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The White Rock Dam project is designed to improve river connectivity
for recreational boaters and aquatic species. (Fuss & O’Neil)
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Bids for the work are being sought this month and a
contract should be awarded in early January, according to Scott Comings,
associate director of the Rhode Island Chapter of The Nature Conservancy. His
organization is overseeing the project, along with a host of partners.
It will be designed so that a berm in front of the canal
could be crested during flood conditions to allow more water downstream, to
reduce the chance of upstream flooding. The height of that berm is still in the
design phase.
The water level below the dam will remain unchanged,
according to Fuss & O’Neill Engineering firm associate Nils Wiberg, but
upstream levels will drop.
Wiberg spoke at a Dec. 2 public hearing. There were about
55 people in attendance who asked a variety of questions about the dam
removal’s impact on the river and recreational boating.
The engineer told the group that because the level of the
river is so flat in that area, the water levels will drop from 2.5-3 feet at
the dam, from 1.5-3 feet upstream at the Boom Bridge Road Bridge and up to a
half-foot at Potter Hill Dam, more than a mile upstream.
Wiberg said tests on the sediment behind the dam show the
usual levels of volatile organic compounds (VOCs) and metals expected to be
found in a river system dotted with mills, but he noted that the levels are all
below exposure levels established by the state Department of Environmental
Management. That means the sediment will stay on site and be used to form the
new river channel or distributed in the old canal and capped with rock.
If the sediment had exceeded exposure levels, Comings and
Wiberg said it would have meant a costly mitigation process.
White Rock would be the third dam on the Pawcatuck River
earmarked either for removal or modification. The effort is designed to improve
fish passage on the 20-plus-mile waterway that stretches from Worden’s Pond in
South Kingstown to Little Narragansett Bay in Westerly. The work also is
expected to reduce flooding above the site.
About 15 miles upstream, the Kenyon Dam and Lower
Shannock Falls Dam have been removed, while a fish ladder was built at historic
Horseshoe Falls.
A total of $1.9 million in federal Sandy money is
available for the White Rock Dam project, according to Comings. He said that
the engineering contract is for $313,793, and will cover three phases of the
dam’s removal and the first phase of study regarding Bradford Dam, about 6
miles upstream.
The owner of the White Rock Dam is Griswold Textile
Print, 84 White Rock Road, just downstream of the structure. Comings said The
Nature Conservancy and a legal representative of the company are working out
details of the project. The company makes high-end decorative fabrics and
employs 18 people. It doesn’t currently use the dam.
The first dam at the site was built in 1770 and was
timber and rock crib construction. Subsequent structures were built over the
years, said Wiberg, until it was washed out in the flooding associated with the
Hurricane of 1938. The existing cement dam was built in 1940, he said.
All of the structures were designed to hold back water to
feed the granite-stoned sluiceway adjacent to it. The water flow was once used
to power the historic mill. While Griswold Textile formed in 1937, the building
it occupies is much older.
Wiberg said the canal suffered a breach in the 1960s,
resulting in the water flowing back into the river below the dam. Otherwise the
water would have just flowed along the canal and discharged further downstream.
Aerial photos from the 1930s show the existence of a gate at the mouth of the
canal and footbridge across it. Those features have been lost.
Comings said the goal of all these projects is for better
river connectivity. Fish are sometimes unable to swim up the sluiceway/ “It is
a barrier (to fish) the majority of the year,” he said.
Bradford Dam has a fish ladder that was improved several
years ago. The entrance to the ladder was changed to enable fish to find it
easier. But Comings said, “There are some things we could improve in Bradford.”
It’s because the dams in Bradford and Potter Hill have
fish ladders that the White Rock Dam was given priority.
The state is currently raising and stocking herring in
Worden’s Pond and Watchaug Pond in Charlestown. The hope is to imprint the fish
born in those ponds. After they swim downstream to the sea to mature, it’s
hoped they would return to spawn in 2016 and restart a cycle that was lost
years ago because of dammed-off waters.
During construction, access to White Rock Dam is expected
to be via the Cherenzia Company’s property. Cofferdams will be built above and
below the dam to dewater it. The canal will be used to carry the water around
the site.
Wiberg said the new channel will not be straight, but
will meander and have rock features to slow the water and provide resting
places for fish as they move upstream.
Several people at the recent hearing questioned the
impact on recreational boaters. Wiberg said a portage path will be available
during construction to accommodate boaters. Details on how that will work
haven’t been finalized.
While water levels will drop in the area, project
officials say the flows should be adequate for fish migration during the
spring.
