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

Table 1
Factors InfluencIng culvert performance.

Inlet Outlet
Factor Control Control
Headwater
Elevation X X
Inlet Area X X
Inlet Edge
Configuration X X
Inlet Shape X X
Barrel Roughness X
Barrel Area X
Barrel Shape X
Barrel Length X
Barrel Slope * X
Tailwater
Elevation X

*Barrel slope affects inlet control per-
formance to a small degree, but may be
neglected.

streambed scour and bank erosion in the
vicinity of the culvert outlet. Minor
problems can occasionally be avoided by
increasing the barrel roughness. Energy
dissipators and outlet protection devices
are sometimes required to avoid excessive
scour at the culvert outlet. When a
culvert is operating under inlet control
and the cui vert barrel is not operating
at capacity, it is often beneficial to
flatten the barrel slope or add a rough-
ened section to reduce outlet velocities,

6. Performance Curves. A performance
curve is a plot of headwater depth or
elevation versus flow rate. The resulting
graphical depiction of culvert operation
is useful in evaluating the hydraulic
capacity of a culvert for various head-
waters. Among its uses, the performance
curve displays the consequences of higher
flow rates at the site and the benefits
of inlet improvements.

In developing a culvert performance
curve, both inlet and outlet control
curves must be plotted. This is necessary
because the dominant control at a given
headwater is hard to predict. Also,
control may shift from the inlet to the
outlet, or vice-versa over a range of
flow rates. Figure 1-15 illustrates a
typical culvert performance curve. At
the design headwater, the culvert operates
under inlet control. With inlet improve-
ment the culvert performance can be in-
creased to take better advantage of the
culvert barrel capacity.

I
Outlet Control /

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Design ..L-
Headwater 'I
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Figure I-IS--Culvert performance curve.

D. Economics.

The hydraulic design of a culvert
installation always includes an economic
evaluation. A wide spectrum of flood
flows with associated probabilities will
occur at the culvert site during its
service life. The benefits of construc-

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