CARBONDALE — A flood warning by the National Weather Service in Paducah was still in effect for Jackson, Union and Alexander counties as of Thursday, but the worst of the flooding — which threatened levees, forced scores of roadway closures and led several counties to declare states of disaster — appeared to be over.
Emergency management officials expressed relief as river levels fell deeper into moderate flood stage. On Wednesday, the Chester Bridge was reopened to traffic.
Jackson County Emergency Management Agency Coordinator Derek Misener said there had been only one report of home damage in all of Jackson County.
“We’re very happy with the way this incident turned out,” Misener said.
Alexander County Engineer Jeff Denny said some roads remain closed, but that the water is receding.
“It’s getting better every day,” Denny said.
But officials said it was hard to ignore the fact that severe flooding is becoming more and more frequent.
“I think it’s safe to say these incidents appear to be trending toward more frequency and severity as time goes on,” Misener said.
Murphysboro Emergency Management Agency Director Brian Manwaring said the Big Muddy River crested just under the all-time record crest of 40.5 feet set in 2011.
Rainfall in a warmer world
Warmer global temperatures have increased evaporation from Earth’s oceans. That leads to higher levels of humidity, which in turn produces heavier rainfall and severe flooding, said Justin Schoof, a climatologist and the chair of Geography and Environmental Resources at Southern Illinois University Carbondale.
“This is one example of something that we theorize should happen in a warmer world: we should be getting more heavy rain events. But it’s also an example of something that we’ve already observed happening,” Schoof said.
Precipitation has increased in the past century by as much as 20 percent in some Midwest locations, according to the 2014 National Climate Assessment.
This month’s flooding event was twofold: it was both localized (flash flooding swamped roads after heavy rains) and large-scale (rivers swelled beyond their banks).
Attributing a single weather event to climate change is tricky, because there are always going to be natural variances in weather.
But there are statistical techniques employed by scientists to estimate the probability of a weather event’s occurrence. These are called “recurrence intervals.”
One example is the so-called 100-year flood. The term doesn’t actually denote a flood that occurs once every 100 years; instead, it refers to a flood that has a 1 percent chance of occurring in a given year.
It's safe to say that this month’s heavy rainfall had a low probability of occurring.
“We got somewhere between 10 and 12 inches in four days, and a little bit more than that in some places. That’s the kind of event that you would only expect about a 1 in 200 chance in any given year, so it is quite a rare event,” Schoof said.
The Mississippi River at Cape Girardeau crested at 45.99 feet on May 6, according to NWS. That’s the sixth-highest observation on record. The all-time highest occurred just last year: Jan. 2, 2016.
“So you have these events that you would only expect a one in 100 chance of happening in a given year actually happening far more frequently than that,” Schoof said.
Climate change is compounding another problem stemming from the fact that rivers are now highly engineered.
“If you constrain a river, so you put up levees and wing dikes and all these different things that help with transportation and river management, then basically what you do is you cut the river off from its floodplain, so that when you get a large flood pulse like we’ve had in the last couple of weeks, then the water has nowhere to go but up,” Schoof said.
Jonathan Remo, assistant professor and undergraduate program director in the Department of Geography and Environmental Resources at SIUC, is an expert in river management and flood mitigation.
“The short version of this story is, our flood control infrastructure is severely underfunded,” Remo said.
Of all the levee systems in Southern Illinois, roughly half have received a “U,” or “Unacceptable” rating from the U.S. Army Corps of Engineers.
“What that means, in English as opposed to engineering-speak, is that these levees have the potential to not operate or to potentially fail before they reach their designed capacity,” Remo said.
Southern Illinois’ levees were constructed by the federal government in the 1950s and 1960s and were then turned over to local public sponsor operations and maintenance. Most have never seen any major maintenance since construction.
“That’s a huge issue in Southern Illinois,” Remo said.
As flooding increases in frequency, it could lead to some tough decisions — like in 2011, when the Army Corps of Engineers detonated a levee to flood Missouri farmland in order to save the historic town of Cairo.
“If we can’t afford these levee systems, what are we gonna do?” Remo said. “I think at some point in the future we’re going to have to have an uncomfortable conversation, of how do we support these systems, or do we kind of pull back in our effort to mitigate floods in certain areas. We haven’t gotten to that point yet because it’s politically fraught with controversy, because there are going to be winners and losers if we change the existing system.”