m diameter tunnels

driven through the left abutment.

WAPDA considers the under water dyke and the four tunnelsan unprecedented option, the example of which does not

exist else where in the World. Moreover, this option would

in no time adversely affect the downstream hydropower

Project of Ghazi Barotha and Chashma and kill them much

earlier.

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Dredging is an Excavation activity or operation usuallycarried out at least partly underwater, in shallow seas orfresh water areas with the purpose of gathering up bottomsediments and disposing of them at a different location.

One of the most popular methods in dealing with reservoirsedimentation

It is the most controversial issue in reservoir storagerehabilitation.

It has both environmental benefits and hazards.

One of the major environmental concerns is that dredging resuspends sediments which may release toxic substances.

If the removed sediments contain hazardous material,disposal of the dredged material may be a major problem.

Sediment Dredging

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The dredging option in case of Tarbela reservoir is not only

prohibitive in cost but also is without any precedence and

impractical.

Any dredging proposal to be effective must provide for

removal and disposal of 550,000 tons of sediments every

day.

Realistically, the target is unattainable even if hundred of

dredgers and ancillary equipment are deployed over the

reservoir stretch of 50 Sq. Km. to work round the clock.

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Heightening the dam can also be considered

but it will only increase in the live storage equal

to few years of sediment inflow.

There are some shortcomings attached to thismethod like:

The gates of the service and auxiliary spillways

would then also be modified.

The stability of the dam must also be studied

before taking out the process.

Heightening of the dam

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It is the last option left when the production and

benefits from the dam will become less than the

management cost of the project.

But as our country is in a crucial financialposition, this option should be kept under the

pillow.

Replacement of existing dam byanother

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Reservoir capacity at Tarbela is being lost at the rate of about 0.132

BCM per year.

The predicted rate of sediment inflow in the reservoir was 0.294

billion m3 annually which envisages that the sediment load will fill up

the capacity of Tarbela reservoir up to 90% in 50 years.

After that, the dam will continue to provide only about 1.2 billion m3of live storage.

The usable life of the dam is considered to be 85 years now but the

usable storage will gradually decline over this time.

Flushing of sediments at Tarbela through the existing tunnels during

low reservoir level is on the average 7% of total incoming sediment.

The remedial action in terms of dredging & excavation and disposal of

upstream deposits are not economical. Studies for sediment flushing

with the existing outlets should continue at Tarbela.

Conclusions

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Tarbela Reservoir

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