Atlanta floods extremely rare

US Geological Survey: 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 streamgaging 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 streamgaging 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 streamgages – 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 three feet to 1068 feet by Sept. 25 and returned to full pool in October. Allatoona Lake rose to 853.25 feet on Sept 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….

This is a satelite image showing wide spread flooding in Lithia Springs, Georgia just west of downtown Altanta. NASA