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

higher than the actual velocity at the
outlet. Normal depth in common culvert
shapes may be calculated using a trial
and error solution of the Manning equa-
tion. The known inputs are flow rate,
barrel resistance, slope and geometry.
Normal depths may also be obtained from
design aids in publications such as HDS
No.3. (23)

In outlet control, the cross section-
al area of the flow is defined by the
geometry of the outlet and either critical
depth, tail water depth, or the height of
the conduit. (figure IIl-14)

Critical depth is used when the tail-
water is less than critical depth and
the tail water depth is used when tail water
is greater than critical depth but below
the top of the barrel. The total barrel
area is used when the tail water exceeds
the top of the barrel.

B. Performance Curves.

Performance curves are representa-
tions of flow rate versus headwater depth
or elevation for a given flow control
device, such as a weir, an orifice, or a
culvert. A weir constricts open channel
flow so that the flow passes through
critical depth just upstream of the weir.
An orifice is a flow control device,
fully submerged on the upstream side,
through which the flow passes. Performance
curves and equations for these two basic
types of flow control devices are shown
in figure III-IS.

When a tail water exists, the control
device may be submerged so that more
than one flow-versus-elevation relation-
ship exists. Then, the performance curve
is dependent on the variation of both
tailwatcr and hcadwater. In the case of
a weir or orifice, the device is called
a submerged weir or a submerged orifice,
respectively. For some cases, submergence
effccts have been analyzed and correction
factors have been developed. (21,22,24)

Culvert performance curves are similar
to weir and/or orifice performance curves.

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In fact, culverts often behave as weirs
or orifices. However, due to the fact
that a culvert has several possible control
sections (inlet, outlet, throat), a given
installation will have a performance
curve for each control section and one
for roadway overtopping. The overall
culvert performance curve is made up of
the controlling portions of the individual
performance curves for each control sec-
tion.

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1, Inlet Control. The inlet control
performance curves are developed using
either the inlet control equations of
appendix A or the inlet control nomographs
of appendix D. If the equations of
appendix A are used, both unsubmerged
(weir) and submerged (orifice) flow head-
waters must be calculated for a series
of flow rates bracketing the design flow,
The resultant curves are then connected
with a line tangent to both curves (the
transition zone). If the inlet control
nomographs are used, the headwaters corre-
sponding to the series of flow rates are
determined and then plotted. The transi-
tion zone is inherent in the nomographs.

2. Outlet Control. The outlet control
performance curves are developed using
equations (I) through (7) of this chapter,
the outlet control nomographs of appendix
D, or backwater calculations. Flows
bracketing the design flow are selected.
For these flows, the total losses through
the barrel are calculated or read from
the outlet control nomographs. The losses
are added to the elevation of the hydraulic
grade line at the culvert outlet to obtain
the headwater.

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If backwater calculations are performed
beginning at the downstream end of the
culvert, friction losses are accounted
for in the calculations. Adding the
inlet loss to the energy grade line in
the barrel at the inlet results in the
headwater elevation for each flow rate.

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3. Roadway Overtopping. A perfor-
mance curve showing the culvert flow as
well as the flow across the roadway is a
useful analysis tool. Rather than using

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