Prepared in cooperation with the Georgia Department of Transportation Preconstruction Division Office of Bridge Design Historic Flooding in Georgia, 2009 OOpen-File Report 2010–12300 U.S. Department of the Interior U.S. Geological Survey Cover. Powder Springs Creek at Powder Springs, Cobb County, Georgia, September 21, 2009. Photo by Brian E. McCallum, USGS. Historic Flooding in Georgia, 2009 By Anthony J. Gotvald Prepared in cooperation with the Georgia Department of Transportation Preconstruction Division Office of Bridge Design Open-File Report 2010–1230 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2010 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 this and other 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., 2010, Historic flooding in Georgia, 2009: U.S. Geological Survey Open-File Report 2010–1230, 19 p. iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Purpose and Scope .......................................................................................................................................3 Description of Storms Associated with the Floods ..................................................................................3 Description of the Floods ..............................................................................................................................5 Flood Stages and Streamflows ..........................................................................................................6 Annual Exceedance Probabilities ......................................................................................................7 Summary........................................................................................................................................................10 Acknowledgments .......................................................................................................................................10 References Cited..........................................................................................................................................10 Figures 1–3. Maps showing— 1. A total of 69 Georgia counties that were declared disaster areas because of flooding in 2009 .............................................................................2 2. Daily rainfall totals in Georgia during March 27–29 and April 1–3, 2009 .............3 3. Daily rainfall totals in Georgia from September 16 through 22, 2009 ....................4 4. Photograph showing U.S. Geological Survey hydrographers measuring the Sweetwater Creek flow over Interstate 20 near Atlanta, Georgia, September 2009 .......................................................................................5 5–6. Maps showing— 5. Location of the 238 U.S. Geological Survey streamgages in Georgia that record peak flow, 2009. .........................................................................................6 6. Annual exceedance probabilities of the 238 U.S. Geological Survey streamgages in Georgia that record peak flow, 2009 ..............................................8 Tables 1. T-year recurrence intervals with corresponding percent annual exceedance probabilities for flood-frequency flow estimates ..........................................1 2. Peak stages and flows at streamgages in Georgia with a peak flow that exceeded the 10–percent annual exceedance probability, 2009 ............................12 iv Conversion Factors and Datums Multiply By To obtain Length inch 2.54 centimeter (cm) inch 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area square mile (mi2) 259.0 hectare (ha) square mile (mi2) 2.590 square kilometer (km2) Volume cubic foot (ft3) 28.32 cubic decimeter (dm3) cubic foot (ft3) 0.02832 cubic meter (m3) Flow rate cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = ( °F – 32 ) / 1.8 Vertical coordinate information is referenced North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83). Elevation, as used in this report, refers to distance above the vertical datum. Historic Flooding in Georgia, 2009 By Anthony J. Gotvald Abstract In past flood reports, flood frequencies were expressed as recurrence intervals for selected flood quantiles, such as the “100-year flood.” The use of recurrence-interval terminology Heavy rains in southern Georgia during March 27– is now discouraged by the U.S. Geological Survey (USGS) April 3, 2009, and in northern Georgia during because it sometimes causes confusion for the general public. September 16–22, 2009, caused severe flooding and The term is sometimes interpreted to imply that there is a set widespread damages to residential, public, and commercial time interval between floods of a particular magnitude when, structures. Of the 159 counties in Georgia, 69 were declared in fact, floods are random processes that are best understood disaster areas because of flooding. The heavy rainfall by using probabilistic terms. While a flood identified by a in southern Georgia resulted in severe flooding in the specific recurrence interval (T-year) is statistically expected Satilla–St. Marys and upper Ochlockonee Basins and to occur, on average, once during the specified period, it may caused approximately $60 million in damages to the public actually occur multiple times during the period or not at all. infrastructure. The heavy rainfall in northern Georgia The terminology associated with flood-frequency resulted in severe flooding on many streams within the estimates is undergoing a shift away from the T-year recurrence upper Chattahoochee, Altamaha, and Coosa–Tallapoosa interval to the annual exceedance probability (AEP) as a Basins and caused 10 deaths, evacuation of thousands of percentage (P-percent). The use of AEP is now recommended residents, and approximately $500 million in damages. because it conveys the probability, or odds, of a flood of a The U.S. Geological Survey computed annual given magnitude being equaled or exceeded in any given year. exceedance probabilities of the peak flows in 2009 at For example, a 1-percent AEP flood (formerly known as the 238 streamgages throughout the State. Record peak flows “100-year flood”) corresponds to the flow magnitude that were recorded at 40 streamgages for the respective periods has a probability of 0.01 of being equaled or exceeded in any of record as a result of the heavy rainfall during the two given year. The percent, P, is computed as the inverse of the multiday events. The peak flows at 33 streamgages T-year recurrence interval multiplied by 100. T-year recurrence exceeded the 1-percent annual exceedance probability intervals with corresponding AEPs are shown in table 1. (100-year recurrence interval), and 19 of these exceeded the 0.2-percent annual exceedance probability (500-year recurrence interval). Table 1. T-year recurrence intervals with corresponding percent annual exceedance probabilities for flood-frequency flow estimates. T-year P-percent annual Introduction recurrence interval exceedance probability Flood data are needed by Federal, State, and local 2 50 agencies to make informed decisions in meeting mission 5 20 requirements related to flood-hazard mitigation, planning, 10 10 and response. For example, the Federal Emergency 25 4 Management Agency (FEMA) needs timely information 50 2 on the magnitude and probability of floods to help respond to flood damage, direct emergency response management, 100 1 protect infrastructure, provide recovery guidance, and plan 200 0.5 for future flood events. 500 0.2 2 Historic Flooding in Georgia, 2009 Rainfall in southern Georgia during March 27–April 3, infrastructure damage occurred to roads, culverts, bridges, and 2009, resulted in floods that exceeded the 1-percent AEP a water-treatment facility (U.S. Geological Survey, 2009). at six USGS streamgages, one of which exceeded the Prolonged rainfall in northern Georgia from September 0.2-percent AEP. Because of the magnitude of and damages 16 to September 22, 2009, resulted in floods that exceeded from this flood, a Presidential Disaster Declaration, the 0.2-percent AEP at 18 streamgages. More than 20 inches FEMA–1833–DR (Federal Emergency Management Agency, of rain fell in parts of northern Georgia during this period. A 2009a), was declared for 46 counties (fig. 1) on April 23, 2009. Presidential Disaster