In this article we will discuss about:- 1. Introduction to Reservoir Sedimentation 2. Measurement of Sediment Load 3. Density Currents 4. Useful Life of a Reservoir 5. Method for Determining Useful Life of a Reservoir 6. Control of Sedimentation.

Contents:

  1. Introduction to Reservoir Sedimentation
  2. Measurement of Sediment Load
  3. Density Currents
  4. Useful Life of a Reservoir
  5. Method for Determining Useful Life of a Reservoir
  6. Control of Sedimentation of Reservoirs

1. Introduction to Reservoir Sedimentation:

All rivers carry certain amount of sediment (or sediment load) which is produced due to erosion in their catchment areas.

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The amount of sediment in a river depends on the extent of erosion in the catchment area which depends on the following factors:

(i) Nature of soil in the catchment area

(ii) Topography of the catchment area

(iii) Vegetal cover

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(iv) Intensity of rainfall

If the soil in the catchment area of a river is loose and easily erodible, the river will bring in large amount of sediment. On the other hand a river will carry less sediment if the soil in the catchment is hard and not easily eroded. Further the catchment areas with steep slopes will give rise to high velocities of flow and hence cause more erosion of the surface soil.

As such rivers having catchment areas with steep slopes will bring in more sediment. Similarly the catchment areas having poor or practically no vegetal cover will be more easily eroded and hence rivers having such catchment areas will carry more sediment.

On the other hand the catchment areas having thick vegetal cover will produce less sediment. Again higher intensity of rainfall in the catchment area will cause greater runoff and more erosion, thus more sediment will be produced.

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The sediment load carried by a river may be divided into the following two parts:

(i) Suspended load

(ii) Bed load

The suspended load (or suspended sediment) is that part of the sediment load which does not move in contact with the bed of the river but is held in suspension against gravity by the vertical component of the eddies in the turbulent flow. It consists of relatively finer material which remains dispersed throughout the flow cross-section of the river.

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The bed load is that part of the sediment load which moves in contact with the bed of the river. It consists of relatively coarser material.

When the sediment laden water of the river approaches the reservoir, the velocity and turbulence are greatly reduced due to which the larger suspended particles and most of the bed load are deposited as a delta at the head of the reservoir. The finer particles remain in suspension for a longer time and are deposited farther down in the reservoir. However, some of the very fine particles may be carried with water discharged through the outlets or the spillway.

The deposition of the sediment in the reservoir is known as reservoir sedimentation or reservoir silting. In order to allow for such deposition of sediment in a reservoir (or silting up of reservoir) certain percentage of the total storage is usually left unutilised is called dead storage.

The dead storage is usually not more than one-fourth of the total capacity of the reservoir. The water surface is never allowed to be dropped below the level of the dead storage. As such all the outlets drawing water from the reservoir are provided above the level of the dead storage.

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2. Measurement of Sediment Load:

The suspended sediment load of a stream is measured by taking the samples of water at various depths. The samples are weighed and filtered to remove the sediment. The sediment is then dried and weighed.

The sediment load is expressed in parts per million (ppm) which may be obtained by dividing the weight of the sediment by the weight of the water sample (i.e., the weight of the sediment and water in the sample) and multiplying it by 106. However, there is no practical device available for the measurement of bed load, which is estimated to be about 15% of the suspended sediment load.

The average annual sediment transported by the stream may be obtained by multiplying the weight of the sediment transported per unit volume of water by the average annual inflow of the stream.

3. Development of Density Currents in Reservoir:

Density current may be defined as a flow of one fluid under another fluid of a slightly different density under the effect of gravity. In many reservoirs density currents are usually developed, especially during floods when rivers carry heavily sediment-laden or muddy water. The two fluids involved in the development of a density current in a reservoir are the sediment-laden water and the water already stored in the reservoir which is relatively clear due to the deposition of the sediment.

When a sediment-laden water enters a reservoir it plunges downwards and moves towards the dam along the bed of the reservoir as a density current below the reservoir water. The reservoir water being relatively clear, has a slightly lower density, than that of the sediment-laden water.

It is because of this density difference, the sediment- laden water of the density current does not mix readily with the reservoir water but maintains its identity for a considerable time. However, in addition to sediment the density difference may also result from dissolved salts or minerals, or from temperature difference which may also develop a density current.

