TM 5-820-4/AFM 88-5, Chap 4
cient entrances incorporate such geometric features
not be installed in material classified as OH or OL
as elliptical arcs, circular arcs, tapers, and para-
according to the Unified Soil Classification System as
bolic drop-down curves. In general elaborate inlet
presented in MIL-STD 619. Although bituminous
designs for culverts are justifiable only in unusual
coatings can be applied to aluminum-alloy pipe, such
circumstances.
coatings do not afford adequate protection (bitumi-
nous adhesion is poor) under the aforementioned
f. Outlets and endwalls must be protected against
corrosive conditions. Suitable protective coatings for
undermining, bottom scour, damaging lateral ero-
aluminum alloy have been developed, but are not
sion and degradation of the downstream channel.
economically feasible for culverts or storm drains.
The presence of tailwater higher than the culvert
For flow carrying debris and abrasives at moderate
crown will affect the culvert performance and may
to high velocity, paved-invert pipe may be appro-
possibly require protection of the adjacent embank-
priate. When protection from both corrosion and
ment against wave or eddy scour. Endwalls (outfall
abrasion is required, smooth-interior corrugated-
headwalls) and wingwalls should be used where
steel pipe may be desirable, since in addition to pro-
practical, and wingwalls should flare one on eight
viding the desired protection, improved hydraulic
from one diameter width to that required for the
efficiency of the pipe will usually allow a reduction in
formation of a hydraulic jump and the establishment
pipe size. When considering a coating for use, per-
of a Froude number in the exit channel that will in-
formance data from users in the area can be helpful.
sure stability. Two general types of channel instabil-
Performance history indicates various successes or
ity can develop downstream of a culvert. The con-
failures of coatings and their probable cause and are
ditions are known as either gully scour or a localized
available from local highway departments.
erosion referred to as a scour hole. Gully scour is to
be expected when the Froude number of flow in the
d. The capacity of a culvert is determined by its
channel exceeds that required for stability. Erosion
ability to admit, convey, and discharge water under
of this type maybe of considerable extent depending
specified conditions of potential and kinetic energy
upon the location of the stable channel section rela-
upstream and downstream. The hydraulic design of
tive to that of the outlet in both the vertical and
a culvert for a specified design discharge involves
downstream directions. A scour hole can be ex-
selection of a type and size, determination of the
pected downstream of an outlet even if the down-
position of hydraulic control, and hydraulic computa-
stream channel is stable. The severity of damage to
tions to determine whether acceptable headwater
be anticipated depends upon the conditions existing
depths and outfall conditions will result. In consider-
or created at the outlet. See chapter 5 for additional
ing what degree of detailed refinement is appropri-
information on erosion protection.
ate in selecting culvert sizes, the relative accuracy of
the estimated design discharge should be taken into
g. In the design and construction of any drainage
account. Hydraulic computations will be carried out
system it is necessary to consider the minimum and
by standard methods based on pressure, energy,
maximum earth cover allowable in the underground
momentum, and loss considerations. Appropriate
conduits to be placed under both flexible and rigid
formulas, coefficients, and charts for culvert design
pavements.
Minimum-maximum cover require-
are given in appendix B.
ments for asbestos-cement pipe, corrugated-steel
pipe, reinforced concrete culverts and storm drains,
e. Rounding or beveling the entrance in any way will
standard strength clay and non-reinforced concrete
increase the capacity of a culvert for every design
pipe are given in appendix C. The cover depths
condition. Some degree of entrance improvement
recommended are valid for average bedding and
should always be considered for incorporation in
backfill conditions. Deviations from these conditions
design. A headwall will improve entrance flow over
may result in significant minimum cover require-
that of a projecting culvert. They arc particularly
ments.
desirable as a cutoff to prevent saturation sloughing
and/or erosion of the embankment. Provisions for
h. Infiltration of fine-grained soils into drainage
drainage should be made over the center of the head-
pipelines through joint openings is one of the ma-
wall to prevent scouring along the sides of the walls.
jor causes of ineffective drainage facilities. This
A mitered entrance conforming to the fill slope pro-
is particularly a problem along pipes on relatively
duces little if any improvement in efficiency over that
steep slopes such as those encountered with bro-
of the straight, sharp-edged, projecting inlet, and
ken back culverts.
Infiltration of backfill and
may be structurally unsafe due to uplift forces. Both
subgrade material can be controled by watertight
types of inlets tend to inhibit the culvert from flow-
flexible joint materials in rigid pipe and with wa-
ing full when the inlet is submerged. The most effi-
tertight coupling bands in flexible pipe. The re-
3-8 Change 1
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