184 EXPEEIMENT STATION EECOED. 



Tests were conducted in 10 States on channels ranging in size from small ' 

 ditches carrying less than 1 second-foot up to canals carrying over 2,600 

 second-feet. The materials of these channels comprise wood, concrete, earth, 

 rubble masonry, cobblestones, and a few special combinations. Test data 

 were also obtained from other sources in cases where in the author's opinion 

 there was not sufficient evidence obtained from his own experiments to war- 

 rant the drawing of conclusions. 



The following conclusions are drawn from a study of the experiments and 

 the data as assembled: " Kutter's formula is applicable to the design of any 

 open channel. The recommendations of the earlier writers concerning the 

 values of n to be chosen were in the main correct. Any weakness was due to 

 the fact that there was not sufficient distinction made between the various 

 categories and that materials of construction are now used which were not 

 covered by the tests from which early deductions were made. The influence 

 of curves was not as a rule included. Concrete lining covered but one value of 

 n, whereas in practice there are many shades of roughness, all applicable under 

 the general head of concrete. The factor n must include all the influences 

 which tend to retard velocity. The principal of these influences are undoubt- 

 edly (a) rubbing friction between the water and the containing channels, and 

 (6) vegetable growth extending into the main body of the water. ... Of 

 secondary importance but nevertheless deserving of careful consideration in 

 about the order named are the following: (c) Angles and shari? curves in the 

 alignment, {(l) influences which tend to disturb parallel filaments of current, 

 . . . (e) sand and gravel, . . . [and] (/) the prevailing wind direction. . . . 

 There is a tendency toward a lower value of n as the velocity and hydraulic 

 radius increase. ... A value of n must be chosen that will apply to the 

 canal in question at the critical period of the season. ... In the design of 

 earth channels having a trapezoidal form when constructed, the value of [the 

 hydraulic radius] should be computed on the basis that the canal takes an 

 elliptical form within a short time and thereafter maintains this shape unless 

 altered artificially." 



The values recommended for n in Kutter's formula, which are applicable for 

 velocities up to about 5 ft. per second and with hydraulic radii to about 2 ft. 

 are as follows: 



For concrete lining "(1) n=0.012 for the highest grade of material and 

 workmanship and exceptionally good conditions. The surface of the lining 

 to be as smooth to the hand as a troweled sidewalk. The expansion joints to 

 be so well covered that they practically fulfill the same condition. The climate 

 and water to be such that moss does not accumulate to any great extent. The 

 water to be practically free from shifting material. The alignment to be com- 

 posed of long tangents, joined by spiraled curves, while the interior of the 

 channel must be of uniform dimensions, true to grade throughout the cross 

 section. (2) 7i=0.013 for construction as in type (1), but with curves as in 

 the usual mountain canyon. Same construction and alignment as in type (1), 

 but with small amount of sand or debris in water. Construction as in type 

 (3), but in very favorable alignment or for water that carries a small amount 

 of fine silt that will eventually form a slick coat. (3) ?t=0.014 for linings 

 made by good construction under favorable conditions. The surface to be as 

 left by smooth-jointed forms or to be roughly troweled. Joints to be good, 

 but causing some retardation. Alignment about equal in curves and tangents, 

 M'ith no spirals between. The bed to be clean and sides free from rough 

 deposits, . . . (4) «=0.015 for construction as in type (3), but with sharp 

 curves and clean bottom or moderate curves and much debris on the bottom 

 but clean-cut sides. (5) n=0.01Q for concrete as constructed by the average 



