HYDRAULIC DESIGN OF BRIDGES WITH RISK ANALYSIS

By V.R. Schneider and K.V. Wilson

CONTENTS

Introduction
Mechanics of study
    Design philosophy
        Design discharge
        The base flood
        Freeboard
        Economic analysis
    Risk analysis
        Definition
        Usefulness of risk analysis
        Procedure
The report
    Report outline
    Introduction
        Acknowledgment
        Purpose
        Project description
    Data collection
        Flood plain and channel geometry
        Land use
        Hydraulic and hydrologic data
        Geologic data
            Surface geology
            Channel morphology
            Soils information
            Scour history
        Cost data
            Capital cost
            Backwater damage
            Traffic interruption
            Embankment repairs
    Data analyses
        Magnitude and frequency of floods
        Hydrographs
        Natural water-surface profiles
        Flow distribution
        Special considerations
        Alternative bridges considered in risk analysis
        Water-surface profiles
    Risk analysis
    Summary and conclusions
Summary
References
Appendix A - Example Report - Leaf River near Collins, MS
Appendix B - Example Report - Leaf River near Hattiesburg, MS

FIGURES

1. Graph showing the variation of expected cost with exceedance probability

2. Graphs showing a summary of the results of the risk analysis and the selection of the design discharge, bridge length, and embankment elevation

3. Graph showing the variation of construction costs with bridge length and embankment height

4. Graph showing the variation of backwater damage with bridge length and embankment height

5. Graph showing the variation of traffic interruption costs with bridge length and embankment height

6. Graph showing the variation of erosion damage potential with bridge length and embankment height

7. Graph showing the variation of design discharge with bridge length and embankment height

TABLES

1. Summary of the risk analysis for one bridge length and embankment elevation

2. Typical report outline

3. Summary of typical data collected during a land-use survey

ABSTRACT

This maual presents example studies and reports which implement the philosophy that the total stream crossing including the approack fills in the flood plains and all necessary waterway openings should be designed and constructed for the least total expected cost to the public. The total expected cost includes the capital investment in the highway, expected replacement and repair costs as a result costs as a result of flood damages, expected user costs from traffic interruption and detour, and expected backwater drainages during the service life of the highway. Techniques for making engineering and economic studies for the least cost designs are presented along with suggestions for managing the time and work required for such studies. A unique design flood is defined for each bridge as the flood whose upstream stage is equal to the lowest elevation of the approach fill or bridge deck. Two example reports were prepared to illustrate the application of the method to a rural site (low backwater damage) and an urban site (high potential traffic interruption and backwater damage).


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For questions or comments, contact K. Van Wilson.