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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II. DESIGN CONSIDERATIONS

A. Hvdrol02v.

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1. General. Hydrologic analysis in-
volves the estimation of a design flow rate
based on climatological and watershed
characteristics. This analysis is one
of the most important aspects of culvert
design. Since statistical uncertainties
are inherent in hydrologic analysis, the
results of the analysis are not as accurate
as the results of the hydraulic analysis
of a culvert. Nonetheless, both of these
analyses are required, and the hydrologic
study must be performed first. FHWA
Hydraulic Engineering Circular (HEC)
Number 19, Hvdrol02v. is an excellent
reference for gaining information and
insight into most of the hydrologic methods
mentioned in this publication. (11)

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A statistical concept often associated
with hydrologic analysis is the return
period, The term return period is used
when refcrring to the frequency of occur-
rence of rare events such as floods.
Mathematically, the return period is the
reciprocal of frequency. For example,
the flood which has a 5 percent chance
of occurring (frequency) in any given
year also has a return period of 20 years;
i.e., 1/0,05 = 20 years. In other words,
this flood event will be exceeded on the
a verage of once every 20 years over a
long period of time, Hence, the 20 year
flood event is likely to be exceeded
five times during a 100-year period,
These events will be randomly spaced
over the 100 years.

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Large and expensive culvert
installations may warrant extensive hydro-
logic analysis. This increased level of
effort may be necessary in order to per-
form risk analysis and/or storage routing
calculations, Risk analysis requires
the computation of flows for several
different return periods, Storage routing
calculations require the definition of
the entire flood event or hydrograph,

Considerable study of the use of risk
analysis in culvert design has occurred
over the past 10 to 20 years. Risk anal-
ysis balances the culvert cost with the
damages associated with inadequate culvert
performance. These studies have been
fruitful in relating culvert design to
economic theory and in defining the mo-
netary consequences of both over-design
and under-design. The limitations of
culvert design based solely on arbitrary
return periods have been duly exposed in
the process.

Storage routing is the attenuation of
the flood flow due to the storage volume
upstream of the culvert. Risk analysis
studies often include storage routing as
an integral part of the culvert sizing
process. Consideration of storage routing
in these studies often reduces the design
culvert size, Hence, storage routing
has been included as an optional part of
the design procedure presented in this
manual.

2. Peak Design Flow. As a flood wave
passes a point along a stream, the flow
increases to a maximum and then recedes.
The maximum flow rate is called the peak
flow. The peak flow has been, and con-
tinues to be, a major factor in the cul-
vert design process,

In traditional culvert design, a struc-
ture is sized to pass a peak flow from
one side of the roadway embankment to
the OthOE with an acceptable headwater
elevation. The magnitude of the peak
flow is dependent upon the selection of
a return period. The assignment of a
return period is generally based on the
importance of the roadway and flood damage
poten tial.

For gaged sites, statistical analyses
can be performed on the recorded stream
flow to provide an estimated peak design
flow for a given return period. The

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