Dolloff Dam Update April 10, 2025

Corey Clark, Chief Engineer of the Dam Bureau, NH DES, has this report:

We have an update on the situation at Dolloff Dam.  Since my last message we have completed the following tasks:

  • Lowered the water level to 5.1 feet below the summer elevation of 250.4 feet.
  • Taken survey measurements of the sinkhole and all downstream leakage.
  • Instrumented two downstream weirs that measure leakage for around the clock measurements.
  • Instrumented the piezometers (aka monitoring wells) within the dam to measure water levels around the clock.
  • Cleaned and patched voids in concrete joints at the interface of the upstream concrete wall and upstream concrete buttresses.
  • Instituted bi-weekly site visits by dam safety engineers.
  • Started refilling the impoundment.

As for my last bullet, those of you on Pawtuckaway Lake have likely observed the level of the lake increase over the past few days. While we still don’t have all the answers regarding why the sinkhole developed or if the sinkhole should just be monitored or repaired, we are cautiously trying to capture the recent rain events to fill the lake. As you can see in the graph below, we are now just above the elevation in which we first observed the sinkhole and started lowering the lake. We are continuing to visit the site regularly to see if any changes are observed as we increase the water elevation. However, if we observe that the sinkhole is increasing in size or if the structural stability of the dam is questioned then we will again start lowering the elevation of Pawtuckaway Lake to reduce the risk of an emergency situation from developing.

Moving forward the Dam Bureau will be performing the following:

  • Continue monitoring the dam with bi-weekly site visits.
  • Continue monitoring data from weirs and piezometers.
  • Continue working with our engineering consultant to evaluate the sinkhole and leakage data to determine if the sinkhole should be monitored or repaired.
  • Dive inspection of the upstream side of the dam to evaluate potential areas of water intrusion. This could take place as early as next week.
  • Drilling in the dam to develop temporary and long-term solutions to the ongoing leakage and sinkhole development issue. This will take place this summer and will likely need to be done via a barge.
  • Continue adding stoplogs at Dolloff Dam to increase the elevation of Pawtuckaway Lake.

As previously stated, the last bullet is contingent on making sure that the dam is performing as expected and there is no increased threat to downstream individuals. In the attached plot you can see that we still have 3.5 feet of pond increase until we reach the summer elevation.

Additionally, we ask that you and members of the public refrain from exploring the dam by foot or by water. We are more than happy to answer any questions, but we want to make sure that nobody gets injured while exploring the site.

Removal of Burnham’s Dam

    Corey Clark, Chief Engineer of the Dam Bureau, NH DES, has a new report, this time about Burnham’s Marsh in the State Park:

I would like to make you aware that last week the Dam Bureau received a phone call from Pawtuckaway State Park about the conditions and concerns surrounding the level of the water at Burnhams Marsh within the State Park. (See map above, to the left of Big Island.)

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The Dam Bureau visited the site and  observed that the dike (it’s actually a dam, not a dike) at the north end of Burnhams Marsh was overtopping and in a state of failure due to the presence of a beaver dam constructed on top of the dam. Even though Burnhams Dike (NHDES dam number D184013) is within Pawtuckaway State Park (in yellow on the map above), it is owned by NH Fish and Game, so the responsibility for maintaining and repairing the dam lies with the NHDES Dam Bureau. On Friday of last week, I too visited the site to evaluate the condition of the dam and determine what could be done to alleviate the current threat to the main road in Pawtuckaway State Park.

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After visiting the site and discussing the situation internally, the Dam Bureau has applied for and received Emergency Authorization through the NHDES Wetlands Bureau to remove Burnhams Dike. Due to the ongoing overtopping of the dike, it can no longer function safely and is at threat of failure. The embankment is severely eroded and without major repairs will continue to be structurally unstable. Additionally, the corrugated metal outlet pipe in the outlet works at the base of the dike is also on the verge of collapse and can no longer safely discharge water. If the dike were to fail, debris and sediment would inundate Pawtuckaway Lake near the Fundy Cove boat ramp/channel immediately downstream of Burnhams Dike.

 

While the removal of the dike will impact the water level of Burnhams Marsh and alleviate the current water inundating the main access road through Pawtuckaway State Park’s, it will also allow for the marsh to return to the natural configuration before the dike was constructed. The Dam Bureau will remove as much of the dike as possible so that any remaining portions of the manmade structure will no longer have the ability to impound water to a magnitude that could threaten infrastructure or Pawtuckaway Lake and is no longer a jurisdictional dam.

The Dam Bureau intends on starting this work during next week and completing the work during the week of April 21st -25th. This tight timeframe is necessary to alleviate the current situation, but also the work can be completed while Pawtuckaway Lake is lower than normal for this time of year.

