Atlanta Floods Extremely Rare

"Given the relatively short length of stream gauging records (less than 100 years), the US Geological Survey cannot accurately characterise the probability due to its extreme rarity," said Robert Holmes, USGS National Flood Program Coordinator.

"Nationwide, given that our oldest stream gauging records span about 100 years, the USGS does not cite probabilities for floods that are beyond a 0.2 percent (500 year) flood."

"If a 0.2 percent (500-year) flood was a cup of coffee, this one brewed a full pot," said Brian McCallum, Assistant Director for the USGS Georgia Water Science Center in Atlanta. "This flood overtopped 20 USGS stream gauges - one by 12 feet. The closest numbers we have seen like these in Georgia were from Tropical Storm Alberto in 1994. This flood was off the charts."

The rains returned water levels in the region's two largest reservoirs, Lake Lanier and Allatoona Lake, to pre-drought levels. Lake Lanier rose by more than 3 feet to 1068 feet by September 25 and returned to full pool in October. Allatoona Lake rose to 853.25 feet on September 23, more than 13 feet over full pool of 840 feet.

"The flooding in Atlanta is certainly near the top of the list of the worst floods in the United States during the past 100 years," said Holmes. "For comparable drainage areas, the magnitude of this flood was worse than the 1977 Kansas City flood, which caused tremendous destruction and loss of life. It is a testament to the diligence of county officials and emergency management teams that more lives were not lost in Georgia."

Significant property losses, however, were a near certainty from this event. According to the National Weather Service, some locations recorded up to 20 inches of rain within 24 hours. Culverts and sewers are not usually designed for events of this magnitude because they are so rare and it is cost prohibitive.

"Applying rainfall frequency calculations, we have determined that the chance of 10 inches or more occurring at any given point are less than one hundredth of one percent", said Kent Frantz, Senior Service Hydrologist for the National Weather Service at Peachtree City. "This means that the chance of an event like this occurring is 1 in 10,000."

For this analysis, USGS reviewed high-water-mark surveys and indirect peak discharge computations throughout the flood-affected region. Scientists gather these data from the field during floods and in their immediate aftermath to supplement or, in this case, to provide data after a gage is destroyed.

The USGS maintains a network of nearly 300 stream gauges in Georgia, which provide data in real time. Data from these stream gauges are used by local, state and federal officials for numerous purposes, including public safety and flood forecasting by the National Weather Service. A map of these gages and graphs of discharge for the last seven days is available online. Users can access current flood and high flow conditions across the country at the USGS WaterWatch Web site. More information on USGS flood-related activities is available at the USGS Surface Water Information Web site.