Flood risk average this year


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Melting snow has left intersections in front of the Pentacrest resembling beaches as the snow melts, leaving piles of sand and pools of water in its place.

Since October, particularly cold weather has left the ground frozen to unusual depths of 2 to 3 feet, meaning the soil is less able to absorb water. This leads to greater and quicker runoff, a possible flood factor in the event of heavy rainfall.

“The historical odds for a quick warm-up that would melt snow cover in a hurry and for a heavy rain event steadily increase the later we go into the spring,” said State Climatologist Harry Hillaker. “Thus, the longer we keep snow on the ground and the longer the soil stays frozen, the higher the odds there will be a big rain and a fast snowmelt to put a lot of water in the rivers in a short time.”

Potential risk for flooding from the frost is balanced out by what Jeff Zogg, senior hydrologist at the National Weather Service in Des Moines, calls “competing factors.”

“What we’re seeing is that the risk is overall average,” Zogg said.

“From a hydrological standpoint, that’s like the entire watershed being paved in concrete,” he said.
Iowa City Public Works Director Rick Fosse said the city prepares itself every year for possible flooding from frozen ground runoff.

Fosse said the city has numerous systems in place to ensure water runs off smoothly.

“If you go around town, we’ve gone around to open up the intakes to make sure that water gets into them,” he said. “We also have staff on call to deal with possible ice dams.”

Hillaker said future risk of flooding depends largely on the amount of precipitation.

“The biggest factor would be the future rainfall, which is not something that can be foreseen with accuracy more than a week to 10 days ahead of time,” he said.

Although temperatures are rising, the frozen ground will persist.

“One thing to note is that the frost will continue to penetrate even deeper as long as air temperatures are averaging below freezing, which they will be this weekend and next week,” Hillaker said.

Temperatures have been below average every month since October. Temperatures in November and December, which Hillaker listed as ripe for the frost, were 3.5 and 5.7 degrees below average, respectively, according to the National Weather Service.

Zogg said a careful eye on the future rainfall is necessary as the transition from winter to spring continues to monitor any change in the flood risk.

“It is important for people to realize that, although the risk is average this year, it’s important to remain vigilant,” Zogg said.

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