348 fry. SEDIMENTATION IN RESERVOIRS [Ch. 20 



termined. These determinations may be basic in evaluating the eco- 

 nomic feasibility of projects on heavy silt-bearing streams. 



FACTORS THAT INFLUENCE RESERVOIR SEDIMENTATION 



The amount of silt that is brought to a reservoir on any stream is 

 influenced by the watershed characteristics above the reservoir, such as 

 the geology, types of cover, and the climate that prevails over the 

 area. The amount of sediment that remains in the reservoir is a func- 

 tion of the retention time of the water in the reservoir. The life of a 

 reservoir is dependent on the ratio of the reservoir capacity in acre- 

 feet to the watershed area in square miles. Where this ratio is small, 

 the life of the reservoir will be relatively small. Where the ratio is 

 large, with other conditions being the same, the life of the reservoir 

 will be correspondingly long. 



In the design of many reservoirs, provision is made for dead storage 

 and live storage. The former ordinarily is considered to provide space 

 for the deposition of sediment for a considerable period of years. It 

 is important in the life of a project to determine whether sediment de- 

 posits in the dead-storage space or whether it deposits in the live space 

 and thereby encroaches on the purposes for which the reservoir was 

 built. 



The level at which a reservoir is operated is an element of significance 

 in reservoir sedimentation. Some reservoirs, usually single-purpose, 

 are operated with relatively constant levels. Multiple-purpose reser- 

 voirs that utilize storage space jointly at different seasons of the year 

 are operated at other than constant levels and with a range in reservoir 

 elevations dictated by the various purposes for which the reservoir was 

 built. This variation in water level in a multiple-purpose reservoir is 

 significant in the deposition of silt in a reservoir and also in the move- 

 ment of silt through the reservoir. At the higher reservoir elevations 

 the silt is first deposited in the live storage space, but, as the reser- 

 voir is drawn down and succeeding storms occur, this sediment is 

 flushed down into the lower elevations of the reservoir and eventually, 

 after a number of cycles, is likely to find its way into the portion of 

 the reservoir originally provided for dead storage. 



In a series of reservoirs, such as, for example, that constructed by 

 the Tennessee Valley Authority on the Tennessee River and its tribu- 

 taries, the deposition in succeeding downstream reservoirs is more and 

 more influenced by the depositions that occur in the upstream reser- 

 voirs. Naturally, the first upstream reservoir traps the greatest amount 

 of sediment, and only the finer material passes on to the next down- 



