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Sunday, July 14, 2013

Divers take plunge as Jackson Falls Dam work begins in downtown Nashua

There will be a few more days before the banks of the Nashua River downtown become visible as work finally begins on Jackson Falls Dam.

On Friday, divers from Commercial Divers Plus, of Lancaster, were in the water around Margaritas Mexican Restaurant, working in and around a massive tube some 8 feet in diameter that carries water from one side of the Jackson Falls Dam to the other, turning an electricity-producing turbine as it goes. ...

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There will be a few more days before the banks of the Nashua River downtown become visible as work finally begins on Jackson Falls Dam.

On Friday, divers from Commercial Divers Plus, of Lancaster, were in the water around Margaritas Mexican Restaurant, working in and around a massive tube some 8 feet in diameter that carries water from one side of the Jackson Falls Dam to the other, turning an electricity-producing turbine as it goes.

In a process that included having a diver maneuver a gigantic plug as big as a bathtub, they sealed off the tube at both ends. It wasn’t easy.

“The visibility might be 5 feet, 6 feet … with a light,” said Steve Gallipeau, owner of Commercial Divers Plus, who has done underwater construction work for decades. “You’re weightless –
it’s like working in outer space. You go down there and try to turn a bolt; you turn, the bolt doesn’t move. … The nice thing is, you never need a ladder.”

After the turbine tube is pumped dry, the 1-megawatt turbine will be removed and the tube reopened, allowing the river to flow out, sharply dropping the water level upstream from the dam.

A Telegraph story Friday said incorrectly that such a drop had already begun, but neglected to factor in the preparatory turbine removal.

The big drop in river levels probably won’t get going until next week.

The work was supposed to start a few weeks ago, but the heavy rains in June made the river too high for it to proceed.

The fall in water level will expose at least 7 feet of the upstream side of the concrete dam. The total dam is about 33 feet high and roughly 180 feet wide.

Much of the exposed portion of the dam will be removed and replaced with concrete topped by adjustable “crest gates” that allow more specific control of upstream water levels. That work will take many weeks; it must be finished by Oct. 1 under current schedules.

The dam work is being done as part of the Cotton Mill Square project, which will turn a former mill building into 109 market-rate and subsidized low-income apartments.

Lowering and altering the dam is necessary to make that development eligible for federal tax credits by removing it from the 100-year floodplain.

The Jackson Falls Dam was built in the 1830s and upgraded many times to power Jackson Mills, where Margaritas is now housed.

Nashua bought the dam in the 1980s, and it has generated hydropower since then, with part of the river flowing through a long tube that has a turbine at one end.

The crest gates, controlled by inflatable tubes, allow much more control to deal with flash floods and should reduce the chance of water levels rising upstream, including near the Cotton Mill development.

Similar crest gates exist on the large Merrimack River Dam in Lawrence, Mass., and have produced debate on a large dam in Lowell, Mass., where opponents decry their appearance in the Lowell National Historical Park.

The Cotton Mill project will also add 1,200 feet to the Nashua Riverwalk.

David Brooks can be reached at 594-6531 or dbrooks@nashua
telegraph.com. Also, follow Brooks on Twitter (@GraniteGeek).