GUILFORD — Residents overwhelmingly preferred rehabilitating the Sweet Pond dam over replacing the 1920s-era structure with a new dam during a public comment meeting last week.
Engineers from firm DuBois & King and members of the Department of Forests, Parks, Recreation (FPR) presented five potential scenarios for the dam at Sweet Pond State Park.
Last year, engineers from the state Dam Safety Section deemed the structure unsafe. The state prefers demolishing dams like Sweet Pond, since removal saves the state money and reduces the hazard of loss of life or property damage downstream if such dams are breached.
A number of Guilford residents committed to saving the pond, the town's only swimming and recreational area, have worked with employees of FPR for almost a year. Residents told engineers that Sweet Pond has been dammed since the 1700s.
Ethan Phelps, parks regional manager with FPR, told the audience at the Feb. 9 meeting that the state was pleased with residents taking proactive steps to deal with the dam's unsafe condition.
As a precaution, the state drained Sweet Pond last summer to relieve pressure created by the equivalent of 18 acres of water from the historic dam.
Alternatives
The five design alternatives included taking no action, rehabilitating the existing dam, replacing the structure, lowering the water level and rehabilitating the dam to a lower height, and removing the dam.
“Taking no action is not a viable option,” said Shawn Patenaude, P.E., senior dam engineer with DuBois & King.
According to Patenaude, the dam's level of instability means that the dam could collapse. “Although there's no current problem, there's telltale signs of instability,” he said.
The Sweet Pond dam's dry laid masonry construction acts as an earth embankment that shows signs of crumbling, said Patenaude.
In addition, the dam's foundation sits on weathered bedrock. Water flows through the horizontal layers of the rock acting as a lubricant so the unanchored dam “floats.”
Water can also seep into the dam's dry laid masonry, Patenaude warned. The downstream side of the dam bulges out because the dam's internal materials are “starting to bleed through the rocks.”
Patenaude told the audience of almost 40 residents that the next option was rehabilitating the dam in place, for an estimated $330,000 with a 50-year design life.
Engineers would anchor the dam and cast a new concrete wall (unnoticeable once the pond refilled) on the upstream side, one that would withstand the water's weight and remove pressure from the older dam.
By contrast, said Patenaude, replacing the dam with a “concrete gravity dam” would cost about $631,000 with a design life of 75 years. He described a gravity dam as a “mass of concrete anchored into the bedrock.”
Audience members rejected the remaining two alternatives: making the level of Sweet Pond and its dam low enough that it would no longer meet the state's definition of “dam,” or removing the structure outright.
Timothy Morton, stewardship forester with FPR, reminded audience members that the state has the final say on the dam's future. Even if FPR chooses to rehab or replace the Sweet Pond dam, there is no guarantee the state will permit or fund either option, he said.
Of the 31 audience members who voiced a preference, about 25 said they wanted to see the dam rehabilitated. People cited maintaining the dam's historic character, the lower project cost, and the potentially easier permitting process as reasons for their choice.
Residents learned the state wanted to remove Sweet Pond dam March 2011 during a public meeting to present the Long Term Management Plan (LTMP) for Windham County's four state parks.
State Dam Safety Section engineers had informed FPR the Sweet Pond dam had earned a “poor condition” rating because it was unstable and the spillway deficient. The dam also has a “High Hazard” rating because of its location upstream from six houses and the potential loss of life and infrastructure if the dam breaks.
“Older dams aren't built to the same standards as today's [dams],” said Dam Safety Engineer Steve Bushman in a separate interview.
Bushman, who works for the state's Dam Safety Section, said the two dam-break analyses the engineers performed showed the potential for significant risk to people and houses downstream.
The “sunny day” analysis, which studies the damage of a normal water level in the pond, revealed three downstream residences would be “inundated” with water. The second analysis, “storm day,” reviews the worst-case scenario like a 100-year flood and demonstrated that water would damage six residences.
The LRMP stated that the dam's safety rating over the past few years has bounced between fair and poor, despite extensive repairs from 1986 to 1988.
FPR will take written comments on DuBois & King's five design alternatives until Friday, March 9. Comments may be sent to Morton at [email protected].
According to Morton and Phelps, Parks Management will review comments and consult with fellow Agency of Natural Resources departments before finally selecting a “preferred alternative.”
Staff will then recommend that choice to FPR Commissioner Michael C. Snyder, who will make the final selection.
To read a copy of DuBois & King's dam analysis, visit: www.vtstateparks.com/htm/sweetpond.htm.