4. Useful Life of a Reservoir:

The deposition of sediment gradually reduces the available storage capacity of a reservoir. As such with more and more deposition of sediment in a reservoir when its useful capacity is reduced so much that it is not able to serve the required purpose, then the useful life of the reservoir is considered to be over.

The sedimentation of a reservoir is measured in terms of trap efficiency. The trap efficiency is defined as the percent of the inflowing sediment which is retained in a reservoir. The observations on various reservoirs have indicated that the trap efficiency is a function of the ratio of reservoir capacity to total inflow, i.e.-

Figure 3.19 shows a plot of trap efficiency versus capacity-inflow ratio on the basis of the data obtained from existing reservoirs. It may be observed from Fig. 3.19 that the trap efficiency is low for reservoirs having small capacity- inflow ratio and it gradually increases as the capacity-inflow ratio increases. Thus if a reservoir of small capacity is constructed on a stream having a large inflow rate, it will have a small capacity-inflow ratio and a low trap efficiency.

This is so because most of the inflow will be discharged to the downstream so quickly that the suspended sediment will hardly have any time to settle in the reservoir. On the other hand if a reservoir of large capacity is constructed on the same stream it will have a higher capacity-inflow ratio and a higher trap efficiency because in this case water will be retained for a longer duration and will thus permit most of the suspended sediment to settle in the reservoir.

Further the trap efficiency of a reservoir will be more in the beginning but it will decrease as the capacity of the reservoir will be reduced due to sedimentation. Thus complete silting up of the reservoir may take a very long time. However, in general when about 80% of the useful capacity of a reservoir gets silted up the useful life of the reservoir is considered to be over.

For economic justification of a reservoir it is essential to determine its probable useful life, for which the following method may be adopted.

5. Method for Determining Useful Life of a Reservoir:

1. Determine the required capacity of the reservoir. Also determine the average annual inflow of the stream and calculate the capacity-inflow ratio. For this value of capacity-inflow ratio obtain the trap efficiency from Fig. 3.19.

2. Divide the total capacity of the reservoir into suitable interval say 10%. Assuming that 10% of the reservoir capacity has been silted up, the available capacity is reduced to 90% of the total capacity. Calculate the capacity-inflow ratio by considering the reduced capacity (i.e., 90% of the total capacity) and the same average annual inflow as taken in step (1). Again for this value of capacity- inflow ratio obtain the trap efficiency from Fig. 3.19.

3. Find the mean of the trap efficiencies obtained in steps (1) and (2) which may be taken as the average value of the trap efficiency for this 10% capacity of the reservoir which is assumed to have been silted up.

4. Determine the average annual sediment transported by the stream. Multiply the average annual sediment by the mean trap efficiency determined in step (3) to obtain the amount of sediment deposited annually in the 10% capacity under consideration. Convert the sediment deposited from weight units to volumetric units such as hectare-metre by considering a suitable value of specific weight of the sediment deposited.

5. Divide the 10% capacity of the reservoir by the volume of sediment deposited annually as determine in step (4) to obtain the number of years which will be taken to fill this 10% capacity of the reservoir by sediment.

6. Repeat the same procedure and obtain the number of years which will be taken to fill each of the next 10% capacities of the reservoir, i.e., by considering 80%, 70% ,60%, 50% etc., as the available capacities until the available capacity of the reservoir is reduced to 20%. The sum of the years which will be taken to fill each of the 10% capacities of the reservoir will give the probable useful life of the reservoir.

Illustrative Example shows the above described method for computing the useful life of a reservoir.

Example:

A reservoir has a capacity of 6 million cubic metre and a drainage area of 300 sq.km. The annual inflow is equivalent to 400 mm of runoff from the given drainage area and annual sediment transported by the stream is equivalent to a weight of 12 million newton per sq.km. of drainage area. The sediment has an average specific weight of 15000 newton per cubic metre.

Determine the number of years it will take to reduce the capacity to 5 million cubic metre. Take trap efficiency as 90%.

Solution:

6. Control of Sedimentation of Reservoirs:

In order to increase the useful life of a reservoir it is necessary to control the deposition of sediment in the reservoir.

The various methods which may be adopted to control the deposition of sediment in a reservoir may be divided into the following two groups:

(i) Pre-Construction Methods:

Pre- construction methods are those methods which may be adopted before and during the construction of a dam and creation of a reservoir.

