TM 5820-4/AFM 88-5, Chap 4
it is necessary to protect roadway embankments
be of clear span, if at all practicable, in order to
against formation of rivulets and channels by con-
preclude serious problems attending debris lodg-
centrated flows. Materials used in such construc-
ment and to minimize channel construction and
tion include portland-cement concrete, asphaltic
maintenance costs.
concrete, stone rubble, sod checks, and prefabri-
g. It is important that storm runoff be controlled
cated concrete or metal sections, Typical of the
over as much of the contributing watershed as
latter are the entrance tapers and embankment
practicable. Diversion channels, terraces, check
protectors made by manufacturers of corrugated
dams, and similar conventional soil conserving
metal products. Downspouts as small as 8 inches
features will be installed, implemented, or im-
in diameter may be used, unless a considerable
proved to reduce velocities and prevent silting of
trash problem exists, in which case a large size
channels and other downstream facilities. When
will be required. When frequent mowing is re-
practicable, unprotected soil surfaces within the
quired, consideration will be given to the use of
drainage area will be planted with appropriate
buried pipe in lieu of open paved channels or ex-
erosion-resisting plants. These parts of the drain-
posed pipe. The hydrologic and hydraulic design
age area which are located on private property or
and the provision of outfall erosion protection will
otherwise under control of others will be consid-
be accomplished in accordance with principles
ered fully in the planning stages, and coordinated
outlined for similar component structures dis-
efforts will be taken to assure soil stabilization
cussed in this manual.
both upstream and downstream from the con-
c. Curbs are used to deter vehicles from leaving
struction site.
the pavement at hazardous points as well as to
h. Engineering criteria and design principles re-
control drainage. The two general classes of curbs
lated to traffic, size, load capacity, materials, and
are known as barrier and mountable and each has
structural requirements for highway and railroad
numerous types and detail designs. Barrier curbs
bridges are given in TM 5-820-2/AFM 88-5, Chap-
are relatively high and steep faced and designed
ter 2, and in AASHTO Standard Specifications for
to inhibit and to at least discourage vehicles from
Highway Bridges, design manuals of the different
leaving the roadway. They are considered unde-
railroad companies, and recommended practices
sirable on high speed arterials. Mountable curbs
of AREA Manual for Railway Engineering.
are designed so that vehicles can cross them with
varying degrees of ease.
3-4. Curb-and-gutter sections.
d. Curbs, gutters, and storm drains will not be
provided for drainage around tank-car or tank-
and adjacent areas must be rapidly and econom-
truck unloading areas, tank-truck loading stands,
ically removed before it becomes a hazard to traffic.
and tanks in bulk-fuel-storage areas. Safety re-
Water falling on the pavement surface itself is re-
quires that fuel spillage must not be collected in
moved from the surface and concentrated in the
storm or sanitary sewers. Safe disposal of fuel
gutters by the provision of an adequate crown.
spillage of this nature may be facilitated by pro-
The surface channel formed by the curb and gut-
vision of ponded areas for drainage so that any
ter must be designed to adequately convey the
fuel spilled can be removed from the water sur-
runoff from the pavement and adjacent areas to
face.
a suitable collection point. The capacity can be
computed by using the nomograph for flow in a
35. Culverts.
triangular channel, figure 3-2. This figure can also
be used for a battered curb face section, since the
a. A drainage culvert is defined as any structure
battering has negligible effect on the cross sec-
under the roadway with a clear opening of twenty
tional area. Limited data from field tests with clear
feet or less measured along the center of the road-
water show that a Manning's n of 0.013 is appli-
way. Culverts are generally of circular, oval, el-
cable for pavement. The n value should be raised
liptical, arch, or box cross section and may be of
when appreciable quantities of sediment are pres-
either single or multiple construction, the choice
ent. Figure 3-2 also applies to composite sections
depending on available headroom and economy.
comprising two or more rates of cross slope.
Culvert materials for permanent-type installa-
tions include plain concrete, reinforced concrete,
b. Good roadway drainage practice requires the
corrugated metal, asbestos cement, and clay. Con-
extensive use of curb-and-gutter sections in com-
crete culverts may be either precast or cast in
bination with spillway chutes or inlets and down-
place, and corrugated metal culverts may have
spouts for adequate control of surface runoff, par-
either annular or helical corrugations and be con-
ticularly in hilly and mountainous terrain where
3-5
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