Raised Bridge and Road Resolve Iowa City's Flooding Problem
Iowa City’s Dubuque Street, a key artery that parallels the Iowa River, had a history of flooding—but in recent years floods have been worse with a much bigger impact.
The road, which carries more than 25,000 vehicles a day between Interstate 80 and the downtown business district and University of Iowa, was closed 54 days during the great flood of 1993.
Water levels exceeded the 100-year flood stage in 2008 for about five days, forcing closure of the nearby Park Road bridge across the river. In the summers of 2013 and 2014, historic river levels closed the street for about three weeks.
Locally heavy rains, 1-in. plus per hour, are known to flood and close Dubuque Street’s northbound lanes. The street had an elevation of about 644 ft, but the 100-year and 500-year flood plains near the road have upper edges of about 651 ft and 653 ft, respectively.
One solution was The Iowa City Gateway project, a $40.5-million project completed in August 2018 that raised a 3,500-ft stretch of the road up to 10 ft above the 100-year flood level, and elevated the Park Road bridge 1 ft above the 200-year flood level.
“Since the roadway and bridge were elevated, we have, on a number of occasions, exceeded the 11,700 cubic feet per second that had typically closed Dubuque Street,” Melissa Clow, special projects administrator for Iowa City’s engineering division, told ENR. Flow would have to exceed 31,010 cfs to close the artery, she says. During the flood of 1993, the maximum flow was 28,200 cfs.
“We cannot say that we have fixed the problem, but we have greatly reduced our risk of flooding,” Clow says. Dubuque Street did not flood after heavy rains last summer following installation of a sanitary sewer trunk line along the road.
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The Park Road bridge has not yet been tested, Clow says. “It can pass the flows that were seen on the Iowa River in 2008,” she notes, adding it was also designed to withstand flood waters against the structure in order to maintain traffic.
A National Institute of Building Science study on natural hazard mitigation found that the project produced $456 million in benefits—mostly related to lives saved from drowning.
