September's flood on Sweetwater Creek was significantly larger than a 500-year flood event — a flood so big that it's given only a 0.2 percent chance of happening in any year. A 100-year flood has a 1 percent chance of happening in any year.

The National Weather Service defines North Georgia rainfall severity as follows for a 24-hour period:

100-year flood: (1 percent chance) 7.2 inches

500-year: (0.2 percent chance) 8.2 inches

1,000-year: (0.01 percent chance) 8.7 inches

Austell's soggy history

The Cobb County city has experienced at least a 100-year flood several times since 1990.

1964: First U.S. Army Corps of Engineers study of Austell flood problems

1978: Second corps study of Austell floods

February 1990: Austell floods to just below the 100-year mark

March 1990: Austell floods again, reaching the 100-year flood mark

May 1990: Austell Mayor Joe Jerkins asks the corps to come back

1991: Third corps study of Austell floods

1992: Federal Emergency Management Agency develops flood map used until 2008

1995: Fourth corps study of Austell floods

July 2005: Austell floods to just below the 500-year flood mark

December 2008: FEMA maps partially updated from 1992

September 2009: Austell floods; water is so high the U.S. Geological Survey says it was worse than a 500-year flood

Source: National Weather Service