Culvert versus Bridge Hydraulics for Larger-Span or Short Culverts

A bridge versus culvert hydraulics approach may be taken to estimate the headwater of larger-span (or equivalently short) culvert-like structures, and design manuals often argue that the former should be technically preferred. A laboratory study was undertaken to investigate short and long box culverts, including roughness effects, with special attention paid to short-structure cases with entirely open-channel flow. The data were analyzed using discharge coefficients and approximate analytical models. Simulations were conducted with the HEC-RAS bridge model (with mainly default or recommended parameter values and options) and HY-8 to assess the predictive performance and benefits, if any, of a bridge versus culvert hydraulics approach. Observations support the presence of shortness and roughness effects, but do not support any significant advantage of a bridge versus culvert hydraulics approach. On the contrary, the performance of HY-8 was found to be superior to that of the HEC-RAS bridge model for rough-bed cases, perhaps fortuitously, due to its often conservative choice of parameter values.