Structure that failed in dam breach must be used again

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The dewatering structure at Brown Bridge dam. Record-Eagle/Glenn Puit
The former site of Brown Bridge pond. Record-Eagle/Glenn Puit

Structure that failed in dam breach must be used again

It can't be fully inspected until it again reroutes river

By Glenn Puit gpuit@record-eagle.com The Record Eagle Tue Nov 06, 2012, 06:14 AM EST

TRAVERSE CITY — A lead investigator in a breach at the Brown Bridge dam said a construction device that failed and caused the Boardman River to flood must be used again to reroute the river before it can be fully inspected.

Byron Lane of Michigan's Dam Safety Unit leads the investigation into what caused Brown Bridge Pond to empty into the Boardman on Oct. 6 while construction workers labored to remove the dam.

At the time, workers were using a construction device adjacent to the dam known as a dewatering structure. The device was supposed to slowly lower the pond over about three weeks, but instead waters breached the structure and rushed into the river in less than six hours. Extensive flooding down river damaged 53 properties.

Lane said the investigation into the cause of the breach requires an inspection of the base of the dewatering structure, as well as the "subsurface" below it, but "a good portion of the structure that failed is still underwater."

Project engineers recently submitted a plan to draw down the water surrounding the dewatering structure, but first, he said, they will have remove the entire dam and powerhouse. That means the Boardman River will have to be redirected back through the dewatering structure — the device that failed — even though investigators haven't yet had a chance to inspect it.

"We are reasonably comfortable this will work," Lane said. "It's important for us to know why this dam failed, and we are comfortable with what (project engineers) are proposing. It's not an ideal situation ... ideally we could get the water through there some other way."

For now, the river is running through what amounts to a hole, Lane said.

"There's basically a hole in the dam, in the embankment, where the water is flowing through," Lane said. "It's not designed and it's not reinforced. It's not the type of situation where we want the water to go through long-term."

Lane said the river will be forced back through the dewatering structure to facilitate the dam and powerhouse removal. Then the river will be directed back through where the dam sits, and investigators will finally get to look at the dewatering structure.

"The river has got to go somewhere," Lane said. "It's coming down at 200 cubic feet per second, and somehow the water has got to go down through. We can't shut off the river."

It could take up to two months to implement the full plan to remove the dam and get a look at the dewatering structure, meaning identifying a formal cause of the breach will take even longer.

Lane hopes investigation findings will be a lesson for other communities that tackle the complexities of removing aging dams. Brown Bridge Dam is one of three scheduled for removal on the Boardman.

"It's obviously disturbing and upsetting," Lane said of the breach. "Everyone involved is disappointed and upset. We will try to find out what happened and put that in our lessons learned category because dams will continue to be removed.

"Engineers and contractors around the country will learn from this," Lane said. "It's not just a Michigan thing. Dams are being removed all over the country."