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Barr Engineering makes recommendation to dredge Lake Mitchell sediment for $25 million

A harmful algae warning issued at Lake Mitchell is shown in 2017.

Posted on May 8, 2023

A leader of the engineering firm that’s been analyzing Lake Mitchell’s phosphorus-rich sediment over the past two years made a recommendation on Monday to dredge the body of water at a cost of $25 million.

Eric Lund, an engineer with Barr Engineering, broke down the lake improvement recommendations during Monday’s Mitchell City Council meeting at City Hall. With the scattered areas of high concentrations of phosphorus-packed sediment along the 693-acre lake bottom and nutrients flowing into the lake via Firesteel Creek, Lund urged city leaders to take an “adaptive approach” to restoration plans.

“The dredging, you’re going to pay for how much sediment you take out of the lake. The volume and thickness, as well as where you are in the lake, is really important,” Lund said of the mechanical dredging process.

According to Lund, the $25 million mechanical dredging would remove roughly 70% of the surface area sediment playing a role in the algae woes that have hampered the lake’s water quality for decades.

The distance from the disposal site and areas being dredged in the lake is a key factor that impacts the cost of the mechanical dredging project, Lund explained.

Firesteel Park, located on the west side of the lake, has been identified as the disposal site where crews would dump the dredged sediment. Based on the sediment sample evaluations Barr Engineering conducted over the past year, the northern portions of the lake near the disposal site is dealing with higher concentrations of phosphorus.

“We’re looking at maybe $10 to $25 a yard with dredging … Areas closer to that (disposal site) are going to be cheaper to dredge,” he said.

Among the costly portions of the recommended mechanical dredging project is the $3.5 million drawdown structure that would be used to control the water levels for dredging the sediment and potentially infusing parts with alum treatments.

Lund also provided city leaders with a different lake improvement plan that solely entailed alum treatments to the lake bed over the course of several years. The alum treatment plan, which would be an alternate method to lake restoration rather than dredging, had a price tag of roughly $16.6 million. It’s a near $9 million cost reduction compared to mechanical dredging.

“With alum, you’re not taking it out, you’re immobilizing the phosphorus and sediment so it can’t release into the water column. You’re treating the lake bed,” he said of the alum treatment process.

According to Lund, a barge would be used to apply the alum along the lake bed. A drawdown of the water would also not be necessary if the council opted to pursue an alum treatment method.

Considering the proposed location of the disposal site being near the north side of the lake where higher levels of phosphorus exist, Lund pointed to mechanical dredging as a “cost effective” method to address the sediment on the northern side of the lake compared to alum treatment.

However, he said turning to alum for portions of the lake that are further south could be another option for city leaders to consider.

“These areas as you get farther away from the disposal site in the historical channel with thick areas of deposited sediment, you start to get the inverse and it becomes really costly to dredge those areas, whereas you can treat them with alum for a significantly lower price,” Lund said.

While city leaders have clearer options to ponder moving forward, one major issue remains: Firesteel Creek unloading phosphorus into the lake.

Lund urged city leaders to develop a long term plan for lake restoration work that entails approaches to improving the impaired 350,000-acre Firesteel watershed. Regardless of the in-lake solutions that are implemented, Lund said the nutrient loads funneling into the lake from Firesteel Creek will be a problem.

“Until the Firesteel Creek input is significantly reduced, we’re going to have a problem whether we 100% dredge it or 100% alum treat it,” he said. “The Firesteel Creek itself – historical channel – has 1- to 4-plus feet of problematic soft sediment in it.”

The city has been focused on cleaning up the Firesteel watershed. The first of what city officials hope to be more wetlands is inching closer to construction along 35 acres of land next to the creek.

The recommendations presented on Monday worked for council member Susan Tjarks, who dubbed the dredging plan as the “most cost effective” path forward to restore the lake.

“Listening to this approach, I feel like this is exactly the way we need to do it to maximize our dollars on the dredging part by focusing on areas where it’s most needed,” she said. “To me, this is the most cost effective use of a lot of money.”

Public Works Director Joe Schroeder said the city is planning to bring an SRF loan application in front of the council in early June, which is when the council would make its big decision on whether to move forward with a dredging project. The SRF loan application would be used to fund the project.


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