Posted on May 18, 2022
The final phase of the Lake Mitchell dredging design will soon be getting underway.
With the first phase of the lake dredging design in the books, the Mitchell City Council unanimously approved the final phase of the lake dredging design during Monday’s meeting. Public Works Director Joe Schroeder said the $767,500 final design will provide a blueprint for the project to begin as early as fall 2023, if the council moves to approve dredging the lake that’s estimated to cost around $15 million.
According to Schroeder, the main scope of the phase II dredging design will entail selecting the method of the drawdown of the lake water for mechanical dredging to occur, evaluating ways to breach the spillway dam, examining the water quality and sediment.
“If we’re digging into the dam, we want to know what that’s all going to entail. This portion of the project is the most labor intensive. One of the big issues with the drawdown is the plans for the existing spillway are lacking,” Schroeder said. “There is going to need to be a lot of effort looking at that structure.”
Schroeder noted there are a couple options to achieve the drawdown of the lake, but he indicated the most likely option is creating a hole in the spillway dam on the north side of the lake or the concrete retaining walls along the sides of the spillway.
In response, Schroeder said the wetland project will be getting underway this summer, indicating it could be complete prior to a future lake dredging project beginning.
McCardle’s question on whether it makes sense to begin dredging prior to the wetland being completed came after a group of Dakota Wesleyan University students recently presented a Lake Mitchell project that they claimed showed dredging would be “pointless without implementing a wetland” along Firesteel Creek to filter out the phosphorus and sediment flowing into the lake.
With the long history of algal blooms plaguing Lake Mitchell’s water quality, Schroeder said the second phase of the design will dig deep into examining the sediment along the lake bottom that’s also spurred algae woes.
Eric Lund, an engineer with Minnesota-based Barr Engineering, has been leading the lake dredging design and fielded questions from the council during Monday’s meeting.
In response to an inquiry about the potential methods crews could use to dradown the lake water, Lund said his team of engineers will explore an idea to construct a gate structure to manipulate the lake water levels.
“We will put together an evaluation that takes those options being as simple as pumping and siphoning the water, to doing a more robust option with one being a gate structure,” Lund said, noting the installation of a gate structure could provide the city with a “long-term benefit” to manipulate Lake Mitchell’s water elevation.
Council member Susan Tjarks addressed some concerns about the impact that dredging the lake will have on the fish population.
The blunt reality of drawing down the lake water for dredging to occur will likely cause “a fish kill off,” Lund said. However, Mayor Bob Everson noted the South Dakota Game, Fish and Parks has shown interest in potentially restocking the lake with fish following dredging, if the council approves dredging the lake.
“The GF&P is committed to helping improve the fishing on the lake, and I would assume they would start with restocking it as well,” Everson said.
The fish evaluation may reveal more information on the impact fish have on the lake’s sediment and water quality, Lund said.
“They go out and do a net survey that has to be in the early part of the spawning season that tells you how many there are, the types and sizes and age. Then we can look at that and ask ourselves, ‘Is the population of the raw fish enough that it could also be creating turbation issues in the sediment itself, and if we do a partial removal would it sort of stir them back up?’” Lund said of the carp and fish evaluation. “Getting some of the war fish out of there is one tool in the toolbox when dealing with water quality issues.”
When the phase II dredging design is completed – which Schroeder anticipates to be March 2023 – it will pave the way for the city to bid the project and begin dredging the lake in fall 2023.
While the city initially budgeted roughly $661,000 for the final dredging design, the total cost surpassed the budgeted amount by roughly $105,000. Schroeder said the additional $105,000 will be taken out of the city’s future capital project fund.
Some key outcomes of the phase I dredging design included determining mechanical dredging as the effective method to dredge the sediment from the lake bottom and identifying an ideal location for the spoil site in Firesteel Creek Park on the west side of the lake. In addition, Lund recommended the city to dredge roughly 50% of the sediment in the lake as part of phase I of the dredging design.
Another new area that the final design will explore is how to maintain enough water in the lake for POET, Lakeview Golf Course and other city-owned areas that rely on lake water for irrigation. There is currently an agreement in place that allows POET to utilize Lake Mitchell water at its ethanol plant north of Mitchell.
Considering the 350,000-acre Firesteel watershed unloads heavy amounts of phosphorus into the lake that fosters algal blooms, creating more wetlands along the creek has also come into focus among city leaders. Over the past few decades, the Firesteel watershed has been listed as impaired for failing to meet the Department of Agriculture and Natural Resource’s nutrient-related narrative.
