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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More Variable and Uncertain Water Supply: Global Warming’s Wake-Up Call for the Southeastern U.S.

The second major drought of the last decade is a wake-up call for the Southeast United States, showing the region’s vulnerability due to its reliance on scarce supplies of fresh water.

The region has been operating under the best-case water availability for the last 50 years, during which drought conditions were relatively rare. But, the region has historically experienced regular droughts. Global warming is the future wildcard, potentially causing both more extremely dry periods and more heavy rainfall events. At the same time, warming-induced sea-level rise will increase the risk of saltwater intrusion into important groundwater aquifers.

A new report from National Wildlife Federation offers the latest scientific research on global warming and water supplies, competition for resources, demographic factors, and how to better prepare for managing the region’s water availability challenges.

“Since 1960, the region’s population doubled and water use for municipalities, irrigation, and thermoelectric power more than tripled. The Southeast is one of the fastest growing parts of the country,” said Amanda Staudt, Climate Scientist for National Wildlife Federation.

In fact, 58 of the 100 fastest growing counties in the nation are in the nine states of the Southeast. The report includes information about Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, Tennessee, and Virginia.

More Variable and Uncertain Water Supply: Global Warming’s Wake-Up Call for the Southeastern U.S. details how:

Water supplies in the Southeastern United States will be more variable and uncertain in the coming decades;
Rapidly expanding population, irrigation, and thermoelectric power use has increased water demand;
Recent droughts underscore the Southeast’s vulnerability;
The astonishing biodiversity of the Southeast is at risk; and
The Southeast should plan for increasing variability in water supplies.
Strategies for meeting the increasing demand for water in the Southeast have not typically accounted for the regular occurrence of drought, as illustrated by recent droughts. During 2007 alone, crop losses are estimated at more than $1.3 billion and wildfires ravaged 600,000 acres in Georgia and Florida.

Climate changes will affect water supplies to communities and put the amazing biodiversity of the Southeast at risk. The river basins of the Southeast are globally renowned for fish, mussels, salamanders and other freshwater species, many of which are already imperiled. Climate change—and the increasingly extreme weather patterns it brings—now poses new threats to these species.

“Global warming presents new challenges for managing America’s water resources, especially in our southeastern states,” added Dr. Staudt. “To prevent the worst impacts of climate change and limit the impacts on communities and wildlife, we must reduce global warming pollution.”

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Georgia State to Host Regional Geomorphology, Hydrology Research Conference October 25

Georgia State University’s Department of Geosciences will present a one-day research conference on Saturday, Oct. 25 which will bring together advanced undergraduate and graduate students, faculty and professionals.

The first-annual Greater Atlanta Geomorphology and Hydrology Research Conference will be held from 8 a.m. to 4:30 p.m. in Room 718 in the General Classroom Building at Decatur Street and Peachtree Center Avenue.

The forum is designed for those involved in the study of geomorphology (the study of landforms and the processes that shape them), hydrology (the study of water resources and water movement throughout the Earth) and related fields to present research in a friendly setting, and to enhance regional collaboration in these fields, as described by Jordan Clayton, a Georgia State assistant professor in Geosciences.

“This is a great way for participants to meet others working in these research areas, and to get valuable feedback on ongoing projects,” said Clayton, whose research interests include the impact of land-use changes on streams and other hydrologic systems.

Participants include Georgia State students and faculty, as well as researchers from the U.S. Geological Survey, Georgia Tech, Kennesaw State University, the University of Alabama, the University of Georgia, and Golder Associates Inc.

For more information about the conference, contact Clayton at 404-413-5791 or jclayton@gsu.edu. Further information, including research abstracts, is available online at http://monarch.gsu.edu/jclayton/gaghrc/index.htm.

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