50 BULLETIN 126, TJ. 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 fast-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 Oiiand 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 

 will successfully withstand velocities up to 20 feet per second, the 

 fall necessary to produce such velocities must be considered, since 



i Eng. News, 07 (1912), No. 1, p. 20. 



