TM 5820-4/AFM 885, Chap 4
h. For most open channel confluences, proper
the center of the roadway of more than 20 feet
design can be accomplished satisfactorily by com-
between undercopings of abutments or spring lines
putations based on the principle of conservation
of arches, or extreme ends of the openings for mul-
tiple boxes; it may include multiple pipes where
of momentum. If the channel flows are supercrit-
ical, excessive waves and turbulence are likely to
the clear distance between openings is less than
occur unless a close balance of forces is achieved.
half of the smaller contiguous opening.
In such confluences, minimum disturbances will
b. Sufficient capacity will be provided to pass
result if the tributary inflow is made to enter the
the runoff from the design storm determined in
main channel in a direction parallel to the main
accordance with principles given in chapter 2.
flow, and if the design depth and velocity of the
Normally such capacity is provided entirely in the
tributary inflow are made equal to those in the
waterway beneath the bridge. Sometimes this is
main channel. Further, even though minimum
not practical, and it may be expedient to design
disturbances appear likely under such design con-
one or both approach roadways as overflow sec-
ditions, it must be remembered that natural flood-
tions for excess runoff. In such an event, it must
flows are highly variable, both in magnitude and
be remembered that automobile traffic will be
distribution. Since this variability leads to unbal-
impeded, and will be stopped altogether if the
anced forces and accompanying turbulence, a need
overflow depth is much more than 6 inches. How-
may well exist for some additional wall height or
ever, for the bridge proper, a waterway opening
freeboard allowance at and downstream from the
smaller than that required for 10-year storm run-
confluence structure.
off will be justifiable.
i. Side inflows to channels generally enter over
c. In general, the lowest point of the bridge su-
the tops of the walls or in covered drains through
perstructure shall clear the design water surface
the walls. If the main channel is earth, erosion
by not less than 2 feet for average flow and trash
protection frequently is required at (and perhaps
conditions. This may be reduced to as little as 6
opposite) the point of entry. If the sides of a chan-
inches if the flow is quiet, with low velocity and
nel through an erosible area are made of concrete
little or no trash. More than 2 feet will be required
or other durable materials and inflows are brought
if flows are rough or large-size floating trash is
in over them, care must be taken to insure positive
anticipated.
entry. There are two methods of conducting storm
d. The bridge waterway will normally be alined
water into a concrete-lined channel. Entry of large
to result in the least obstruction to streamflow,
flows over the top is provided by a spillway built
except that for natural streams consideration will
as an integral part of the side slope while smaller
be given to realinement of the channel to avoid
flows are admitted to the channel by a conduit
costly skews, To the maximum extent practicable,
through the side slope. Gating of conduit is not
abutment wings will be alined to improve flow
required at this location because any pending is
conditions. If a bridge is to span an improved trap-
brief and not damaging. Where covered tributary
ezoidal channel of considerable width, the need
drains enter, examination must be made to see
for overall economy may require consideration of
whether the water in the main channel, if full,
the relative structural and hydraulic merits of on-
would cause damaging backflooding of the tribu-
bank abutments with or without piers and warped
tary area, which would be more damaging than
channel walls with vertical abutments.
temporary stoppage of the tributary flow. If so,
means for precluding backflow must be employed;
e. To preclude failure by underscour, abutment
this can often be accomplished by a flap gate at
and pier footings will usually be placed either to
the drain outfall, and if positive closure is re-
a depth of not less than 5 feet below the antici-
quired, a slide gate can be used. If flow in the main
pated depth of scour, or on firm rock if such is
channel is supercritical, the design of side inlet
encountered at a higher elevation. Large multi-
structures may require special provisions to min-
span structures crossing alluvial streams may re-
imize turbulence effects.
quire extensive pile foundations. To protect the
channel against the increased velocities, turbu-
lence, and eddies expected to occur locally, re-
33. Bridges.
vetment of channel sides or bottom consisting of
a. A bridge is a structure, including supports,
concrete, grouted rock, loose riprap, or sacked con-
erected over a depression or an obstruction, such
crete will be placed as required. Criteria for se-
as water, a highway, or a railway, having a track
lection of revetment are given in chapter 5.
or passageway for carrying traffic or other mov-
ing loads, and having an opening measured along
f. Where flow velocities are high, bridges should
3-4
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