Wentworth Pond drawdown reduced in scope, DES staff exploring dam restoration or replacement

ALTON — A temporary drawdown of Wentworth Pond, affecting the Merrymeeting River and Lake Watershed, has been reduced 12 feet to 2.5 feet, after town leaders and residents voiced concerns to New Hampshire Department of Environmental Services staff at a hearing Aug. 11.

This drawdown is intended to identify effects of repairing or rebuilding the Alton Power Dam, which has been deemed in “poor” condition.

Town leaders and members of the public were dismayed by what they learned at the hearing. Many said they were blindsided by the information, with concerns about the area’s environment and property values being affected.

“The public and the board’s interests and concerns are aligned,” Town Administrator Ryan Heath said. “I feel the community is united in their efforts to preserve the dam. I think ultimately, if it is feasible, the town will pursue ownership of the dam.”

Heath said the town is looking into all avenues after hearing from Corey Clark, chief engineer of the NH DES Dam Bureau, about the project.

Public hearing

The drawdown came before the selectboard in the spring, proposing to lower of 2.5 feet of Wentworth Pond behind Alton Power Dam. This was for data collection, passive restoration, and to provide for additional storm discharge capacity at the site.

Clark said a couple issues came up in the spring, with one being sediment testing. Given ice conditions and timing, it needed to be postponed, so they could collect sediments efficiently. There were also concerns about downstream drinking water wells for the waterworks, which he said are the gravel pack wells near the baseball field.

Since the postponement, NH DES staff have been collecting sediment samples, which are currently being reviewed at the state laboratory. Additional wetland and wildlife studies have been done and are under review, received at the end of July.

Two production wells have been monitored, and another well near the public works department is being monitored. A data logger is at the site, as well. Clark said they have baseline data, and NH DES staff are working with the water department to get baseline data, so when the pond is drawn down on Sept. 2, they can identify any impacts.

“That drawdown is going to be a little bit different than what we proposed this spring,” he said.

In the spring, NH DES staff were going to remove the stoplogs. There are 2.5 feet of stoplogs across the top of the spillway, which would have been removed to keep the pond at that elevation.

To find out what will happen with the drinking water wells, Clark told the board they would be completely lowering the water body. Clark said they would remove the stoplogs, then open the low-level gate to bring the water down.

“It will probably be somewhere around 12 feet down from its current water surface at the dam,” Clark said. “That will translate upstream a little differently, given natural constrictions that are up by the Route 40 bridge, and further upstream.”

Clark said the water will remain at that level for one to two weeks, and they will collect “good data” in the monitoring wells to see if there is an impact to the drinking water supply.

“We will be surveying a number of different areas, starting from the dam, all the way up to Route 28, to figure out what’s going to happen to the marsh and the river system as a result of lowering the impoundment completely,” Clark said.

After the two-week period, the gate would be closed, and the pond brought up to the 2.5-foot drawdown mark. He said there would be noticeable impacts throughout September and into October, which should settle later in the fall.

Clark’s intention is to come back to the board at the end of the year with findings. He said the contract with the Dam Bureau’s consultant expires, as well as its funding, at the end of the year.

“After the end of this year, it will probably be a little bit of a delay until we can get some capital appropriation money, through the state Legislature, for continuing additional studies and design, and ultimately going to construction” Clark said.

Clark said, in June, he presented to the state Capital Budget Committee for a request of $750,000 to continue the design and get a shovel-ready project for Alton Power Dam and Jones Pond Dam. There was another ask for $750,000 to finish the Merrymeeting Dam Design.

Clark noted there was a request for $7 million to eventually reconstruct the gates at Lakeport Dam, as well as the measuring flume that records velocities coming out of Lake Winnipesaukee.

Selectboard, Water Dept concerns

Selectboard member Drew Carter said the project doesn’t seem reasonable to him, especially since results are still not available from sediment testing.

Clark said there were results which showed high metal concentrations downstream, albeit low, but common for industrial areas. He said these will all factor into the decision-making process of future efforts of repairs or rebuilding a dam.

