Ch. 21] CONTROL OF SILTING 371 



accumulated silt is flushed out periodically. An example of the gravity- 

 type of desilting basin is to be found at the head of the Interstate 

 Canal on the North Platte project in eastern Wyoming. 



Mechanical desilting basins operate by allowing water to pass into 

 chambers where rotating scrapers feed the settled silt into a central 

 collecting trench, from which it flows through sludge-collecting pipes 

 into the river. An example of the mechanical type of desilting basin 

 is that for the Ail-American Canal at Imperial Dam near Yuma, 

 Arizona. This is a comparatively new development, and further ex- 

 perience and experimentation should develop still more efficient de- 

 silting apparatus. 



The second problem, the reduction of the cost of removal of silt 

 from the canals, is of major interest when measures to reduce or elim- 

 inate the silt carried by the irrigation water are too costly or im- 

 practicable. 



Reduction of the cost of maintenance can be accomplished through 

 lining, through other improvements in canal design, and through im- 

 proved designs of cleaning machines. As previously mentioned, lining 

 is a problem of economics — to determine how much money can be 

 spent for that purpose. If the cost of the lining is within the economic 

 limit, it should be installed. If not, it will be more economical to 

 continue periodic cleaning of a canal in an unlined condition, and to 

 devote attention to improving the efficiency of cleaning machines. 



Other improvements in design of irrigation systems should reduce 

 the cost of maintenance arising from sedimentation. In lined canals 

 along steep hillsides, where the accumulation of silt results from storm- 

 water inflows from adjacent lands, cleaning can be eliminated or re- 

 duced by building overpasses or covers over the canal to prevent 

 silt-laden storm water from entering, or by the construction of under- 

 drain culverts. Storm-water culverts under canals must be designed 

 to have sufficient velocity to keep silt moving to preclude the neces- 

 sity for cleaning the culverts. In modern design, where a canal 

 crosses a draw or storm-water drain, inverted pipe siphons are gen- 

 erally provided. 



Cleaning of irrigation canals is currently performed in several dif- 

 ferent ways: (a) by hand, (b) by draglines, and (c) by other ma- 

 chines. Cleaning by hand, with shovels or other hand tools, is usually 

 expensive but is sometimes employed where labor is plentiful and the 

 condition of the canal, owing to narrow or unstable banks, does not 

 permit use of a heavy machine. 



The standard dragline does a fairly efficient mechanical job of re- 

 moving silt from the canals and restoring the original canal section. 



