EROSION AND SILTING 



345 



sluggish flow favours the growth of aquatic plants, while any change of 

 velocity from high to low in a stream carrying material in suspension, 

 causes a deposition of a portion of the material, and a consequent silting 

 up of the channel. In order to prevent deposit in small sewers or 

 drains, a mean velocity of not less than 3 feet per second is necessary. 

 For sewers from 12 to 24 inches diameter the velocity should not be less 

 than 2*5 feet per second, while with larger sizes than this the velocity 

 may be reduced to 2 feet per second. 



Mr. R. G. Kennedy, 1 from observations on a large number of Indian 

 irrigation canals, concludes that there is a certain critical velocity at 

 which a long canal will maintain its channel in silty equilibrium. This 

 velocity is given by v = c h ' 64 feet per second, where c has the following 

 values : 



Light sandy soil 

 Coarse sandy soil 

 Sandy loam . 

 Coarse silt 



c = -82. 



90. 



99. 



1-07. 



Where a main canal supplies or is supplied by feeders, the various 

 depths and velocities of flow should be adjusted to suit this relationship 

 in order that there may be no silting or erosion in the main or feeder 

 canals. 



If v be any other velocity, and if q and q be the amounts of silt 

 carried respectively at v and v. 



= Vo 2 approximately. 



Taking c = '84, the following table shows suitable mean velocities of 

 maximum flow for equilibrium of such channels in sandy soil : 



Ganguillet and Kutter give the following as thn safe bottom and mean 

 velocities, but state that these are probably too small rather than too 

 large : 



" Proc. Inst. C.E.," vol. 119, 1874-5, p. 281. 



