The partial dam failure occurred after three days of heavy rains that left the Minnesota River at its third highest flood height since at least 1881, according to Brennan Dettmann, a meteorologist with the National Weather Service based in the Twin Cities.
In the Mankato area, where the dam is located, 7 to 8 inches of rain fell over three days, based on an analysis by Kenny Blumenfeldsenior climatologist at the Minnesota State Climate Office.
“That elbow of the Minnesota River got hit really hard,” he said, adding that in southern Minnesota, rains that heavy would have between a 0.5% and 2% chance of happening each year.
Bill McCormick, who led Colorado’s dam safety program from 2011 to 2021, said the extreme rains have increased tension across the country.
“We are having more frequent and intense storms that are testing aging infrastructure. Spillways and dams that may not have experienced as many storms in a given year are now experiencing more storms,” he said. “All these old systems are being tested more and more.”
Housing development is also increasing danger levels for some dams, McCormick added, as people settle in once-rural areas where dams built for farmland now protect subdivisions.
Hiba Baroud, assistant professor of civil and environmental engineering at Vanderbilt University, said the partial failure of the Rapidan, among others, should prompt lawmakers to take a hard look at how to bolster dam infrastructure and marshalling repairs.
“We really need to think proactively to project potential scenarios for all dams in the U.S. and start prioritizing which dams need to be rehabilitated or improved to avoid a situation like this,” she said, “instead of witnessing a major event and using that as a warns about this particular dam.”
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