CONCRETE LINING FOR IRRIGATION CANALS. 49 



S3, 000 as a general average for the value of each second-foot saved by 

 lining. 



(2) Increase in carrying capacity. — The volume carried by a canal 

 in earth is as a rule much less than that carried by a concrete-lined 

 canal of the same dimensions and grade. This is due to the smoother 

 perimeter of the latter, and its greater uniformity in cross section, 

 alignment, and grade. The discharge of a typical canal in earth 

 having a mean velocity of 2.5 feet per second and a coefficient of 

 friction (n in Kutter's formula) of 0.0225, may be increased from 25 

 to 80 per cent by lining with concrete. A gain of 25 per cent in the 

 volume carried is readily obtained as the result of lining, but to 

 secure a gain of 80 per cent involves the construction of first-class 

 lining and conditions favorable to the maximum discharge of water 

 in such channels. 



(3) Reduction of charge for operation and maintenance. — On many 

 systems, particularly where the canal follows a side hill, much diffi- 

 culty is encountered from breaks on the lower bank when the canal 

 is crowded to its full capacity or when an opening may be made by a 

 gopher or other burrowing animal. A concrete lining should pre- 

 vent such breaks except in cases where the water overtops the bank 

 due to stoppage or other causes. Faulty location of the canal and 

 weak places developing later can very often be largely corrected 

 by a good concrete lining. Where the original grade is such that 

 scour occurs, or where excessive curvature causes cutting of the 

 sides, a similar remedy may be used. Maintenance charges also 

 will be materially reduced by the lessening of weed growth and the 

 prevention of the shifting of the channel through scouring. In 

 some systems the fall of the country is too great to be taken up by 

 the grade of the canal and many drops are required which may 

 form a considerable proportion of the cost and necessitate high 

 maintenance expenses. The use of a concrete lining frequently 

 permits a sufficiently high grade to be used so that no drops are 

 needed, the saving in these structures paying part of the cost of 

 the lining. 



(4) Insurance against damage to crops. — As the losses from the 

 lack of water at critical times during the irrigation season are often 

 much greater than the actual cost of repairs, a portion of the cost 

 of any canal lining may be considered as an insurance against such 

 accidents. An instance of this occurred on the Turlock Canal, of 

 California, in 1910, when a break thought to have been due to a 

 gopher hole caused 1,000 feet of the main canal on a steep side hill 

 to be washed out. The canal was out of service for six weeks during 

 the period when water was most needed for crops. The actual 

 cost of repairs was $20,000, but the estimated damage to crops was 

 $1,000,000. 



48307°— Bull. 126—14 4 



