As heavy rainfalls increase, how can Penacook Lake’s dam adjust?

  • Marco Philippon, city water treatment plant superintendent, at the spillway for the Penacook Lake dam.

  • Marco Philippon, city water treatment plant superintendent, stands at the structure at the top of the dam built in 1872. GEOFF FORESTER—Monitor staff

  • Marco Philippon, city water treatment plant superintendent, at his office at the plant on Hutchins near the dam of Penacook dam. GEOFF FORESTER—Monitor staff

  • The 2500 acre Penacook Lake that provides water for the city of Concord. GEOFF FORESTER—Monitor staff

  • The small spillway for the dam. GEOFF FORESTER—Monitor staff

  • Marco Philippon, city water treatment plant superintendent, stands at the structure at the top of the dam built in 1872. GEOFF FORESTER—Monitor staff

  • The 2500 acre Penacook Lake that provides water for the city of Concord. GEOFF FORESTER—Monitor staff

  • Marco Philippon, city water treatment plant superintendent, takes a tour of the dam built in 1872. GEOFF FORESTER—Monitor staff

  • Marco Philippon, city water treatment plant superintendent, stands at the structure at the top of the dam built in 1872. GEOFF FORESTER photos / Monitor staff

  • During the Mothers Day flood of 2006, water poured over the spillway of the city dam. Courtesy of Marco Philippon

  • The pink area on this map shows where water would go if it overtopped the dam and spillway at the east end of Penacook Lake, on the left of this map, and flowed down to the Merrimack River, on the right. The blue area shows areas that would be flooded. City of Concord

Monitor staff
Published: 6/17/2020 6:32:11 PM

Because Penacook Lake in Concord has no development along its banks and is almost entirely hidden from view, it’s easy to forget how big it is. Here’s a reminder: On a sunny day it loses a million gallons to evaporation alone, which is about one-fifth the amount being used by all of Concord.

Large size is a good thing, of course, since the lake provides drinking water for the city and some nearby communities. But it would become a bad thing if the earthen dam that holds back the water should ever fail.

According to engineering studies, in that case some 15 homes and other buildings would be inundated by up to 1.5 billion gallons of water rushing downhill toward the Merrimack River on the other side of North State Street. That includes the Mill Place West apartments in a former mill that straddles the lake’s outlet, Rattlesnake Brook.

Another 18 buildings, among them West Congregational Church, are in the “secondary inundation zone” and would be badly harmed.

That isn’t an immediate concern since the 250-foot-wide dam has been there since 1872, when the lake was created by joining Forge Pond with Long Pond, and continues to pass all inspections. But as extreme rain events become more common in the climate change era, what was good enough in the past may not be good enough in the future.

“The (New Hampshire) Dam Bureau has gone out to all high-hazard dams and said … we need you all to take another look with new information that we have on 100-year and 500-year rain events. Now that we’ve been living her for a while, and collecting data, what we thought was 100-year and 500-year isn’t,” said Marco Philippon, superintendent of the city’s water treatment plant alongside the dam, on Hutchins Street.

The big question: “Can your spillway handle this new environmentally changed amount?”

Spillways

Most dams have spillways, openings that are lower than the dam but higher than the usual water level. When the lake or pond rises the spillway releases water before it can pass over the dam itself, known as overtopping. Overtopping eats away at the structure of dams, particularly earth-and-rock dams like Penacook Lake’s, and is the main cause of dam failures.

As a result, having a big enough spillway is vital to dam safety. But what is big enough?

That’s the question being faced by Penacook Lake dam and 31 other of New Hampshire’s “high hazard dams,” a term that means there are “potential life implications” if they fail. They are eligible for federal money under the Rehabilitation of High Hazard Potential Dams program from the Department of Homeland Security.

The dams were rated “poor” but in Penacook Lake’s case that’s not because the structure is in bad shape. It means that it’s not clear the spillway can handle the amount of water that would be produced under new guidelines for extreme events.

Those new guidelines for Concord say the dam should be able to handle 8 inches of rain within 24 hours. This is a huge amount: Only six times in 154 years has the city seen even half that much, and the most rain ever recorded in Concord within 24 hours is 5.98 inches, according to National Weather Service records.

But there is great concern that climate change will upend these records. Warm air holds more moisture than colder air, so as global temperatures there is more likelihood of heavy rainfall. One estimate published this week in Proceedings of the National Academy of Sciences, said that by 2100 North America could be seeing what are now 100-year events every 2½ to 3 years.

What to do about it

The Dam Bureau is reviewing city documents now to see if their engineers agree with analysis of how much water the spillway can handle. After the numbers will come decisions about what, if anything, to do.

The spillway dates back to 1974, when it was built along with the city’s first water treatment plant.

It proved its worth in the the Mothers Day flood of 2006, when Penacook Lake rose to within inches of overtopping the dam. So much water poured through the spillway that it overflowed the granite-lined channel and filled up the grassy embankment around it, but the dam was safe and water never flowed onto Hutchins Street or nearby buildings.

But even though the spillway worked as it should, that event showed a weakness of the design, Philippon said.

Dams and spillways are controlled by boards on the top that can be removed to allow more water to pass through. These boards are often 3 or 4 feet tall, but on Concord’s spillway they’re just 18 inches tall. That provides very little flexibility for releasing water in advance when storms are coming.

One option would be to enlarge the embankment around the spillway to provide more area for water to collect safely. That would be difficult because the spillway is squeezed between the water treatment plant and Hutchins Road.

Another solution is to raise the dam, which is only nine feet fall, by perhaps three feet. Then the spillway could be raised and the current embankment deepened, providing more flexibility and more water storage.

Concord is lucky in that unlike many cities that depend on lakes, such as Manchester, it owns all the land around its water supply. That’s why there is no development along the banks: The city has blocked it to keep the water clean.

No development

Penacook Lake was ringed by summer camps through the 1940s when they were removed because of pollution from outhouses, and St. Paul’s School long ago moved its rowing team to Turkey Pond, although the concrete boat ramps remain at the lake’s western end. No boating or swimming is allowed on Penacook Lake, the main reason it is one of the few large lakes in New Hampshire that have not been infested with Eurasian milfoil.

If the water level was allowed to rise two or three feet because of a higher dam, all that would happen is that the roughly 6 miles of banks around the lake would be more flooded. That would be no problem, with one exception: Lake View Drive and the intersection with Long Pond Road at the lake’s southwestern end would also be flooded.

“We would have to raise the road,” said Philippon.

After the Dam Bureau approves the city’s calculations about the effects of the new 8-inch standard for extreme precipitation will have on water amounts, engineers will model what that could do to the dam, spillway and surrounding watershed if it happens when the lake is full.

“Then it gets modeled, (figuring) here’s what it can handle, here’s the gap. Then we get to the second phase, which is do we or don’t we have to do anything, and what? … Which is more expensive, which is less expensive, which works best?”

There’s no great rush to make a decision, since final plans don’t have to be approved until 2021.

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


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