TECHNIQUES for ESTIMATING MAGNITUDE and FREQUENCY of FLOODS in RURAL BASINS of GEORGIA by Timothy C
Total Page:16
File Type:pdf, Size:1020Kb
TECHNIQUES FOR ESTIMATING MAGNITUDE AND FREQUENCY OF FLOODS IN RURAL BASINS OF GEORGIA By Timothy C. Stamey and Glen W. Hess U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 93-4016 Prepared in cooperation with GEORGIA DEPARTMENT OF TRANSPORTATION Atlanta, Georgia 1993 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director For additional information Copies of this report can be write to: purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Books and Open-File Reports Section 3039 Amwiler Road, Suite 130 Federal Center Peachtree Business Center Box 25425 Atlanta, GA 30360-2824 Denver, CO 80225 CONTENTS Abstract 1 Introduction 2 Purpose and scope 2 Previous studies 2 Climatological factors influencing floods 7 Method of study 7 Flood-frequency data analysis 8 Pearson Type III analysis 8 Coefficient of skewness 9 Regression model 9 Basin and climatic characteristics 10 Regional regression analysis 11 Accuracy of the regional relations 13 Limitations 15 Use of flood-frequency relations 16 Gaged sites 16 Ungaged sites 17 Sites draining more than one region 18 Example application 18 Flood frequency for large streams 20 Ocmulgee River 20 Oconee River 20 Altamaha River 23 Flint River 24 Summary 24 Selected references 26 Glossary 28 Appendix I - Alphabetical listing of streamflow sites in Georgia 30 Appendix II - Alphabetical listing of streamflow sites outside of Georgia 40 111 ILLUS RATIONS Figures 1.-4. Maps showing location of streamflow s ites used in flood regionalization studies in: 1. Northwest Georgia 3 2. Northeast Georgia 4 3. Southwest Georgia 5 4. Southeast Georgia 6 Figure 5. Map showing location of physiographi : provinces, regional flood-frequency boundaries, and selected reservoir gaging station* 12 6. Graph showing the relation of 100-year flood discharge to drainage area in flood-frequency regions in Georgia 14 Figures 7-10. Graphs showing relation of flood discharge to drainage area for selected recurrence-interval floods in region 3 for the: 7. Ocmulgee River 21 8. Oconee River 22 9. Altamaha River 23 10. Flint River 25 TABLES Table 1. Flood-frequency discharge data for ru ral streams in Georgia [in back of report] 43 2. Regional flood-frequency relations for rural streams in Georgia 11 3. Accuracy of regional flood-frequency relations 1'or rural streams in Georgia 15 IV CONVERSION FACTORS, ACRONYMS, AND VERTICAL DATUM Multivl\ To obtain Length inch (in.) 25.4 millimeter foot (ft) 0.3048 meter mile (mi) 1.609 kilometer Gradient foot per mile (ft/mi) 0.1894 meter per kilometer Area square mile (mi ) 2.59 square kilometer Volume acre-foot (acre-ft) 1.233 x 103 cubic meters Flow cubic foot per second (ft3/s) 0.02832 cubic meter per second Velocity foot per second (ft/s) 0.3048 meter per second ACRONYMS GDOT Georgia Department of Transportation IACWD Interagency Advisory Committee on Water Data USGS U.S. Geological Survey VERTICAL DATUM Sea Level:--In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929-a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. V TECHNIQUES FOR ESTIMATING MAGNITUDE AND FREQUENCY OF FLOODS IN RURAL BASINS OF GEORGIA By Timothy C. Stamey and Glen W. Hess ABSTRACT Methods of estimating the magnitude of floods having recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-years are described for rural streams in Georgia that are not affected by regulation, tidal fluctuations, or urban development. Flood-frequency discharge data also are presented for the 2- to 500-year recurrence intervals for 426 streamflow sites used in the regional analysis. Annual peak-discharge data through September 30,1990 were analyzed for 426 streamflow sites on rural streams in Georgia, and adjacent parts of Alabama, Florida, North Carolina, South Carolina, and Tennessee, having 10 or more years of record. Flood frequencies were computed by fitting the logarithms of the annual peak-discharge data to a Pearson Type III distribution curve. Peak-discharge data were used in multiple least- squares linear-regression analyses with basin and climatic variables to develop regional flood-frequency relations. Analyses indicate that the drainage areas of the basins are the most significant variables; therefore, drainage area is the only basin characteristics included in the regional regression relations. Four regions were delineated that have distinct flood-frequency-discharge characteristics for streams draining from 0.1 to 3,000 square miles, and flood-frequency relations were developed for each of these regions. The average standard error of prediction of the regional flood-frequency relations ranged from 26 to 38 percent for the 2- to 500-year recurrence-interval floods. Individual relations of flood magnitude and frequency to drainage area are described for sites along the mainstems of the Ocmulgee, Oconee, Altamaha, and Flint