Flood risk below average in Iowa

BY LI DAI | APRIL 08, 2015 5:00 AM

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For the past 10 years, Iowa City has seen some of the worst flooding in its history, but for 2015, officials say the spring flood risk is below average.

The National Weather Service in Des Moines determines the flood risk up to three months in advance. According to the current National Weather Service flood outlook, the risk is slightly below normal probabilities of spring flooding in eastern Iowa.

“The flood risk across Iowa over the next three months is near normal to below normal,” said Jeff Zogg, a senior hydrometeorologist at the Weather Service in Des Monies.

Zogg said a normal risk of flooding doesn’t mean a zero percent chance of floods in eastern Iowa.

“Historically, flooding typically occurs in Iowa from the late winter into early spring,” he said. “Thus, normal would include some flooding.  Although we do not see indications of widespread, severe flooding, isolated to scattered flooding may occur due to runoff from thunderstorms and heavy rainfall.”

According to the Weather Service website, the Iowa River has less than a 50 percent chance of flooding near Iowa City.

“Taking things further, the flood risk along the Mississippi and Missouri Rivers is below normal,” Zogg said. “It is generally near normal on the rivers within Iowa that drain to both the Mississippi and Missouri Rivers.”

He said that during this period of the year — moving from late winter into early spring — the factors that go into determining the flood risk are snowpack, ground frost, soil moisture, and present river levels.

“Snowpack is nonexistent across Iowa, which would tend to decrease the flood threat,” Zogg said.

He said soil moisture is also near normal to below normal, which itself can decrease the risk. Also, river levels continue to stay normal or below normal, which, when combined, end up in an overall lower risk.

Heavy rainfall potential is also considered as a year-round factor.

“During the late winter and early spring season, the combination of snowmelt and rainfall help to drive any flooding that occurs,” Zogg said. “Once the snow is gone, we are left with only the rainfall to consider.  Thus, outside of this time of year, we typically use the rainfall potential to help determine the flood threat.”

Zogg said with all of these factors coming together, over the next three months, flood risk is near or below normal.

“River levels are highly impacted by rainfall amounts and intensity at this time of year,” State Climatologist Harry Hillaker said. “Thus, the ability to accurately predict river levels is dependent upon accurate predictions of rainfall.”

He said future flooding depends almost entirely upon future rainfall.

“Flooding potential along small streams and creeks is highly dependent upon short-term intense rainfall events in the local area,” Hillaker said. “Flooding potential along major rivers depends upon widespread and frequent heavy rainfall over all or much of the watershed.”

He said frozen soil greatly increases flood risk because frozen soil is largely impervious to water.

Iowa City Public Works Director Ron Knoche said the city has staff and equipment available to respond to floods, despite the below average risk.

“The city has a storm-sewer system to collect the rain,” he said. “If a large rain event occurs that overwhelms the storm-sewer system, there are creeks and drainage ways to drain the storm water to the Iowa River.”

Knoche said the threat of possible ice dams no longer exists for this year, but the city does have staff and equipment to respond.

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