FLOOD01360

• County: Statewide
• Title: Hydraulic Design of Highway Culverts
• Date: 9/1/1998
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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A method of increasing inlet perf or-
mance is the use of beveled edges at the
entrance of the culvert. Beveled edges
reduce the contraction of the flow by
effectively enlarging the face of the
culvert. Although any beveling will
help the hydraulics, design charts are
available for two bevel angles, 4S degre-
es and 33.7 degrees, as shown in figure
IlI-3.

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The larger, 33.7-degree bevels require
some structural modification, but they
provide slightly better inlet performance
than the 4S-degree bevels. The smaller,
4S-degree bevels require very minor struc-
tural modification of the culvert headwall
and increase both inlet and outlet con-
trol performances. Therefore, the use
of 4S degree bevels is recommended on
all culverts, whether in inlet or outlet
control, unless the culvert has a groove
end. (The groove end provides about
the same performance as a beveled edge.)

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3) Hydraulics of Inlet Control.
Inlet con trol performance is defined by
the three regions of flow shown in Figure
III-4: unsubmerged, transition and sub-
merged. For low headwater conditions,
as shown in figures III-I-A and III-I-B,
the entrance of the culvert operates as
a weir. A weir is an unsubmerged flow
control section where the upstream water
surface elevation can be 'predicted for a
given flow rate. The relationship between
flow and water surface elevation must be
determined by model tests of the weir
geometry or by measuring prototype dis-
charges. These tests Or measurements
are then used to develop equations for
unsubmerged inlet control flow. Appendix
A contains the equations which were devel-
oped from the NBS model test data.

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For headwaters submerging the culvert
entrance, as are shown in figures III-I-e
and III-I-D, the entrance of the culvert
operates as an orifice. An orifice is
an opening, submerged on the upstream
side and flowing freely on the downstream
side, which functions as a control sec-
tion. The relationship between flow and
headwater can be defined based on results

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from model tests. Appendix A contains
the submerged flow equations which were
developed from the NBS test data.

The flow transition zone between the
low headwater (weir control) and the
high headwater flow conditions (orifice
control) is poorly defined. This zone
is approximated by plotting the unsub-
merged and submerged flow equations and
connecting them with a line tangent to
both curves, as shown in figure 1II-4.

OVERALL INLET
CONTROL CURVE ~

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SUBMERGED (ORIFICE) FLOW

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... -c?"UNSU81ltERGEO(WE1R) FlOW

FLOW

Figure III-4--Inlet flow control curves.

The inlet control flow versus headwater
curves which are established using the
above procedure are the basis for con-
structing the inlet con trol design
nomographs. Note that the approach velo-
city head can be included as a part of
the available headwater in the inlet
control relationships.

4) Inlet Depressions. The inlet
control equations or nomographs provide
the depth of headwater above the inlet

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