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The Rapidan Dam southwest of Mankato should withstand recent historic flooding on the Blue Earth River, but the near-miss underscores the risk for an old and vulnerable structure that county officials have wrestled with removing for years.

"This brings that conversation of what are we going to do with the dam, a 114-year piece of infrastructure, more on the table than it was four days ago," said Kevin Paap, who chairs the Blue Earth County Board.

It was the second-strongest flood ever recorded at the dam, with the rushing waters peaking Monday when officials advised of a possible "imminent failure." By Tuesday morning, concern had ebbed despite the water gates clogged with debris, a channel carved around the dam's edge and a threatened home seen teetering on the edge of a damaged riverbank.

On Tuesday night, Blue Earth County officials announced that part of that home had been "undercut enough to have fallen into the river." Safety officials were "monitoring for downstream impacts," the county said in a statement.

Destructive floods in 2019 and 2020 had previously left the hydroelectric plant inoperable and county officials grappling with spending at least $15 million to bring the dam into working order — with little hope of making enough money from selling power — or shelling out $82 million to remove it. Barr Engineering looked at the conundrum in 2021 and concluded "doing nothing was not considered a viable alternative in this study because doing nothing would probably result in the structure deteriorating to a point that may result in the structure failing."

Blue Earth opted against the repairs then and has been in a holding pattern since, just keeping up with safety standards. The dam passed a federal inspection in May, but county engineer and Public Works director Ryan Thilges said Tuesday he had "significant concern[s]" with raging water impacting the stability of the dam and its long-standing issues.

"It's basically an albatross around the county's neck," said John S. Gulliver, a professor emeritus at the University of Minnesota's College of Science and Engineering.

'Fast and furious' flood

Debris was what sent water flowing over and around the dam, county officials said.

As floodwaters rose in southern Minnesota in the past few days, the county faced considerably more trees and debris than it usually might because of drought, Thilges said. In the past few years, dry weather killed off trees that did not travel to and pass through the dam amid low water levels.

Blue Earth's usual method for clearing such an obstruction was off the table because a contractor with a key piece of equipment was hours away. Plus, trying to remove the debris would have been a safety risk with the high water.

"It all came just fast and furious," Thilges said.

Water spilled over the top of the dam and swerved around the west end of the structure. That took out an Xcel Energy substation, cutting power for hundreds until the company restored electricity late Monday night. The water's new path around the dam, however, relieved some pressure on the structure, and by Tuesday morning county officials said they believed the dam would hold.

Aaron Lavinsky
Video (00:50) Quick-flowing water erodes the earth around the Rapidan Dam near Mankato on Monday.

They also said a wall of water would not swamp Mankato if the structure fell. Because the dam isn't operating, it's not holding a glut of water in its reservoir that a failure would unleash. Thilges said the county believed in the event of a failure, areas 2 miles downstream would see water levels rise about 2 feet, and about 3 to 4 inches in Mankato.

The greater concern was environmental. Sediment stuck above the dam would harm the river ecosystem, Gulliver said, since it includes agriculture pollution. That's one reason why dam removal is so expensive, he added.

Gregory Baecher, a professor of civil and environmental engineering at the University of Maryland, said floating debris can be fatal for a dam and is an expensive "pain in the neck" to manage.

Baecher said just about all dams have debris problems, but it's more concerning in places with lots of trees. Dam operators can take actions such as installing floating barriers known as log booms upstream.

"What happens a lot [is] that you've got accidents or incidents that are expensive to repair," Baecher said. "It doesn't happen a lot that dams fail because of debris, but they can."

From afar, Baecher said the issue isn't because of "poor management."

"It doesn't look like the dam itself was at fault," he said. "It just looks like there was too much water, in this case."

Significant concerns remain

Still, Baecher said the dam troubles might raise questions about county officials' planning if the flood was a "50-year storm" rather than, say, a once-in-a-thousand-years event.

Thilges said the biggest flood recorded at the dam in 1965 is considered a 500-year storm, with a flow rate at 43,100 cubic feet of water per second. He didn't know the exact characterization for this week's event, which peaked Monday at 34,800 cubic feet per second.

The dam's location and original construction also played a role in the partial failure of the west abutment. Meanwhile, the dam's fair condition also maintains a risk of a bigger failure.

The 87-foot-high dam, built in 1910, is on erodible sandstone bedrock in a flood-prone area. The river is now scouring away some of that bedrock, county officials said. Thilges said studies of the dam have found risks in water wearing a concrete slab at the bottom of the dam that calms falling water.

"That's a significant concern right now, of additional scour downstream of the dam potentially impacting the stability of the dam," he said.

Blue Earth, along with state and federal officials, is trying to gain a better understanding of the impact, though closer inspection that requires boats or underwater diving aren't possible yet.

Even before this weekend, the county had no plans to restart the dam because the potential electricity sales were not worth the cost. Instead, Blue Earth officials decided to surrender its license with federal regulators and turn oversight to state officials before making a decision on whether to decommission the dam with input from the Minnesota Department of Natural Resources.

In the meantime, Blue Earth said it conducted routine maintenance and safety inspections. The May inspection by the Federal Energy Regulatory Commission found no major problems in need of immediate action, though it did say there were issues like concrete cracks and scouring that Blue Earth should monitor.

Paap, the County Board chairman, said the dam's survival, in some ways, showed its strength and resilience. He added state and federal government could potentially cover the high cost of removing the dam.

Sullivan said there are always competing interests that make dam removal complicated, but Baecher advocated for setting money aside, since removing the dam is "probably the right thing to do" when the county isn't using it.

Baecher also said extreme rain events are increasing with climate change, and dam safety risks are rising across the country because more people are moving into hazardous places.

"With climate change, all those historical data are becoming less and less useful," he said. "Because the past is no longer prologue if things are changing."