The Great Flood of '93 could happen again, but scientists don’t agree on the odds | St. Louis Public Radio

The Great Flood of '93 could happen again, but scientists don’t agree on the odds

Jan 2, 2019

Scientists who have studied the historic 1993 flood agree that a similar event could strike the St. Louis region again. But they disagree on how likely it could occur.

No other flood has reached the 49.6-foot crest that the Mississippi River rose to in St. Louis in the summer of 1993. But scientists have observed that heavy rains and high flood levels are becoming more frequent, potentially due to climate change. In the last five years, floods in 2013, 2016 and 2017 have claimed spots on the National Weather Service’s top 10 historic crests in St. Louis.

The U.S. Army Corps of Engineers estimates that the '93 flood was a 350-year flood, a flood that has nearly a 0.3 percent chance of happening in any given year. However, researchers at Washington University say the Corps has greatly underestimated flood probabilities.

The '93 flood is really a 50-year flood, if one considers how much flooding has worsened in Missouri, Bob Criss, a geologist at Washington University, said. A 50-year flood has a 2 percent chance of happening in any given year.

The 1993 flood’s “levels have been superseded all over our state, except for the region around St. Louis and parts of the lower Missouri River by subsequent floods,” Criss said.

Major flooding in early 2016, for example, broke the 1993 flood record in Cape Girardeau.

“We’ve had three more so-called 10-year floods since 2008 in St. Louis,” Criss said. “How do you have three of these floods in less than a 10-year period? These are very simple calculations. It’s not hard to show that something’s incorrect.”

Having accurate estimates for flooding is important, Criss said, because that could help avoid floodplain development that contributes to constriction of rivers and, therefore, flooding.

Officials at the Corps of Engineers’ St. Louis District maintain that the 350-year estimate determined by a 2004 study of the Mississippi River is correct. Criss’ calculations focus more on flood stage, the height water levels reach to cause significant flooding, which is a metric better used for coastal areas, said Don Duncan, a hydraulic engineer at the Corps’ St. Louis District. The Corps’ calculations use flow, or the volume of water passing through a specific point along the river, because that better reflects what drives flooding along the Mississippi River, he said.

However, Duncan noted that the Corps’ statistical methods don’t have a way to account for climate change.

“The science isn’t quite there to do quantitative values on such a large basin such as the Mississippi,” Duncan said. “So, if we had a way to do that adjustment, we would do it.”

A study published this year concluded that nearly 500,000 people in Missouri are vulnerable to river flooding. It also suggested that climate change, a major factor of more frequent heavy rains, could raise flood risks to communities.

Scientists speculate that a large portion of the Midwest that includes St. Louis has been going through a “wet period,” said Jonathan Remo, a geography professor at Southern Illinois University, Carbondale.

“We don’t quite have it all worked out why that is, but we know probably since '93 until at least 2017, this part of the country has been really wet,” Remo said.

But if a flood at least as severe as the one in '93 were to happen in the near future, the Corps would be most concerned with how well communities would be able to defend themselves.

Dave Busse, chief engineer at the Corps’ St. Louis district, recalled how anxious he and others in the agency’s Water Control Office were about the levees in the metro area 25 years ago.

“There was a lot of worry about that because they had never been tested,” he said.

The long rains that summer also soaked many levees, causing stability issues. Many “major levees” were able to withstand the crest when it arrived in August, Busse said. But levees aren’t the final solution to flooding.

“We like to tell everyone there are no perfect levees,” he said. “Eventually, all levees will be overtopped or fail. I don’t know how long that will be, if that’s tomorrow or if that’s a thousand years from now. There is nobody that has a perfect crystal ball that can tell you when that next big flood comes and we need to be prepared.”

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