FLOOD01242

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Title: Hydraulic Design of Improved Inlets for Culverts
• Date: 9/1/1981
• Prepared For: USDOT
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Bevel-Edged Inlets

The first degree of inlet improvement is a beveled edge. The
bevel is proportioned based on the culvert barrel or face dimension
and operates by decreasing the flow contraction at the inlet. A
bevel is similar to a chamfer except that a chamfer is smaller and
is generally used to prevent damage to sharp concrete edges during
construction.

Adding bevels to a conventional culvert design with a square-
edged inlet increases culvert capacity by 5 to 20 percent. The
higher increase results from comparing a bevel-edged inlet with a
square-edged inlet at high headwaters. The lower increase is the
result of comparing inlets with bevels with structures having
wingwalls of 30 to 45 degrees.

Although the bevels used herein are plane surfaces, rounded
edges which approximate the bevels are also acceptable.

As a minimum, bevels should be used on all culverts which
operate in inlet control, both conventional and improved inlet
types. The exception to this is circular concrete pipes where the
socket end performs much the same as a beveled edge. Examples of
bevels used in conjunction with other improved inlets are shown
in Figures5 and 6. Culverts flowing in outlet control cannot be
improved as much as those in inlet control, but the entrance loss
coefficient, ke, is reduced from 0.5 for a square edge to 0.2 for
beveled edges. Therefore, it is recommended that bevels be used
on all culvert entrances if little additional cost is involved.

Side-Tapered Inlets

The second degree of improvement is a side-tapered inlet
(Figure 5). It provides an increase in flow capacity of 25 to 40
percent over that of a conventional culvert with a square-edged
inlet. This inlet has an enlarged face area with the transition
to the culvert barrel accomplished by tapering the sidewalls. The
inlet face has the same height as the barrel, and its top and bottom
are extensions of the top and bottom of the barrel. The intersection
of the sidewall tapers and barrel is defined as the throat section.

Side-tapered inlets of other configurations were tested, some
with tops tapered upward but with sidewalls remaining an extension
of the barrel walls, and others with various combinations of side and
top tapers. Each showed some improvement over conventional culverts,
but the geometry shown in Figure 5 produced superior performance.

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