Public Safety

Don't be so quick to wish away the snow - too warm, too fast could raise flood risks

We may want warm weather fast, but slow snow melt is better to reduce flooding along Cedar River

Water from Lake Macbride flows over the spillway into the Iowa River at a First Day Hike at Lake Macbride in Solon on Tuesday, Jan. 1, 2019. Parks nationwide hosted guided hikes in an effort to encourage people to get outside and start the new year in nature. (Rebecca F. Miller/The Gazette)
Water from Lake Macbride flows over the spillway into the Iowa River at a First Day Hike at Lake Macbride in Solon on Tuesday, Jan. 1, 2019. Parks nationwide hosted guided hikes in an effort to encourage people to get outside and start the new year in nature. (Rebecca F. Miller/The Gazette)

While Eastern Iowans tired of cold may long for a string of 50- to 60-degree days, that kind of warm up would greatly increase risks of major flooding.

As it is, the probability of the Cedar River reaching major flood stage at Cedar Rapids this spring is seven times the historical risk, according to a National Weather Service forecast released Thursday. This is a worse outlook than two weeks ago, when the weather service said Cedar Rapids’ major flood risks were three times normal.

“Essentially, we have just built the snow pack in the northern parts of the Cedar River basin over the last two weeks,” Meteorologist Jessica Brooks said Thursday. “The longer that snow sits there, the higher potential for a rapid snowmelt. The faster that snow melts, the worse our conditions are going to get.”

The Cedar River has a 66 percent likelihood of reaching its major flood stage of 16 feet this spring, compared with the 9 percent historical average, the weather service’s Quad Cities bureau reported. Vinton, Palo and Conesville also have significantly elevated odds of major flooding.

Cedar Rapids has dealt with river crests as high as 17.94 feet in recent years with relatively minor impact to the general public, the city’s Public Works Director Jen Winter said Thursday.

“Last fall, we had several major flood stage threats that we handled successfully through our interim response plan, which outlines key flood protection measures at various flood stages,” Winter said in an email. “These tactics include both above-ground measures (earth levees and sand-filled HESCO barriers) and below ground measures (closure of storm sewer gates to prevent backup from the river), and can be implemented within three to four days.”

In preparation for this spring, the city has inventoried supplies, inspected equipment and held ongoing flood response training for staff, Winter said. Permanent protection measures, like the NewBo/Sinclair levee, have reduced the role of temporary protection in those areas, she said.

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Iowa City’s risks of major spring flooding still are on par with normal, mostly because Coralville Lake can be used to control water flow into the Iowa River. But the Iowa River at Marengo has a 38 percent likelihood of reaching major flood stage of 19 feet, compared to 6 percent historical average, the weather service reported.

Brooks said the two major factors pushing flood potential on the Cedar River are 99 percent ground saturation and moisture levels in snow in the northern part of the river basin.

“Almost all of that area has 2 to 4 inches of liquid in the snow,” which is equivalent to 2 to 4 inches of rain, Brooks said.

“Into southern Minnesota, some areas have more than 4 inches,” she said. “That’s a lot of water that has to go into the Cedar basin.”

If the snow can melt gradually, Eastern Iowa rivers will be better able to handle the flow, Brooks said. Temperatures in the 30s and 40s this weekend, accompanied by rain, could melt some of the snow without causing a torrent.

“Personally, I would love it to get warm, but I know that would be a very bad thing for our flooding situation,” Brooks said.

l Comments: (319) 339-3157; erin.jordan@thegazette.com

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