Engineers suggest multi-million-dollar dredging project for Lake Mitchell, with work spanning four years
With the council recently allocating $665K for the final design to be completed, the city is closer than ever to dredge the lake as a way to improve the water quality for future use.
MITCHELL — City leaders have a clearer picture on dredging Lake Mitchell in hopes of reducing the algae woes that have been plaguing the body of water for decades.
During Monday’s Mitchell City Council meeting, a team of engineers leading the preliminary design unveiled their findings that suggested dredging the lake mechanically as the most effective method.
To mechanically dredge the 693-acre lake, Eric Lund, project leader with Barr Engineering, said it would cost between $13 million and $15 million and span over roughly four years, depending on weather and water flow rates.
“We really based our scope on the results of a lot of the previous good work that had been done. The conclusions were that the water quality is impaired by both internal in-lake sources and the external watershed load coming in from Firesteel Creek,” Lund said.
Roughly a year ago, the city council tabbed Minnesota-based Barr Engineering to complete the preliminary design at a cost of $339,000. Over the past year, city leaders have met on a bi-weekly basis with the engineering firm to receive updates on the project.
After project leaders took samples of sediment at various areas around the lake, Lund said the results showed there were elevated levels of phosphorus found in the sediment along the lake bottom. However, he said there are still some uncertainties with the volume of phosphorus-rich sediment that is one of the root causes of the algal blooms the lake experiences on a yearly basis.
Previous lake studies indicated that about 50% of its phosphorus load flows into the body of water through Firesteel Creek, while the other half is in the lake itself. Particularly, the sediment.
While the dredging design is aimed at developing an approach to remove the sediment from the lake, the city is also working on a wetland project along Firesteel Creek with the goal of drastically reducing the phosphorus flowing into the lake from the creek.
Lund said the goal would be to remove roughly half of the soft sediment sitting along the lake bottom.
Breaking down the project’s rough timeline, Lund said the next step in the process would be developing the final dredging design and drawing down the lake water to expose the soft sediment along the lake bottom for it to dry out.
“While the lake is drawn down, you start the sediment investigation. Then you would design, bid and permit the dredging project,” Lund said, noting that phase of the project could take around one year. “Then you complete the dredging project, restore the disposal site and refill the lake.”
While there are several drawdown levels that the city could pursue, the engineers suggested a 20-foot drawdown of the lake that would temporarily expose about 550 acres of the lake bottom. Lund said the west side of the lake has been identified as the area with the most soft sediment, and the 20-foot drawdown would leave the entire west side of the lake dry for crews to remove the sediment.
“There are two components to it. The first is to bring the lake down to expose that sediment and let it dry out. The second part is how you keep the lake down, so the contractor can complete their work,” Lund said. “The control of water is an important factor because it affects a lot of things like the duration of the project.”
To lower the water level for crews to mechanically dredge the lake sediment, Lund pointed to the method of breaching the spillway dam on the north side of the lake as the most effective option. However, breaching the spillway would raise the costs of the project a little over $2.7 million, which would bring the cost of the project to about $15.7 million.
“You would actually remove concrete chunks from the spillway itself. You could get a very high flow rate,” Lund said. “It would provide the most long-term benefit. You could reconstruct it with a gate structure that you could use to manipulate the lake elevation.”
Managing the water levels while the lake is drawn down depends on the flow of water coming into the lake from Firesteel Creek, Lund said. As Lund put it, dredging the lake will be a “risk management” project.
“The control of the water is an important factor because it affects a lot of things like the duration of the project,” he said.
Although estimating the water flow each year will be challenging, Lund said project leaders are using past averages, including years when the area saw more precipitation and dry spouts.
Firesteel Park chosen as spoil site
After examining about a dozen potential areas along the lake that could serve as spoil sites, Firesteel Park emerged as the most ideal spot to dispose of the dredged sediment and material.
Using Firesteel Park as the spoil site would also allow the city to create a sledding and snowboarding hill, which was an idea former Parks and Recreation Director Nathan Powell drummed up during lake dredging discussions.
Eric Prunty, an engineer with Brosz Engineering, said construction crews would use traditional earthwork equipment, such as bulldozers and excavators, to remove the sediment along the lake bottom and haul it to the spoil site.
“We anticipate the contractor would build one or two ramps out of the lake to get it up to Firesteel Park. The material would be dumped, spread and dried out,” Prunty said. “The making of a sledding hill in time could be made with the materials.”
For Firesteel Park — which sits along the west side of the lake — to serve as the spoil site, Prunty said crews would need to remove trees and close off a portion of West Harmon Drive to regularly haul the dredged sediment to the Firesteel Park spoil site.
“The site would need to be cleared of the trees and top soil would be removed for final soil stabilization capping of the dredged material. The on site earth material would be used to construct embankment berms around the perimeter,” Prunty said.
If the city moves forward with a lake dredging project, it would mark the second time the man-made body of water is dredged. In 1980, Joe Kippes, a leader of a nonprofit organization that’s raising funds to support dredging, said the lake was dredged with the purpose of increasing water capacity.
While talks of dredging the lake have resurfaced over the past few years for different reasons, no plans materialized from those past discussions until now. With the council recently allocating $665,000 for the final dredging design to be completed by Barr Engineering, the city is closer than ever to dredge the lake as a way to improve the water quality for future use.