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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For the side-tapered inlet, there are two possible control
sections: the face and the throat. Hf' as shown in Figure 5,
is the headwater depth based upon face control. Ht is the head-
water depth based upon throat control.

The advantages of a side-tapered inlet operating in throat
control are: The flow contraction at the throat is reduced; and,
for a given pool elevation, more head is applied at the throat
control section. The latter advantage is increased by utilizing
a slope-tapered inlet or a depression in front of the side-tapered
inlet.

Slope-Tapered Inlets

A slope-tapered inlet is the third degree of improvement. Its
advantage over the side-tapered inlet without a depression is that
more head is available at the control (throat) section. This is
accomplished by incorporating a FALL in the enclosed entrance
section (Figure 6).

This inlet can have over 100 percent greater capacity than a
conventional culvert with square edges. The degree of increased
capacity depends largely upon the amount of FALL available between
the invert at the face and the invert at the throat section. Since
this FALL may vary, a range of increased capacities is possible.

Slope-tapered inlets of alternate designs were considered and
tested during the research. The inlet shown in Figure 6 is recommended
on the basis of its hydraulic performance and ease of construction.
As a result of the FALL concentrated between the face and the throat
of this inlet, the barrel slope is flatter than the barrel slope of a
conventional or side-tapered structure at the same site.

Both the face and throat are possible control sections in a
slope-tapered inlet culvert. However, since the major cost of a
culvert is in the barrel portion and not the inlet structure, the
inlet face should be designed with a greater capacity at the allowable
headwater elevation than the throat. This insures that flow control
will be at the throat and more of the potential capacity of the barrel
will be utilized.

Performance Curves

To understand how a culvert at a particular site will function
over a range of discharges, a performance curve, which is a plot of
discharge versus headwater depth or elevation, must be drawn. Figure
7 is a schematic performance curve for a culvert with either a
side-tapered or slope-tapered inlet.

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