Hydraulic Design of Highway Culverts - DOT On-Line Publications

E. Tapered Inlet Face Control Performance Curves
Either a side-tapered inlet with an upstream sump or slope-tapered inlet design may be used if a
FALL is required at the throat control section of a tapered inlet. The minimum face design for
the tapered inlet is one with a performance curve which does not exceed the design point.
However, a "balanced" design requires that full advantage be taken of the increased capacity
and/or lower headwater gained through use of various FALLs. This suggests a face
performance curve which intersects the throat control curve either: (1) at the design headwater
elevation, (2) at the design flow rate, (3) at its intersection with the outlet control curve, or (4) at
other points selected by the designer. These options are illustrated in Figure C-5 by points a
through e representing various points on the throat control performance curves. The options
are:
1. Intersection of face and throat control performance curves at the design headwater
elevation (points a or b).
2. Intersection of face and throat control performance curves at the design flow rate (points a, c
or d). This option makes full use of the FALL to increase capacity and reduce headwater
requirements at flows equal to or greater than the design flow rate.
3. Intersection of the face control performance curve with throat control performance curve at
its intersection with the outlet control performance curve (points b or e). This option results in
the minimum face size which can be used to make full use of the increased capacity available
from the FALL at the throat.
4. Variations in the above options are available to the designer. For example, the culvert face
can be designed so that culvert performance will change from face control to throat control at
any discharge at which inlet control governs. Options 1 through 3, however, appear to fulfill
most design objectives. Generally, the design objective will be to design either the minimum
face size to achieve the maximum increase in capacity possible for a given FALL, or the
maximum possible decrease in the required headwater for a given depression for any discharge
equal to or greater than the design discharge.
Figure C-6 illustrates some of the possible tapered inlet designs for a specific design situation.
The dimensions of the side-tapered inlet are the same for all options. This is because
performance of the side-tapered inlet face nearly parallels the performance of the throat and an
increase in headwater on the throat by virtue of an increased FALL results in an almost equal
increase in headwater on the face. Depressing the throat of a culvert with a side-tapered inlet
requires additional barrel length.
Face dimensions and inlet length increase for the slope-tapered inlet as the capacity of the
culvert is increased by additional FALL on the throat. No additional headwater depth is created
at the face by placing additional depression on the throat. However, use of a greater
depression at the throat of a culvert with a slope-tapered inlet does not increase the barrel
length.
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