EDITOR'S NOTE: Whether manmade or evidence of weather's mercurial nature, unprecedented wet and dry spells are posing new challenges.
After suffering through drought in 2003 and 2004, Georgia's Douglas County experienced record flooding when hit with Tropical Storm Cindy and Hurricane Dennis the next year. Two fairly uneventful years passed. Then the pattern repeated: record-setting dry spells in 2007 and 2008 followed by flooding last September that killed seven people.
Climatologists expect the number of such dramatic swings to increase. In June 2009, the U.S. Global Change Research Program's Global Climate Change Impacts in the United States found that:Precipitation has increased 5% over the last 50 years.The amount of rain falling in the heaviest downpours has increased 20% over the last century.Many types of extreme weather events have become more frequent and intense during the past 50 years.
Unless they've experienced a flash flood firsthand, few people realize how powerful just a few inches of water are. Here, Douglasville-Douglas County Water and Sewer Authority engineer Johnny Barron shares what happened last fall as residents fled the deluge via the roads that criss-cross 200 square miles.
It was almost midnight on Sun., Sept. 20, 2009, and I was pacing the living room floor waiting for the power to come back on.
It had been raining all evening. Although I couldn't track rainfall on the Internet, I knew this was “the big one.” I decided to get dressed and go into the office where I work as an engineer for the Douglas County stormwater utility in Georgia.
My first sign of trouble was the flooding on Highway 5. Five cars were stalled in three feet of water and the fire department was setting up a barricade. I'm familiar with the computer models used to map the local floodplains, so I knew the highway would be underwater during a 100-year flood. But I'd always had a hard time imagining that such a small creek could ever get that high.
The National Weather Service later reported that a persistent low pressure system stalled over Mississippi, bringing moisture from the Gulf of Mexico across the Southeast. Local meteorologists spoke of a “training effect” in which multiple cells of precipitation followed one after another across certain areas. The result was record-breaking rainfall.
Our own network of rain gauges measured 24-hour rainfall amounts of 15 to 21 inches in the western half of the county, with the most intense portion of the storm dumping more than five inches in one hour. The rainfall far exceeded the 500-year rainfall amount of 8.2 inches of rain in a 24-hour period.
Rainfall events above the 500-year storm event are difficult to classify. The chance of exceeding 10 inches of rainfall in a 24-hour period in north Georgia is one in 10,000, according to the National Weather Service.
“If the 500-year storm was a cup of coffee, this one brewed a full pot,” says Brian McCallum, assistant director for the U.S. Geological Survey (USGS) Georgia Water Science Center.
The runoff from this event created a flash flood, the likes of which have never been seen in Douglas County. The USGS stream gauge on Dog River measured a peak flow of 59,900 cubic feet/second, dwarfing the estimated 500-year peak flow of 19,800 cubic feet/second. The flood level was 12 feet higher than the predicted 500-year flood elevation.
Fifty-three-year-old Wayne Hicks left his house just after 3 a.m. to help a friend whose house was threatened by rising waters. As Hicks drove his Dodge pickup truck along Banks Mill Road, the darkness and heavy rainfall obscured his view of the swollen Mobley Creek flowing over the road.
“I have a big truck. I didn't think there would be a problem,” he recalls. “The next thing I knew, the truck was moving sideways. I immediately thought to roll down my window so that I could get out.”
As he was clinging to a tree, Hicks tried to warn an oncoming motorist, but the roar of the water was too loud and he was concealed by the darkness. “There was nothing I could do to help,” he says. “All I could do was watch the Jeep float by.”
Frequent lightning illuminated his surroundings and helped Hicks plan his escape. With each flash, he identified the next branch to grab. He continued moving tree to tree until he was able to pull himself from the water.
He was fortunate: The driver of the Jeep that floated past was 44-year-old Debra Hooper. Her body was found a week later. She was one of seven people killed by the flooding.
The flash flood rose so quickly and was so widespread that there wasn't enough time for county officials to properly barricade flooding roads. Even where barricades were erected they weren't always effective. In some places, they were washed away as the water continued to rise. In others, motorists drove around them. Even if time and resources had allowed, many areas were inaccessible to county work crews because of the flooded roads.
Poor visibility compounded the problem. I saw it firsthand: The rainfall was so heavy that my headlights were inadequate. I could only see 10 to 20 yards ahead. At the edge of the flood, the water looked shallow. Because of my familiarity with the floodways, I knew where the rushing water was even if I couldn't see it. Sadly, the average driver was unaware.
That night dozens of cars washed off roads. County emergency services rescued 15 people. Others, like Wayne Hicks, rescued themselves. The death toll would undoubtedly have been higher if not for the efforts of emergency services and public works employees who worked all night.
By morning, almost 200 public roads were damaged, and more than 140 roads were closed. Emergency officials were urging people to stay home. Even so, traffic was a nightmare. The water and gas utilities were crippled as dozens of gas pipes and water mains washed out with the roads. Several utility poles fell over as the roads washed away, leaving thousands of people without power.
In all, the damage to infrastructure is expected to exceed $20 million in Douglas County alone. Private property damage was also severe. Dozens of dams washed out and many more were damaged. Many private driveways were rendered impassible. An estimated 300 families were displaced and more than 4,000 applied for federal assistance.
For all of the damage, though, nothing is more regrettable than the loss of seven lives on county roads.
— Barron (email@example.com) is an engineer for the Douglasville-Douglas County (Ga.) Water and Sewer Authority.