COMPUTATION OF BACKWATER AND DISCHARGE AT WIDTH CONSTRICTIONS OF HEAVILY VEGETATED FLOOD PLAINS by V.R. Schneider, J.W. Board, B.E. Colson, F.N. Lee, and L. Druffel Prepared in cooperation with the Federal Highway Administration and the State Highway Departments of Mississippi, Alabama, and Louisiana 1977 Water-Resources Investigations 76-129 TABLE OF CONTENTS Abstract Introduction Methods of computing discharge and backwater     U.S. Geological Survey method         Discharge         Backwater     Federal Highway Administration method         Backwater Data collection     Peak discharge measurement     Valley cross sections     Water-surface elevations     Bridge geometry     Manning's roughness coefficient Analysis of data     Computing the natural profile     Measurement of H1*     Measurement of h3*     Comparison of U.S. Geological Survey and Federal Highway Administration methods     Energy loss evaluation Theoretical analysis     Energy losses in approach reach     Energy losses in flow expansion reach     Proposed method to compute backwater and discharge Results Conclusions Future work Example of backwater calculations using proposed method References FIGURES 1. Definition sketch of the variables used in computing backwater and discharge by the Survey method 2. Graph showing the water-surface elevation on the downstream side of the right embankment, West Fork Amite River near Busy Corner, MS 3. Graph showing the comparison of measured backwater to that computed by the Survey method 4. Graph showing the comparison of measured backwater to that computed by the FHWA method 5. Ratio of the average flow path length in the approach reach and the width of the constricted section as a function of the geometric constriction ratio and the ratio of the distance to the approach cross section and the width of the constriction 6. Ratio of the distance to the approach section and width of the constricted section as a function of the geometric constriction ratio 7. Summary of data for computing backwater by the proposed method 8. Comparison of results of computing backwater by the Survey, FHWA, and proposed methods 9. Summary of data for computing discharge by the proposed method 10. Summary of data for evaluating the effect of distance to the approach section on the accuracy of the computed backwater and discharge 11. Summary of data from the verifcation files for evaluating the effect of distance to the approach section on the accuracy of computed backwater and discharge 12. Percent errors in computing backwater and discharge as a function of approach section location 13. Cross-section properties for section 1 ABSTRACT The U.S. Geological Survey, cooperated with the Federal Highway Administration and the State Highway Departments of Mississippi, Alabama, and Louisiana, to develop a proposed method for computing backwater and discharge at width constrictions of heavily vegetated flood plains. Data were collected at 20 single opening sites for 31 floods. Flood-plain width varied from 4 to 14 times the bridge opening width. The recurrence intervals of peak discharge ranged from a 2-year flood to greater than a 100-year flood, with a median interval of 6 years. Measured backwater ranged from 0.39 to 3.16 feet (0.12 to 0.96 meters). Backwater computed by the present standard Geological Survey method averaged 29 percent less than the measured, and that computed by the currently used Federal Highway Administration method averaged 47 percent less than the measured. Discharge computed by the Survey method averaged 21 percent more than the measured. Analysis of data showed that the flood-plain widths and the Manning's roughness coefficient are larger than those used to develop the standard methods in current use and the accurate computation of backwater and discharge depends on improving the method of computing the energy loss. With the proposed method for computing backwater and discharge, the contracted and natural water-surface profiles are computed using standard step-backwater. The energy loss terms in the step-backwater procedure are computed as the geometric mean of the energy slopes and the representative flow distance between the ends of the reach. An estimate of the average flow path was derived from potential flow theory for the approach reach while an empirical method based on the straight-line distance between the bridge and a valley cross section one-bridge opening width downstream was developed for the computing backwater was 1 percent. The mean error using the proposed method for computing discharge was 3 percent. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.