370 golze. PROBLEMS OF IRRIGATION CANALS [Ch. 21 



at the same time — prevents seepage and prevents erosion, the latter 

 reducing sedimentation in canals. 



The principal question involved is one of cost. Although reinforced 

 concrete is almost the perfect lining, it is nearly always the most 

 costly. Research has been conducted and is still going on, and more 

 is required, to find other materials, including other forms of concrete, 

 with substantially the same properties as reinforced concrete but lower 

 in cost. 



If the value of water can be approximated, it then becomes a matter 

 of simple mathematics to determine the amount that can be economi- 

 cally spent on sealing a canal for any given cross section. For ex- 

 ample, assume a 50-mile canal designed to carry 1,320 cubic feet 

 per second with a velocity of 4.72 feet per second. Its cross-sectional 

 area = 1,320/4.72 = 280 square feet. With side slopes of 1%:1 and a 

 40-foot bottom, the radius = 4.6 and the wetted perimeter = 280/4.6 

 = 61 feet. 



With a 1-foot freeboard on each side, the total perimeter = 63 feet. 



If the canal in question has an annual inflow of 200,000 acre-feet 

 and 25 percent of that inflow can be recovered by sealing, and if the 

 water is considered to be worth $2 per acre-foot, the annual saving 

 will be worth $100,000. At 3 percent interest, over a 40-year period 

 (assuming the life of a lining material to be 40 years), $100,000 will 

 retire an investment of $2,311,479. 



The canal owner can afford, therefore, to spend $2,311,479 to seal 

 the 50-mile canal, or $46,230 a mile, or $8.76 per foot of canal. 



The allowable cost per square yard for lining will then be equal 

 to (8.76 X 9) /63 = $1.25. 



In other words, a canal lining that will be good for 40 years and 

 will not exceed $1.25 per square yard installed must be found. 



This example, of course, may not give sufficient weight to the ex- 

 treme need for additional water now developing generally through- 

 out the West. A close study of the economic factors influencing the 

 value of water may result in the assignment of higher values, thereby 

 justifying a greater expenditure for lining. 



When the cost of overcoming sedimentation by eliminating silt at 

 its source is prohibitive or impractical, the inflow of silt into canals 

 may be reduced by the construction and operation of desilting basins. 

 These may be of two types: (a) gravity, and (b) mechanical. Grav- 

 ity-type desilting basins are usually found at the beginning or head 

 of irrigation canals. They consist of a stilling basin where the flow 

 of water is reduced to the point that the bulk of silt suspended in 

 the water is dropped. Through wasteways to the adjacent river, the 



