CHANNEL CONNECTING TWO RESERVOIRS OR LAKES 91 



discharge. The distance upstream that the effect is appreciable depends 

 upon the proportions of the channel, its roughness, and the pool level. 

 It may vary from a few hundred feet to a number of miles. It should 

 be noted that the velocity head of the stream at the end of the channel 

 is lost, the energy going into eddies in the reservoir pool. 



Entrance to a uniform channel. The discharge into a long uniform 

 channel leading from a reservoir or lake depends upon the following 

 factors, which are listed in order of importance. 



Channel slope critical or steep : 



1. head upon the crest. 



2. shape of the entrance, contraction. 



3. friction loss in the entrance. 



Channel slope mild: 



1. normal flow in the channel at total head equal to the head on 

 the crest minus the 



2. friction loss in the entrance. 



Whether the slope is steep, critical, or mild depends, in turn, upon the 

 discharge, so that we have a vicious circle. Problems are solved by 

 the method used for computing flow from an adverse slope to a sus- 

 taining slope. Friction loss over the adverse slope is replaced by 

 friction loss in the entrance section, which may be taken into account 

 by applying an appropriate velocity coefficient, as is done for orifices 

 and short tubes. 



If the entrance to a channel of mild slope is not well rounded, eddies 

 will develop, and the resulting energy loss will be comparatively high. 

 A poorly rounded entrance to a channel of steep slope may cause con- 

 traction, with actual separation from the walls, and a corresponding 

 decrease in the discharge. It is of interest to note that any instability 

 in the flow at the entrance of channels with critical or steep slope is 

 likely to cause disturbances similar to the inversions of a free jet, 

 decreasing the expected capacity by encroaching upon the freeboard. 



Channel connecting two reservoirs or lakes. Two large reservoirs 

 or lakes are connected by a channel. The water level in each reservoir 

 is subject to slow changes, and the flow in the connecting channel may 

 be in either direction. It is desired to know, for any given combination 

 of reservoir elevations, what the discharge in the channel will be. The 

 rate of change of the lake elevations is so slow that flow in the channel 

 may be considered to be steady flow, at any instant. 



Consideration of the method to be used in summarizing the ultimate 

 solution of the problem is advisable before starting the detailed compu- 



