Hydraulic Design of Highway Culverts - DOT On-Line Publications

arrel for pipes or the flow per meter (foot) of span width for box culverts. For multiple barrels
with identical hydraulic characteristics, the total discharge is assumed to be divided equally
among the barrels. An iterative procedure or development of a combined performance curve is
required for culverts with dissimilar barrels or invert elevations. The discharge of the component
barrels must add up to the total peak design flow at a common headwater elevation. For
multiple barrel installations with bevel-edged inlets, the bevels are sized on the basis of the total
clear width of the barrels. No more than two barrels may be used with tapered inlets using the
design procedures of this manual.
E. Economics
The ideal culvert selection process minimizes the total annual cost of the installation over the life
of the roadway. The annual cost includes capital expenditures, maintenance costs, and risks
associated with flooding. An initial analysis of the choice between a culvert and a bridge is
necessary. If a culvert is selected, a comparison of the available shapes and materials would
follow. Durability, maintenance, and replacement costs are factored into the selection process
along with the initial in-place construction cost. The results are then utilized to evaluate the
design flood based on total annual cost. This procedure is referred to as a risk analysis.
Unfortunately, many of the factors required in such an analysis are not well defined, making it
difficult to justify and perform except for expensive installations.
Figure VI-22--Multiple Barrel Culverts with One Low Flow Barrel
1. Service Life. The desired service life of the culvert should be considered in the selection
process. If the culvert is in a location where replacement or relining would be impractical, the
service life of the culvert should equal the service life of the highway. If rehabilitation is feasible,
or if it is determined that the highway will be rebuilt in a relatively short time, a culvert with a
shorter service life should be selected. The service life of the culvert should match the
installation. There is no need to pay for an "eternal" culvert where a short lived one would
suffice, and vice-versa.
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