(6) The Froude number of flow in the channel

slope of an alluvial channel most adaptable to the

is slightly in excess of the value of 0.16 previously

terrain would have to be accomplished by means of

estimated to be satisfactory with a depth of flow of

one or more check dams and/or drop structures.

5 feet, but it is within the range of 0.12 and 0.20

(8) Assume that there is a source of stone for

considered to be satisfactory for preventing either

supply of riprap with an average dimension of 3

severe erosion or deposition of medium to very

inches. The feasibility of a riprap-lined trapezoidal

coarse sand. However, should it be desired to

channel with 1V-on-3H side slopes that will convey

convey the design discharge of 400 cubic feet per

the design discharge of 400 cubic feet per second

second with a Froude number of 0.16 in a trapezoi-

with depths of flow up to 5 feet can be investigated

as follows. The equation, F = 1.42(d50/D)1/3, or

dal channel of 25-foot base width and 1V-on-3H

Q%gB5 and F, respectively, can be used in conjunc-

side slopes, the values of 0.0225 and 0.16 for

figure D-4 can be used to estimate the Froude

number of flow that will result in failure of various

tion with the figure D-3 to determine correspond-

sizes of natural or crushed stone riprap with

ing values of SB1/3]/n2 (0.72) and D/B (0.21) re-

various depths of flow. The maximum Froude

quired for such a channel. Thus, a depth of flow

number of flow that can be permitted with average

equal to 5.25 feet, and a slope of 0.000154 foot per

size stone of 0.25-foot-diameter and a flow depth

foot would be required for the channel to convey

of 5 feet is 0.52. Similarly, the maximum unit

the flow with a Froude number of 0.16.

discharge permissible (33 cubic feet per second per

(7) The slopes required for either the rectan-

foot of width) can be determined by the equation,

gular or the trapezoidal channels are extremely

moderate. If a steeper slope of channel is desired

q = 8.05 d501/3 D7/6

(eq D-3)

for correlation with the local topography, the feasi-

bility of a lined channel should be investigated as

or figure D-5. For conservative design, it is recom-

well as the alternative of check dams or drop

mended that the maximum unit discharge be limited

structures in conjunction with the channel previ-

to about two thirds of this value or say 22 cubic

ously considered. For the latter case, the difference

feet per second per foot of width for this example.

between the total drop in elevation desired due to

Thus, an average channel width of about 18.2 feet

the local topography and that permissible with the

is required to convey the design discharge of 400