Possible causes of flood raise questions

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Above is a 1920s era construction drawing of the Brown Bridge Dam. On the left of the drawing is a "Proposed Diversion Channel" and investigators are now trying to determine if the diversion channel was ever constructed and if it caused a breach at the dam on Oct. 6 that drained the Brown Bridge Pond and flooded the Boardman River.

Possible causes of flood raise questions

Theories include a buried chute or a bad structure

BY GLENN PUIT gpuit@record-eagle.com The Record Eagle Fri Nov 09, 2012, 06:14 AM EST

TRAVERSE CITY — Could a chute buried underground for nearly 100 years be the cause of the October Brown Bridge Dam breach that flooded the Boardman River?

Or, was the breach instead caused by an ill-advised engineering and construction plan?

Interviews conducted by the Record-Eagle this week make clear that both scenarios could be possible causes for the Oct. 6 breach that caused the Brown Bridge pond to empty into the Boardman River as construction crews labored to remove the dam. Resulting flooding swelled the Boardman River by as much as five feet, threatening bridges and damaging 53 properties.

At the center of the investigation into the cause of the breach is the failure of a construction device known as a dewatering structure. Built into an earthen embankment next to the dam, it was supposed to slowly lower the pond over about three weeks. Instead, the pond rushed into the river in less than six hours.

This week, Sandra Sroonian — a senior project engineer on the Brown Bridge Dam removal — showed the Record-Eagle an old construction drawing of the Brown Bridge Dam from the 1920s. Next to a sketch of the dam is a drawing of a chute or channel, and the words "Proposed Diversion Channel."

The dam's original builders presumably considered the proposed diversion channel as a way to reroute the river around the dam construction site when it was first built in 1921.

However, Sroonian said that it's not known whether the diversion channel was ever built. If it was, it would have sat underneath the earthen embankment where the dewatering structure was built.

"It says 'proposed,'" Sroonian said of the old construction plans. "We aren't 100 percent certain it was even constructed."

After the pond raced past the dewatering structure Oct. 6, old steel sheet piling that project officials were previously unaware of was observed near its entryway, Sroonian said. It's not clear yet if the old sheet piling, situated some 14 feet below the water's surface, could be part of an old diversion channel.

"We don't know," Sroonian said. "Everything is speculation until we dry it out."

Project planners had only the old drawing to go on in designing the dewatering structure. No other records provided to planners by the city of Traverse City contained any information about an old diversion channel.

"You do your best due diligence," Sroonian said.

Mike Walton is co-owner of Molon Excavating, the construction company that built and installed the dewatering structure. When asked about the old sheet piling, he said, "We know it turns and goes into our dewatering structure."

"There is certainly some historical stuff (down there) we were not aware of," said Walton, who declined to speculate on a cause of the breach.

Byron Lane, head of the Michigan Department of Environmental Quality's safety unit, called attempts to discern a cause of the breach as "speculation" at this point.

One expert not involved in the Boardman project or investigation, Frank Christie, has 50 years of experience in dam construction and design. He questions why project planners ever considered building a dewatering structure into an earthen embankment to begin with, calling it "a very questionable practice" because it would disturb the dirt and allow water to find a pathway out.

"The failure of earthen embankments is caused by water seeping through them, so the first thing to do is don't build something that would cause seepage to occur," Christie said. "I would never consider doing anything like that."

When told of the old construction drawing showing a proposed diversion channel, Christie said the sheet piling that has emerged could be materials the original dam builders used to close off a diversion channel in 1921.

"If that's where the diversion channel was, you definitely would not want to go into that area and start mucking around in the embankment," Christie said. "If that's what the drawing shows, you'd want to be careful to stay away from it completely or find out what was there before you went through extensive work."

Christie said a more viable plan to drain a reservoir would be to send the water through the existing dam structure, then lower remaining waters by chipping away at the dam's concrete spillway.

He called the breach at the Brown Bridge dam "very remarkable."

"Dam removal in the United States is not something new," Christie said, adding, "I can't remember ... a single one that has failed during removal."

Lane said the dewatering structure was approved by DEQ officials prior to the project commencing.

Walton said dewatering structures have been used "many times in many places successfully" in dam removals.