Minnesota authorities can’t do much to prevent potential bridge collapse

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MINNEAPOLIS — Rushing waters of the Blue Earth River have already left a trail of debris and destruction along the banks of a southern Minnesota dam that partially collapsed last week, but officials acknowledged Tuesday that the structure most in danger may be the nearby bridge.

The County Road 9 bridge is in danger of collapse, and officials said they have little recourse. The threat to the bridge intensified after a series of heavy rains and flooding hit the Midwest for days. Water levels in the Blue Earth River rose dramatically, testing the structural integrity of the dam. The dam held, but the specter of collapse did not diminish.

Today, the nearly 40-year-old bridge, which residents use to cross the dam from rural areas to nearby towns, is at risk of collapse if the weather does not cooperate.

“Unfortunately, we are at the mercy of Mother Nature at this point,” said Ryan Thilges, Blue Earth County Public Works Director. “We are very concerned about the potential for a partial or complete failure of the bridge.”

Thilges stood atop a hill east of the Rapidan Dam near the city of Mankato, Minnesota. He was flanked by Minnesota Gov. Tim Walz and other officials who visited the dam to check on the status of the flooding and recovery efforts.

Authorities are cautiously monitoring the dam and bridge, noting that the still-swollen river has radically changed the area.

“I think the concern is whether the bridge is going to be structurally damaged by this and whether it will need to be replaced?” Walz said.

Floodwaters carved a new channel around the dam, cutting deep into a steep bank, toppling power poles, destroying a substation, engulfing a home and forcing the demolition of a beloved store. The conditions made it too dangerous for authorities to get close enough to the bridge for a thorough inspection, but they have already identified worrying signs of damage.

The river is carrying large amounts of sediment, making the bridge’s support pillars, built on sandstone bedrock, unstable. Authorities have been able to stabilize at least one pillar, but they have not been able to reach the others.

The situation was complicated by “a huge amount of trees that fell into the river” Sunday, Thilges said. The dead trees, the result of drought in recent years, collided with the bridge, and some are hanging from the pillars. The county has been unable to find contractors who feel safe enough to remove the debris.

“Nobody was willing to send their operator out and risk their lives to try to push these trees through,” Thilges said.

The floods caused millions of dollars in damage to bridges, homes and roads in Minnesota, Iowa, Nebraska and South Dakota. The dam drew attention after officials initially said it faced an “imminent threat” of collapse.

Rapidan Dam is more than a century old, having been completed in 1910. Although it was built to generate electricity, it has been damaged by several floods in recent decades. An April 2023 assessment by the National Inventory of Dams found Rapidan to be in poor condition, and officials have been studying the possibility of removing it.

A federal disaster declaration has been approved for Blue Earth County, and local officials said the additional resources will be critical to rebuilding efforts. But those projects could be complicated by a sensitive landscape where relief efforts can sometimes worsen decline, Thilges said.

“I’ll be completely honest: All of the solutions we’ve come up with have had negative effects that are almost as serious, if not worse, that could further affect the stability of the dam, or cause damage to the bridge or further erosion,” he added. “We need Mother Nature to give us a break.”