Hydraulic Design of Highway Culverts - DOT On-Line Publications

D. Economics
The hydraulic design of a culvert installation always includes an economic evaluation. A wide
spectrum of flood flows with associated probabilities will occur at the culvert site during its
service life. The benefits of constructing a large capacity culvert to accommodate all of these
events with no detrimental flooding effects are normally outweighed by the initial construction
costs. Thus, an economic analysis of the tradeoffs is performed with varying degrees of effort
and thoroughness.
1. Benefits and Costs. The purpose of a highway culvert is to convey water through a
roadway embankment. The major benefits of the culvert are decreased traffic interruption time
due to roadway flooding and increased driving safety. The major costs are associated with the
construction of the roadway embankment and the culvert itself. Maintenance of the facility and
flood damage potential must also be factored into the cost analysis.
Figure 1-16--Risk analysis benefit
versus cost curve
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2. Analysis. Traditional economic
evaluations for minor stream crossings
have been somewhat simplistic. Culvert
design flows are based on the importance
of the roadway being served with little
attention given to other economic and site
factors. A more rigorous investigation,
termed a risk analysis, is sometimes
performed for large culvert installations.
The objective of the risk analysis is to find
the optimum culvert capacity based on a
comparison of benefits and costs (Figure I-
16). The designer should be aware of the
risk analysis process and consider using it
to analyze alternatives where flood
damage is large or culvert cost is
significant.