352 fry. SEDIMENTATION IN RESERVOIRS [Ch. 20 



Base Survey of Ranges 



After ranges have been located on the map, elevations must be taken 

 along each range to give a cross section of the reservoir at each range. 

 The ends of the proposed ranges are first monumented in the field with 

 a permanent type of monument adequately referenced so that they can 

 be found several years later. 



In canyon-type reservoirs such as those of Norris and Fontana reser- 

 voirs in the Tennessee Valley, where the sides of the valley are steep 

 and the water depth is great, an accurate profile should be taken by 

 surveying along each range prior to the filling of the reservoir. This 

 base profile before filling is preferable to a profile by echo sounding 

 immediately after filling, although, in reservoirs of very rugged topog- 

 raphy, echo sounding may be the only feasible method for obtaining 

 the original section. If there is timber along the line of the silt range, 

 this should be cleared if it is intended to sound with a lead line in the 

 future. Where echo-sounding equipment is to be used for subsequent 

 surveys, the clearing is not essential. 



In reservoirs located in terrain where the topography is less steep, 

 the depth of water is less and the width is greater than in canyon-type 

 reservoirs, it is much more economical to determine the profile along 

 the silt range by soundings after the reservoir is filled than it is to do 

 so by ground-survey work prior to filling. Bank work to supplement 

 the sounding surveys along the ends of the range may be necessary to 

 develop the section above the existing stage of water when the sound- 

 ing is done. 



Subsequent Reservoir Surveys 



The surveys either prior to or immediately after the filling of a 

 reservoir furnish the base for comparison with data from resurveys 

 made after an appropriate interval of years following closure of the 

 reservoir. Such resurveys enable determination to be made of the 

 volume and rate of reservoir sedimentation and the manner in which 

 the sediment is deposited and moves through the reservoir. 



Since resurveys usually must be made with water in the reservoir, 

 one form or another of sounding is used. For very shallow water 

 depths, a sounding pole equipped with a base plate may be used. Be- 

 yond the range of a sounding pole, the general practice until the past 

 few years has been to make soundings by conventional lead-line meth- 

 ods. Although the lead line will still be useful for surveys of reser- 

 voirs where more modern methods are not available or justified eco- 



