Hydraulic Analysis and Design of Pipe Culverts: USGS versus FHWA

Culvert performance curves relate the flow through a culvert to the headwater and tailwater elevations. These curves are calculated differently by the USGS and Federal Highway Administration (FHWA), leading to the possibility of significant discrepancies in predicted culvert performance under some flow conditions. The FHWA methodology generally leads to conservative culvert designs relative to designs based on the USGS equations, and the degree of conservatism is the focus of this paper. The largest discrepancies in estimated headwater elevations for given flow rates occur under high-headwater and low-tailwater conditions, and these discrepancies are due in large part to the uncertainty in whether Type 5 or 6 flow will exist. The results of this paper show that for horizontal culverts with square-edge entrances and smooth surfaces the overestimation in the headwater depth can range from 9–45% for headwater depths in the range of 1.5–3 culvert diameters, with lesser overestimations of 7–30% in cases of rounded entrances. The amount of overestimation will be less for high-roughness culverts. In cases for which the culvert is not horizontal, the overestimation, if any, will depend on the slope of the culvert, length-to-diameter ratio, and roughness.