Text B

In every stream some suspended particles are carried by water flow along the river bed. They are called the bed load. Since the specific gravity of soil materials is about 2.65, the suspended particles tend to settle to the channel bottom but upward currents counteract the gravitational settling. When water reaches reservoir, the velocity and turbulence are greatly reduced. The larger suspended particles and most of the bed load are deposited at the head of the reservoir. Smaller particles remain in suspension longer and deposit farther down the reservoir, although the very smallest particles may remain in suspension for a long time and some may pass the dam with water discharged through sluiceways, turbines or spillways.

All reservoirs are filled with sediment. If the sediment inflow is large compared with the reservoir capacity, the useful life of the reservoir may be very short. Small water-supply reservoirs can be filled with sediment during the first year after their completion. Reservoir planning must take into account data on the probable rate of sedimentation in order to determine whether the useful life of the proposed reservoir will be sufficient to warrant its construction.

Actually reservoir sedimentation cannot be prevented but it may be retarded. One way of doing it is to select a site where the sediment inflow is low. After a site has been selected it is necessary to construct such a reservoir the capacity of which would be large enough to ensure a prolonged useful life of the reservoir. Although trap efficiency of large reservoirs is high it does not increase linearly and the useful life of a large reservoir is longer than that of a small reservoir if all other factors remain constant.