TM 5-820-3/AFM 88-5, Chap. 3
values of SB1/3/n2 and Q%gB5 are determined as 30
Froude number of flow of 0.8, the corresponding
for S and Q based on n = 0.015 and B = 1 foot
yields
and 0.275, respectively. Solving these regulations
Q%gB5 to be from 3 to 30 and 0.085 to 0.275,
Greater widths of hydraulically efficient rectangular
channels would convey greater discharges, but
respectively. The relations between discharge and
slopes flatter than 0.00675 foot per foot would be
channel width for subcritical rectangular channels
required to prevent the Froude number of flow
with a depth-to- width ratio of 0.5, a slope of 0.001
from exceeding 0.8. Therefore, a rectangular chan-
foot per foot, and a Manning's n of 0.015 can be
nel of the most efficient cross section and a slope as
plotted as shown in figure D-7 to select the 11.5-
steep as 0.01 foot per foot are not practical for
foot-width of channel required to convey the design
subcritical conveyance of the design discharge and
discharge of 400 cubic feet per second.
(17) As a check, the exact value of SB1/ 3/n2
the example problem. A similar analysis for any
shape of channel would result in the same conclu-
can be calculated to be 10.1 and used in conjunc-
sion; stable subcritical conveyance of the design
tion with a D/B ratio of 0.5 and figure D-6 to
straining parameters, Q%gB5 = 0.16 and F = 0.47,
discharge on a slope of 0.01 foot per foot is not
obtain corresponding values of the remaining con-
feasible.
(15)
Assuming that the average slope of
required to satisfy all of the dimensionless relations
the local terrain was about 0.001 foot per foot for
for rectangular channels. The actual discharge
the example problem, practical subcritical paved
capacity of the selected 11.5-foot-wide channel
channels could be designed as discussed in
with a depth of 5.75 feet can be calculated based on
paragraphs (16) through (19) below.
these relations to ensure the adequacy of the
(16)
Based on the desired range of Froude
selected design. For example, based on the
numbers of flow (0.25 to 0.8) in a rectangular
magnitude of the discharge parameter (0.16), the
channel of efficient cross section (D/B = 0.5),
channel should convey 407 cubit feet per second:
figure D-6 indicates the corresponding range of
values of the restraining parameters SB1/3/n2 and
Similarly, based on the Froude number of flow to
0.47, the channel should convey a discharge of 422
cubic feet per second:
Therefore, the 11 .5-foot-wide channel is sufficient
(18) A similar procedure would be followed
for subcritical conveyance of the design discharge
to design a trapezoidal channel with a depth-to-
of 400 cubic feet per second and, based on figure
width ratio of 0.3, a slope of 0.001 foot per foot,
D-1, is sufficient for transporting materials as large
and a Manning's n of 0.015 utilizing figure D-3.
as average size gravel.
For example, in order to maintain a Froude number
D-10
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