The Army Corps of Engineers placed the 90-year-old dam in the second-highest category as a safety risk after inspectors voiced concerns that water might be seeping under the dam between Minneapolis and St. Paul.
An aging concrete dam on the Mississippi River in the Twin Cities has been classified as "unsafe or potentially unsafe" after an engineering team raised a concern about whether water is seeping under its foundation, the U.S. Army Corps of Engineers said Wednesday.
The concern arose during a national safety review of more than 600 corps-owned dams to help identify high-risk dams needing repairs.
Engineers in the corps' St. Paul office said an inspection team found no evidence that Lock & Dam No. 1, also known as the Ford Dam, has a seepage problem -- which could cause the dam to fail -- but uncertainty led to the dam being placed in the "urgent" category. "This particular dam being 90-some years old has a very old design that we don't use anymore," said Michael Bart, engineering chief and dam safety officer for the corps' St. Paul district.
He said engineers hope to answer seepage questions by examining soil data collected during the dam's last rehabilitation, completed in 1981.
"They just ask some questions," Bart said of the corps teams, which are brought in from other districts. "If we don't have analysis to back that up, that's what puts it in that category."
The corps placed the dam in Category 2, with the potential to fail, the second-highest risk category on the corps' 5-level safety scale.
Failure of the dam could halt navigation on the upper river and, under some conditions, flood the riverside areas of Hidden Falls Park in St. Paul and Minnehaha Park in Minneapolis, just downstream from the dam. No homes are at risk.
Even if a seepage hazard is confirmed, the risk to the public is low, Bart added.
"It is not a life-safety risk, it is economic," he said of the possibility of dam failure. "... We would not be able to navigate upstream of the dam."
The highest-risk dams are classified as Category 1, Urgent and Compelling, meaning that its risk of failure is extremely high or near certain. No corps dams have been given this rating in Minnesota.
Seepage is common with dams and needs to be controlled to prevent the structure from being compromised. If Lock & Dam No. 1 turns out to have a seepage problem, the corps would study what work needs to be done, Bart said. If it doesn't have a problem, the classification would be changed to a lower-risk category, he added. The corps also is studying whether seepage occurs at the powerhouse on the east side of the dam.
David Rydeen, dam safety program manager for the St. Paul district, said officials didn't notify the cities of St. Paul and Minneapolis, which own the parkland downstream of the dam, about the safety rating.
Two other dams in the state are rated in the same "Urgent" category -- Lock & Dam No. 3 near Red Wing, which is scheduled for a $70 million renovation, and Orwell Dam on the Otter Tail River near Breckenridge, where the corps is replacing wells that reduce groundwater pressure affecting the discharge channel, according to corps engineers.
Bart said navigation dams such as Lock & Dam No. 1, because they back up less water than flood-control dams, typically do not cause serious flooding if they fail. He added that it is relatively easy to lower the pool behind a lock and dam if a concern would arise about the structure.
"Generally speaking, it is not a wave of water that is going to come out of these structures," he said.
The emergency plan for Lock & Dam No. 1, issued in 1988, but still considered valid, says failure of the dam under normal pool conditions would cause the river below the dam to suddenly rise nearly 9 feet, posing a danger to people in boats and along the shore. The flow would not be a hazard farther downstream, the plan says.
Under overcast sky Tuesday, Nate Herzog of Richfield stood on the sandy bank of Hidden Falls Park, casting into the slow-moving water. His friend Andrew Ruona stood nearby. Neither was particularly concerned about the dam's safety rating.
"I guess if I were down here at the moment it broke, that would not be good," said Herzog, as he baited his hook with a leech.
David Shaffer • 612-673-7090