346 



HYDEAULICS AND ITS APPLICATIONS 



Actually, as indicated by Kennedy, the safe mean velocity depends also 

 upon the depth. More recent work shows that at medium depths 

 through firm loamy soil a mean velocity of 3*0 to 3*5 feet per second is 

 safe, while with fine well-rammed gravel or loose rock this may be 

 increased to from 5 to 7 feet per second. In a concrete-lined channel 

 faced with cement, the maximum safe velocity with water which carries 

 solid material in suspension is about 9 feet per second. A higher 

 velocity wears and roughens the bottom until the roughness thus 

 produced reduces the velocity sufficiently to prevent further erosion. 

 With an ordinary brick or heavy dry-laid rubble channel, the velocity 

 should not exceed 15 feet per second, any higher velocity necessitating a 

 carefully-laid facing of heavy masonry with cemented joints. 



ART. 98. GAUGING OF FLOW IN STREAMS AND OPEN CHANNELS. 



Many methods are available for obtaining the discharge of a stream, 

 these differing widely in the accuracy of their results and the cost and 

 difficulty of their application. The method to be adopted in any case 

 depends largely on the degree of accuracy required and on the size of the 

 stream. 



The accuracy of a discharge measurement, whatever be the method 

 adopted, depends greatly on the physical characteristics of the stream at 

 the point of measurement. If possible this should lie on a straight 

 reach and away from the influence of a bend, the bed should be per- 

 manent and not strewn with boulders, and the slope and wetted 

 perimeter such that at all stages of the stream the velocity at all parts 

 of the section may be easily measurable. The banks should be sufficiently 



