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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IlL CULVERT DESIGN

A. Culvert Flow.

1. General. An exact theoretical analysis
of culvert flow is extremely complex because
the flow is usually nonuniform with regions
of both gradually varying and rapidly varying
flow. An exact analysis involves backwater
and drawdown calculations, energy and mo-
mentum balance, and application of the results
of hydraulic model studies. For example, the
U.S. Geological Survey has defined 18
different culvert flow types based on inlet and
outlet submergence, the flow regime in the
barrel, and the downstream brink depth. (20)
Often, hydraulic jumps form inside or down-
stream of the culvert barrel. In addition, the
flow types change in a given culvert as the
flow rate and tail water elevations change.

In order to systematically analyze culvert
flow, the procedures of this publication have
been developed, wherein the various types of
flow are classified and analyzed on the basis
of control section. A control section is a
location where there is a unique relationship
between the flow rate and the upstream water
surface elevation. Many different flow
conditions exist over time, but at a given time
the flow is either governed by the inlet
geometry (inlet control); or by a combination
of the culvert inlet configuration, the
characteristics of the barrel, and the tail water
(outlet control), Control may oscillate from
inlet to outlet; however, in this publication,
the concept of "minimum performance"
applies. That is, while the culvert may
operate more efficiently at times (more flow
for a given headwater level), it will never
operate at a lower level of performance than
calculated.

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The culvert design method presented in
this publication is based on the use of design
charts and nomographs. These charts and
nomographs are, in turn, based on data from

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numerous hydraulic tests and on theoretical
calculations. At each step of the process,
some error is introduced. For example,
there is scatter in the test data and the
selection of a best fit design equation involves
some error. Also, the correlation between the
design equations and the design nomographs
is not exact. Reproduction of the design
charts introduces additional error. Therefore,
it should be assumed that the results of the
procedure are accurate to within plus or
minus ten percent, in terms of head.
Additional information on the precision of the
design charts is provided in appendix A.

Table 1 .in chapter I shows the factors
which must be considered in culvert design
for inlet and outlet control. In inlet control,
only the inlet area, the edge configuration,
and the shape influence the cui vert
performance for a given headwater elevation.
The headwater elevation is calculated with
respect to the inlet invert, and the tailwater
elevation has no influence on performance.
In outlet control, all of the factors listed in
table 1 affect culvert performance,
Headwater elevation is calculated with respect
to the outlet invert, and the difference
between headwater and tail water elevation
represents the energy which conveys the flow
through the culvert.

2. Types of Control. A general
description of the characteristics of inlet and
outlet control flow is given below, A culvert
flowing in inlet control has shallow, high
velocity flow categorized as "supercriticaI."
For supercritical flow, the control section is
at the upstream end of the barrel (the inlet).
Conversely, a culvert flowing in outlet
control will have relatively deep, lower
velocity flow termed "subcritical" flow. For
sub critical flow the control is at the
downstream end of the culvert (the out-
let), The tail water depth is either

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