A historic flood swept down the Lower Mississippi River in May 2011.
Along the way, the river swelled to record levels at the cities of Vicksburg and Natchez, both in Mississippi, while nearing records in Memphis, Tennessee, and Greenville, Mississippi. The flooding caused an estimated $2-4 billion in damage.
The flood was comparable to the Great Mississippi River Flood of 1927. And, according to new research from University of Wisconsin-Madison engineers published in the journal Science Advances, floods like it in the Lower Mississippi River Basin may become much more common as the 21st century progresses.
Daniel Wright, an associate professor of civil and environmental engineering, says the research focuses on how storm clustering drives flooding in the Lower Mississippi River Basin. Because the lower basin—which is made up of parts of Mississippi, Louisiana, Arkansas, Tennessee, Missouri and Kentucky along the Mississippi River—can handle so much water, it usually takes the confluence of several storms to trigger flooding.
“The Lower Mississippi cares about really big storms, and in particular, multiple really big storms occurring one after the other,” Wright says. “It takes weeks, if not months, for these floods to build. It’s hard, especially in the Lower Mississippi River Basin, to figure out the probability of these rare floods because we have, at best, 100 years of data to use. If we’re talking about high-quality rainfall data, much less than 100 years’ worth.”
So the researchers, led by Wright’s former student Yuan Liu (PhD CEE ’25), who is now a postdoctoral researcher at University of Oxford in England, created a model, StormLab, that can simulate flood-causing storm sequences. Now, Wright says, instead of 50-100 years of flood data to work with, the model gives researchers thousands of years of simulated data. For this study, they used StormLab to create 7,600 years of high-resolution rainfall predictions across the entire 1.2 million square miles of the Mississippi watershed.
From there, the group selected the biggest floods in the dataset and analyzed them using a flood-simulation model to determine what type of extreme storm sequences, or “clusters,” produced those floods.
The researchers classified the storm clusters as isolated, which is an individual storm occurring somewhere in the Mississippi River watershed; spatial clustering, which is a single storm with heavy precipitation than can stretch across multiple drainage basins in the watershed; temporal clustering, in which two or more successive storms bring heavy rains to a drainage basin; and compound clustering, which is a mix of temporal and spatial clustering.
“You can imagine a scenario where you have snowmelt from the Rocky Mountains that’s coming down and into the Mississippi River,” Wright says. “At the same time, you have a storm that hits around Ohio, so that’s more water coming in. And then there’s another storm that hits lower down the river, so you have water from all these storms coming in at the same time. It’s a big timing question about how everything fits together from the different tributaries.”
Wright says isolated extreme storms are the most common type seen today. However, as weather patterns shift over time, the model predicts that compound clusters will become the most common type of storm clustering.
The 2011 Mississippi River floods are an example of the flooding that can follow in the aftermath of extreme storm compounded clustering. Two powerful storm systems swept through the United States in April 2011. One of the systems, responsible for the April 25-28, 2011, Super Outbreak (the largest tornado outbreak ever recorded), exacerbated an ongoing period of heavy rain, leading to some areas across the South and the Ohio River Valley getting more than a foot-and-a-half of rain in a week.
The 2011 flooding was so severe that the U.S. Army Corps of Engineers opened the massive Morganza Spillway for only the second time since its completion in 1954 (the spillway was first opened for flooding in 1973), flooding thousands of square miles in Louisiana’s Atchafalaya Basin to protect New Orleans from inundation.
“We estimated that in 2011, that flood had a return period of about 112 years, or less than a 1% chance of happening in a given year,” Wright says. “By 2050, that same flood could be expected to happen every 23 years. By 2100, it was down to every eight years, or more than a 10% chance of happening in a given year.”
That could have major implications for infrastructure built to protect against the Mississippi River’s worst flooding. The Mississippi River and Tributaries Project, authorized by the Flood Control Act of 1928, is designed to withstand a hypothetical worst-case flooding scenario based on a sequence of extreme storms from January 1937, February 1938 and January 1950 happening three days apart.
Wright says the resultant flood had a return period of 58,000 years, making it vanishingly unlikely to ever occur. However, the model predicts that by 2100, that return period drops dramatically to 150 years—still unlikely, but only a little less likely to happen than 2011’s flood.
“You can see a 150-year return period and think that’s a long time. But what that really means is that over the average person’s lifetime, there’s roughly a 50-50 chance that a flood at least that big will occur” Wright says. “The infrastructure we’re building today will still be around in 2100. It’ll still be around 100 or more years from now, so we’ve got to consider how we’re prepared for the future.”
Top photo via iStock