Campus News

Perfect storm

First published study of Atlanta floods in 2009 may hold clues about recent flooding in other U.S. cities

September is normally a hot, dry month around Atlanta. The first hints of autumn usually don’t arrive until the end of the month, if then. So it was a surprise last fall when record rainfall turned much of the metro area and north Georgia into a lake, plunging such attractions as Six Flags Over Georgia underwater and in places exceeding flood levels expected only once every 500 years.

Now, in what is likely the first scholarly published study of the floods, a team of climatologists, meteorologists, geologists and hydrologists led by UGA has shown that a convergence of record-setting events combined to cause tens of millions of dollars in damages and at least 10 deaths.

And while the future of such floods is unclear, as are possible ties to global warming, conditions may be ripe for a reoccurrence, said Marshall Shepherd, lead author of the study published in the online edition of the Bulletin of the American Meteorological Society.

“More frequent or intense flooding events, coupled with expanding impervious surfaces like roads and parking lots, will affect the ecosystem and the very fabric of societal infrastructure,” said Shepherd. “We are thus going to need revolutionary designs, management and policies if we are going to mitigate the impact of future events.”

Other authors of the paper are Thomas Mote, John Dowd and Mike Roden, who with Shepherd are in the Franklin College of Arts and Sciences; Pamela Knox of the Office of the State Climatologist; Steven McCutcheon of the U.S. Environmental Protection Agency; and Steven Nelson of the National Weather Service in Peachtree City.

Recent studies have noted that regions of the southeastern U.S. face an increasing vulnerability to climatic extremes because of population growth and the addition of impervious surfaces. The floods this year in Oklahoma City and Nashville point out how serious the issue has become, and even in a sparsely populated area of Arkansas, authorities reported at least 19 dead in a flash flood in June.

But even the knowledge of an increased threat couldn’t have prepared metro Atlanta leaders for what befell them last September.

“Just as an example, the U.S. Geological Survey measured the largest flow ever recorded on Sweetwater Creek near Austell, which has a streamflow record dating back to August 1904,” said Shepherd. “And a
climatological assessment by the National Weather Service showed that September 2009 was fifth wettest in Atlanta’s history.”

While newscasters have turned the phrase “perfect storm” into one of the most-used clichés of the age, no two words describe better what led to the Atlanta flooding, the new research shows.

First was the unrelenting rainfall, caused by an odd combination of events. Prior to the record rainfall in north Georgia, a low-pressure system stalled over parts of a three-state area, pulling moisture from the Gulf of Mexico. Many days of rainfall nearly saturated the soil and filled streams, rivers and reservoirs over the Southeast. Remnants of two tropical storms in the Pacific and Atlantic (and evaporation from the Gulf of Mexico) generated some of the wettest air observed in the Southeast. Instability in the atmosphere and the mountains north of Atlanta produced “training” or repeated heavy rainfall.

The spark that set the floods afire may have been the concrete and paving of the metro area. The water, unable to soak into the ground, overfilled sewers and drainage and flooded roads, schools, neighborhoods, Interstate-20 and Six Flags Over Georgia, all designed to rarely be submerged.

Satellite data, multi-sensor Doppler radar estimates, traditional National Weather Service gauges and a new community volunteer network defined the causes of flooding during the event.

“Though the meteorological set-up was unique, the role of Atlanta’s impervious surfaces and rapid continued growth should not be missed,” said Shepherd. “This increase in roofs, roads and parking is known to alter runoff intensity and, in the future, make flooding potentially just as bad and in many cases worse.”

Another possibility is that large cities may initiate or alter storms through heat-island, convergence and pollution effects. How (or if) this played a part in the Atlanta floods will be the subject of future research by Shepherd, who plans to recreate the event using computer simulations, though without Atlanta included.

“The September 2009 flooding that definitively ended the drought in Georgia is consistent with the Intergovernmental Panel on Climate Change projections that the frequency and severity of extreme hydroclimate events such as droughts and floods will likely increase,” the study stated-bad news for areas like Atlanta that have had enough already.