50 BULLETIN 126, U. S. DEPARTMENT OF AGRICULTURE. 
SUITABLE GRADES FOR LINED CANALS. 
In deciding upon suitable grades, cross sections, and alignment 
for lined canals, one has to take into account the two types of canals 
which are lined. One of these is represented by the canal in opera- 
tion designed for earth, the other by the new canal designed for 
concrete lining. 
The grade which is suitable for a canal in earth is not the most 
economical grade for the same canal when lined with concrete. It 
is not, however, feasible to make any material change in the grade 
of an old canal preparatory to lining it. The irregularities can and 
should be removed so as to secure a uniform gradient, but more 
than this can not be done without changing the location. 
The discussion of suitable grades must therefore be confined in 
this report to new canals intended to be lined before carrying any 
large percentage of their maximum capacity. Disregarding all 
other features and considering only the most economical method 
of conveying water, the steep grade with its correspondingly high 
mean velocity is best. The fact is now fairly well established that 
water can pass over a concrete surface at a high velocity without 
injurious effects. It is only when fas1>flowing water strikes against 
concrete or is obstructed by it that damage is likely to result. Mr. 
A. P. Davis, chief engineer of the Reclamation Service, cites a case 1 
in which a concrete chute on the south canal of the Uncompahgre 
project, discharging 300 cubic feet per second at a velocity of over 
20 feet per second for one year, not only showed no perceptible wear, 
but it had acquired a growth of slimy moss over the concrete surface 
subject to this velocity. 
In the summer of 1913 Justin T. Kingdon, of this office, made an 
examination of a concrete chute (PI. I, fig. 2) on the main canal of 
the Orland project, Orland, Cal. The canal maintains a fairly 
constant flow throughout the season. It was measured shortly 
after the observation and found to be discharging 84 second-feet of 
water which at the bottom of the chute had a velocity of 17 feet per 
second, and like the south canal of the Uncompahgre project, this 
concrete showed no wear on its wetted area, and the growth of slimy 
moss which it had acquired was especially noticeable over that 
portion of the surface subject to the highest velocity. 
It would therefore appear that the permissible velocity in lined 
canals depends largely on considerations other than damage to 
the lining. The mention of three of these causes may serve to 
make this statement clear. Assuming that a concrete-lined canal 
mil successfully withstand velocities up to 20 feet per second, th( 
fall necessary to produce such velocities must be considered, sinc< 
> Eng. News, 67 (1912), No. 1, p. 20. 
