TM 5820-4/AFM 885, Chap 4
tion, stilling basin, or impact device may be re-
in water surface and increase in velocity, this type
quired.
of outlet should be used without riprap only if the
material in the outlet channel can withstand ve-
44. Drop structures and check dams. Drop
locities about 1.5 times the velocity in the culvert.
structures and check dams are designed to check
At such an outlet, side erosion due to eddy action
channel erosion by controlling the effective gra-
or turbulence is more likely to prove troublesome
dient, and to provide for abrupt changes in chan-
than is bottom scour.
nel gradient by means of a vertical drop. The
(b) Cantilevered culvert outlets may be used
structures also provide satisfactory means for dis-
to discharge a free-falling jet onto the bed of the
charging accumulated surface runoff over fills with
outlet channel. A plunge pool will be developed,
heights not exceeding about 5 feet and over em-
the depth and size of which will depend on the
bankments higher than 5 feet provided the end
energy of the falling jet at the tailwater and the
sill of the drop structure extends beyond the toe
erodibility of the bed material.
of the embankment. The check dam is a modifi-
(2) Transition type. Endwalls (outfall head-
cation of the drop structure used for erosion con-
walls) serve the dual purpose of retaining the em-
trol in small channels where a less elaborate
bankment and limiting the outlet transition
structure is permissible. Pertinent design fea-
boundary. Erosion of embankment toes usually
tures are covered in chapter 5, TM 58203/AFM
can be traced to eddy attack at the ends of such
885, Chapter 3.
walls. A flared transition is very effective, if pro-
portioned so that eddies induced by the effluent
45. Miscellaneous structures.
jet do not continue beyond the end of the wall or
overtop a sloped wall. As a guide, it is suggested
a. A chute is a steep open channel which pro-
that the product of velocity and flare angle should
vides a method of discharging accumulated sur-
not exceed 150. That is, if effluent velocity is 5 feet
face runoff over fills and embankments. A typical
per second each wingwall may flare 30 degrees;
design is included in chapter 6, TM 58203/AFM
but if velocity is 15 feet per second, the flare should
885, Chapter 3.
not exceed 10 degrees. Unless wingwalls can be
b. When a conduit or channel passes through or
anchored on a stable foundation, a paved apron
beneath a security fence and forms an opening
between the wingwalls is required. Special care
greater than 96 square inches in area a security
must be taken in design of the structure to pre-
barrier must be installed. Barriers are usually of
clude undermining. A newly excavated channel
bars, grillwork, or chain-link screens, Parallel bars
may be expected to degrade, and proper allowance
used to prevent access will be spaced not more
for this action should be included in establishing
than 6 inches apart, and will be of sufficient
the apron elevation and depth of cutoff wall. Warped
strength to preclude bending by hand after as-
endwalls provide excellent transitions in that they
sembly.
result in the release of flow in a trapezoidal sec-
(1) Where fences enclose maximum security
tion, which generally approximates the cross sec-
areas such as exclusion and restricted areas,
tion of the outlet channel. If a warped transition
drainage channels, ditches, and equalizers will,
is placed at the end of a curved section below a
wherever possible, be carried under the fence in
culvert, the transition is made at the end of the
one or more pipes having an internal diameter of
curved section to minimize the possibility of ov-
not more than 10 inches. Where the volume of flow
ertopping due to superelevation of the water sur-
is such that the multipipe arrangement is not fea-
face. A paved apron is required with warped end-
sible, the conduit or culvert will be protected by
walls. Riprap usually is required at the end of a
a security grill composed of 3/4-inch-diameter rods
transition-type outlet.
or 1/2-inch bars spaced not more than 6 inches on
(3) Stilling basins. A detailed discussion of
center, set and welded in an internal frame. Where
stilling basins for circular storm drain outlets can
rods or bars exceed 18 inches in length, suitable
be found in chapter 7, TM 58203.
spacer bars will be provided at not more than 18
inches on center, welded at all intersections. Se-
curity grills will be located inside the protected
b. Improved channels, especially the paved ones,
area. Where the grill is on the downstream end of
commonly carry water at velocities higher than
the culvert, the grill will be hinged to facilitate
those prevailing in the natural channels into which
cleaning and provided with a latch and padlock,
they discharge. Often riprap will suffice for dis-
and a debris catcher will be installed in the up-
sipation of excess energy. A cutoff wall may be
stream end of the conduit or culvert. Elsewhere
required at the end of a paved channel to preclude
the grill will be permanently attached to the cul-
undermining. In extreme cases a flared transi-
4-5
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