PLIA Sponsors Spring Cleanup on Route 156

A crew of volunteers for the PLIA did more than just talk trash – they removed it from a stretch of highway along Route 156 on Saturday morning, April 5. Our local police department provided an assist by sending out two notices advising motorists to be on the alert for these volunteers and to drive with caution.
As these photos illustrate, workers showed up in force with great can-do attitudes, and in no time they had cleared many bags of litter thrown onto the roadside over the winter. Just look at their haul!

The PLIA organizes three such events every year, so if you missed this one, there will be another this summer – look for announcements. Removing trash from the environment prevents motor oil, battery fluid, tires, and other pollutants from entering our watershed. We love our town and want to keep it “Clean and Scenic”!

More Dolloff Dam News from the NH Dam Bureau

PLIA President Tom Duffy recently spoke with Corey Clark, Chief Engineer of the Dam Bureau, NH DES. Here is what he learned about Dollof Dam and its repairs:

Dolloff Dam has been under study from 2020, when leakage under and through the dam was discovered. Monitor “test” wells were installed to determine the cause and amount of the leakage more accurately.  When water was raised to “full pond”, wells showed higher leakage. When the lake was lowered, test wells showed less leakage, demonstrating that water has been seeping through the dam. The dam bureau also noted  gaps in the large blocks on the water side of the dam. The immediate plan is to lower the lake down the full 7 feet that is possible at the dam. This will expose gaps in the blocks  on the lake side of the dam to allow them to be patched with concrete.
It is estimated that ALL THIS WORK SHOULD BE COMPLETED BY the end of the first week of April if they are able to start work by April 1. This is contingent on getting the lake sufficiently lowered by that time.
Boards will then be put back into the dam and the test wells will be monitored as the lake refills. Based on the data from the test wells, the dam bureau will determine if the leakage has been decreased and if the sink hole is growing or is stabilized.
At some point this spring, divers will check the bottom of the dam from the lake side for additional gaps and suspicious areas.
A second monitoring station is measuring the leakage from the bottom of the dam where there is a sealed door acting as a plug at the very bottom of the dam. There are monitoring “weirs” there and data is collected hourly to determine the amount of leakage from this area and to see if the leakage increases as the lake fills.
In addition to this immediate work on Dolloff Dam, be aware that 4 locations on Pawtuckaway are under review by the NH DES for more extensive renovation.  Those locations are the two dams on the lake, Dolloff and Drowns, and the two dikes, Gove Dike and Drowns Dike.  At some, as yet undetermined time in the future, the dams and dikes may be subject to major repair and large, perhaps total, drawdown of the lake may be necessary.

Doloff Dam Repairs May Be Needed, Lake to be Lowered Two Feet

On Monday this week (March17) the NH Department of Environmental Services observed a sinkhole on the crest of Dolloff Dam on Pawtuckaway Lake. A small depression was observed in the vicinity of the sinkhole last year but over the course of the winter this depression appears to have developed into a sinkhole. A sinkhole can be a significant issue on any dam and warrants immediate investigation. A sinkhole in a dam is primarily caused by water running through the soil material in the dam and that water eroding the internal dam material. If left unchecked, this could lead to increasing erosion and loss of the structural stability of the dam.

 

 

In the case of Dolloff Dam, leakage has been historically observed on the downstream side of the dam in the area of the sinkhole, however, this leakage has never been associated with internal erosion. While NHDES does not feel that the stability of Dolloff Dam is of concern at this point in time, more investigation is necessary to determine if that leakage is creating a situation where material is being moved from within the dam.

During a site visit on Tuesday (March 18), several voids were observed in the concrete on the upstream side of the dam which may be leading to water flow through the dam. NHDES will be lowering Pawtuckaway Lake, possibly as much as to the historic winter drawdown elevation of seven feet, so that these voids can be filled with cement. While this will help with the situation, NHDES does not believe that this will solve the issue. 

The lowering of Pawtuckaway Lake will occur with the use of the discharge mechanisms at Dolloff and Drowns Dam and will likely take one to two weeks. At that point, NHDES will mobilize to repair the concrete which will take approximately one week. Once repairs are completed, and further investigation is conducted, NHDES intends on starting to refill Pawtuckaway Lake to the full summer elevation. However, if further investigation reveals that there is a risk of further sinkhole development if the impoundment is increased, then NHDES may have to keep the impoundment at its lowered elevation. As many of you know, NHDES is currently working with an engineering consultant to review the deficiencies of all four Pawtuckaway Lake Dams and to come up with an overall rehabilitation design for all four structures. NHDES will be working with that consultant to perform further investigations, such as diving and camera work, to prepare a plan for refilling Pawtuckaway Lake.
Thanks to Corey for keeping us in the loop!

 

 

More Repair Work on Mendums Pond Dam

             

(Ed Note:  When water needs to be released to increase the flow in the Lamprey River System, the dam is opened alternately in Pawtuckaway Lake and Mendums Pond. When water from Mendums Pond is not available, as in this drawdown, all the needed water must come from Pawtuckaway Lake)

In 2015, Mendums Pond was drawn down 20 feet below full lake so that major repairs could be made to the Mendums Dam.  This repair took over a year to complete and Mendums did not reach full lake level until 2017. You can read the NH DES bulletin about that activity here.

