Rapidan Dam partial failure: What happens if the dam collapses?

Officials are continuing to monitor the Rapidan Dam in southern Minnesota after a partial failure on Monday.

The dam suffered a side breach due to high water flows which sent water rushing over the west side of the dam, destroying an Xcel Energy substation.

The high flow of water has eroded dirt from the bank of the river near the dam, leaving one home dangling over the edge.

Is there a major danger to residents?

Speaking Tuesday, Blue Earth County officials clarified the risk presented if the dam were to completely fail.

Officials made it clear that Rapidan is considered a low-risk dam. They say online posts have exaggerated the danger if there were a colossal failure of the dam.

"There is a mistruth out there that people believe if the dam had a catastrophic failure, that a wall of seven-foot water would be coming," explained Blue Earth County Emergency Manager Eric Weller. "The water level in a catastrophic event would not be that significant."

Blue Earth officials say modeling – at a flow of about 42,000 cubic feet per second – shows Mankato would see a brief increase of about three or four inches. Within two miles of the dam, it could be about a two-foot rise.

Since the 2019 flood, the dam hasn't been generating electricity. Meaning, it's been "run of river flow" – meaning all the water that comes to the dam passes through, and the dam doesn't hold back a lot of water.

Officials say the bigger concern in a total failure would be sentiment build-up on the upstream side of the dam being released, which could cause some environmental impacts.

How it started

Officials said Tuesday that the first notification about high water flows along the Blue Earth River dam came late last week. A second notification was sent on Sunday as the river flow continued to increase.

After the first round of notifications, officials closed the bridge deck to members of the public and the county park campground downstream from the dam.

On Sunday afternoon, crews inspecting the dam reported additional debris accumulating on the upstream side of the dam. Officials say they contacted a contractor, who had done work at the dam in the past, to come and move the debris. However, the specialized excavator that is used in the process was five hours away and the contractor was concerned about safety due to the high river flows.

By 1:30 a.m. on Monday, water had begun overlapping the dam. At 3 a.m., officials made another round of notifications under the Emergency Action Plan.

"As flows peaked, there was partial failure of the west abutment," officials say. "This resulted in the loss of an Xcel substation causing power outages."

A park building was also destroyed in the water breach.

(FOX 9)

Where things stand now

Water flows have abated some on Tuesday, dropping from peaks at 34,800 cubic feet per second on Monday to 33,000 cubic feet per second.

Water flowing around the west side of the dam continues to erode soil on the west river bank.

The county has not issued any large-scale evacuation order, but assessments of the dam are continuing. Right now, officials say the risk of a full failure has diminished.

Officials say this is already the second-largest flood on record at the Rapidan Dam, behind the 2019 flood that also caused damage to the dam.

Xcel Energy worked Monday to restore power to nearby residents in the area of the dam.