Hydraulic Design of Highway Culverts - DOT On-Line Publications

The mitered face slope-tapered inlet design is more complicated than the vertical face design. A
hypothetical face section is located downstream of the weir crest formed where the extension of
the fill slope meets the stream bed. The face section is defined by a perpendicular line
extending to the FALL slope from the top edge of the inlet, neglecting bevels.
The slope-tapered inlet is the most complex inlet improvement recommended in this manual.
Construction difficulties are inherent, but the benefits in increased performance can be great.
With proper design, a slope-tapered inlet passes more flow at a given headwater elevation than
any other configuration.
Slope-tapered inlets can be applied to both box culverts and circular pipe culverts. For the latter
application, a square to round transition is normally used to connect the rectangular slopetapered
inlet to the circular pipe.
C. Hydraulics
1. Inlet Control. Tapered inlets have several possible control sections including the face, the
bend (for slope-tapered inlets), and the throat. In addition, a depressed side-tapered inlet has a
possible control section at the crest upstream of the depression. Each of these inlet control
sections has an individual performance curve. The headwater depth for each control section is
referenced to the invert of the section. One method of determining the overall inlet control
performance curve is to calculate performance curves for each potential control section, and
then select the segment of each curve which defines the minimum overall culvert performance
(Figure IV-6).
Figure IV-6—Inlet control
performance curves (schematic)
If the dimensional criteria of this publication are
followed, the crest and the bend sections will not
function as control sections over the normal
range of headwaters and discharges. The crest
of the depression may function as a control
section for very low flows and headwaters but
this is generally not of importance in design.
Figure IV-6 depicts performance curves for each
of the potential inlet control sections and the
overall inlet control performance curves.
The design procedures for tapered inlets include
checks on crest lengths for both depressed sidetapered
inlets and slope-tapered inlets with
mitered faces. As long as) the actual crest
length exceeds a certain minimum value, there
is no need to construct a crest performance
curve. Also, if the bend section is located a
minimum distance of D/2 upstream of the throat
section, the bend will not control and the bend
section performance curve does not need to be
calculated.
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