Iowa Experts Say Serious Flood Frequency May Increase

December 17, 2008

Time will tell if the record floods of 2008 are an aberration or the beginning of a trend. But some experts say the likelihood of serious floods in Iowa is increasing in frequency.

Eugene Takle is a professor of atmospheric science and agricultural meteorology at Iowa State University. He said that if he had to hazard a guess, the possibility of serious flooding is increasing.

“I’d say the likelihood of this type of event is increasing,” Takle said.

His colleague at Iowa State, Elwynn Taylor, said he believes increases in Iowa’s annual precipitation, coupled with anticipated precipitation increases linked to global warming, greatly increase the likelihood of more frequent and severe flooding.

Taylor says that since 1950, Iowa’s annual precipitation has increased by 10 percent, which in turn has doubled the amount of water carried by rivers in the state.

It took a previously unthinkable chain of weather events to swell the Cedar River to a 31.12-foot-tall, 1.6 mile-wide tsunami that swamped much of downtown Cedar Rapids on June 13.

Taylor said a 10 percent increase in precipitation equates to roughly 3 more inches per year, making rivers six times more prone to flooding because it all runs off.

What had been considered a 100-year flood is now a 17-year flood, Taylor said.

Though the parameters of a 100-year and 500-year floods have increased dramatically, the odds of another flood like the one in June are about 1 in 250 in a given year, said Witold Krajewski, a water resources engineer at IIHR Hydroscience & Engineering at the University of Iowa.

Krajewski and his colleagues start with the same historical data used by the Geological Survey but subject it to a different analysis that considers the role of drainage patterns in the landscape.

“Yes, there was a lot of rain, but the rain was not as extraordinary as the flood,” Krajewski said.

Krajewski said the timing and spacing of the rainfall, combined with drainage patterns within the Cedar Rapids watershed caused the runoff to converge just upstream of Cedar Rapids on June 13.

That created a “traffic jam” Krajewski said, which led to the large-scale flooding.

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