Hydraulic Design of Highway Culverts - DOT On-Line Publications

C. Inlet Control Performance Curves
Next, inlet control performance curves should be drawn for the inlet edge configurations
selected. These edges may include square edges, beveled edges, or the throat section of a
tapered inlet. A depression may also be incorporated upstream of the inlet control section to
lower the inlet control headwater elevation. To construct the inlet control performance curves,
perform the following steps:
Figure C-2--Box Culvert Outlet Control Performance Curves
1. Calculate the inlet control headwater for the culvert barrel selected based on outlet control.
2. Determine the required face invert elevation to pass the design discharge by subtracting the
headwater depth from the design headwater elevation.
a. If the inlet invert elevation is above the stream bed elevation at the control section, the invert
should be lowered to the stream bed. The culvert will then have a capacity exceeding the
design flow with the headwater at the design headwater elevation.
b. If the required invert elevation is below the stream bed elevation at the face, the invert must
be depressed using a FALL.
c. If, in the designer's judgment, the required FALL is excessive, the inlet geometry must be
improved or a larger barrel must be used.
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