Historic flooding in northern Georgia, September 16-22, 2009

A primary mission of the U.S. Geological Survey (USGS) is the measurement and documentation of the magnitude and extent of hydrologic hazards, such as floods, droughts, and hurricane storm surge. USGS personnel were deployed to document historic, widespread flooding that occurred throughout the Atlanta metropolitan area and northwestern Georgia in the early fall of 2009. The floods were created by prolonged rainfall that occurred during September 16–22, 2009, with an especially intense period of rainfall during the late evening of September 20. The National Weather Service (NWS) reported that the southeastern United States had above-normal precipitation from August into early September, resulting in saturated soil conditions making the region extremely flood prone. Precipitation totals were the sixth highest on record for the month of September for the region (National Weather Service, 2010).

Monetary losses reported from this flood were nearly $220 million in flood-damage claims, with 17 counties declared Federal disaster areas. Eleven fatalities were attributed to this flood, with many of them occurring during early morning hours when drivers attempted to cross flooded roads. Significant damage to hundreds of roads and bridges across the region also severely impacted traffic conditions for months after the event (National Weather Service, 2010).

The September 2009 flooding brought the highest levels ever recorded at many USGS streamgages across the region. Particularly hard hit was the Chattahoochee River Basin, west of Metropolitan Atlanta. At USGS gaging station 02336000 on the Chattahoochee River at Atlanta, Georgia, a peak flow of 40,900 cubic feet per second (ft/s) and a peak stage of 28.12 feet (ft) were recorded on September 21. This was the highest stage in 81 years of continuous record at the gage. The September 22 peak flow represented between a 1- and 0.5-percent annual exceedance probability (100- to 200-year) flood. Station 02337000 on Sweetwater Creek near Austell, Georgia, recorded a stage of 30.82 ft with a peak flow of 31,500 ft/s, which resulted in an exceedance probability much greater than the 0.2-percent (500-year) flood level and caused the closure of Interstate 20 for more than a day. Hardest hit was the Dog River Basin in Douglas County, where floodwaters rose 12 ft above USGS streamgage 02337410. The Dog River crested at a stage of 33.83 ft with a streamflow estimate of 59,900 ft/s, which was almost six times the 100-year flood. Upstream from Atlanta, Lake Sidney Lanier rose almost 4 ft in a 48-hour period following the storm and is credited for preventing record flood levels in Atlanta (James Hathorn, U.S. Army Corps of Engineers, oral commun., November 6, 2009).

Significant flooding occurred in the headwaters of the Altamaha River Basin to the east of Atlanta. At USGS station 02207120 on the Yellow River at GA 124, near Lithonia, Georgia, the highest ever peak stage determined from surveys was 27.47 ft on September 22, with a peak flow of 16,500 ft/s.

The Coosa and Tennessee River Basins also were flooded in many locations. The USGS streamgages on Allatoona, Noonday, and Butler Creeks—all tributaries to Allatoona Lake— recorded peak flows with magnitudes far greater than 0.2-percent (500-year) floods. Although these flows caused Allatoona Lake to rise to more than 13 ft above the full-pool level, the dam still saved the downstream community of Rome, Georgia (James Hathorn, U.S. Army Corps of Engineers, oral commun., November 6, 2009).

During this historic flood, more than 50 USGS streamgages throughout north Georgia recorded flows above the highest ever measured, and 20 USGS streamgages were knocked out of operation by floodwaters. USGS field personnel performed flood measurements to extend streamflow ratings that allowed for more accurate NWS forecasts. All 20 streamgages were restored to operational status within 5 days, however.

By Brian E. McCallum and Anthony J. Gotvald. For more information, contact the Georgia Water Science Center at 770-903-9100 or http://ga.water.usgs.gov.

Lessons learned from this flood include the need for more effective communication of the latest river information by Federal agencies with floodthreatened communities. Communicating the flood threat in an easy, accessible manner would have helped emergency managers and the public greatly during this flood. In response, the USGS developed WaterAlert (http://water.usgs.gov/wateralert) to send notifications of flood events by way of text and e-mail. Also in development are real-time flood-inundation maps to give the hydrograph spatial context by way of a map-based product.

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