TM 5-820-3/AFM 88-5, Chap. 3
(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
D-3
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