Hydraulic Design of Highway Culverts - DOT On-Line Publications

C. Culvert Hydraulics
A complete theoretical analysis of the hydraulics of a particular culvert installation is timeconsuming
and difficult. Flow conditions vary from culvert to culvert and they also vary over time
for any given culvert. The barrel of the culvert may flow full or partly full depending upon
upstream and downstream conditions, barrel characteristics, and inlet geometry.
Research by the National Bureau of Standards (NBS) sponsored and supported by the Federal
Highway Administration (FHWA), formerly the Bureau of Public Roads (BPR), began in the early
1950s and resulted in a series of seven reports. (see references 4, 5, 6, 7, 8, 9, and 10) These
reports provided a comprehensive analysis of culvert hydraulics under various flow conditions.
These data were used by the BPR staff to develop culvert design aids, called nomographs.
These nomographs are the basis of the culvert design procedures in HEC No. 5, HEC No. 13,
and this publication.
The approach presented in HEC No. 5 is to analyze a culvert for various types of flow control
and then design for the control which produces the minimum performance. Designing for
minimum performance- ignores transient conditions which might result in periods of better
performance. The benefits of designing for minimum performance are ease of design and
assurance of adequate performance under the least favorable hydraulic conditions.
1. Flow Conditions. A culvert barrel may flow full over all of its length or partly full. Full flow in
a culvert barrel is rare. Generally, at least part of the barrel flows partly full. A water surface
profile calculation is the only way to accurately determine how much of the barrel flows full.
a. Full Flow. The hydraulic condition in a culvert flowing full is called pressure flow. If the crosssectional
area of the culvert in pressure flow were increased, the flow area would expand. One
condition which can create pressure flow in a culvert is the back pressure caused by a high
downstream water surface elevation. A high upstream water surface elevation may also
produce full flow (Figure I-11). Regardless of the cause, the capacity of a culvert operating
under pressure flow is affected by upstream and downstream conditions and by the hydraulic
characteristics of the culvert.
Figure I-11--Culvert Flowing Full
(No tailwater at outlet end)
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