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Three dozen NH dams in ‘poor’ condition and eligible for federal aid

  • Penacook Lake, the source of Concord's drinking water, is at a much lower level than usual in late September, 2016.

Concord Monitor
Published: 5/21/2020 9:43:59 PM
Modified: 5/21/2020 9:43:48 PM

Thirty-nine dams in New Hampshire are eligible for federal funding because they’re in poor shape and would create a “high hazard” if they failed.

“These dams ... are by no means unsafe,” said Steve Doyon, chief dam safety engineer for the New Hampshire Department of Environmental Services of the state dams eligible for the program. “They can do their jobs under conditions even where you get a significant amount of rainfall.”

The question is how well historical trends about rainfall totals can measure risk — that is, whether the definition of “significant” is shifting. Warmer air holds more moisture, so global warming means rainstorms have the potential to release more water than might otherwise be expected.

This is the second year of what is known as the Rehabilitation of High Hazard Potential Dams program from the Department of Homeland Security. Money can be used for repairs or even removal of dams, as well as such things as updating floodplain management plans or moving buildings or roads out of harm’s way. The program covers 65% of the cost, with the remainder provided by the state or town.

Last year’s first round of funding produced three projects totaling about $700,000, involving two state-owned dams — Goose Pond in Canaan and Mendums Pond in Nottingham — as well as Upper Wilson Pond dam, owned by the town of Swanzey.

Dams eligible this year include Penacook Lake in Concord, owned by the city; and Pittsfield Mill and Northwood Lake dams, both owned by the state.

Doyon said it was likely the state would apply this year for aid to projects related to Milton 3-Ponds in Milton and Pawtuckaway Lake dam in Nottingham.

New Hampshire has more than 900 dams. Most of them are very small, often left over from the days of water-powered mills or built as retention basins around developments, and privately owned.

Almost 150 are considered high-hazard, meaning their failure would pose a risk to human life. No high-hazard dam has been built in many years, although occasionally a dam’s status is changed because of nearby development.

“Every inspection we do, and we inspect dams on a regular basis, we look downstream to see if anything has changed. If there was a meadow that is now a housing development, the dam may have gone from low hazard to a high hazard, and now we have to apply the high hazard standard,” said Doyon.

The most important measure of a dam is how much water can pass safely through it. If extreme rains raise the level of the water behind the dam faster than the water can pass through then the level might overtop the dam, which can erode the structure and lead to failure.

A high hazard dam must be able to handle 2½ times the outflow of a 100-year rainfall event for that area, Doyon said. In most of the state that 100-year event is 6 to 8 inches in a 24-hour period. That is an extreme amount, since getting even 4 inches in 24 hours is rare in New Hampshire.

“It’s possible but not very likely,” said Doyon.

The state can take action if serous problems are encountered, including forcing the owner to lower the water level, but deciding what action to take isn’t always straightforward.

“We don’t want to apply the same standard to every dam owner if it’s not feasible … or not necessary,” said Doyon.

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