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USGS Virginia Water Science Center

U.S. GEOLOGICAL SURVEY
SIR 2005-5254


Comparison of Peak Discharge and Runoff Characteristic Estimates from the Rational Method to Field Observations for Small Basins in Central Virginia

U.S. Geological Survey

By: D. C. Hayes and R. L. Young.

This report is available as a pdf.


Abstract

Various types of drainage structures are necessary to protect human life, highway settings, and the flood-plain environment from surface runoff. The design of a drainage structure requires hydrologic analysis of precipitation amount and duration, peak rate of runoff, and the time distribution of runoff from a given basin.

Many hydrologic methods are available for estimating peak flows from a basin, and no single method is applicable to all basins. The Rational Method is commonly used to estimate the design-storm peak discharge. The concepts of the Rational Method are sophisticated and considerable engineering knowledge is required to select representative hydrologic characteristics, such as time of concentration and runoff coefficient, which will result in a reliable design discharge. Validation of the Rational Method is difficult because direct measurement of some hydrologic characteristics, for example, time of concentration and runoff coefficient, is not easily accomplished.

Eight small basins in central Virginia ranging from 2.5 to 52.7 acres were selected for comparison of design characteristics to observed hydrologic data. Design estimates of drainage area, time of concentration, and runoff coefficients were used to estimate the design-storm peak discharge with the Rational Method. The basins were instrumented with monitoring devices to determine instantaneous discharge and measure discrete depths of precipitation from storms. These data were analyzed to estimate times of concentration and runoff coefficients for individual storms. Times of concentration and runoff coefficients were estimated directly from hyetograph and hydrograph data and by the Rational Hydrograph Method. The Rational Hydrograph Method (RHM) is a mathematical and statistical model where in the observed hydrograph is compared to predicted hydrographs developed with the Rational Method using the hyetograph data and paired combinations of times of concentration and runoff coefficients.

Design estimates of time of concentration for eight study basins generally were longer than the estimates derived directly from the observed (hyetograph and hydrograph) data, and, therefore, underestimated peak discharges and are considered less conservative. In contrast, design estimates of time of concentration generally were shorter than the estimates derived from the RHM, and, therefore, overestimate peak discharges and are considered more conservative.

Design estimates of runoff coefficient for eight study basins generally were larger than the runoff coefficients derived either by solving the rational equation for the runoff coefficient from the observed data or by the RHM, and, therefore, overestimate peak discharges and are considered more conservative.

Design estimates of peak discharge were compared to discharges computed for each study site using the median values of the times of concentration and runoff coefficients as input values for the Rational Method. Design peak-discharge values at seven of the eight study basins generally were greater than the discharges computed from the median values of time of concentration and runoff coefficients determined from the storm data and are considered more conservative. However, rainfall intensities and duration measured during storms generally had less than or equal to a 2-year recurrence interval when compared to local intensity-duration-frequency curves. Only a few storms generated intensities and durations near the 10-year recurrence interval. It is expected that design peak discharges based on a 10-year recurrence interval would be greater than discharges based on data collected from higher frequency storms.

Design estimates of peak discharge for the design storm frequency and observed peak discharges and rainfall intensities for eight basins in central Virginia were compared to observed peak discharges at similar-sized basins across the United States and separately to observed peak discharges at similar-sized basins in Virginia and surrounding states.

A curve drawn over the range of the maximum observed runoff for 1,025 streamflow-gaging stations from across the United States defines the upper boundary for small basins (less than 400 acres). The maximum observed runoff was 10.2 inches per hour (in./hour) for basins smaller than 256 acres. The maximum observed runoff from the 122 storms analyzed at eight study basins was 3.6 in./hour, and the greatest average rainfall intensity for storms analyzed was 6.60 in./hour. Curves also were drawn over the range of flood-frequency estimates of the 10-, 25-, 50-, and 100-year peak flows for 596 streamflow-gaging stations across the United States with 10 or more years of annual peak-flow data. The curves define the upper boundaries of flood-frequency estimates for small basins. Similar regional curves for maximum observed runoff and flood-frequency estimates were developed from records from streamflow-gaging stations in Virginia and surrounding states.

Data collected and analyzed for this study confirm the nonuniformity of precipitation in time and space, and are evidence for the validity of the assumption that unsteady runoff conditions are generated from varied precipitation, overland flow, and subsurface stormflow. Runoff characteristics determined using different methods from multiple storms validate, to a degree, use of the Rational Method for peak-flow design computations. Further validation would require a flood-frequency analysis of annual peak-flow data.

Table of Contents

Abstract

Introduction

Purpose and Scope

Description of Study Basins

Runoff

Factors Affecting Runoff

Sources of Runoff

Peak Discharge Estimates from the Rational Method

Rational Method

Design Computations

Parameter Estimates from Storm Data

Data Collection

Time of Concentration and Runoff Coefficient Estimation

Data analysis

Discharge Computations

Comparison of Design Computations and Parameters Estimated From Storm Data

National Peak-Flow Data

Maximum Observed Runoff and Flood-Frequency Envelope Curves

Data Analysis 

Comparison of Design Computations and Envelope Curves

Summary and Conclusions

Acknowledgments

References Cited 


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