Q: With the Rapidan Dam being over 110 years old and repairs mounting, I was wondering what would happen if it collapsed during a flood? How bad would be the flooding downstream?

A: Blue Earth County Public Works Director Ryan Thilges doesn’t sound particularly worried about a catastrophic failure of the county-owned hydroelectric dam, but that doesn’t mean Thilges and others haven’t considered the possibility.

“It would require a very massive flood, I think, for that to happen or some very extenuating circumstances,” Thilges said.

The dam’s condition is monitored by the county, by the private power company that operates the dam, and by the Federal Energy Regulatory Commission.

The emergency action plan in place requires additional monitoring during times of flooding, along with procedures for closing recreational use of the Blue Earth River in the vicinity of the dam and evacuating the county campground just downstream from the dam in any situation where there was concern about the structure’s integrity.

“As long as the campground is evacuated and there’s nobody on the river, the danger to anyone’s life and even property is limited,” according to Thilges.

Because of that, the greatest concern involving a failure of the dam is environmental.

Most visitors to the dam, near County Road 9 just west of Rapidan, probably look at the reservoir and envision a tremendous amount of water stored behind it. It’s a mirage.

The dam, built between 1908 and 1910, is 87 feet high from its top to the streambed on the downstream side. But the reservoir is no longer anywhere close to 87 feet deep. During average flows, there might be 10 feet of water in the reservoir, Thilges said.

“The rest is accumulated sediment,” he said.

Runoff from farmland into the Blue Earth River has long carried substantial amounts of silt and soil. When the water is moving quickly, the sediment moves with it. But when the dam was built, most of the water came to a standstill behind the structure, the sediment sank to the bottom, and nearly the entire reservoir is now filled with the stuff. So there’s no need to worry about a towering wall of water roaring downstream to Mankato, a dozen river miles away.

That’s why a dam collapse is mostly an environmental concern. A substantial amount of that accumulating sediment would, over time, end up flowing downstream to the Minnesota River at Sibley Park, to the Mississippi River and eventually to the Gulf of Mexico.

That in itself would degrade water quality, but the sediment also has agricultural chemicals attached, including chemicals that have been banned for decades but still potentially reside deep in the sediment behind the dam from their arrival as long as 111 years ago.

All of that said, Thilges isn’t losing sleep over the possibility of a dam collapse. Repairs continue to be made, federal and county officials are keeping a close eye on the dam. And he noted that the structure has survived some challenging weather over the years, including just three springs ago, when the dam withstood the second-largest flood on record.

Q: When are they going to finish old Highway 66?

A: As reported elsewhere in today’s Free Press, the final segment of that project is resuming later this month and is to be completed no later than October 2022.

The reader’s question isn’t surprising, considering how long the highway — now known as Blue Earth County Road 1 — has been under construction. The first segments were upgraded in 2016.

But Thilges notes it hasn’t been under construction every year since then. The county took years off when Highway 22 was closed for major upgrades, not wanting two of the three primary north-south routes in the county closed at the same time.

The project was also on hiatus in 2020 for a different reason.

The delay in starting the final phase of construction came partly from environmental permitting delays for segments of the road near the river or wetlands. But letting the project sit for more than a year also had a cost-saving component. Instead of removing and replacing waterlogged soils deep below the roadbed, the county employed a much less expensive process of piling massive amounts of earth above those areas, which — throughout 2020 and the first half of 2021 — squeezed the moisture out, creating an acceptable base for constructing the road.

That seems to have worked as intended, Thilges said, “and we’re comfortable moving forward.”

The contractor has until October 2022 to finish the work. Thilges seemed tempted to predict County Road 1 will reopen for good much sooner than that but ultimately just said it would be done “in 2022.”

“I’ve got to play it safe,” he said. “We’ve got a very capable contractor, but I also know we have some very challenging conditions.”

Correction: The original version of this story stated that the Rapidan Dam withstood a major flood in the spring of 2019, calling it the largest on record. Thilges said that flood was actually the second-largest on record.

Contact Ask Us at The Free Press, 418 S. Second St., Mankato, MN 56001. Call Mark Fischenich at 344-6321 or email your question to mfischenich@mankatofreepress.com; put Ask Us in the subject line.

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