52 BULLETIN" 194, U. S. DEPARTMENT OF AGRICULTURE. 
have a medium-smooth bottom, 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 con- 
dition, clay and lava-ash soil takes about this value. 
IV. 71 = 0.025 for canals where the retarding influence of moss, 
growths of dense grass near the edges, or scattered cobbles begins to 
show. The value of n 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 smalJ ditch 
to serve but one or two farms. 
V. 71 = 0.030 for canals subject to heavy 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 
in 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 channels having side 
slopes of 1 \ to 1 in 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 principle used in 
plotting figures 4 and 5. 
The second set shows related values of velocity, slope, the friction 
factor ri, 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 
in the design of chute drops and covers slopes from 0.005 where figure 
