A simple method for estimating flood flow around bridges

The software program HEC-RAS from the Hydrologic Engineering Center (HEC) of the US Army Corps of Engineers is probably one of the most commonly used methods in the world for computing water surface profiles in rivers. The energy method (one of the four bridge subroutines within HEC-RAS) computes the bridge backwater (the upstream surface increase) by applying standard step calculations five times from the end of the expansion reach up to the beginning of the contraction reach, using two different transition loss coefficients and different reach lengths. The aim of this study was to estimate the backwater in a less cumbersome and practical way, without sacrificing accuracy. A one-step energy method is suggested, based on a comprehensive set of laboratory bridge backwater data from compound channels. The proposed method gives an absolute mean error of 10% when applied to these laboratory data and an absolute mean error of 25% when applied to field data collected by the United States Geological Survey including actual flood profiles through many bridges.