Magnitude and Frequency of Floods in the Suwannee River Water Management District, Florida U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 9 6-4176 Prepared in cooperation with the Suwannee River Water Management District Magnitude and Frequency of Floods in the Suwannee River Water Management District, Florida By G.L. Giese and M.A. Franklin U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 96-4176 Prepared in cooperation with the SUWANNEE RIVER WATER MANAGEMENT DISTRICT Tallahassee, Florida 1996 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. For additional information write to: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Center 227 North Bronough Street, Suite 3015 Box 25286 Tallahassee, Florida 32301 Denver, CO 80225-0286 CONTENTS Abstract ............................................................................................................................................................................. 1 Introduction ....................................................................................................................................................................... 1 Purpose and Scope....................................................................................................................................................... 2 Previous Studies .......................................................................................................................................................... 3 Physical Setting ........................................................................................................................................................... 3 Station Analysis................................................................................................................................................................. 3 Regional Analysis ............................................................................................................................................................. 4 Improved Frequency Estimates By Use Of Weighting ..................................................................................................... 8 Gaged Sites.................................................................................................................................................................. 8 Ungaged Sites on Gaged Streams................................................................................................................................ 8 Summary ........................................................................................................................................................................... 9 References ......................................................................................................................................................................... 9 Appendix 1. Station, regional, and weighted T-year flood estimates for the Suwannee River Water Management District .................................................................................................................................................... 14 Figures 1. Map showing location of Suwannee River Water Management District and stream-gaging stations used for measuring peak discharges ................................................................................................................................ 2 2. Map showing physiographic regions within the Suwannee River Water Management District ............................ 4 3. Graphs showing elation of flood discharge to drainage area for selected frequencies on the Suwannee River....................................................................................................................................................... 5 4. Graphs showing relation of flood discharge to drainage area for selected frequencies on the Santa Fe River ......................................................................................................................................................... 5 Tables 1. Flood-frequency regression model for the Suwannee River Water Management District ..................................... 6 2. Probability that a flood of a given recurrence interval will be exceeded during indicated time period ................. 7 CONVERSION FACTORS AND ABBREVIATIONS Multiply By To obtain inch (in.) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) square mile (mi2) 2.590 square kilometer (km2) cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) Contents iii SYMBOLS Symbol Meaning Ag Drainage area for gaged site, in square miles Au Drainage area for ungaged site, in square miles B1T, B2T, B3T Partial regression coefficients CT Regression constant Cv Coefficient of variation DA Drainage area, in square miles EY Accuracy for a flood estimate, in equivalent years of record KT Pearson Type III deviate LE Channel length, in miles LK Total area of lakes and ponds, in percent of drainage area M Mean of logarithms of the annual peaks N Number of items in a data set n Time interval, in years P Exceedance probability PN Probability of at least one exceedance within the specified time interval Qg Estimate from log-Pearson Type III distributions of T-year flood, in cubic feet per second QR Estimate from regression equation of T-year flood for gaged site, in cubic feet per second Qru Regional estimate of T-year flood from regression equation for ungaged site, in cubic feet per second QT Estimate of the T-year flood from log-Pearson Type III distribution in cubic feet per second Qu Adjusted estimate of T-year flood for ungaged site, in cubic feet per second Qwt Weighted estimate of T-year flood at gaged site, in cubic feet per second R Multiple correlation coefficient S Standard deviation of the logarithms of annual peaks SEP Standard-error of prediction SL Channel slope, in feet per mile T Recurrence interval, in years X1, X2, X3... Independent variables in linear regression iv Symbols List Magnitude and Frequency of Floods in the Suwannee River Water Management District, Florida By G.L. Giese and M.A. Franklin ABSTRACT District) (fig.1) provide vital water-related func- tions. These functions include water- quality treat- Flood-frequency statistics for 2-, 5-, 10-, ment, water supply, flood water conveyance and 25-, 50-. 100-, 200-, and 500-year recurrence attenuation, fish and wildlife habitat, and recre- intervals, based on three methods of analysis, are ational and economic uses. These systems depend presented for 25 continuous-record and seven on the maintenance of the natural variability of the peak flow partial-record gaging stations in the hydrologic cycle as reflected by the magnitude, Suwannee River Water Management District. The duration, and timing of changing streamflow, rising first method, for gaged stations, utilizes station and falling water levels of lakes, rivers, and aqui- records; the second method, for ungaged sites, fers, and interaction of surface and ground waters. Alterations to the natural hydrologic regime by utilizes regional regression analysis; and the third human activities may have adverse effects on the method uses a weighted combination of the natural systems and their functions. However, many station and regional values. Because the weighted aspects of natural-system requirements are poorly values utilize two more or less independent known and must be better understood in order to estimates of the peak flow statistic, they are establish minimum flow and water-level require- considered more accurate than the station ments that will allow adequate water to balance estimates or the regression estimates alone. Also, present and future needs of the natural system with the use of another weighting scheme to improve those of the human population. Quantification of the estimates of flood frequency statistics at ungaged natural hydrologic regime that has shaped the cur- sites is demonstrated. rent natural system is basic to developing this The karstic nature of much of the understanding and achieving this balance. Accordingly, in 1994, the USGS (U.S. Geo- Suwannee River Water Management District logical Survey) and the SRWMD entered into a significantly attenuates flood peaks in some cooperative agreement wherein the USGS agreed to streams by providing substantial subsurface provide, in the course of a long-term program of storage when river stages are high. At such times, investigation, the hydrologic information or tools springs discharging into rivers may reverse flow needed for the SRWMD to establish minimum flow temporarily and become sinks. and water-level requirements for surface and ground waters of the SRWMD. This report, dealing only with flood magnitudes and frequency, is one of INTRODUCTION a planned series of studies intended to accomplish this goal. In recent years there has developed a better The primary motivation for this particular awareness that natural systems such as wetlands, report is to document the high-flow regime of the flood plains, native ecological communities, and natural hydrologic system of the SRWMD. This aquifer recharge areas within the 7,640-square-mile information is available to researchers studying min- SRWMD (Suwannee River Water Management imum