IOWA CITY — It was an up-and-down day for flood-wary Johnson County, and it may remain that way for weeks.
After telling area officials Wednesday morning that Coralville Lake may top its emergency spillway — something that has only ever happened during the devastating floods of 1993 and 2008 — the Army Corps of Engineers later in the day updated its calculations and said water will come close to going over but should stay in the lake
Even with that forecast, flooding is already occurring downstream and is expected to worsen as more water is let out of the lake.
The corps, which oversees the man-made reservoir, now predicts the lake will reach 711.5 feet above sea level on July 9. The spillway sits at 712 feet. The lake was at 702.4 feet Wednesday afternoon.
All eyes will be on the sky and weather forecasts during the coming weeks. Any significant rainfall in the Iowa River basin, which feeds into Coralville Lake, would threaten the spillway, said Dee Goldman, Coralville Lake operations manager.
“It’s dicey right now,” he said. “It’s a hit-or-miss.”
Communities downstream of Coralville Lake dealt with the same thing last June, when it was predicted lake levels would rise above the spillway. The lake ultimately crested at just above 708 feet, however.
This year, water levels far upstream from Coralville Lake are having a significant effect. At midday Wednesday, the Iowa River in Marshalltown as at a record height, and that water takes five days to reach the lake, Goldman said.
The outflow of water leaving Coralville Lake will increase over the coming days. Wednesday morning, it was 8,000 cubic feet per second. On Thursday, it is projected to be 18,000 cfs, and Goldman believes the reservoir gates will be fully open at that outflow.
When water goes over the spillway, officials can no longer control it and significant flooding is likely along the Iowa River south of the lake, including in Iowa City, Coralville and on the University of Iowa campus
With that threat looming, area officials started taking preventive measures. That includes preparing pumps and looking at computer models of various flooding scenarios, said Terrence Neuzil, who serves as Johnson County’s Emergency Management spokesman and is chairman of the county’s Board of Supervisors.
The National Weather Service in the Quad Cities is forecasting the Iowa River in Iowa City will reach 24.2 feet on Thursday, which is in the “moderate” flood stage. That forecast only takes into account rain expected in the next 24 hours.
The county supervisors on Wednesday issued mandatory evacuation orders for 11 small riverside neighborhoods in rural areas. During last year’s flooding, the county required residents to leave some of those same areas.
Iowa City officials are warning residents and businesses in several flood-prone neighborhoods to be on alert. Recommended evacuations were expected on Normandy Drive and Taft Speedway, the city said in a news release. Mandatory evacuations are possible in the coming days or weeks, it said.
Rick Fosse, Iowa City’s public works director, said it’s important for people in the flood plain to have a plan because for at least the next couple of weeks the area will be vulnerable to storms that come through the Iowa River basin.
“The threat will not be gone on Monday,” he said.
In addition to closing buildings, relocating services, suspending transit operations, and blocking roadways and paths, the University of Iowa has erected a 12-foot HESCO barrier outside Mayflower Residence Hall and 4-foot barriers along the Iowa River’s east and west banks.
Crews also on Wednesday were busy clearing construction materials and leveling the ground for HESCO barriers near the Iowa Memorial Union and the Iowa Advanced Technology Laboratory, according to UI spokesman Stephen Pradarelli. And sandbagging was underway at the water plant intake and near the power plant on campus.
A flood wall was being installed at the Art Building West.
As to what this year’s flooding could mean for incoming students this fall, Pradarelli said it’s too soon to tell.
The only damage on campus so far has been downed tree limbs and fallen panels off a scoreboard at Kinnick Stadium from a storm that blew through Monday, and Pradarelli said value estimates are not yet known.
Coralville is installing a 1,300-foot-long HESCO barrier along Clear Creek behind Second Street, said Ellen Habel, assistant city administrator. Temporary flood panels will go up along the Iowa River behind Iowa River Power Restaurant, she said.
With a section of Dubuque Street in Iowa City expected to close late Wednesday due to flooding, vehicles getting on and off Interstate 80 will have to find alternate routes. Many drivers are likely to use First Avenue in Coralville, and there are partial lane closures there over the interstate for a construction project, Habel said.
Johnson County Sheriff Lonny Pulkrabek has banned boating, swimming and other recreation on the Iowa River, creeks and streams south of the dam.
“Things are going to get bad again,” Pulkrabek predicted.
A key difference between this year and 2008 is Iowa City, Coralville and the UI have completed many flood-mitigation projects, including pump stations, levees, berms and more. Also, hundreds of homes in flood-prone neighborhoods were bought out and demolished.
“We’re fortunate that this community is a lot better prepared than we were in 2008,” Neuzil said.
But homes remain in river neighborhoods, and some flood projects are unfinished. One of those is a levee by the Cole and Thatcher mobile home parks in Iowa City. The city was installing temporary barriers 8 feet high to protect the homes, Fosse said.
With homes being evacuated, the American Red Cross serving Greater Iowa is opening an emergency shelter at the Johnson County Fairgrounds, 4265 Oak Crest Hill Rd. SE., Iowa City.
United Way of Johnson & Washington Counties activated its Emergency Volunteer Center and Disaster Call Center to coordinate volunteers. People interested can sign up at the center, which is at the county fairgrounds, or get more information by calling (319) 337-8657.
The Gazette’s Vanessa Miller and Lee Hermiston contributed to this report. l Comments: (319) 339-3175; firstname.lastname@example.org