BACKWATER AND DISCHARGE AT HIGHWAY CROSSINGS WITH MULTIPLE BRIDGES IN LOUISIANA AND MISSISSIPPI

By B.E. Colson and Verne R. Schneider

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 83-4065

Prepared in cooperation with MISSISSIPPI STATE HIGHWAY DEPARTMENT

Jackson, Mississippi
1983

CONTENTS

Abstract
Introduction
Purpose and scope
Methods for computing discharge
Methods for computing backwater
Data collection
    Peak discharge measurement
    Valley cross sections
    Water-surface elevation
    Bridge geometry
    Manning's roughness coefficient
Analysis of data
    Computation of natural profile
    Measurements of h1*
    Measurements of h3*
    Stagnation points
    Computation of discharge
    Computation of backwater
    Errors in computed backwater and discharge
Discussion of results
Summary and conclusion
References

ILLUSTRATIONS

1. Definition sketch showing the variables used in computing backwater and discharge
2. Definition sketch showing the variables used in computing backwater and discharge at a multiple opening constriction
3. The comparison of measured and computed flow division points (stagnation points on the upstream side of the embankment)
4. The comparison of the measured and computed discharge
5. The comparison of the computed and measured h for method I
6. The comparison of the computed and measured h for method II
7. The comparison of the computed and measured h for method II
8. The comparison of the computed and measured backwater, h1*, for method II
9. The comparison of the computed and measured backwater, h3*, for method I
10. The comparison of the computed and measured backwater, h3*, for method II

TABLES

1. Site location and flood date
2. Summary of site data
3. Summary of data for computing backwater and discharge
4. Summary of measured and computed backwater and discharge
5. Comparison of measured and computed backwater

ABSTRACT

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 computing 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 foot with a bias of -0.25 foot, suggesting that the method underestimates backwater. The other method gave a root mean square error of 0.39 foot with a bias of -0.03 foot. The results indicate that the method developed for single-opening highway crossings can be applied to the multiple bridge crossings.


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