TM 5-820-3/AFM 88-5, Chap. 3
CHAPTER 5
STORM DRAINS AND CULVERTS
The most efficient entrances would incorporate one
5-1. General.
or more of such geometric features as elliptical
The storm-drain system should have sufficient ca-
arcs, circular arcs, tapers, and parabolic drop-down
pacity to convey runoff from the design storm
curves. Elaborate inlet design for a culvert would
within the barrel of the conduit. Hydraulic design of
be justifiable only in unusual circumstances. The
the storm-drain system is discussed in TM 5-820-
rounding or beveling of the entrance in almost any
4/AFM 88-5 chapter 4. A drainage culvert is a
way will increase the culvert capacity for every
relatively short conduit used to convey flow
design condition. These types of improvements
through a roadway embankment or past some other
provide a reduction in the loss of energy at the
type of flow obstruction. Culverts are constructed
entrance for little or no additional cost.
from a variety of materials and are available in
e. Entrance structures (headwalls and wing-
many different shapes and configurations. Culvert
walls) protect the embankment from erosion and, if
hydraulics and diagrams, charts, coefficients, and
properly designed, may improve the hydraulic
related information useful in design of culverts are
characteristics of the culvert. The height of these
shown in TM 5-820-4/AFM 88-5 chapter 4.
structures should be kept to the minimum that is
consistent with hydraulic, geometric, and structural
5-2. Headwalls and endwalls.
requirements. Several entrance structures are
a. The normal functions of a headwall or wing-
shown in figure 5-1. Straight headwalls (fig 5-la)
wall are to recess the inflow or outflow end of the
are used for low to moderate approach velocity,
culvert barrel into the, fill slope to improve en-
light drift (small floating debris), broad or unde-
trance flow conditions, to anchor the pipe and to
fined approach channels, or small defined channels
prevent disjointing caused by excessive pressures,
entering culverts with little change in alignment.
to control erosion and scour resulting from exces-
The "L" headwall (fig 5-lb) is used if an abrupt
sive velocities and turbulences, and to prevent ad-
change in flow direction is necessary with low to
jacent soil from sloughing into the waterway open-
moderate velocities. Winged headwalls (fig 5-1c)
ing.
are used for channels with moderate velocity and
b. Headwalls are particularly desirable as a
medium floating debris. Wingwalls are most
cutoff to prevent saturation sloughing, piping, and
effective when set flush with the edges of the cul-
erosion of the embankment. Provisions for drainage
vert barrel, aligned with stream axis (fig 5-id) and
should be made over the center of the head-wall to
placed at a flare angle of 18 to 45 degrees. Warped
prevent scouring along the sides of the walls.
wingwalls (not shown) are used for well-defined
c. Whether or not a headwall is desirable de-
channels with high-velocity flow and a free water
pends on the expected flow conditions and em-
surface. They are used primarily with box culverts.
bankment stability. Erosion protection such as
Warped headwalls are hydraulically efficient
riprap or sacked concrete with a sand-cement ratio
because they form a gradual transition from a
of 9:1 may be required around the culvert entrance
trapezoidal channel to the barrel. The use of a
if a headwall is not used.
drop-down apron in conjunction with these wing-
d. In the design of headwalls some degree of en-
walls may be particularly advantageous.
trance improvement should always be considered.
5-1
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