TRAVERSE CITY — An email sent to a state official describes an engineering design modification to the Brown Bridge Dam removal project two days before a breach flooded dozens of Boardman River residential properties.
The Record-Eagle obtained the emails this week. They document an exchange between Sandra Sroonian, an engineer who supervises the Brown Bridge Dam removal project, and Lucas A. Trumble of the Michigan Department of Environmental Quality's Hydrologic Studies and Dam Safety Unit.
Sroonian wrote to Trumble on Oct. 3 and requested a design change to the dam removal project. The change was necessary because construction workers unexpectedly encountered a “prior/historic structure” buried deep underground at the dam site, she said.
The discovery was made as workers built a channel-like device adjacent to the dam known as a dewatering structure, which was supposed to allow for a slow, gradual draw-down of Brown Bridge pond through an earthern embankment adjacent to the dam.
Sroonian told Trumble that as workers built the dewatering device in the embankment they drove sheet piling into the ground and struck a buried “historic structure” at the spot where the dewatering device was supposed to go.
“While driving sheet piling in the vicinity of the drawdown structure, resistance was unexpectedly high, requiring use of heavier equipment than anticipated,” Sroonian wrote to Trumble. “During subsequent excavation of the area at the dam where the drawdown discharge channel will be constructed, the cause of the high resistance was visibly observed.”
Sroonian said she and others saw parts of the buried historic structure sticking up from the bottom of the planned channel. She described what she saw as “cobbles/boulders embedded in stiff clay along with foundation materials, including steel sheet pile from a prior/historic structure.”
The discovery prompted Sroonian to ask for permission to make a change to the project design. She requested and received permission from Trumble to raise the elevation of a proposed concrete slab below the dewatering device by two feet, without having to modify a project permit issued by the DEQ.
Trumble agreed to the change in an Oct. 4 email, and noted "modifications to the drawdown structure do not require a revision to the permit."
The dewatering device failed the morning of Oct 6. Brown Bridge Pond raced into the Boardman River, boosted the river level several feet and flooded to properties along the river. An investigation into the breach's cause continues.
Byron Lane leads the Michigan Dam Safety Unit and is Trumble's supervisor. He also supervises the breach probe.
On Thursday he said "We don't think so" when asked if it appeared the design modification caused the dam breach. But Lane said officials have discussed a potential role played by the old structure beneath the dewatering device.
“We’ve certainly talked about that as part of the investigation," he said. "It will be addressed in our report.”
Lane said his office awaits lab test results on soil borings before finalizing a report on the breach's cause.
The Record-Eagle in October reported on an old construction drawing of the Brown Bridge Dam from the 1920s that showed the original dam designers planned to build a chute or channel next to the dam. The channel is marked on old construction drawing with 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 built in 1921.
Sroonian told the Record-Eagle in October that project leaders were aware there might be an old, underground diversion channel, given the construction drawings. But they did not know for sure if the channel had been built.
"It says 'proposed'," Sroonian said of the old construction plans. "We aren't 100 percent certain it was even constructed."
One expert on dam safety and construction, Frank Christie of Onekema, questioned the use of a dewatering device to drain a pond for a project of the Boardman's magnitude. If project managers suspected they'd hit an old diversion channel during dewatering device construction, they should have immediately stopped the project to study the situation, he said.
"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."
Chuck Lombardo, spokesman for the Boardman River Dams Implementation Team, said project leaders look forward to the DEQ's final report on the breach cause.
“We have all along been hesitant to speculate," Lombardo said. "They are looking at the historic structure and that’s just one of many things they are looking at."