Hydraulic Design of Highway Culverts - DOT On-Line Publications

conditions. Variations in the concrete mix, such as higher cement content, help to reduce the
deterioration of concrete culverts subject to alkaline soils and water. Concrete tends to perform
better than metal in clay or organic muck. In areas of severe acidity, such as acid mine
drainage, concrete box culverts have been protected by fiberglass linings.
Bituminous or fiber-bonded coatings on metal culverts may require special consideration. The
designer should ascertain that the this coating will in fact increase the service life. Delamination
is the primary mode of failure and can occur due to sunlight exposure and abrasion. Damage to
the coatings during handling and placing is another consideration. Polymer coatings appear to
overcome some of these deficiencies. They have excellent corrosion resistance properties and
are generally more abrasion-resistant, less subject to damage in handling and placement, and
have fewer manufacturing flaws.
J. Culvert Hydraulic Programs
Numerous calculator and computer programs now exist to aid in the design and analysis of
highway culverts. These programs possess distinct advantages over traditional hand calculation
methods. The increased accuracy of programmed solutions represents a major benefit over the
inaccuracies inherent in the construction and use of charts and nomographs. In addition,
programmed solutions are less time consuming. This feature allows the designer to compare
alternative sizes and inlet configurations very rapidly so that the final culvert selection can be
based on economics. Interactive capabilities in some programs can be used to change certain
input parameters or constraints and analyze their effects on the final design. Familiarity with
culvert hydraulics and the traditional methods of solution provides a solid basis for designers to
take advantage of the speed, accuracy, and increased capabilities available in culvert hydraulics
programs.
The logic, capability, and output of programmed solutions vary depending mainly upon
computer storage capacity and design function. Most programs analyze the performance of a
given culvert, although some are capable of design. Generally, the desired result of either type
of program is to obtain a culvert which satisfies hydrologic data and site conditions by
considering both inlet and outlet control. Results usually include the barrel size, inlet
dimensions, performance data, and outlet velocity. Some programs are capable of analyzing
side-tapered and slope-tapered inlets. Often the analysis or design of the barrel size is for one
barrel only; multiple barrels are designed by apportioning the design discharge between
barrels. The larger computer programs may contain such desirable features as
backwater calculations, performance curves, hydrologic routines, and capabilities for routing
based on upstream storage considerations.
The various culvert design hydraulic programs now available will not be covered in this manual.
It would be impossible to cover all of the available culvert hydraulics programs because they are
numerous and many are proprietary. However, users should be cautioned to review all
programs and design procedures for conformance with the culvert design processes
outlined in this manual and particularly in Appendix A.
See the FHWA website, http://www.fhwa.dot.gov/bridge/hydsoft.htm, for calculator and culvert
design computer programs available to the public.
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