These methods are as indicated below:

(a) Selection of Reservoir Site:

The reservoir sedimentation can be controlled by selecting such a site for the reservoir where the sediment inflow is low. Thus if alternative sites for a reservoir are available then a site which excludes runoff from an easily erodible part of the catchment area should be preferred to the one which includes this runoff. Similarly if tributary of a main river carries more sediment, then the site for a reservoir on the main river should be on the upstream of the confluence of this tributary and the main river, in order to exclude the water from this tributary entering the reservoir.

(b) Reservoir Design:

A reservoir of small capacity on a river having large inflow rate, the rate of sedimentation is less than that for a reservoir of a large capacity on the same river. Thus the rate of sedimentation of a reservoir may be controlled by designing the reservoir in such a way that instead of providing the entire designed capacity at the first instance the capacity of the reservoir may be increased in stages.

For this initially a reservoir of small capacity may be created by constructing the dam upto a certain lower height. Later on when a portion of the reservoir gets silted up its capacity may be increased by raising the dam to the desired height. However, in this case some provision will have to be made in the design of the dam so that it may be raised in stages.

Further an adequate number of outlets may be provided in the dam at different elevations, so that the flood water which is usually heavily sediment- laden may be discharged to the downstream without much of the sediment being deposited in the reservoir.

(c) Control of Sediment Inflow:

The inflow of sediment to a reservoir can be controlled by constructing check dams and by providing vegetation screens.

A check dam is a small dam usually constructed across a stream to trap most of the coarser sediment transported by the stream. Thus inflow of sediment can be controlled by constructing check dams across those tributaries which might be carrying relatively more sediment. The check dams may also be constructed across gullies to retain some sediment and prevent it from entering the stream.

Vegetation screens may be developed by promoting the growth of vegetation in the catchment as well as at the entrance to the reservoir. The vegetation screens would trap a large amount of sediment if flood waters pass through them before entering the reservoir and hence help in reducing the sediment inflow.

(ii) Post-Construction Methods:

Post-construction methods are those methods which may be adopted after the construction of a dam and creation of a reservoir.

These methods are as indicated below:

(a) Control of Sediment Deposition:

The deposition of sediment in a reservoir can be controlled to some extent by designing and operating the outlets in a dam in such a manner that the water having higher sediment content is discharged to the downstream through the outlets.

The sediment content of water in the reservoir is relatively high during and immediately after flood and if most of the water can be discharged to the downstream at such times, the amount of sediment trapped in the reservoir will be reduced. Moreover the concentration of sediment is more at some levels and if water can be withdrawn from these levels through the outlets provided there the deposition of sediment in the reservoir will be reduced.

(b) Removal of Sediment Deposits:

The sediment already deposited in a reservoir may be removed either by excavation, dredging, etc., or by scouring through sluices in the dam. The removal of the deposited sediment by excavation, dredging, etc., is however not feasible in most of the cases except in the case of some very small reservoirs. There are certain limitations in the use of sluices for removing the deposited sediment.

For scouring the deposited sediment the sluices are to be provided near the bottom of the dam which involves a lot of structural problem and hence except for some small dams such sluices are usually not provided. Moreover for scouring of the deposited sediment through sluices considerable amount of water is lost.

Further it is also found that the sluices are not much effective in the removal of the deposited sediment because the water flowing through the sluice scoops out a narrow channel for itself on the upstream side of the sluice and leaves the bulk of the deposited sediment undisturbed.

However, to some extent the scouring action may be made more effective if simultaneously with scouring the deposited sediment is loosened and pushed towards the sluices by mechanical means. The sluices are therefore generally used for allowing the flood waters to escape to the downstream with minimum detention in the reservoir so that much deposition of sediment does not take place.

(c) Erosion Control in the Catchment Area:

The control of erosion in the catchment area will reduce the inflow of sediment to the stream and consequently there will be less inflow of sediment to a reservoir created on the stream. The erosion in the catchment area may be controlled by the use of the methods of soil conservation within the area.

The various methods of soil conservation are afforestation, control of deforestation, regressing and control of grazing, control on terrace cultivation and provision of contour bunds, checking gully formation by providing small embankments, providing check dams in the gullies formed, etc. These methods of soil conservation are quite effective in the control of sedimentation of reservoirs but are costly and would show appreciable results only after considerable time.

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