TM 5-820-3/AFM 88-5, Chap. 3
(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
D-4
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