TM 5-820-3/AFM 88-5, Chap. 3
where CL is an empirical coefficient between 2 and
7, as shown in figure 6-2. The stilling basin length
Thus, crest elevation will be 5-4.1 = 0.9 feet above
and end sill height can be determined from the
channel invert and distance from crest to down-
design curves in figure 6-2. Optimum performance
streams channel invert, h , will be 4+0.9=4.9 feet.
of the basin is obtained when the tailwater-critical
depth ratio is 1.25 to 1.67. However, the basin will
Critical depth, dc:
function satisfactorily with higher tailwaters if the
depth of tailwater above the weir does not exceed
0.7 dc. The stilling basin walls should be high
enough to prevent the tailwater from reforming
over the walls into the stilling basin. Riprap pro-
tection should be provided immediately down-
stream from the structure. Guidance provided in
paragraph 5-4k can be used for design of the riprap.
From figure 6-2:
e. A design illustrating the use of the above in-
formation and figure 6-2 is shown in the following
example. Design a drop structure for a discharge of
250 cubic feet per second in a trapezoidal channel
with a 10-foot base width and side slopes of IV on
3H, and a depth of flow of 5 feet. The amount of
drop required is 4 feet. If the crest is placed at
invert of the channel, the head on the crest, H, will
be equal to the depth of flow, 5 feet.
Width of Crest, W:
The tailwater depth will depend on the channel
configuration and slope downstream from the
structure. If these parameters are the same as those
of the approach channel, the depth of tail-water will
be 5 feet. Thus, the tailwater/dc ratio is 5/2.73 =
Since the base width of the channel is 10 feet, the
1.83 which is greater than 1.67 recommended for
weir crest should be made 10 feet long and raised
optimum energy dissipation. However, the
up to maintain a depth of 5 feet upstream. If the
tailwater depth above the crest (5.0- .49 = 0.10)
width determined above would have been greater
divided by critical depth (2.73) is (0.1/2.73=0.04)
than 10 feet then the greater width would have had
much less than 0.7 and the basin will function sat-
to be retained and the channel expanded to
isfactorily.
accommodate this width.
Riprap design:
f. With width of crest equal to 10 feet determine
head on the crest:
6-6
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