Major Midwest Flood Risk Underestimated by as Much as Five Feet, Study Finds

As floodwaters surge along major rivers in the midwestern United States, a new study from Washington University in St. Louis suggests federal agencies are underestimating historic 100-year flood levels on these rivers by as much as five feet, a miscalculation that has serious implications for future flood risks, flood insurance and business development in an expanding floodplain.

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

Published this month in an advance online issue of the Journal of Earth Science, the findings are important, Criss said, because many of the nation's flood-control river levee systems are not engineered to withstand floods that rise much higher than the projected 100-year flood level.

Any flood that rises even a few inches over the top of a 100-year levee has the potential to cause a catastrophic breach of the flood control system, he warns.

Based on complicated equations currently used by key federal agencies, such as the Federal Emergency Management Agency (FEMA) and the U.S. Army Corps of Engineers (USACE), the official 100-year flood level is a key national index of potential flood severity. Levees are commonly designed to withstand floods at "100-year" levels and "100-year" flood zones are delineated on detailed flood insurance maps produced by FEMA.
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Global warming and the resulting increase in extreme weather cycles has only added to the flooding risk in recent years, he said.

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