Erosion and sedimentation effects of reservoirs

Land surface is eroded by natural phenomenon such as precipitation, overland and channel flow, freeze-thaw action, wind and gravitation as well as by human activity. The magnitude (or rate) of erosion by water depends on rainfall regime, soil type and aggregation, land slope, catchment size and vegetational cover. The effects of these factors on the rate of erosion are discussed either qualitatively or quantitatively in this paper. Erosion of land surface results in the production of loose particles, termed sediments, which are carried downstream by overland flow. Some of these sediments reach streams and are transported to lower elevations as suspended sediments (suspended or wash load) or as sediments of the bed (bed load). Formulae or analytic methods to estimate suspended and bed loads of streams are discussed briefly, their limitations are emphasized and practical means of sediment measurement are described. Construction of a dam in a basin results in trapping a major portion of sediment carried by a stream (also called sediment yield of a basin) and also causes a severe erosion of the river channel downstream. These effects are exemplified by a case study which reveals the dangers involved in using empirical formulae to estimate the sediment yieldmore » of a basin.« less