“It seems to me, if it is one to two weeks, and it takes you a couple weeks to get results, well, that’s too bloody late,” Carter said.

Carter asked why the Dam Bureau decided on a drawdown, and the news “surprised everybody” when the town was notified. Clark said it was based on concerns about drinking water supply. He said even if the Alton Power Dam is reconstructed, the pond will be drawn down in that time. It was drawn down in 1986, when the dam was reconstructed, as well.

“Without doing this, there’s no way to know whether there are going to be impacts upstream,” Clark said.

Board Chair Paul LaRochelle said people are very concerned with their property values and how the state will address this. Clark said they do not get involved with property values.

Selectboard member Richard Shea said with testing incomplete, this sounds like a “test run.” He also said Clark said funding is gone at the end of the year, and it seems like the state is now rushing the project through so they can spend money while they have it, instead of lining up for next year to fight for funds. He said it seems like they are “rushing headstrong without the facts.

“I think that’s not right for this town,” Shea said.

Clark said while he is sorry it seems that way, that it is somewhat the case, because federal funds need to be spent.

Clark said the concern is the dam is so deficient that if a catastrophic event occurs, including what he called a “thousand-year storm,” the dam may not hold. While some in the meeting audience said there were big storms in recent years, and the dam was fine, Clark noted a storm brought 11 inches of rain to Albany. A thousand-year storm threshold is defined at 10.9 inches, Clark said.

Water Superintendent Courtney Mitchell said it was a “sad response” by the state, and it is the state’s job to keep up with their infrastructure, just as it is hers to keep up with Alton’s.

“Clearly, the state has dropped the ball,” Mitchell said.

Post-meeting response

“Where we are at right now is that we have several avenues that legal is pursuing,” Heath said Monday morning. “They are all different, between the classification of a high-hazard dam, to changing regulations of a thousand-year flood, to the state rushing the drawdown, and to potential damages that could be suffered.”

Heath said since the state owns the Alton Power Dam, there is a good chance it could be a “tie goes to the runner” scenario.

“The best thing I have been able to tell residents now is that we are under a clock, and actively pursuing every angle all at once,” Heath said.

Heath said since the entire Merrymeeting River and Lake Watershed will be affected, he has also been in contact with selectboard members and state representatives from New Durham. He heard members of the Merrymeeting Lake Association have retained counsel, as well.

Updated drawdown changes

The drawdown begins Wednesday, and will be the originally discussed 2.5 foot drawdown rather than the temporary drawdown of 12 feet.

Clark sent a letter to the Alton Selectboard on Monday, writing there were key reports unavailable on Aug. 11 from NH DES and Fish and Game. After reviewing that data, the decision was made.

Clark stated in the letter that hydraulic analyses conducted for the dam’s discharge capacity show the dam overtops by about 1.8 feet during a thousand-year storm, and nearly overtops during a 100-year storm. These conditions, he said, would likely lead to the dam failing with an “uncontrolled downstream release, and inundation of Letter S Road and downstream low-lying residential and commercial areas.”

“The dam is classified as high hazard and is documented to be in poor condition, with insufficient spillway capacity and multiple structural deficiencies,” the letter states. “Lowering the impoundment by 2.5 feet increases the discharge capacity and provides additional freeboard during storm events. This reduces, though does not eliminate, the risk of overtopping and potential failure while longer-term decisions regarding rehabilitation or removal are pursued.”

The temporary 12-foot drawdown discussed will be “postponed” until a stoplog bay structure is installed.

A row of stoplogs will be removed at a maximum of one row per day across the dam, until all are removed, or the water surface is at or near the bottom row. While the low-level gate is operational, there is a risk of downstream sediment moving if the gate is opened.

“For that reason, the low-level gate will remain closed, unless needed in an emergency, until a temporary stoplog-bay-type structure can be constructed upstream or downstream of the gate,” the letter states.

If installation can’t be completed before winter, the 12-foot drawdown will take place in spring or fall of 2027.