Hydraulic Design of Highway Culverts - DOT On-Line Publications

2. Comparisons Between Culverts and
Bridges. Economic considerations are of
primary importance in deciding between the
use of a bridge or a culvert at stream
crossings where either will satisfy hydraulic
and structural requirements. The initial cost
for a culvert is usually much less than for a
bridge. The use of increased headwater at a
culvert installation normally permits the use
of a smaller opening (Figure VI-23). This
advantage must be balanced with the
possible flood damages associated with an
increased headwater, especially at higher
discharges. Maintenance costs for culverts
may result from erosion at the inlet and
outlet, sedimentation and debris buildup, and
embankment repair in case of overtopping.
Bridge maintenance is more costly, however,
including such aspects as maintenance of
the bridge deck and superstructure, erosion
around piers and abutments, and possible
sediment and debris accumulation.
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Figure VI-23--Bridge versus culvert
at same location
Safety, aesthetics, and environmental considerations are also involved in the choice of a bridge
or culvert. Safety considerations for culverts include the use of guardrails or safety grates.
Bridge decks often constrict shoulder and median widths and are subject to icing which can
present traffic safety problems. A bridge may be considered more aesthetically pleasing in
traversing a scenic valley or canyon. Environmental considerations such as fish and wildlife
passage may also favor a bridge over a culvert.
Hydraulic Design Series Number 1, "Hydraulics of Bridge Waterways," (21) may be used in the
hydraulic analysis of highway bridges to approximate backwater on floodplains with sparse
vegetation. For bridges on wide heavily vegetated floodplains, other techniques developed by
the United States Geological Survey and the United States Corps of Engineers should be used.
3. Comparisons Between Materials and Shapes. Cost comparisons between various
materials and shapes vary with region and with time. It is recommended that costs for culverts
of equal hydraulic capacity be compared periodically to help guide material selection.
Requesting alternative bids for several acceptable materials is economically beneficial on most
projects.
Detailed economic analysis of culvert material selection requires site-specific considerations.
Structural strength is a concern under high fills. Steep channel slopes produce high exit
velocities which are further accelerated by using smooth pipes. Acidic drainage will promote
corrosion of some materials. Certain materials can not withstand the attack of abrasive bed
loads. Water tightness at joints may be an important consideration. All of these factors have an
impact on the annual cost of the culvert based upon the selected material.