RURAL ENGINEERING. 185 



gang of laborers, using forms that leave prominent lines at the cracks, no 

 finish coat being applied. Bed to have the usual small amount of rock frag- 

 ments and patches of sand and gravel. Average amount of curvature. . . . 

 (6) n=O.On for roughly coated linings with uneven joints. This value also 

 is applicable where rough deposits accumulate on the sides and conditions of 

 alignment are poor. (7) «=0.018 for very rough concrete with sharp curves 

 and deposits of gravel and moss. A broken gradient, irregular cross section, 

 and the like, contribute to such a high value of «." 



For wooden flumes "(1) ii.=0.012 for well-constructed, clean flumes with 

 surfaced lumber for both siding and battens. All lumber to run longitudinally. 

 Alignment to consist of long tangents, with gentle curves between. Construc- 

 tion to be such that the grade line will remain uniform, preventing sags and 

 wavy alignment. ... (2) n=0.013 for well-constructed, clean flumes of sur- 

 faced lumber and battens, following mountain contours, where the alignment will 

 consist of about equal gentle curves and tangents. ... (3) »=0.014 for flumes 

 of very smooth interior, but with many bends or sharp curves. ... (4) 

 n=0.015 for flumes of unplaned lumber, but otherwise as of type (2), . . . 

 (5) n=0.01Q for flumes of type (4) where sharp bends rather than curves are 

 installed. For flumes lined with rough roofing material and for the ordinary 

 grade of construction on a flume that is built and generally left to care for 

 itself." 



With reference to masonry lined channels, it is stated that " if [the bottom] 

 be of smooth concrete and the sides are reasonably smooth,, then the value of n 

 approximating that in average concrete may be used." 



For earth channels "(1) w=0.016 for excellent conditions of earth channels. 

 The velocity to be so low that a slick deposit of silt may accumulate or the 

 natural material be such as to become smooth when wet. The influence of 

 vegetation at the edges to be a minimum. The water to be free from moss 

 and other aquatic growth. The alignment to be free from bends and sharp 

 curves. (2) n=0.02 for well-constructed canals in firm earth or fine, packed 

 gravel where velocities are such that silt may fill the interstices in the gravel. 

 The banks to be clean-cut and free from disturbing vegetation. The align- 

 ment to be reasonably straight. (3) w=0.0225 . . . for the average well-con- 

 structed canal in material which will eventually have a medium smooth bot- 

 tom, with graded gravel, grass on the edges, and average alignment or silt at 

 both sides of the bed and scattered stones in the middle, or a smooth bottom 

 with an average amount of grass and roots forming the sides. Hardpan In 

 good condition, clay, and lava-ash soil take about this value. (4) w=0.025 

 for canals where the retarding influence of moss, growths of dense grass near the 

 edges, or scattered, cobbles begins to show. ... (5) n—O.OS for canals subject 

 to heavy growths of moss or other aquatic plants. Banks irregular or over- 

 hanging with dense rootlets. Bottom covered with large fragments of rock 

 or bed badly pitted by erosion. Values of n between 0.025 and 0.03 also cover 

 the condition where the velocity is so high that cobbles are kept clean and 

 unpacked in the center of the cftnal, but silt deposits near the sides." 



For cobble bottom canals it is stated that " where the cobbles are graded in 

 size and well packed the value of n is about 0.027, but the value rapidly in- 

 creases as the larger rocks predominate and the lack of graded sizes prevents 

 packing," 



The values of n for metal flumes are given in a previous report by Cone, 

 Trimble, and Jones (E. S. R., 30, p. 885). 



Two sets of curve charts to aid in the design of irrigation channels are also 

 included. An appendix contains abstracts of descriptions of canals on which 

 similar tests have been made in recent years by other agencies than this oflSce. 



