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

.

\~

e

.

(:.

e

Use of FALL Upstream of Side-Tapered Inlet

A depression may be utilized upstream of the face of a side-
tapered inlet. As illustrated in Figures 9 and 10, the depression
may be constructed in various ways, as an extension of the wing-
walls, or by a paved depression similar to that used with side-
~apered pipe culvert inlets, shown in Figure 16. The only require-
ments are: the plane of the invert of the barrel be extended
upstream from the inlet face a minimum distance of D/2, to provide
a smooth flow transition into the inlet; and, the crest be long
enough to avoid undesirably high headwater from crest control at
design discharges. Chart 17 may be used for checking crest
control if the fall slope is between 2:1 to 3:1. The length of
the crest, W, may be approximated, neglecting flow over th~ sides
of sloping wingwal1s. This provides a conservative answer.

Performance Curves

Figure 12 illustrates the design use of performance curves
and shows how the side-tapered inlet can reduce the barrel size
required for a given discharge. (The detailed calculations for
Figure 12 are given in Example Problem No.1). Performance
curve No. 1 is for a double 7 ft. x 6 ft. conventional culvert
with 90 degree wingwal1s (headwall) and 1:1 bevels on both the
top and side. This conventional inlet will be the "standard" to
which curves for the improved inlets may be compared.

The hatched performance curve is for a double 6 ft. x 5 ft.
box culvert with a side-tapered inlet with no FALL upstream. It
is a composite of the throat and face control curves. The outlet
control curve was also computed, but falls outside of the limits
of the figure. This indicates that further increases in capacity
or reduction in.headwater are possible. Face.control governs to
a discharge of 375 cfs, and throat cont.rol.'for larger discharges.
Thus, the barrel dimensions (throat size) cOntrol the designs at
high discharges, which should always be the case. In this example,
the size of the culvert was reduced from a double 7 ft. x 6.ft.
box to a double 6 ft. x 5 ft. for the same allowable headwater.
Use of an upstream FALL would reduce the barrel size still further
to a size comparable to that required with a slope-tapered inlet.

Double Barrel Design

As shpwn in the above example, double barrel structures may
be designed with improved inlets. The face is proportioned
on the basis of the total clear width as described for bevels.

13-25