Backwater and discharge at highway crossings with multiple bridges in Louisiana and Mississippi

Data were collected for nine floods in Mississippi and Louisiana at eight stream crossings having two to six separate bridge openings. Discharge through each bridge, water surface profiles, valley cross sections, and bridge geometry were measured. The multiple openings were divided into equivalent single-opening cases by apportioning interior embankments in direct proportion to the area of openings on either side. Using existing procedures for computer discharge, the bias in computed discharge was 2 percent with a root mean square error of 18 percent. Backwater was computed by two current U.S. Geological Survey methods that use the average flow path in the friction loss term for the approach. One method gave a root mean square error of 0.34 ft. with a bias of -0.25 ft., suggesting that the method underestimates backwater. The other method gave a root mean square error of 0.39 ft with a bias of -0.03 ft. The results indicate that the method developed for single-opening highway crossings can be applied to the multiple bridge crossings. (USGS)