Proposed dam design shows flexibility
Des Moines Water Works listened to concerns about building a new dam on the Raccoon River, and now plans to propose what might be a first-of-its-kind design—part rock rapids, part inflatable gates.
Though the focus of the project is to increase the water yield for its west treatment facility at Maffit Reservoir, it’s also necessary to avoid harming the river’s fish and keeping the river accessible for recreation, said Randy Beavers, assistant general manager for Des Moines Water Works.
“There are really three issues that we’re trying to satisfy in this process, if we do proceed with the low-head dam,” he said. “What we’re looking at is a composite structure that could serve all these needs. We would have a rock chute for roughly a third of the structure to accommodate the fish and the paddlers, and then there would be inflatable gates over the other two-thirds.”
Currently, not enough water can be collected at the Water Works’ west treatment facility, he said, putting an unnecessary burden on its main Fleur Drive plant.
“In the nearly four years that the plant’s been in operation, less than half the time are we able to pump water to the full capacity of the plant,” Beavers said. “In summertime, when pumpage is high, the river and the shallow ground water collector pools tend to be lower. We’re only able to pump roughly 40 percent of the needed water into the treatment plant during those summertime conditions.”
Beavers said the solution is to build a dam about a half mile downstream from Intestate 35 and Iowa Highway 5, which would allow the water level to be higher near the collector wells, allowing the west plant to operate at full capacity.
Martin Konrad, executive officer in the fisheries bureau at the Iowa Department of Natural Resources, said that he expressed concerns in January when he learned of Water Works’ proposal to build a concrete low-head dam on the Raccoon River. He warned that the dam might endanger the river’s fish.
“That type of structure is not friendly to fish movement or conducive to water recreation,” Konrad said. “Our warm-water fish need rest areas to get up and over a dam.”
River recreation enthusiasts also came forward to express dismay over the disruption of the stretch of river they had grown accustomed to paddling. Nate Hoogeveen, president of the Iowa Whitewater Coalition, wanted Water Works officials to be aware of the dangers, sometimes potentially fatal, that dams can create for boaters.
Hoogeveen said that it would be more cost-efficient to modify the dam design now rather than wait until something goes wrong and then address the problem. Water Works officials listened and are designing possibly a first-of-its-kind composite dam for its next presentation to the public a couple of months from now.
The rock portion of the dam would be similar to rock rapids found in whitewater areas, Beavers said. Its slope would be consistent with the natural swimming habits of the fish.
“We have, I think a 20-to-1 slope that we need to meet for fish migration,” Beavers said. “They can swim upstream 20 feet horizontally for every one-foot rise. We want to see if we can build that in with this new kayak passage.”
The inflatable portion of the dam could also be lowered to allow for fish migration in the key months of April through June, Beavers said. When deflated, only a two-foot concrete base will be noticeable on two-thirds of the dam. When the rubber bladders are inflated, this portion of the dam would stand seven feet tall. The rock chute would permanently stand at six feet. in height.
The details of the design are being fleshed out by Water Works engineers for an upcoming presentation to the Army Corps of Engineers and the DNR. It appears as if the proposal will be well-received on the DNR side.
“It’s an improvement over the original one; there’s no doubt about that,” Konrad said. “It should allow for more fish movement at the time of the year that we’re most concerned about.”
“Environmentally, we don’t feel that it’s going to have a negative environmental impact,” Beavers said. “We feel that we can get the fish issue resolved working with the DNR, and it would give the boating community a recreational asset in a way.”
If the design receives approval from the river’s stakeholders and passes the public comment stage, work could start in the near future.
“The soonest that something could be built is next year,” Beavers said. “We’ll know by fall whether we’ll have any funds budgeted next year to try to build this.”
The cost of the project hasn’t been determined, since it is still in the proposal stage, but Beavers predicts that the additional cost due to the modifications will be minimal.
“I don’t know that it’s going to add all that much (cost),” Beavers said. “There might be a little additional cost for hauling the rock in to build this canoe chute as opposed to pouring the concrete foundation, but I’d be surprised if it’s more than a 10 percent additional cost to build this canoe chute.”