Ch. 21] CAUSE OF SILTING 365 



to reduce or eliminate sedimentation, and (2) how to reduce the cost 

 of removal. The first is of prime concern to engineers and geologists; 

 the second chiefly to engineers. 



Before attempting an analysis of these problems to see what can 

 be done about them, it is desirable to consider the effects of sedimenta- 

 tion of canals in order that the practical application of the problems 

 can be understood. There are five major effects of canal sedimenta- 

 tion. The first three of these can be classed as adverse; they are: 

 (a) changes in hydraulic properties of the canals; (b) providing a base 

 for weeds and water plants; (c) increasing the maintenance costs. 

 Two of the effects can be classed as favorable, and they are: (a) seal- 

 ing of crevices and porous areas in the canals; (b) provision of a source 

 of fertilizer to lands beyond the farm turnout. 



A brief discussion of each of these effects is important to an under- 

 standing of the problems and the solutions sought. It is apparent 

 that deposits of silt in a canal will change its hydraulic properties. 

 If the grade is changed, the velocity of the canal is altered. If the 

 cross section is changed, the area of the canal is changed. Both 

 changes affect discharge or carrying capacity. Almost always the 

 velocity and cross section are reduced so that the net effect is cumu- 

 lative; that is, the volume of water carried by the canal is reduced. 



For example, examine the effect of six inches of silt deposit on the 

 bottom of representative canals. For a canal with a width of 6 feet 

 and a depth of 3 feet, r = 1.86, n = 0.0225, s = 0.00040, v = 1.95, 

 the design capacity Q is 52.7 cubic feet per second. With a silt de- 

 posit of 6 inches, Q reduces to 37.6 cubic feet per second, a loss of 

 28.7 percent in carrying capacity. Similarly, 6 inches of silt will re- 

 duce the initial capacity of a larger canal from 126 to 100 cubic feet 

 per second, or 20.6 percent, and that of a main canal from 458 to 400 

 cubic feet per second, or 12.7 percent. 



It is evident from these simple computations that the control and 

 removal of silt from irrigation canals are important, particularly 

 in the smaller canals. If the silt cannot be removed from the water 

 before it enters the canal system, ways must be found to remove the 

 deposits as cheaply as possible from the canals. If the canals are not 

 kept relatively clean, sufficient irrigation water cannot get through, 

 and crop damage results. But, if the expense of removal is too great, 

 it will exceed the capacity of the farmer to pay, again leading to eco- 

 nomic failure. The cost of cleaning canals balanced against the cost 

 of desilting works will often show that the desilting works are a 

 cheap investment. 



