3.1.2 Sedimentation and control of sedimentation

Sedimentation or silting of reservoirs means the deposition of silt and clay i.e. fine particles of soil at the bottom of reservoir. This silt is brought with runoff or flood water. They are in suspended condition while they are flowing with runoff or flood water. When these fine particles reach reservoir and water becomes still they try to settle down at bottom due to gravity. The coarser ones get deposited earlier due to weight.

Factors affecting sedimentation

· Catchment area – In fan shaped catchments deposition of finer soil fractions is more than fern shape catchments

· If the slope is steep, more particles are carried with water and high velocity of soil may erode more soil.

· Climatic condition – like dryness, heat, rain helps in production of silt.

· Nature of surface soil – If surface soil is loose, then silting is more.

Pre construction measures

· Selection of dam site – if catchment is less erodible silting will be less

· Construction of dam in stages – life of reservoir can be prolonged by constructing dam in stages first the dam is built lower and raised subsequently when some of its capacity get silted up.

· Construction of check dams – check dam can be constructed across river streams contributing major sediment load.

· Vegetation screens – vegetation growth is promoted of entrance of reservoir as well as in catchment to trap large amount of sediment cheap and good method.

· Construction of under sluices in dam – the dam is provided with opening in its base so as to remove more silted on d/s side.

Post construction measures

· Removal of post flood water – sediment content is more in flood water hence effort is to remove post flood water

· Mechanical stirring of sediment – deposited sediment is scoured and distributed by mechanical means, so as to push it towards sluices.

· Erosion control & soil conservation – this includes all general methods of soil conservation & reduce erosion. This is most effective method.