Despite that major rebuild, in 2014, the dam bureau felt that more major work needed to be done and the lake was again drawn down 20 feet in the fall of 2024. Here is a quote from the NH DES Dam Bureau website.

“Mendums Pond Dam in Nottingham will have a deep drawdown this fall for reconstruction of the downstream part of the dam. The project is split into 3 phases and when each phase is completed, the pond may be filled incrementally. Worst case scenario for 2025 will be the pond at complete drawdown and best-case scenario is to gain some pond through the summer and fall. Variables such as weather, availability and timing of subcontractors, etc. can impact the schedule and is challenging to forecast”

Cyanobacteria Problem on Lake Kanasatka

Lake Kanasatka, in Moultonborough, has had a serious issues with phosphorus loading and the resulting cyanobacteria blooms for several years. In 2023, for example, a cyanobacteria warning was issued by the state that lasted from late summer through the fall, 121 days in total.

The following articles detail the issues facing that lake and the costly solution to solution to that problem.  (Ed Note: Even with this treatment, Kanasatka continued to have cyanobacteria blooms last year, although with a much diminished duration.)

 

 

Restoring Lake Kanasatka FAQ Bulletin

Lake Kanasatka Alum FAQ

Alum Treatment on Lake Kanasatka has successfully eliminated Cyanobacteria Blooms

Lake Kanasatka Feted for Reversing Cyanobacteria

 

Enfield Adopts New Septic Rules for Waterfront Property

The Enfield NH Selectboard recently adopted new septic regulations for waterfront property in the town that require septic inspections every six years and pump-out every three years to reduce the inflow of phosporus into the town’s waterbodies.

Enfield Septic Rules for Waterfront Property

Understanding Warrant Article 18

 

The Town will be asking for a loan from the State to fund a watershed study for Pawtuckaway Lake but the loan principal will be forgiven when the watershed study is finished so the Town will not owe any money. All the interest that accrues during the term of the loan will be paid for by the PLIA.

Read the following paragraphs for additional information.

What Does it Mean?

The Warrant Article, if passed, allows the Town to apply for a Clean Water State Revolving Fund loan. The funds will ultimately be used to pay for an updated watershed plan for Pawtuckaway Lake. Even though the loan, if granted, will have the principal ultimately forgiven, state law requires that the Town must have the authority and ability to repay as if the loan were not going to be repaid. That is the reason why the Warrant Article mentions that the Town would be authorized to issue bonds to repay the principal of the loan even though it would not be needed in this case. The Warrant Article also states that the PLIA will pay any and all interest on the loan. It is estimated that the total interest would be around $1500-$2000. It also mentions, in addition to doing the watershed plan, the possibility of doing some mitigation actions. If done, these would be quite small activities, more in the nature of feasibility studies than the relatively major mitigation activities that might be specified in the watershed plan.

What is it For?

The PLIA would like to update our watershed plan which was last updated in 2008. While a watershed plan has many parts, in essence its purpose is to determine the factors which affect the health of a water body and recommend mitigation actions to improve it. While we have several reasons for wanting this update, the primary reason is to determine the factors which are leading to an increasing level of phosphrous in our lake. While any excess nutrients in the lake are a problem, phosphorous in particular, can lead to bigger and more frequent cyanobacteria blooms. We had two such blooms last summer leading to recommendations to stay out of the lake for periods of time since they can cause serious illness and possibly even death. With an updated plan we can understand the most effective mitigation actions and actually get a start on the major mitigation work.

Why a Loan?

Developing watershed plans is a complex activity and requires hiring experts to do the work in conjunction with information from local residents. The cost to update our plan is estimated to be between 65K and 100K. We hope to get the necessary funding from a federal program, the Clean Water State Revolving Fund (CWSRF), which is administered separately by each state. In NH these loans have to be given to a town rather than a lake association or other non-governmental body.

How Does it Work?

As stated above, the way NH administers the program is to provide the loan to towns. These loans are like lines of credit in that interest accrues only as the money is requested by the town. When the purpose for the loan, namely an updated watershed plan, is complete, the Town informs the State that the loan should be closed. Having fulfilled the loan purpose, the State then forgives the principal. The net result is that the only thing that is owed to the State is the interest on the loan.

Summary

The Town will be asking for the money directly but the PLIA in consultation with the Town will engage a consultant to actually perform the work. The key here is that the Town is required by the terms of the loan program to be the middle man but the Town incurs no financial obligation other than a tiny amount of administrative work. Of course, none of this is a guarantee that we will qualify for the loan even if approved by the town voters. If the Warrant Article is approved, we will work with the Town to provide a pre-application to the State by May 31st. After a vetting process to determine loan candidates, the actual loan applications are due in the fall.