MANKATO – The good news coming out of Tuesday’s Blue Earth County board meeting? It will cost far less than thought to remove the Rapidan Dam.
The bad news? It’s going to cost more to replace the nearby bridge.
In the months following the 2024 summer flooding which saw the failure of the dam, Blue Earth County Public Works Director Ryan Thilges originally quoted the cost of the dam removal at around $88 million and the bridge replacement at $18 million. Now, after doing some more research into the project, Thilges says the costs will be around $35 million and $27 million respectively.
With all the changes, the total cost of the project stands at about $62 million; a figure less than the initial estimate for the dam removal alone.
Thilges says these estimates are a little more stable, but that they are still preliminary as only about 30% of the design work has been done.
“We’ve refined the cost estimate compared to the initial estimate we had to make after the flood,” Thilges said. “The initial estimate was made right away after the flood without knowing some of the intricacies of the work.”
Two of the main reasons for the cost changes, both to the dam and to the bridge, revolve around what the new bridge will be made of. Originally, the county planned on using H-piles – support pillars in the shape of an H – to support the bridge. However, concerns over the changing river elevation and the cost of armoring the piles with unnatural rock formations such as riprap led the county to change that plan.
“We changed those H-piles to a drilled shaft foundation. It’s much more robust and provides us with higher flood and scour resiliency,” Thilges said.
Being able to remove a lot of rock armoring from the equation is what helped with the dam removal costs. Thilges says that going forward with a more natural stabilization process reduced those costs, as well as helped get the DNR, MPCA and Army Corp of Engineers on the county’s side.
“(Rock armoring) puts a man-made impediment in the river. That’s why (the MPCA) was opposed to it. The DNR had those concerns as well,” Thilges said.
Another factor increasing the cost of the bridge is what the bridge deck is made of. At first, the county was considering epoxy-coated bars for the deck instead of chromium. However, MnDOT may require chromium bars, which come with a cost “three to five times as much” as epoxy-coated ones.
“The intent is to have a longer service life,” Thilges said. “Expoy bars can last 50 to 60 years, maybe even longer, but the bridge below the deck could last even longer than that.”
Thilges says that the current goal is to begin construction work on the bridge by 2026 or 2027.