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 5-4*k *can be used for design of the riprap.

From figure 6-2:

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:

head on the crest: