First published study of 2009 Atlanta floods may hold clues to recent urban flooding in other U.S. cities-warnings, too

September is normally a hot, dry month around Atlanta, Ga. 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 the University of Georgia, has shown that a convergence of record-setting events, perhaps unprecedented in the area’s history, 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 just 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 UGA 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, Ga.

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 (June) and Nashville (May) 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 and commentators 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, of course, was the massive and 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 called “ArkLaTex,” pulling moisture from the Gulf of Mexico. Many days of rainfall nearly saturated the soil and filled many 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 over the Southeast. Inherent instability in the atmosphere and the mountains northwest and north of Atlanta produced “training” or repeated heavy rainfall.

The spark that set the floods metaphorically afire, though, may have been all the concrete and paving of the metro area itself. 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 relatively new community volunteer network, defined the multifaceted 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 so-called 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 authors say in their study—bad news for areas such as Atlanta, Nashville and Oklahoma City, which have had enough already.

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Soggy September floods north Georgia

Heavy rains caused record flooding in north Georgia in mid-September, while other parts of the state experienced and normal to below-normal rainfall.

A large, slow-moving low-pressure system brought extremely humid air into Georgia mid-month. It triggered days of intense rainfall, producing what was estimated to be a 500-year flood around Atlanta, according to the U.S. Geological Survey.

The highest rain totals estimated by National Weather Service radars were 15 inches to 20 inches in Douglas County and other areas east and west of downtown Atlanta.

Twenty-one counties were declared eligible for federal disaster funds. Preliminary damage estimates were $500 million to $1 billion. Thousands of homes were affected, and 500 were destroyed or significantly damaged.

The flood is considered the worst since 1919, when flood waters almost destroyed West Point in western Georgia.

Parts of four interstates were closed. I-20 remained closed for more than 24 hours as flood waters rose 3 feet over the pavement and inundated Six Flags Over Georgia.

Following the rain, Lake Lanier rose 1.5 feet in 24 hours. Lake Allatoona rose almost 9 feet during the week after. More than 130 dams and many bridges now require stability inspections.

Four water-treatment plants in Atlanta were damaged and dumped raw sewage into the Chattahoochee River. A pipe in a levee near Macon broke and discharged millions of gallons of raw sewage into the Ocmulgee River at the end of the month. Citizens were urged to stay out of the flood water. Some communities issued advisories to boil water.

Ten people were killed, most while driving onto roads covered by moving water. Seven deaths were in Douglas County, where the heaviest rainfall occurred. Additional deaths occurred in Carroll, Chattooga and Gwinnett counties.

The highest monthly total from National Weather Service reporting stations was 10.68 inches in Macon (7.42 inches above normal). The lowest was in Alma at 1.59 inches (1.75 below normal). Atlanta received 8.94 inches (4.85 inches above normal), Athens 9.86 inches (6.33 inches above normal), Columbus 5.30 inches (2.23 inches above normal), Augusta 3.63 inches (.15 inch above normal), Savannah 2.43 inches (2.65 inches below normal) and Brunswick 4.57 inches (1.67 inches below normal).

Forty-eight Community Collaborative Rain, Hail and Snow Network observers reported 15 inches or more total rainfall for the month. The highest single monthly total was 22.86 inches near Kennesaw in Cobb County. However, two observers in Douglas County reported overflowing rain gauges. An observer in Lilburn in Gwinnett County reported 21.79 inches.

The highest daily rainfall reported by a CoCoRaHS observer exceeded 11 inches in a few hours near Douglasville on Sept. 21, when the rain gauge overflowed. The Carrollton observer nearest the area of maximum rainfall reported 10.64 inches in 24 hours ending that morning.

The Georgia Automated Environmental Monitoring site at Dallas in Paulding County reported 16.15 inches for the month, including 5.61 inches on the Sept. 20 and 4.54 inches on Sept. 21.

Numerous daily and monthly rainfall records occurred at National Weather Service cooperative observing stations around the state. Dallas and Carrollton broke 30-year records of heaviest daily rainfall. Atlanta broke a daily maximum rainfall of 3.52 inches on Sept. 19, and Macon broke daily records with 2.32 inches on Sept. 17.

No tornadoes were reported. Scattered reports of strong winds or small hail where reported four days resulting in toppled trees or scattered power outages.

The heavy rains in north Georgia caused extensive damage to nurseries, vineyards and hay fields. Many counties reported rot in cotton and insect infestations. Fieldwork came to a stop in many areas. In other areas, the rain was beneficial to crops and harvesting proceeded at a good pace.

Temperatures across the state were near normal in September except in Augusta where the average temperature was 75.6 degrees (1.6 degrees above normal).

By Pam Knox
University of Georgia

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