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Edition:Revised Final Report; July 2003 ¿ April 2005
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Abstract:Temporary ponding of water on the upstream side of a culvert serves to reduce the peak discharge that the culvert
must convey. In most cases the discharge reduction resulting from detention storage is minor and can be neglected in
design. However, in some cases the storage effect is substantial, and accounting for it may result in a more economical
design.
This report presents new methods for quickly estimating the effects of detention storage on the performance of
existing culverts and the sizing of new culverts. These methods, which do not require hydrograph routing, are
applicable to pipe and box culverts operating under inlet control with no overtopping of the roadway. Peak discharges
can be computed by the Rational method, USGS regression equations or any other appropriate method. Water-surface
areas at two or more stages are needed to define an approximate depth-area relationship. The required areas can be
estimated from existing topographic maps or measured in the field by one person with a rotating laser level and a GPS
unit.
Analyses of seven pipe-culvert sites in Johnson County showed that detention-storage design would reduce the
required pipe diameter by at least one increment at five of the seven sites, and by two or more increments at three of
the sites. Similar analyses of ten box-culvert sites showed that detention-storage design would reduce the required
span by more than 10% at three of the ten sites. Our test results indicate that storage effects are less likely to be
significant for large culverts than for small culverts.
The design of a culvert for detention storage rather than peak flow generally requires another survey, extra design
effort and the purchase of additional right-of-way or a drainage easement for the storage area. Detention-storage
design is economically justifiable only if cost savings on the culvert exceeds these added costs. In locations where
storage effects are significant but detention-storage design is not economically justifiable, the culvert should be
designed for peak flow.
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