Hydraulic Design of Highway Culverts - DOT On-Line Publications

a. Economic Considerations. As ponding elevations increase upstream from a culvert,
detrimental economic consequences can occur. Although for major structures it may be
advantageous to perform a flood risk analysis (Chapter 6), site-specific constraints are
sometimes adopted in lieu of a full risk analysis. Such constraints are based on some
designated elevation that is not to be exceeded within a specified return period. This elevation
may correspond to some critical point on the roadway such as the roadway shoulder or the
roadway overtopping elevation. Another criteria might be the flood damage elevation of an
upstream building. Possible loss of life and the importance of the highway are likewise
considered. While all of these factors pertain to risk analysis, a detailed risk analysis is generally
not performed.
b. Regulatory Constraints. The requirements of the National Flood Insurance Program are a
major consideration in culvert design. Most communities are now participating in this program.
The limitation on flood plain construction as it affects the base (100-year) flood elevation is of
primary importance. Depending upon the culvert location, existing floodway encroachments,
and whether there is a specified floodway, the allowable water surface elevation increase varies
from 0 to 0.3 m (1 foot). Regardless of the return period utilized in the culvert design for the
particular roadway, the 100-year return period flood must be checked to ascertain the effects of
the culvert on the base flood elevation (19).
c. Arbitrary Constraints. Some state or local agencies place arbitrary constraints on the
headwater produced by a culvert. For example, the headwater depth may not be allowed to
exceed the barrel height or some multiple of the barrel height. Although these constraints will
severely limit the flexibility inherent in culvert design, they must be followed unless the
controlling agency can be convinced to relax the restrictions or grant an exemption.
C. Summary of Data Needs. Table 3 summarizes the various data needed for culvert design.
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