COMPARISON OF FLOOD FREQUENCY ESTIMATES FROM SYNTHETIC AND OBSERVED DATA ON SMALL DRAINAGE AREAS IN MISSISSIPPI

by B.E. Colson

Prepared by the U.S. Geological Survey in cooperation with the Mississippi State Highway Department

1986

Water Resources Investigations Report 86-4034

CONTENTS

Abstract
Introduction
Flood-frequency analysis
Analysis of observed data
Comparison of results
Summary
Selected references

FIGURES

1. Map showing locations of gaging stations used for comparing observed and synthetic flood frequency

2. Graph showing comparison of synthetic and observed 2-year flood discharge

3. Graph showing comparison of synthetic and observed 5-year flood discharge

4. Graph showing comparison of synthetic and observed 10-year flood discharge

5. Graph showing comparison of synthetic and observed 25-year flood discharge

6. Graph showing comparison of synthetic and observed 50-year discharge

7. Graph showing comparison of synthetic and observed 100-year flood discharge

8. Flood-frequency curves for selected gaging stations based on observed (measured) discharge data

9. Flood-frequency curves for selected gaging stations based on synthetic discharge data

TABLES

1. Gaging stations and period of record

2. Basin characteristics and comparison of observed versus synthetic flood discharges

ABSTRACT

In 1964, the U.S. Geological Survey in Mississippi expanded the small stream-gaging network for collection of rainfall and runoff data to 92 stations. To expedite availability of flood-frequency information, a rainfall-runoff model using available long-term rainfall data was calibrated to synthesize flood peaks. Results obtained from observed annual peak flow data for 51 sites having 16 to 30 years of annual peaks are compared with the synthetic results. Graphical comparison of the 2, 5, 10, 25, 50, and 100-year flood discharges indicate good agreement. The root mean square error ranges from 27 to 38 percent, and the synthetic record bias from -9 to -18 percent in comparison with the observed record. The reduced variance in the synthetic re- sults results is attributed to use of only four long-term rainfall records and model limitations. The root mean square error and bias is within the accuracy considered to be satisfactory.

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