Federal officials: September's flood 'off the charts'

By Mary Lou Pickel

The Atlanta Journal-Constitution

If a 500-year flood is a cup of coffee, what Georgia got in September was the whole pot, the U.S. Geological Survey said Wednesday.

The Sept. 21 flood in Georgia was worse than what's statistically projected to happen once every 100 years -- even worse than every 500 years. It was "extremely rare", "epic" and so "stunning", the U.S. Geological Survey says the flood has defied its attempts to define it. 

"This flood was off the charts," said Brian McCallum, assistant director for the U.S. Geological Survey's Georgia Water Science Center in Atlanta.

The Geological Survey does not quantify floods greater than a 500-year flood because the numbers begin to be too uncertain after that point, McCallum said.

According to the National Weather Service, some locations recorded up to 20 inches of rain from 8 p.m. Sept. 20 to 8 p.m. the following day.

"Normally we get just over 50 inches of rain in one year," McCallum said of metro Atlanta. "We got almost half our annual rainfall in one day in some places."

Scientists called the flooding in Atlanta one of the worst floods in the country in the last 100 years.

Flood waters washed away streamgages used to measure the water flow. In Douglas County, the Dog River flowed over the  streamgage by 12 feet, roaring by at more than 448,000 gallons per second -- well beyond a 500-year flood, McCallum said.

All of that rain returned water levels in Lake Lanier and Allatoona Lake, to pre-drought levels. Lake Lanier rose by more than three feet after the flood and returned to full pool in October. Allatoona Lake rose to 13 feet over full pool.

"It takes a very large flood to end a very historic drought, and we certainly got that in September," McCallum said.