Atlanta Floods Extremely Rare

I am very grateful that my home was not in an area that flooded. Even more so because I both in pain from a pulled muscle, and sick, and in bed.

http://www.sciencedaily.com/releases/2009/11/091106121918.htm

ScienceDaily (Nov. 9, 2009) — The epic flooding that hit the Atlanta area in September was so extremely rare that, six weeks later this event has defied attempts to describe it. Scientists have reviewed the numbers and they are stunning.

"At some sites, the annual chance of a flood of this magnitude was so significantly less than 1 in 500 that, given the relatively short length of streamgauging records (well less than 100 years), the U.S. Geological Survey cannot accurately characterize the probability due to its extreme rarity," said Robert Holmes, USGS National Flood Program Coordinator. "Nationwide, given that our oldest streamgauging 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 streamgauges -- 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."

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"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 from 8:00 pm on Sept. 20 to 8:00 pm the following day. 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."

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