August 01. 2013 10:52PM

Nashua River dam work set to begin

Union Leader Correspondent

The Nashua River has been lowered about 10-feet to allow the construction of an adjustable crest gate at the Jackson Falls Dam. (KIMBERLY HOUGHTON/Union Leader Correspondent)

NASHUA — An old shopping cart, used tires and orange traffic cones are just a few of the items found along the edge of the Nashua River, which is exposed from the water being temporarily lowered.

In the past week, the river has been intentionally lowered about 8 to 10 feet to allow crews to start making significant improvements to the Jackson Falls Dam. As part of the Cotton Mill Square project, a multimillion dollar housing plan, an adjustable crest gate for the 180-feet dam will be constructed to decrease the water level of the Nashua River by about four feet during flooding.

Now that the river has been lowered, an old, temporary cofferdam is exposed near the downtown area of Margarita's Restaurant. According to Thomas Galligani, the city's economic development director, crews from H.L. Turner Group of Concord are in the process of reinforcing the temporary dam.

Once that area is completely pumped out and left to dry, it will become a construction zone within the next day or two, he said. Then workers will begin taking off the top 10-feet of the Jackson Falls Dam.

A new base will be constructed, along with the new, adjustable crest gate, said Galligani.

The existing dam operates as a run-of-river dam, not a store-and-release dam. The new crest gate, however, will use a compressed air piping system and steel plate operation to inflate or deflate the gate automatically when necessary, coordinating with the rise and fall of the river.

"This will take Cotton Mill out of the floodplain, along with countless other properties along the water's edge," Galligani said Thursday. "It is an interesting project because it is so visual."

Despite some of the debris that is now visible along the edge of the river, Galligani said it is better than he expected.

"This is an opportunity to clean some of that up," he said.

On Wednesday, Greg Andruskevich and Joshua Segal of the Mine Falls Park Advisory Committee cleaned up trash that washed ashore because of the lower water levels.

An old shopping cart from Alexander's supermarket, which is no longer in operation, was located by the cleanup crew, along with tires, a bike seat, handlebars, an orange traffic cone and many beer bottles and cans.

According to Andruskevich, individuals have been spotted canvassing the exposed riverbank with metal detectors.

"I met a family out there that found an old Christmas bell during their search. There is just so much junk in there," he said. "The water is the lowest I've ever seen it. Surprisingly, though, it is not smelly at all. The water itself is very clean."

To lower the 100-year floodplain zone and solve future downtown flooding, the city entered into agreements last year with Nashua Hydro Associates and Cotton Mill Square, LLC, for the $1 million dam renovation.

The adjustments to the dam will pave the way for the Cotton Mill Square project to begin, which includes revitalizing a 108-year-old historic building on Front Street, about 100 apartment units, contamination cleanup and a riverwalk.

The separate $26 million mixed-income housing project by developer John Stabile has already received approval from various state and federal agencies.

The Jackson Falls Dam is owned by the City of Nashua, but is leased to Nashua Hydro Associates until 2014. Construction on the dam should be completed by the beginning of October, according to Galligani.