The Swamp Stomp

Volume 15, Issue 37

Washington University in St. Louis recently released a study that suggests federal agencies consistently underestimate historic 100-year flood levels on rivers by up to five feet in some areas. Such miscalculations have severe ramifications for future flood risks, flood insurance, and business development in an expanding floodplain.

PhD Robert Criss, a professor of geology in the Department of Earth and Planetary Sciences in Arts & Sciences and the author of the study, claimed, “This analysis shows that average high-water marks on these river systems are rising about an inch per year—that’s a rate then times greater than the annual rise in sea levels now occurring due to climate change.”

Criss continued to explain that his findings are important because many of the nation’s flood-control river levee systems are not designed to withstand flood levels that rise higher than the projected 100-year flood level, a key national index of potential flood severity. He warned that floods that exceed even a few inches over 100-year levee may potentially lead to cataclysmic events.

The Federal Emergency Management Agency (FEMA) and the U.S. Army Corps of Engineers (USACE) establish official 100-year flood levels by computing a series of complicated equations. Any levees that are constructed to withstand “100-year” levels and “100-year” flood zones are then delineated as such on FEMA’s detailed flood insurance maps.

Criss, however, has researched water flows on major rivers for decades now, and has long argued that any man-made river control systems, such as levees, locks, dams, and navigation-enhancing dikes have steadily elevated the risk of devastating flooding by constricting river channels and preventing floodwaters from flowing naturally into surrounding wetlands and floodplains.

In a 2008 study, Criss demonstrated that flood patterns along the Mississippi River near Hannibal, Missouri, already exceeded drastically the official federal flood risk calculations. Since that study was published, Hannibal has seen floods exceeding the “10-year” flood levels in 2009, 2010, and 2011; as well as experiencing stages officially designated as “50-year” floods in 2013 and 2014.

“Such outcomes are far too unlikely to be attributed to a nearly continuous succession of statistical flukes, and instead must be attributed to faulty calculation of flood risks. Many factors such as climate change and in-channel structures are causing flood levels to rise, so realistic estimation of future flood levels must take these changes into account,” Criss said.

Possibly the most significant part of Criss’ new study is the proposed new statistical equation for the analysis of environmental variables that are changing over time. He claimed, “Official calculations emphasize discharges (flows) in flood risk analysis, yet many compelling reasons show that water levels (stages) should be used instead. Stages are easily understood and are, in fact, the most relevant quantity. If floodwaters are encroaching a home, the owner is concerned about the water level, not about the discharge of the river.”

According to Criss’ equation, which gives more consideration to modern river conditions, the projected high water mark for a 100-year flood event on the St. Louis riverfront would be 51.5 feet—that is over 21 feet above the current flood stage.

Criss said, “The official calculations for the ‘100-year’ flood level at St. Louis are about 5.5 feet too low, primarily because they neglect both the tendency for the flood levels to increase over time and the increased volatility we’re seeing with extreme weather swings.”

Such a difference seriously calls into question the reliability of existing flood control systems. The current system in St. Louis was built to a height of 52 feet, and in 1993 flood levels were only two-and-a-half feet from overflowing the city’s flood wall system.

“In other words, if we experience another flood on the Mississippi of the proportions seen in 1993, it’s hard to say whether the floodwalls protecting St. Louis would be high enough to prevent extensive flooding in the downtown area,” asserted Criss.

Criss’ equation demonstrated similar miscalculations in a number of other Midwestern cities and towns along the Mississippi, Missouri, Ohio, and Illinois rivers. It may be time for these communities to reassess their flood protection systems in order to protect against higher flood crests.