FLOOD01360

• County: Statewide
• Title: Hydraulic Design of Highway Culverts
• Date: 9/1/1998
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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critical depth at the culvert outlet or
the downstream channel depth, whichever
is higher. In a given culvert, the type
of flow is dependent on all of the factors
listed in table 1.

a. Inlet Control.

I) Examples of Inlet Control.
Figure 111-1 depicts several different
examples of inlet control flow. The
type of flow depends on the submergence
of the inlet and outlet ends of the cul-
vert. In all of these examples, the
control section is at the inlet end of
the culvert. Depending on the tailwa-
ter, a hydraulic jump may occur down-
stream of the inlet.

Figure III-I-A depicts a condition
where neither the inlet nor the outlet
end of the culvert are submerged. The
flow passes through critical depth just
downstream of the culvert entrance and
the flow in the barrel is super critical.
The barrel flows partly full over its
length, and the flow approaches normal
depth at the outlet end.

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Figure III-I-B shows that submergence
of the outlet end of the culvert does
not assure outlet control. In this case,
the flow just downstream of the inlet is
supercritical and a hydraulic jump forms
in the culvert barrel.

Figure III-I-C is a more typical design
situation. The inlet end is submerged
and the outlet end flows freely. Again,
the flow is supercritical and the barrel
flows partly full over its length. Crit-
ical depth is located just downstream of
the culvert entrance, :lnd the flow is
approaching normal depth at the down-
stream end of the culvert.

Figure III-I-D is an unusual condi-
tion illustrating the fact that even
submergence of both the inlet and the
ou tlet ends of the cui vert does not assure
full flow. In this case, a hydraulic jump
will form in the barrel. The median
inlet provides ventilation of the culvert
barrel. If the barrel were not venti-

lated, sub-atmospheric pressures could
develop which might create an unstable
condition during which the barrel would
alternate between full flow and partly
full flow.

2) Factors Influencing Inlet
Control. Since the control is at the
upstream end in inlet control, only the
headwater and the inlet configuration
affect the culvert performance. (table
1) The headwater depth is measured from
the invert of the inlet control section
to the surface of the upstream pool.
The inlet area is the cross-sectional
area of the face of the culvert. General-
ly, the inlet face area is the same as
the barrel area, but for tapered inlets
the face area is enlarged, and the control
section is at the throat. The inlet

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Figure 111-2-- Flow contractions
r or various culvert inlets.

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