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October 20, 2012

Dam answers start to flow in

Why the breach happened still is unknown, how is becoming clear

TRAVERSE CITY — City Manager Ben Bifoss used a piece of chalk and an old blackboard to offer what might be the clearest public explanation thus far of what happened to dump the contents of Brown Bridge Pond into the Boardman River.

"I can tell you what happened, but I can't tell you why," Bifoss told Marge Forgione, a Brown Bridge Advisory Committee member who had gathered with others in a pole barn this week to discuss the flood.

Forgione had asked if someone might show her what failed at the dam site.

Bifoss drew a rough, overhead sketch of the Brown Bridge Pond dewatering structure, including the dam, a 15-by-170-foot steel dewatering structure, and four "turbulences" of swirling muddy water that caused serious flooding and erosion inside and outside the structure.

The breach occurred Oct. 6 about 10 a.m., after workers who are removing three Boardman River dams as part of a river restoration project lifted a steel sheet from a temporary wall that separated the 20-foot-deep pond from an 18-to- 20-foot wall of stop logs at the structure's west end.

Project managers intended to move two 7.5-foot stop logs later that day to begin the slow three-week drain of the pond. The first indication of problems came when workers outside the structure saw a three-foot drop in the pond about 10-20 minutes after the metal sheet was removed.

Next, water bubbled and rushed up from the river bottom at the end of the structure.

Then, another "turbulence" appeared between the stop-log wall and the temporary steel sheet wall, Bifoss said.

Outside the structure, two more turbulences began to erode the structure's north side.

On the south side, a 25-foot sheet pile wall that originally extended from the now demolished dam powerhouse broke in two places. That allowed torrents of pond water to race along the structure's south side and into the river.

Molon Excavation general contractor Mike Walton operated the backhoe on the eroding north side. Another backhoe was moved to the south side, and over the next several hours construction crews dumped loads of concrete and rock to stabilize the structure and slow the flow into the Boardman.

Steve Largent, Boardman River program coordinator for the Grand Traverse Conservation District, said the breach and flooding around the north end "could have been twice as bad as it was, if Molon crews hadn't gotten control of it."

The Brown Bridge Advisory group oversees preservation of the city's 1,310-acre quiet area and meets several times each year. It was the group's first regular meeting since the breach.

Meeting attendees asked if the city plans to hire an independent inspector to investigate what caused the breach.

Bifoss said the city had no plans to do so. Inspectors from the project's engineering contractor and other agencies involved in the dam removal are expected to do the inspections.

The Department of Environmental Quality is taking the lead on the inspection process, he added.

City Commissioner Jim Carruthers, an advisory committee member, noted that the DEQ has been deeply involved in the project for years and questioned whether their oversight was a good idea.

David Downer, a river resident and committee member, said the Boardman River took a "big hit" from the breach.

"This is not to be trivialized," Downer said. "This is bad for fish and bad for people. I'm not a fish biologist, but I don't believe that the fish can live day after day of having that silt run through their gills. At my house, the water used to clear after a big rain like we've had this week, but it hasn't cleared."

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