DISTRIBUTION OF VELOCITY IN OPEN CHANNEL 337 



Mean Velocity in a Vertical. 



Actually the position of the filament of maximum velocity in a vertical 

 is not of great importance. That of the filament of mean velocity, which 

 is at a greater depth, is however very important in stream gauging, since, 

 if it be known, the operation of gauging reduces itself to the measurement 

 of a single velocity at this depth in each of a series of verticals distributed 

 across the stream. 



The depth of the latter filament varies from about '5h to 'lh, having 

 the former value in a wide and shallow stream of depth less than about 

 2'0 feet, and the latter value in a smooth wooden or cement channel 

 whose depth is approximately one-half the width. In the great majority 

 of cases in practice it lies between '55h and *65/i increasing with the 

 depth and diminishing with the roughness of the channel. An examina- 

 tion of a large number of river and canal gaugings by members of the 

 U.S. Geological Survey 1 leads to the following as the most probable 

 values of its depth. 





Generally speaking, the velocity at six-tenths depth in any vertical 

 will give the mean velocity in that vertical within 5 per cent, except in 



U. S. Geol. Survey, Water Supply and Irrigation Papers, No. 95. 



H.A. 



