The dams that must not fail
Due to the potential danger, the dams are consistently monitored by a combination of private, state, and in some cases, federal inspectors to ensure that everyone living around them is safe.
Jim Gallagher, chief of the Dam Bureau, said dams are classified as high hazard not because they are structurally unsound, but because of the risk they would pose to the surrounding area should they ever fail. For a dam to be classified as high hazard, only one person's life has to be in danger were it to fail. Once classified as high hazard, the dam's owners must develop and maintain an emergency action plan. Also, the state must inspect the dams at least once every two years.
Gallagher said Nashua's four high-hazard dams, the Jackson Plant Dam, which is owned by the city but leased to Essex Power Services Inc., and the Bowers Dam, Harris Pond Dam, and Supply Pond Dam, which are all owned by Pennichuck Water Works Inc., are in good condition and have responsible owners.
"High hazard does not mean high danger," Gallagher said, while adding, "Pennichuk is a very good, conscientious dam owner. (Essex) is another good dam owner. They both want to keep their dams in good shape for both safety and revenue reasons."
If the Bowers Dam were to go, eight houses and the residents within them would be in danger, making it potentially the most destructive dam in Nashua. The Harris Pond Dam threatens four houses, while the Supply Pond Dam threatens three. If the Jackson Plant Dam were to fail, a Margarita's Mexican Restaurant would be in danger.
John Patenaude, chief executive officer at Pennichuck said his company does not just rely on state inspectors to say that the dams, which are designed for water supply, are safe.
"Our folks go out there in storms, we adjust the overflows so water can come through without jeopardizing the dams. We are conscious of storms and bad weather, and make sure the dams are in good shape," Patenaude said.
Gallagaher said that while the Jackson Plant Dam falls under federal jurisdiction, state inspectors accompany federal inspectors when they visit the dam, and it is in good shape.
Andrew Locke, vice president for Essex, said the dam, which has a one-megawatt facility on-site, is inspected once a year because it falls under federal regulatory jurisdiction. He added that Essex is working with the city to modify the dam to achieve greater control over water levels during flood conditions.
Of the roughly 2,600 dams in the state, 137 are classified as high hazard. Gallagher said the Dam Bureau has a team of four inspectors and one supervisor and an $800,000 annual budget to ensure that dams in the state that pose a threat to lives and personal property are safe. Along with inspecting dams, Gallagher said the Dam Bureau also has a team that repairs and maintains 278 state-owned dams.
In Hudson, there is only one dam classified as high hazard, but the town is currently developing plans to replace the dam with a bridge after being told by state officials the dam wasn't strong enough.
The Pelham Road Culvert, which is owned by the town, is a roadway that doubles as a dam. Department of Environmental Services Dam Bureau Chief Jim Gallagher said that as a result of state inspections, the town was informed the culvert that allowed water to flow through the road embankment does not have enough water discharge capacity, while the road itself is not strong enough to handle a strong flood flowing over it.
Gallagher said if conditions are right, a situation similar to the flood in Alstead in 2005 that washed away an entire road could occur in Hudson.
Instead of strengthening the dam, Hudson officials made the decision to replace it with a bridge, which Gallagher said "is fine with us."
Patrick Colburn, town engineer, said that since the town is enrolled in the state bridge program, the cost to construct the bridge would be shared by the town and the Department of Transportation.
"Certainly high-hazard dams are a concern," Colburn said.
Colburn added that the town has already hired someone to design the new bridge, and Gallagher said he has been informed that the town plans to advertise the project for construction later this year, with an eye toward beginning work in the summer of 2014.