TM 5-820-3/AFM 88-5, Chap. 3
(19) As a check, the exact value of SB1/ 3/n2
of flow between 0.25 and 0.75 in a trapezoidal
channel with side slopes lV on 3H and a depth-to-
was calculated to be 10.2 and used in conjunction
width ratio of 0.3, the constraining parameter of
with D/B of 0.3 and figure D-3 to obtain corre-
rameters, Q%gB5=0 15 and F=0.63, required to
SB1/3/n2 would have to have a value between 2 and
sponding values of the remaining constraining pa-
15 (fig. D-3). The relations between discharge and
base width for these subcritical trapezoidal channels
satisfy the dimensionless relations of trapezoidal
were plotted as shown in figure D-7 to select the
channels. The actual discharge capacity of the se-
12-foot-base width required to convey the design
lected trapezoidal channel with a base width of 12
discharge of 400 cubic feet per second.
feet and a flow depth of 3.6 feet based on these re-
lations would be 425 and 458 cubic feet per
second, respectively.
Therefore, the selected trapezoidal channel is suffi-
under annual discharge conditions. Lime
cient for subcritical conveyance of the design dis-
stabilization can be effectively used to confine clay
charge of 400 cubic feet per second and based on
soils, and soil-cement stabilization may be effective
figure D-1 is sufficient for transporting materials as
in areas subject to sparse vegetative cover. Sand-
large as coarse gravel.
cement and rubble protection of channels may be
b. Having determined a channel that will satisfy
extremely valuable in areas where rock protection
the conditions desired for the design discharge, de-
is unavailable or costly. Appropriate filters should
termine the relations that will occur with the an-
be provided to prevent leaching of the natural soil
ticipated maximum annual discharge and ensure
through the protective material. Facilities for
that deposition and/or erosion will not occur under
subsurface drainage or relief of hydrostatic
these conditions. It may be necessary to
pressures beneath channel linings should be
compromise and permit some erosion during design
provided to prevent structural failure.
discharge conditions in order to prevent deposition
D-11
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