SIMULATION OF FLOOD HYDROGRAPHS FOR GEORGIA STREAMS
by Ernest J. InmanPrepared by the u.s. Geological Survey in cooperation with the Georgia State Department of Transportation
1987
Water-Supply Paper 2317
CONTENTS
Abstract
Introduction
Data base
Basins smaller than 20 square miles
Basins of 20 to 500 square miles
Hydrograph-simulation procedure
Basins smaller than 500 square miles
Basins larger than 500 square miles
Hydrograph-width relation for basins smaller than 500 square miles
Testing of dimensionsless hydrograph
Verification
Bias
Sensitivity
Regression analysis of lagtime
Regionalization
Limits of independent variables
Testing of lagtime regression equations
Verification
Bias
Sensitivity
Application of technique
Summary
Selected References
Metric conversion factors
FIGURES
1-12. Plots of
1. Observed flood hydrograph and unit precipitation from Conley Creek near Forest Park, July 2, 3, 1974
2. Unit hydrograph computed from observed data in figure 1 with runoff of 1.00 inch and lagtime of 1.03 hours
3. Average unit hydrograph from Conley Creek near Forest Park, with correct timing of average center of mass
4. One-fourth, one-third, one-half, and three-fourths-lagtime- duration dimensionless hydrographs for Conley Creek near Forest Park
5. Average one-half-lagtime-duration dimensionless hydrograph for Region 1, and the range of the data from the 16 stations from which it was computed
6. Average one-half-lagtime-duration dimensionless hydrograph for Regions 1, 2, and 3 and the Atlanta urban area
7. Statewide dimensionless hydrograph
8. Observed and simulated hydrographs showing width comparisons at 50 and 75 percent of peak flow for an Atlanta urban station
9. Statewide, Soil Conservation Service, and Stricker-Sauer dimensionless hydrographs
10. Hydrograph-width relation for dimensionless hydrograph
11. Observed and simulated hydrographs for width comparisons at 50 and 75 percent of peak flow for Spring Creek near Iron City
12. Observed and simulated hydrographs for width comparisons at 50 and 75 percent of peak flow for Flint River near Griffin
13. Map of Georgia showing regional boundaries for flood-frequency and lagtime estimating equations
14. Plot of simulated flood hydrograph for Ogeechee River at State Highway 24
TABLES
1. Listing of discharges at 5-minute intervals with peaks aligned for seven unit hydrographs with dates of occurrence and the average unit hydrograph computed for Conley Creek near Forest Park
2. Time and discharge ratios of the statewide dimensionless hydro- graph
3. Relation of discharge ratios to hydrograph-width ratios
4. Differences of hydrograph widths of estimated and observed hydrographs at 50 and 75 percent of observed peak flow, and differences of peak discharge computed from regional regres- sion equations and observed peak dischrge, both discharges being for the same recurrence interval, and the means of these three differences
5. Selected physical characteristics of basins north of the Fall Line
6. Selected physical characteristics of basins south of the Fall Line
7. Selected physical characteristics of Atlanta urban basins
8. Summary of lagtime estimating equations
9. Results of split-sample tests of lagtime equations
10. Sensitivity of computed lagtime to errors in independent variables with the north-of-the-Fall Line equation
11. Sensitivity of computed lagtime to errors in independent variables with the south-of-the-Fall Line equation
12. Sensitivity of computed lagtime to errors in independent variables with the Atlanta urban equation
13. Simulated coordinates of the flood hydrograph for Ogeechee River at State Highway 24
ABSTRACT
Flood hydrographs are needed for the design of many highway drainage structures and embankments. A method for simulating these flood hydro- graphs at ungaged sites in Georgia is presented in this report.
The O'Donnell method was used to compute unit hydrographs and lagtimes for 355 floods at 80 gaging stations. An average unit hydrograph and an average lagtime were computed for each station. These average unit hydrographs were transformed to unit hydrographs having durations of one-fourth, one-third, one-half, and three-fourths lagtime, then re- duced to dimensionless terms by dividing the time by lagtime and the discharge by peak discharge. Hydrographs were simulated for these 355 floods and their widths were compared with the widths of the observed hydrographs at 50 and 75 percent of peak flow. The dimensionless hy- drograph at 50 and 75 percent of peak flow. The dimensionless hydro- graph based on one-half lagtime duration provided the best fit of the observed data.
Multiple regression analysis was then used to define relations be- tween lagtime and certain physical basin characteristics, of these characteristics, drainage area and slope were found to be significant for the rural-stream equations and drainage area, slope, and im- pervious area were found to be significant for the Atlanta urban- stream equation.
A hydrograph can be simulated from the dimensionless hydrograph, the peak discharge of a specific recurrence interval, and the lagtime ob- tained from regression equations for any site in Georgia having a drainage area of less than 500 square miles.
For simulating hydrographs at sites having basins larger than 500 square miles, the U.S. Geological Survey computer model CONROUT can be used. This model routes streamflow from an upstream channel loca- tion to a user-defined location downstream. The product of CONROUT is a simulated discharge hydrograph for the downstream site that has a peak discharge of a specific recurrence interval.
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