(4) The width of a rectangular channel and

IV on 3H required to convey the design discharge

the average width of a trapezoidal channel required

with a 5-foot depth of flow would be 25 feet.

%gB can now be calculated as 0.2 and 0.0225, re-

to convey the maximum rate of runoff of 400 cubic

(5) The values of the parameters D/B and Q/

5

feet per second can be determined by dividing the

design discharge by the permissible unit discharge.

spectively. Entering figure D-3 with these values, it

For the example problem an average channel width

is apparent that corresponding values of 0.95 and

0.185 are required for the parameters of SBa/n2

of 40 feet is required. The base width of a

trapezoidal channel can be determined by

and F, respectively. Assuming a Manning's n of

subtracting the product of the horizontal com-

0.025, a slope of 0.000203 foot per foot would be

required to satisfy the SB1/3/n2 relation for the 5-

ponent of the side slope corresponding to a vertical

displacement of 1 foot and the depth of flow from

foot deep trapezoidal channel with base width of 25

the previously estimated average width. The base

feet and IV-on-3H side slopes.

width of a trapezoidal channel with side slopes of