Ch. 20] LAYOUT OF RANGES 351 



MEASURING SEDIMENT IN RESERVOIRS 



Reservoir-sedimentation surveys determine the amount of material 

 deposited in a reservoir and show where the deposits exist within the 

 reservoir. Successive surveys provide data for tracing the movement of 

 sediment through or within a reservoir. Primarily, the practical objec- 

 tive is to obtain accurate information on which to base estimates, first, 

 of the length of time that will elapse before any of the purposes of the 

 reservoir are interfered with and, second, of the total useful life of 

 the project (Fry, 1948). 



Volumetric measurements of sediment in reservoirs are made by 

 soundings taken to develop the configuration of the reservoir sides and 

 bottom below water surface at the time of the survey. Deposits above 

 water level are measured by bank sections obtained by ground survey- 

 ing methods. 



In small reservoirs and in special situations in larger reservoirs, 

 sufficient soundings are taken below water and elevations are taken 

 above water to permit drawing of contours for the reservoir. Com- 

 putations of the volumes of storage at different elevations based on 

 successive surveys will indicate by the reduction in storage the amount 

 of deposition at any elevation (Eakin and Brown, 1939) . 



In large reservoirs, the contour method is not generally applicable 

 because of the time and expense that would be entailed. In these 

 reservoirs, volumes are determined by soundings made along ranges 

 established usually at the time the project is built (Fry, 1948). 



Layout of Ranges 



Before a reservoir is filled, silt ranges should be carefully located 

 on a map in sufficient number and in proper locations so that sound- 

 ings on these ranges will furnish the necessary data for computation of 

 silt volumes. Range locations should consider important local tribu- 

 taries and their probable sediment characteristics. A closer spacing 

 of ranges is usually more desirable in the upper and shallower end of 

 the reservoir than in the lower and deeper portion. The width of reser- 

 voir is important. If there is a system of reservoirs on one stream such 

 as that on the Tennessee River, the location in the system with respect 

 to protection from upstream reservoirs may be significant. In reser- 

 voirs subject to considerable drawdown, closer spacing of ranges within 

 the drawdown reach is desirable, as the deposition and movement of silt 

 throughout this range are greater than in other parts of the reservoir. 



