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N.H. dams not nearly as big a problem as the dam in California

  • The view from atop the Hopkinton-Everett dam looks down on the spillway into the Contoocook River along Route 127. Issues with the Lake Oroville Dam in California have raised concerns about dams in the Granite State. GEOFF FORESTER photos / Monitor staff

  • The Contoocook River is seen through Rowell’s €Covered Bridge off Route 127.

  • The Contoocook River off of Route 127 that is used as a spillway from the Hopkinton-Everett dam is seen. A few New Hampshire dams have concrete spillways, including those associated with the Hopkinton-Everett flood control dams, but none near the size of Lake Oroville Dam in California. GEOFF FORESTER / Monitor staff



Monitor staff
Thursday, February 16, 2017

The narrowly avoided disaster at the huge Lake Oroville Dam in California has raised the question of whether something similar might happen in New Hampshire, which saw a fatal dam collapse 19 years ago.

“Anything is possible, but we really don’t have a situation like that,” said Steve Doyon, administrator of the Dam Safety and Inspection section of state Department of Environmental Services.

The Lake Oroville dam north of Sacramento, Calif., is 770 feet tall, the highest in the U.S., and has been overwhelmed by record heavy rains. That caused excess water to be released this weekend into an adjacent emergency spillway made of interconnected concrete slabs, which was used for the first time.

Some of the concrete crumbled, leading to fears that the torrent of water would erode the hillside and cause the entire dam to fail. More than 200,000 people living downstream were evacuated for two days until the water level abated slightly.

New Hampshire has more than 900 dams, almost 150 of which are considered high-hazard, meaning their failure would pose a risk to human life. The state has about 900 dams all told, most of them very small.

Even though New Hampshire has been dealing with dry conditions, the threat of flooding is present every spring as rain lands on top of frozen ground. The most severe flooding in the state occurred a decade ago, in 2006 around Mother’s Day and in 2007 in mid-April, causing evacuations but no fatalities.

A few New Hampshire dams have concrete spillways to deal with runoff, including those associated with the Hopkinton-Everett flood control dams, but none are anywhere near the size of Lake Oroville.

The state’s tallest dam is the Moore Dam on the Upper Connecticut River in Littleton, which is 193 feet tall, barely one-quarter the height of the Lake Oroville Dam. Like the Franklin Falls Dam on the Pemigewasset River in Franklin, it is owned and operated by the U.S. Army Corps of Engineers.

The tallest state-owned dam is the Murphy Dam in Pittsburg, which is 106 feet tall.

These dams hold back far less water than Lake Oroville in California, whose lake totals 3.5 million acre-feet, a common measure of storage capacity. By contrast, Doyon said, Lake Francis, which is formed by the Murphy Dam, holds about 100,000 acre-feet.

But it doesn’t take much water for a dam failure to be dangerous, as New Hampshire found in 1996 when the privately owned Meadow Pond Dam in Alton failed, killing a woman whose pickup truck was swept off Route 140 into a ravine. That dam was made of earth and was overwhelmed by heavy rains.

The state regulates 110 dams that it considers “high hazard,” meaning people might die if the dam fails, and the Corps of Engineers regulates a couple dozen more.

The state has about 220 dams that are rated “significant hazard,” which would cause great loss of property and damage to major roads but not necessarily loss of life if they failed, and about 550 that are rated “low hazard,” posing little threat of damage to property, Doyon said.

Many dams in New Hampshire are “run of the river” dams that don’t hold back large amounts of water but instead use the river’s flow to produce power, such as the Penacook Upper Falls Dam and the prominent Amoskeag Falls Dam in downtown Manchester.

Of the dams that hold back water, creating ponds or lakes, many were built after World War II as flood-control projects.

Such construction wound down three decades ago, ending with the Gunnison Lake Flood Control Dam in Goshen, which opened in 1983.

No high-hazard dam has been built in New Hampshire since, although Doyon said some dams have had their status upgraded to high hazard because new development has been built downstream.

“We inspect dams on a schedule: High hazard, every two years; significant hazard, four years; low hazard every six years,” Doyon said. Inspections can be moved up by events and the state Dam Bureau can issue an administrative order mandating repairs, as happened with a downtown levee in the town of Lincoln that was damaged by floods due to Hurricane Irene in 2011.

That situation, like the dangers with Lake Oroville in California, reflect another concern: an increase in what meteorologists call “extreme precipitation events” caused by the changing climate that put more stress on dams than they were designed to handle.

Still, Doyon noted, no dams had serious issues in New Hampshire during some recent major floods, which may reflect heightened standards since the Meadow Pond Dam failure.

But there’s no getting around the fact that many dams are old.

“Many have gone past their service life. That doesn’t mean changes of failure has increased – but when you have an old dam, you’re going to have a few more problems,” Doyon said.

Nationally, dam safety officials have raised concerns about the need to spend more on maintenance for these often-overlooked parts of the country’s infrastructure.

(David Brooks can be reached at 369-3313 or dbrooks@cmonitor.com or on Twitter @GraniteGeek.)