A large portion of Jim Hruska’s house fell into the Blue Earth River at the Rapidan Dam around 9:30 p.m. Tuesday.
Blue Earth County said public works, emergency management and the sheriff’s office are monitoring for downstream impacts.
The house, which was evacuated on Sunday night, was on the west bank of the dam, which started to give way on Monday.
Video of house falling into the river.
The rate of water flow going around the Rapidan Dam had lessened slightly and a bank near the Dam Store seem more stable earlier in the day, but officials are still concerned about whether the sandstone the dam is built on is being washed away, which could jeopardize the structure.
On Tuesday evening the Minnesota River at Mankato was near its predicted crest at 29.5 feet. Mankato staff continue to patrol the levee and flood control system on a 24-hour basis. The levee system is built to handle a river level of 39.5 feet.
When an electric substation at the dam washed away early Monday morning, about 600 people in the area lost power. Xcel Energy brought in an army of workers and all power was restored by late Monday or early Tuesday morning.
Blue Earth County Engineer Ryan Thilges said at a press conference Tuesday morning that the event at the dam was “a partial failure of the west embankment,” not an actual breach of the dam.
County and city officials had to spend a good amount of time Tuesday combating false statements made on Facebook and other social media that said the dam had collapsed and a wall of water was headed to Mankato.
Thilges said they have no way to know if the highly erodible sandstone bed the dam was built on in 1910 is being eaten away, which could still cause the dam itself to fail.
A representative from the U.S. Army Corp of Engineers was on scene Monday and remained there Tuesday. Someone from the Federal Energy Regulatory Commission, which licenses and inspects dams, was to arrive Tuesday to provide technical assistance.
The county had been implementing its emergency action plan for the dam since late last week when water flow hit more than 10,000 cubic feet per second. They closed the nearby campground and began inspecting the dam. The water flow increased to 15,000 cubic feet per second Sunday.
“There was considerable debris under the (highway) bridge,” Thilges said. But they were unable to get at the trees and other debris. There also were some trees stuck in the open gates of the dam. The county called a contractor who has worked at the dam before to see if the company could remove the trees from the dam, but the equipment needed was too far away.
At 1:30 a.m. the water began flowing over the top of the dam and a notice of “imminent failure” was issued as per the emergency plan. Thilges said the notice is given, even if the dam isn’t beginning to break up, to give the handful of downstream residents ample warning in case the dam does fail.
Early Sunday morning, a large amount of tree debris moved from under the highway bridge to the back of the dam. The flow of water also had increased dramatically, reaching 34,800 cubic feet per second. (By Tuesday the flow had reduced some, to 33,000 cubic feet per second.)
The flooding at the dam was the second highest ever, behind only the 1965 flood.
The debris caused the powerful flow of water to shift from going over the dam to the west end and begin carving away the streambank located next to Jim Hruska’s home and his family’s Dam Store.
While the bank was quickly eaten away Monday, with mature trees toppling into the river, the area didn’t appear to still be eroding on Tuesday morning. Thilges said they believe that after all the dirt was washed away the waters got down to bedrock, which shifted the powerful flow more out to the river.
Thilges said that if the dam itself were to break or topple or slide forward, there is no concern about water causing serious flooding downstream or at Mankato-North Mankato. He noted that all the water that comes up to the dam has always passed downstream and that not a lot more volume of water would be added downstream if the dam broke.
Thilges said the dam has always been inspected with the latest done in May by the Federal Energy Regulatory Commission, which found no major problems but some things like scouring that the county should continue to monitor.
A dam failure, he said, would be much more an environmental concern than a flood concern. That’s because there is more than 100 years’ worth of sediment containing a variety of chemicals and pollutants in the river behind the dam. If that sediment broke loose and flowed downriver, the environmental impact would be considerable.
He said they don’t think a lot of that sediment has been released during the current event.
County Emergency Management Director Eric Weller said they had to combat a lot of false information being posted on social media, including that the dam was destroyed and a 7-foot wall of water was headed down the Blue Earth River toward the Minnesota River.