By Gregg Hennigan
IOWA CITY — Even as water went over Coralville Lake’s emergency spillway last summer, what remained under the lake’s surface also was on the mind of many flood victims.
Years of sedimentation buildup have reduced the lake’s capacity by 14 percent. The thinking was, if that material was removed through a process known as dredging, there’d be more room for water in the man-made reservoir, which was built a half-century ago to control flooding.
“There was a lot of talk of dredging the lake in the wake of the flood last year,” said Jerry Anthony, who lived in Iowa City’s flood-ravaged Parkview Terrace neighborhood.
But officials with the Army Corps of Engineers, which oversees the lake, and a University of Iowa expert say dredging the lake would have had a negligible effect.
The UI’s renowned fluids research and engineering laboratory, IIHR-Hydroscience & Engineering, ran a simulation with the corps’ operations model and found that dredging the lake would not have lessened the severity of last year’s flood.
“The sedimentation that has accumulated in the reservoir has very minimal, almost not-measurable effect on the total peak discharge that was released during the flood of last year,” said Larry Weber, director of IIHR.
The reason for that, he said, is that the majority of the sediment is below the conservation pool, which is the target elevation for the reservoir at different times of the year. The corps’ strict guidelines call for the lake to be at 679 feet above sea level in the early spring and 683 feet just before Memorial Day. The lake hit a record 717 feet last June.
So even if the lake is dredged, the water would still typically be at 683 feet, rising from there in the event of flooding.
“Anything below 683 doesn’t matter because the water is always going to be at that elevation,” said John Castle, lake operations manager.
Even dredging the lake and reducing the conservation pool by a few feet wouldn’t help much because the extra storage would quickly fill up in the spring, Weber said. There may be a small benefit in reducing the major-flood-pool elevation, he said, but his lab has not completed that analysis.
What’s more, Castle said, dredging Coralville Lake would be “prohibitively expensive,” perhaps in the hundreds of millions of dollars. There’s also a question of whether the material brought up would have to be treated like hazardous waste because of the presence of herbicides and pesticides brought by runoff, he said. The water is safe to be in now, Castle said, but dredging may stir up some of those chemicals.
Anthony, the Iowa City resident, said he and many of his former neighbors now generally accept that dredging is not a viable solution. But keeping the conservation pool steady — it has been raised by a few feet over the decades — or even lowering it would help some, he said.
He also said there should be policies along the Iowa River watershed to decrease runoff, and he suggested the corps buy land along the river that can flood, which possibly would allow the dam’s outflow to be increased.
Weber, who also is co-chair of the UI’s flood-mitigation task force, said increasing outflow is a strategy that may be worth discussing once the most pressing recovering needs have been addressed.
After May 1, corps guidelines put the maximum outflow at 6,000 cubic feet per second, until the lake reaches 707 feet, at which time outflow is gradually increased. (Outflow hit a record 39,462 cfs last year). After the 1993 flood, there was some discussion of changing it to 8,000 cfs, but that would have required a large study that local governments would have had to help pay for, Castle said, and the study was never done.
Weber’s lab ran a simulation on increasing outflow to 8,000 cfs after May 1 and found it would not have helped in last year’s flood.
The corps also has faced criticism by some that it puts the lake’s recreational uses ahead of flood control.
Castle said the reservoir’s top priority is to reduce flooding. But, he said, there’s no doubt that its most popular use is as a recreation spot.
“So you can’t ignore that fact,” he said. “But we don’t make decisions based on recreation over flood control.”