Finding The Mother Of All Floods

Wednesday, July 31, 2019 - by Scott Fiedler

Massive floods, far larger than February’s record-breaking rainfall, have occurred in the Tennessee Valley. The big questions are: when did they happen, will such large floods occur again and if they do, how often?

A team of researchers from the University of Alabama and Tennessee Valley Authority are on a mission to find evidence of the worst floods in the region’s history.

“We call them paleofloods—floods in the past that now can only be discerned from the sediment deposits they left behind. We’re interested in floods that happened as far back as 10,000 years ago, particularly the largest ones,” explained principal investigator Dr. Lisa Davis, Associate Professor, Department of Geography at the University of Alabama.   

Scientists have kept weather and streamflow data for about 150 years, but extreme flood events can go hundreds, if not thousands, of years between occurrences, according to Davis.

The goal of Davis’ project is to work with TVA engineers to integrate paleoflood data into their flood frequency and dam safety assessments to help these assessments better account for the most extreme floods. She and Ph.D. student Rachel Lombardi are building on thousands of years of paleoflood data available from an earlier collaborative research by collecting paleoflood data from sheltered caves located in bluffs high above the Tennessee River.

Digging for Clues

Out in the field, Lombardi carefully cleans the side of a trench dug on a hill about a mile from the river in Marshall County, Alabama. She’s looking for clues in the sediment to tell her whether floodwaters once covered this area.

“We’re looking for shiny white sparkles of mica,” Lombardi explains. “Mica is a mineral that is washed down from the Great Smoky Mountains. If it’s found here, that would tell us there was possible paleoflood activity.”

Lombardi did not find mica at this test site, so the data indicates no floodwaters reached here. But further analysis back at the lab will help confirm that.

Why are these tiny sparkles so important? “We have to know more about the worst floods to be able to accurately assess the severity of a recent one. How can we say it’s the worst flood in 500 years if we don’t have flood information going back 500 years?” Lombardi says. “For this reason, paleoflood records, combined with instrument and historical observations, provide the most robust context for assessing floods.”

The historic data Davis and Lombardi collect in floodplains and caves directly helps TVA make risk decisions to keep people and property safe.

Research in Wheeler reservoir shows evidence of at least six major paleofloods that occurred between Huntsville and Guntersville during the past 2,200 years. “If one of these paleofloods hit today, and we didn’t have TVA river controls, areas south of Huntsville could be under water,” said Davis.

Paleoflood data has been collected for years in the dry, arid western U.S., but here in the Southeast, the humid climate and abundant rainfall can wash away sediment lines and other indicators. That’s why the team is, literally, digging for clues in many different locations in the Valley.

Economic Benefits of Flood Reduction

Curt Jawdy, TVA lead hydrologist, believes that when Mother Nature is involved, ignoring her is never smart. “We’re learning more every day, but it’s important to continue this research. You can’t underestimate Mother Nature because too much is at stake, so we need to keep looking.”

Over the past 86 years, TVA dams brought prosperity to the region through flood control and river navigation. Since TVA was formed in 1933, its dams have averted over $8.6 billion in flood damages.

Flooding is particularly relevant now, with a flooding event not far back in the rearview mirror. In February, 2019, when the Tennessee River swelled from the wettest month in recent history, TVA’s 49 dams stored at least 3.5 trillion gallons of water, reducing the downstream impacts to the hardest-hit areas in west Tennessee and parts of northern Alabama—areas that saw as much as 13.5 inches of rainfall over a two-week period.

Reducing flood risk is an economic benefit Jawdy believes in. “We have some of the most capable flood routing systems in the country,” he said. “We know that we are vastly reducing flood risk for the Valley—but we need to know more about the truly rare storms. If we can, it will help us to make good decisions if such a storm occurs in the future.  Understanding these floods also help us to make the best investments possible in maintaining our dams.”

In other words, the Tennessee Valley’s flood resilience could turn out to be yet another strong economic development tool. “With this study, we may learn that we need to update our flood plans,” said Jawdy. “And we may be able to go out and say to industries that the Tennessee Valley is one of the safest places to locate your business well into the future.”

Until then, Davis will continue the search for an epic flood.

“We’re proud to be part of the Valley’s future by looking at the past,” said Davis. “By expanding the data set to thousands of years, we can give TVA and communities the confidence that they can keep people and businesses safe.”


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