Shive-Hattery, Fyra Engineering and Barr Engineering were the three firms that proposed their statement of interest plans to dredge Lake Mitchell, pending the City Council’s approval. Each engineering firm was provided with 40 minutes to present their proposals in front of a variety of city officials. While a plan to dredge the lake has yet to be approved, an overwhelming majority of city officials have expressed their support for such a project to address the lake’s long history of algae woes.

Representatives with Barr Engineering, of Minneapolis, opened the presentation by highlighting previous lake dredging projects that the firm has led and deemed a success. Eric Lund, environmental engineer with Barr, said Lake Mitchell shares similarities with Lake Zumbro, which is a 600-acre body of water in southeastern Minnesota that the firm recently dredged as part of a $7.5 million restoration project. The size of Lake Mitchell is roughly 800 acres.

“Similar to Lake Mitchell, it’s a dammed up reservoir on a river, and the primary focus was to increase the water depth since it filled in with sediment overtime and lost its recreation ability,” Lund said.

For the Lake Zumbro restoration project, Lund said they were able to remove roughly 500,000 cubic yards of sediment through hydraulic dredging. Breaking down the cost of the project, Lund said the hydraulic dredging process amounted to $5 million, while constructing the disposal site for the dredged sediment came out to $2.5 million. During previous dredging discussions among city officials, the cost estimates to dredge Lake Mitchell hovered around $10 million, which Lund said is in their “wheelhouse.”

“This disposal site was 2 miles from the nearest point of the lake, so some of the pumping we had to do was over 5 miles,” Lund said. “With modern hydraulic dredging, you can achieve that with no problem. We have really strong relationships with hydraulic dredging contractors, which is a really small pool.”

Lund broke down the difference between hydraulic and mechanical dredging, both of which that city has mulled over. Mechanical dredging is done with more traditional construction equipment to remove the sediment and phosphorus by using excavators and bulldozers, while hydraulic consists of applying a pipeline in the lake that discharges the sediment and phosphorus to a disposal site away from the body of water with pumps.

“Mechanical dredging is going to take much less space, whereas hydraulic can be up to double the actual volume of dredging material, so you might need to have a really efficient dredging disposal site,” Lund said, noting he recommends with hydraulic dredging for Lake Mitchell. “Mechanical is usually more effective on a small job, and hydraulic is more effective on large jobs. There is a really high cost of mobilization for hydraulic dredging but once you get going, it is typically a much lower unit rate on a per cubic yard basis.”

A month ago, the city Parks and Recreation Department pinpointed a roughly 40-acre disposal site next to the lake in Firesteel Park, where city officials are considering turning it into a winter sports area. If the city chose hydraulic dredging, Lund said it wouldn’t be feasible to discharge the sediment in that area due to its size, noting the spoil site for the Lake Zumbro project was roughly 80 acres.

However, the city’s recently acquired 371-acre property formerly owned by the Kelley family that sits along Firesteel Creek roughly 2 miles west of the lake, is an area that Lund said would be ideal for a spoil site.

“I’m really intrigued by the Kelley property. It’s flat, and it’s connected to the creek and you could run a hydraulic pipeline all the way up the creek to the site,” Lund said.

While Barr Engineering is an out of state firm, they would be partnering with Sioux Falls-based Brosz Engineering to complete the dredging project.

Shive-Hattery proposal

Iowa-based Shive-Hattery opened their presentation by providing a deeper look into the sediment on the lake bottom that their representatives deem is largely contributing to the lake’s ongoing algae blooms.

Dendy Lofton, aquatic ecologist with LimnoTech, an environmental engineering firm that would partner with Shive-Hattery on the lake dredging project, pointed to dredging as the best long-term solution to improve the lake and address the phosphorus and sediment loads contributing to the algae. Previous studies on the lake showed 53 percent of the phosphorus that flows into the lake comes from Firesteel Creek, while 47 percent is in the lake itself. According to Loftson, the southern portion of the lake has the highest concentration of sediment.

Based on seven core samples of the sediment that was collected at the bottom of the lake recently, Loftson said the mass of phosphorus that’s in the sediment is “enormous.”

“What that means is both need to be reduced to be able to improve water quality, and we understand you have done a lot of work on that,” Lofton said. “The biggest problem is the diffusive flux of phosphorus from the sediment.”

In deciding between mechanical and hydraulic dredging, Luke Monat, civil engineer with Shive-Hattery, said the biggest factor hinges on the location of the spoil site where the dredged material will be stored.

Daniel Jensen, water resources engineer with Shive-Hattery, said the mechanical dredging spoil sites or basins could provide the city with an opportunity to transform them into recreation areas such as a snow sledding hill. Shive-Hattery is the firm that was contracted in 2020 to design the potential Lake Mitchell jetty and marina project.

Jensen broke down the cost comparisons between mechanical and hydraulic dredging, which he said largely hinges on the spoil sites and the number of them.

“In a hydraulic dredging scenario, you would easily be able to use a single basin in an area such as Firesteel Park and pump that material relatively efficiently and your unit cost would be fairly uniform across the board,” Jensen said. “In a mechanical dredging, we looked at what if we were able to distribute the storage basins more evenly throughout the lake and cut down that haul distance and what it might do to a potential cost.”

Jensen estimated a hydraulic dredging project with one spoil site would cost roughly $7 per cubic yard of dredging, which equates to around $7 to $8 million when dredging 1 million cubic yards of the lake that Shive-Hattey referenced during their presentation. For mechanical dredging with one spoil site like Firesteel Park, Jensen estimated a cost of around $11 to $8.50 per cubic yard. However, if there were three spoil sites near the lake shore, Jensen said it could cost around $4 to $5 per cubic yard, equating to roughly $5 million. Previous

Fyra Engineering returns

After working with the city of Mitchell on developing a potential dredging plan three years ago that never materialized, Nebraska’s Fyra Engineering returned to make its pitch.

Mike Sotak, Fyra’s owner, explained why he believes Lake Mitchell will have to be dredged using both mechanical and hydraulic methods to maximize water improvements.

“Mechanical, hydraulic dredging and sediment capping, I don’t see this project going forward without using all,” Sotak said. “Mechanical dredging helps us understand the soft sediment and make sure we’re getting it all… You have some deeper parts of this lake that are just going to make sense dredging hydraulically.”

Greg Kipp, of Rapid City-based Verax environmental consulting, would be working with Fyra on the dredging project and broke down potential ways of capping the phosphorus-rich sediment in Lake Mitchell.

“We have unique minerals and ways that the phosphorus is bound up here in South Dakota lakes vs. other lakes,” Kipp said. “So we want to make sure that we pay attention to that unique characteristic so that when we do the dredging, we want to make sure that we get everything that we can economically. If there are materials in say the channels of Firesteel Creek that are not economical to get, we want to make sure those are appropriately capped and chemically maintained with alum so they don’t frustrate the lake management.”

Although Sotak stopped short of providing any cost estimates, he said the project’s timeline could range from one to two years. Sotak highlighted the information that his team learned about the lake through its previous consulting work with the city in 2018 as an advantage to lead a successful dredging project.