Rapidan Dam: 114-year-old dam was in poor shape prior to failure

Federal regulators found the Rapidan Dam to be in poor condition in their recent inspections over the last few years.

It’s 114 years old and like a lot of dams across the country, it is starting to act its age.

Failure wasn’t exactly inevitable, but civil engineers say the typical design life for a dam is 50 years and this one was more than twice as old.

They give Minnesota a C grade for dam conditions, which doesn’t inspire a lot of confidence, but it’s at least better than the national grade of D.

Half of Mankato’s electricity came from the Rapidan Dam after its construction in 1910 and 1911.

One man died while building the 475-foot-long structure as he fell off scaffolding near the top and just days after he had carved his name near the sandstone bedrock almost 87 feet below.

As the Rapidan dam buckles a bit in its second century, it gives us a sharp reminder of the looming potential for danger on Minnesota waterways.

"The aging infrastructure is a challenge," said Jason Boyle, the Department of Natural Resources state dam safety engineer. "You know, a lot of the dams in the state were built 80 years ago."

Civil engineers say concrete can last 50 to 100 years in dams, but maintenance is key.

Blue Earth County documents six repair projects since the dam was built, all but one of them since 2002.

High water events in 1965, 2010, 2019, and 2020 did significant damage and Blue Earth County started considering removing the dam as far back as the year 2000.

Federal inspectors deemed it to be in poor condition two years ago.

The Rapidan Dam is classified as having a significant hazard potential because a failure would cause plenty of environmental damage, but nobody lives permanently in the area immediately downstream.

People are more likely to die if a high hazard dam gives way and a 2022 analysis found Minnesota had six high hazard dams in unsatisfactory or poor condition.

"We prioritize the high hazard dams, of course, because they're the ones that, if they would fail, would cause, you know, potential loss of life," Boyle said.

Nationwide, engineers say almost 16,000 high hazard dams are in poor condition, and about 40 dams fail every year.

The Department of Natural Resources says the Lake Bronson Dam is the only one of those left in Minnesota and a repair project is in the early stages.

But they have dozens of smaller projects on a priority list and climate change is making it harder to keep up with regular maintenance.

"The more and more often we see hundred-year floods happening, they're not designed to handle that event," said Boyle. "So that's when then we see the damages."

Blue Earth County Commissioner Kevin Paap told FOX 9 that federal inspectors were back at Rapidan in May, and although they still believed it was in poor condition, they told commissioners there were no major dam safety issues needing immediate action.