DISTRIBUTION OF VELOCITY IN AN OPEN CHANNEL 333 



sides and bottom of an open channel, and from analogy with pipe flow it 

 might be expected that the maximum velocity in any cross section would 

 be found in the surface and at the centre of the stream. 



While this is commonly the case in a broad, rapid, and shallow stream 



b a 



in any other case the filament of maximum velocity is usually found 

 below the surface even with a down-stream wind. Its depth varies with 

 the direction of the wind, with the depth and physical characteristics of 

 the stream, and with the velocity of flow. On a calm day it usually lies 

 at a depth between '1 h and '4 h (where h is the depth of the stream) and 

 for depths above 5 feet, has a mean depth of about '3 h. 



Fig 146, taken from a gauging by Darcy of a rectangular channel *25 



Velocity. Ft. Per Sec. 

 FIG. 147. 



metre deep and '8 metre wide, shows the general distribution of velocity 

 over various vertical sections of a rectangular channel and also the equi- 

 velocity contours in a cross section, while Figs. 147 and 148, show 