Rivers. Flood-frequency relations are based on the log-Pearson Type III analysis of long-term peak-discharge records at gaged sites on the four rivers. INTRODUCTION Flood-magnitude and frequency data are needed for development along and near streams and rivers. The data are important in the design of bridges, culvert embankments, darns, and levees, and serve as a basis for flood-plain regulation and establishment of flood-insurance rates. The U.S. Geological Survey (USGS), in cooper; ition with the Georgia Department of Transportation (GDOT) and other State and local agencies, has conduc ed studied to develop and improve the estimates of the magnitude and frequency of floods on rural Georgia stre ams The principal cooperating agencies that help fund the collection of annual peak discharge and stage data, and other streamflow data in Georgia are Georgia Department of Transportation Highway Division Georgia Department f Natural Resources Environmental Protection Division Georgia Geolog ic Survey Tennessee Valley Aut lority U.S. Army Corps of Engineers Mobile District Savannah District U.S. Department of Commerce National Oceanic and Atmospheric Administration National Weather Service The authors also acknowledge the assistance in the data-jcollection program by the Georgia Power Company. This report is based on peak-discharge data th rough September 1990 collected by the USGS at 426 streamflow sites. The streamflow sites include 357 si es in Georgia (figs. 1-4), with the remaining 69 sites located near the Georgia border in adjacent parts of Alabama, Florida, North Carolina, South Carolina, and Tennessee (Appendix I and II). Purpose nd Scope This report presents and illustrates methods of e jtimating the magnitude and frequency of floods having recurrence intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years on rural Georgia streams. This report is based on flood-frequency analyses of annual peak-discharge data at streamflow monitoring sites through September 1990. This report includes (1) regional relations for estimating the magnitude and frequency of flood discharges on rural, ungaged, non-regulated streams; (2) method for e timating flood-frequency data at or near gaged sites on rural streams; and (3) flood-frequency data on mainstem streams with drainage basins located in more than one region. Previous Studies There are six previously published flood-frequen y reports for Georgia streams. Bunch and Price (1962) used the index flood method that was described by Ca rter (1951) and Dalrymple (1960). Spear and Gamble (1964a, 1964b) and Barnes and Golden (1966) developed flood-frequency regression methods for several states, and used data abstracted from Bunch and Price (1962) or the Georgia portion of the their reports. Golden and Price (1976) described flood-frequency methods for strc ams having drainage areas of less than 20 mi2, and used multiple regression methods to relate peak discharges for flood^ of selected recurrence intervals to drainage areas. Subsequently, Price (1978) prepared a flood-free uency report based on peak-discharge data for 262 sites in Georgia and 46 sites in adjacent states, and develo ed flood-frequency relations using multiple regression methods for streams having drainage from 0.1 to 1,( 000 mi2. Inman (1983 and 1988) described methods for determining flood-frequency relations for urban stream: in Metropolitan Atlanta and other Georgia cities. 85° 34° 33° Base from U.S. Geological Survey State base map, 1970 0 10 20 30 40 50 MILES J _________________I 0 10 20 30 40 50 KILOMETERS EXPLANATION REGIONAL FLOOD-FREQUENCY BOUNDARY PREAMFLOW SITE AND AS LISTED IN TABLE 1 Figure l.~Location of streamflow sites used in flood regionalization studies in northwestern Georgia. 35' NORTH CAROLINA EXPLANATION REGIONAi FLOOD-FREQUENCY BOUNDARY STREAMF^OW SITE AND NUMBER AS LISTED IN TABLE 1 LOCATION OF MAP AREA Base from U.S. Geological Survey State base map, 1970 Figure 2.~Location of streamflow sites used in: ood regionalization studies in northeastern Georgia. 85° 83° 33° 32° - 0 10 20 30 40 50 MILES i'i ri | U i-________I__________ r r 0 10 20 30 40 50 KILOMETERS EXPLANATION REGIONAL FLOOD-FREQUENCY BOUNDARY STREAMFLOW SITE AND NUMBER AS LISTED IN TABLE 1 Base from U.S. Geological Survey State base map, 1970 Figure 3.--Location of streamflow sites used in flood regionalization studies in southwestern Georgia. 83° LOCATION OF MAP AREA EXPLANATION REGIONAL FLOOD-FREQUENCY BOUNDARY STREAMFLOW SITE AND NUMBER AS LISTED IN TABLE 1 0 10 20 30 40 50 MILES I I 0 10 20 30 40 Base from U.S. Geological Survey State base map, 1970 Figure 4.-Location of streamflow sites used in f ood regie nalization studies in southeastern Georgia CLIMATOLOGICAL FACTORS INFLUENCING FLOODS The climatological factors that influence flooding in Georgia vary areally, seasonally, and annually. In winter, continental high-pressure systems periodically move frontal systems through the State, producing rain over large areas.