Magnitude and Frequency of Rural Floods in the Southeastern United States, 2006: Volume 1, Georgia
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Prepared in cooperation with the Georgia Department of Transportation Preconstruction Division Office of Bridge Design Magnitude and Frequency of Rural Floods in the Southeastern United States, 2006: Volume 1, Georgia Scientific Investigations Report 2009–5043 U.S. Department of the Interior U.S. Geological Survey Cover: Flint River at North Bridge Road near Lovejoy, Georgia, July 11, 2005. Photograph by Arthur C. Day, U.S. Geological Survey. Magnitude and Frequency of Rural Floods in the Southeastern United States, 2006: Volume 1, Georgia By Anthony J. Gotvald, Toby D. Feaster, and J. Curtis Weaver Prepared in cooperation with the Georgia Department of Transportation Preconstruction Division Office of Bridge Design Scientific Investigations Report 2009–5043 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Suzette M. Kimball, Acting Director U.S. Geological Survey, Reston, Virginia: 2009 For more information on the USGS--the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Gotvald, A.J., Feaster, T.D., and Weaver, J.C., 2009, Magnitude and frequency of rural floods in the southeastern United States, 2006—Volume 1, Georgia: U.S. Geological Survey Scientific Investigations Report 2009–5043, 120 p. iii Contents Abstract ..........................................................................................................................................................1 Introduction ....................................................................................................................................................1 Purpose and Scope ..............................................................................................................................2 Previous Studies ...................................................................................................................................2 Description of Study Area ...................................................................................................................2 Acknowledgments ................................................................................................................................4 Data Compilation ............................................................................................................................................4 Peak-Flow Data .....................................................................................................................................4 Physical and Climatic Basin Characteristics ...................................................................................4 Estimation of Flood Magnitude and Frequency at Gaged Stations .......................................................5 Flood Frequency ....................................................................................................................................5 Skew Coefficient ...................................................................................................................................8 Generalized-Skew Analysis ................................................................................................................8 Estimation of Flood Magnitude and Frequency at Ungaged Sites ........................................................9 Regression Analysis .............................................................................................................................9 Regionalization of Flood-Frequency Estimates ..............................................................................10 Regional Regression Equations ........................................................................................................11 Accuracy and Limitations ..................................................................................................................13 Analysis of Gaged Basins within Multiple Hydrologic Regions ..................................................20 Application of Methods ..............................................................................................................................20 Estimation for a Gaged Station .........................................................................................................21 Estimation for an Ungaged Site Near a Gaged Station ................................................................22 Summary and Conclusions .........................................................................................................................23 References Cited..........................................................................................................................................23 Appendix. Development of Generalized-Skew Coefficient .............................................................115 iv Figures 1–2. Maps showing— 1. Study area and ecoregions in Georgia, South Carolina, North Carolina, and surrounding States ......................................................................................................3 2. Locations of rural stream-gaging stations in Georgia, South Carolina, North Carolina and surrounding States that were considered for use in the regional regression analysis, 2006 .......................................................................27 3. Graphs showing examples of distributions with zero skew, positive skew, and negative skew ........................................................................................................................8 4. Map showing hydrologic regions and locations of rural streamgaging stations in the study area used in the regional regression analysis .................................................29 5. Graph showing distribution of systematic peak-flow record lengths for rural streamgaging stations used in the regional regression analysis .............................11 6. Maps showing hydrologic regions in Georgia with locations of the rural streamgaging stations used in the regional regression analysis and counties in Georgia .............................................................................................................14 7–8. Graphs showing— 7. Distribution of percentages of the drainage area within a hydrologic region as opposed to drainage areas for the rural streamgaging stations used to develop the regional regression equations ...........17 8. Actual and predicted 1-percent chance exceedance flows for rural streamgaging stations with a portion of the drainage basin within hydrologic region 1 ............................................................................................................20 A1. Map showing locations of streamgaging stations used in the generalized-skew study ...........................................................................................................116 A2. Graphs showing relation of at-site skews to basin drainage area by U.S. Environmental Protection Agency ecoregion ..............................................................117 v Tables 1. Summary of rural streamgaging stations in and near Georgia, South Carolina, and North Carolina that were considered for use in the regional regression analysis, 2006 ................................................................................32 2. Basin characteristics considered for use in the regional regression analysis .................6 3. T-year recurrence intervals with corresponding annual exceedance probabilities and P-percent chance exceedances for flood-frequency flow estimates .........................7 4. Flood-frequency statistics for rural streamgaging stations in Georgia that were considered for use in the regression equations, 2006 ..........................................................56 5. Regional flood-frequency equations for rural ungaged streams with drainage basins that are within one hydrologic region .......................................................13 6. Explanatory variables that are used in the regional regression equations for rural streamgaging stations in Georgia ..................................................................................80 7. Average variance of prediction and standard error of prediction for the regional regression equations ..................................................................................................16 8. Values needed to determine prediction intervals for the regression equations .............18 9. Variance of prediction values for rural streamgaging stations in Georgia that were considered for use in the regression equations .........................................................90 A1. Number of streamgaging stations, by State, used in the study ........................................116 A2. Summary statistics for the Bayesian GLS models for generalized- skew coefficient ........................................................................................................................118