What is Reservoir Sedimentation?
The deposition of sediment in the reservoir is known as “Reservoir Silting’ or ‘Reservoir Sedimentation’.
In the catchment area the soil may get eroded and rocks may get disintegrated due to various reasons. The loose soil and the disintegrated rocks form the sediment which is carried by the river and gets deposited on the reservoir bed near the base of the dam.
The sediment mainly consists of sand and silt.
The process of deposition of sand and silt in the reservoir is designated as reservoir sedimentation. Every river carries certain amount of sediment load.
The sediment particles try to settle down to the river bed due to the gravitational force, but may be kept in suspension due to the upward currents in the turbulent flow which may overcome the gravity force.
Due to these reasons, the river carries fine sediment in suspension as suspended load, and larger solids along the river bed as bed load.
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 remain in suspension for much longer period, and may finally escape from the dam along with the water discharged through the sluiceways, turbines, spillway, etc.
Thus the deposition of sediment in the reservoir is known as reservoir sedimentation or reservoir silting
Sediment Load is of following types:
i) Suspended load
ii) Bed load
i) Suspended load:
The suspended load 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.
ii) Bed load:
The bed load is dragged along the bed of the river.
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.
Causes of Reservoir Sedimentation:
Following are the causes of sedimentation in reservoir:
1) Nature of soil in the catchment area :
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.
2) Topography of the catchment area:
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 more sediment.
3) Cultivation in catchment area:
The intensive cultivation in the catchment area will make the soil loose and rain water will carry alot of sediment to the river. Such rivers will carry more sediment.
4) Vegetation cover in catchment area:
If the catchment area is covered with vegetal cover like grass, plants, forest area, etc. then the erosion of the soil will be controlled and river will carry less sediment.
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.
5) Intensity of rainfall in catchment area:
If the intensity of rainfall in catchment area is high, then it will increase the rate of run-off and more erosion and the river will carry more sediment.
Measures to control Reservoir Sedimentation
In order to increase the useful life of a reservoir it is necessary to control the deposition of sediment in the reservoir.
The following are some of the methods used for the control of the deposition of sediment.
1) Proper Selection of Reservoir site :
The reservoir site should be selected in such a way so that the load of sediment in the flowing water from the tributaries is considerably low.
If it is found that a tributary carries heavy sediment, the reservoir site should be shifted towards the upstream to avoid that tributary.
The reservoir site should be such that it excludes runoff from easily crodable catchment area.
2) Reservoir Design:
The sediment trapped in the reservoir also depends upon the reservoir design. A small reservoir on a big river has lesser trap efficiency.
Hence, at the beginning the reservoir should not be constructed to its full capacity.
If the dam is constructed lower in the first instance, and is being raised in stages, the life of the reservoir will be very much increased.
During the floods, the sediment carried by the river is the maximum.
Hence, sufficient outlets should be provided in the dam at different elevations so that the water carrying heavy sediment may be discharged to the downstream by opening the outlets.
3) Control of Sediment Inflow:
The inflow of sediment to a reservoir can be controlled by constructing check dams and by providing vegetal cover over the catchment area.
These preventive measures will decrease the soil erosion and hence sediment inflow is reduced.
4) Control of Sediment Deposit :
The deposition of sediment in a reservoir can be controlled to some extent by designing and operating the outlets in a dam.
The sediment deposit in the reservoir can be controlled by proper operation of the gates of scouring sluices and the head regulators of the canals etc.
During the floods, when sediment inflow is higher, the scouring sluices must be opened to discharge the silt downstream.
5) Removal of Sediment Deposit:
The sediment already deposited in a reservoir may be removed either by excavation or through the scouring sluices provided at the bottom of the dam by loosening the sediments by mechanical agitator.
6) Erosion Control in the Catchment Area: The control of erosion in the catchment area will reduce the inflow of sediment to the river and consequently there will be less inflow of sediment to a reservoir.
The methods of soil conservation are quite effective in the control of sedimentation of reservoirs.
The various methods of soil conservation are afforestation, provision of control bunds, checking gully formation by providing small embankments, providing check dams in the gullies formed, control of cultivation, regrassing and control of grazing etc. Provision of vegetation screen helps in reducing the ‘sheet erosion’.