CONCRETE LINING FOR IRRIGATION CANALS. 49 
$3,000 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> 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 hning with concrete. A gain of 25 per cent in the 
volume carried is readily obtained as the result of Hning, but to 
secure a gain of 80 per cent involves the construction of first-class 
hning 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 e 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 
