52 BULLETIN 194, U. S. DEPARTMENT OF AGRICULTURE. 



have a medium-smooth bottom, with graded gravel, grass on the 

 edges and average ahgnment 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 formmg the sides. Hardpan in good con- 

 dition, clay and lava-ash soil takes about this value. 



IV. 71 = 0.025 for canals where the retarding mfluence of moss, 

 growths of dense grass near the edges, or scattered cobbles begins to 

 show. The value of ti in earth channels where the maintenance is 

 neglected commences at this value and rapidly goes up. This is a 

 good value to use in the design of small head ditches or a small ditch 

 to serve but one or two farms. 



V. 71 = 0.030 for canals subject to heav^y growths of moss or other 

 aquatic plants. Banks irregular or overhanging with dense rootlets. 

 Bottom covered with large fragments of rock, or bed badly pitted by 

 erosion. Values of n between 0.025 and 0.030 also cover the condi- 

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

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

 For values above 0.030 the channel is much choked with vegetation, 

 very irregular, crooked, overhung with dragging trees and grasses, 

 or there is some other condition that should not be allowed to exist 

 m a well-kept system. 



VALUES OF n FOR COBBLE-BOTTOM CANALS. 



The typical clean-washed cobblestone ditch is very common near 

 the mouths of canyons. Where the cobbles are graded in size and 

 well packed the value of n is about 0.027, but the value rapidly 

 increases as the larger rocks predominate and the lack of graded sizes 

 prevents packing. 



ESTIMATION CHARTS. 



As an aid to the designer of irrigation channels the writer has 

 prepared two sets of curve charts. 



The first set shows values of width and depth of channel as related 

 to area and hydraulic radius. These elements for rectangular chan- 

 nels are shown in figure 4 and for trapezoidal chamiels having side 

 slopes of 1^ to 1 m figure 5. These cover the two shapes most com- 

 monly used, while the engineer working in materials that require 

 other slopes may build up his own charts on the prmciple used in 

 plotting figures 4 and 5. 



The second set shows related values of velocity, slope, the friction 

 factor n, and the hydraulic radius. This set is divided into three 

 general classes. First. For construction in concrete, wood, or steel 

 for ordinary slopes (fig. 6). Second. For the same materials of 

 construction on very steep slopes (fig. 7). The latter chart is for use 

 m the design of chute drops and covers slopes from 0.005 where figure 



