a dam near Mankato, Minnesota, saw its west buttress washed away by river water and a Burlington Northern Santa Fe Railroad bridge, which carries freight across the Big Sioux River between North Sioux City, SD and Sioux City, Iowa, partially collapsed on June 24 after more than 24 hours. of heavy rain
The partial collapse of the west buttress dam of the Rapidan Dam north of Mankato on the Blue Earth River occurred in the early morning hours of June 24 as rapid liquefaction of the earth berm washed it away and damaged a nearby Xcel Energy concrete substation.
“This water is moving very fast. It’s moving at over 40,000 cubic feet per second, which is probably around 10 to 15 miles per hour, and there’s a lot of debris in the water,” said Jeffrey Johnson, director . of public works with the City of Mankato. Officials believed the water had risen as of 10:30 a.m. on June 25, and no further damage to the dam had yet been discovered.
Although the earthen buttress will need to be rebuilt, local officials were cautiously optimistic that further damage could be avoided if the rain subsides. Equipment was not available over the weekend to begin immediate repairs to the buttress as the water was still rising and efforts were focused on protecting the dam. More storms are expected over the next few days.
“The water found the path of least resistance on the west side of the dam,” said Ryan Thilges, county engineer and director of public works for Blue Earth County. He also said a small park building was damaged by water flowing west of the dam, as was a substation that provides power to about 600 homes in the area.
Authorities said they would continue to monitor the situation as water continued to flow around the dam.
“We are working with him [U.S. Army] Corps of Engineers and the National Weather Service in some models so we can get a better idea of what’s going to happen,” Johnson said. “It’s not a catastrophic failure, it’s slowly failing, so I think it’s a good situation to be in . .”
The Federal Energy Regulatory Commission will also participate in the damage assessment. The Corps’ National Dam Assessment rated Rapidan Dam in poor condition in April 2023, ranking its hazard potential as “significant.” The dam is now owned by Blue Earth County and is one of many old hydroelectric dams that have not produced power for years, causing the county to require other funds for dam repair and maintenance.
The county said in a news release that water flow peaked on June 24 at 34,800 cubic feet per second and had subsided to 33,000 cubic feet per second by June 25. be the cause of similar failures by the Association of State Dam Safety Officials.
The railway bridge partially collapses
On the Big Sioux River, a Burlington Northern Santa Fe freight-only rail bridge between North Sioux City, SD and Sioux City, Iowa partially collapsed around 11:00 p.m. on June 24.
“This is the main bridge into Iowa that moves a lot of goods and different materials across the state,” South Dakota Gov. Kristi Noem (R) said at a news conference. “It was a huge loss for us. This will affect us for many, many months.”
The bridge was closed before the river crested at about 44.9 feet.
“We have been monitoring the region through our increased track inspections and had not been operating over the North Sioux City Bridge as a precaution given the conditions,” said Kendall Sloan, BNSF spokeswoman.
He said the middle span of the bridge collapsed and all trains are being diverted through Creston, Iowa.
“We will continue to monitor and inspect conditions in the area and execute recovery operations as necessary,” he said.
More than a dozen Corps river wides in the area have already broken all-time records, some as high as 7 feet.
