University of Iowa study: Floods not markedly bigger, but they are getting more frequent

Flood frequency apparently affected by changes in temps and seasonal rainfall

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IOWA CITY — The central United States — including Iowa — has seen more flood events in recent years, although the magnitude of those events has not increased, according to a new study out of the University of Iowa.

Changes in both seasonal rainfall and temperature across the Midwest appear to be driving the rise in flood frequency, causing “adverse societal consequences” — like decreased food production, displaced communities and residents, and other economic losses reaching the billions.

Those findings might not surprise many in Iowa, who have lived through floods in 1993, 2008, 2011, 2013, and 2014.

But researchers analyzed 50 years of data at 774 stream gauge stations in 14 states in an effort to answer the question, “Is the character of recent flooding truly distinct from the long-term averages, or is it simply an artifact of our relatively short collective memory?”

“Our analysis reveals that the largest flood peaks have not been strongly increasing in this broad belt of the central United States but, rather, the region has been experiencing a greater number of flood events,” according to the study, published Monday in the advance online edition of the journal Nature Climate Change.

UI assistant professor Gabriele Villarini and UI graduate student Iman Mallakpour, both with the Department of Civil and Environmental Engineering, first examined trends in the magnitude of annual maximum daily discharge data for 774 U.S. Geological Survey stream gauge stations across the central United States between 1962 and 2011.

About 20 percent of the stations showed statistically significant changes, with just 13 percent reporting increased flood magnitude.

The findings changed markedly, however, when researchers looked at frequency. About 34 percent of the stations showed an increase in flood events — particularly those stations in North Dakota south to Iowa and Missouri and east to Illinois, Indiana and Ohio.

“It’s not that big floods are getting bigger, but that we have been experiencing a larger number of big floods,” Villarini said.

Researchers also investigated potential causes for the rise in flood frequency, examining variability in rainfall patterns. Like their flood results, researchers didn’t find much change in the magnitude of heavy rainfall, according to the study.

But they did find increases in the frequency of heavy rainfall days.

“The fact that we observe the largest changes in the frequency rather than in the magnitude of heavy rainfall is generally consistent with what we found for flood events,” according to the report.

Those findings are relevant as researchers report a warmer atmosphere can hold more moisture and a “pronounced increase in intense rainfall events is included in models of future climate.”

Explaining changes in precipitation and temperature involves assessing the human impact and “represents a much more complex problem.”

Instead, Villarini told The Gazette, the UI researchers next are planning to assess the potential impact of the changes in flood frequency.

“What are the repercussions on the infrastructure and the economy?” he said. “The damage has been going up, but people are getting more and more used to it, so they’re better prepared.”

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