This report comes from NH DES, where the Dam Bureau has been busy working on dams and dikes in Pawtuckaway:

The removal of Burnham’s Dike is serving an important role in helping to keep the organic matter present in Burnhams Marsh from washing into Fundy Cove, were the dam/dike to have failed. This effort will help protect water quality within Fundy Cove and, therefore, the rest of Pawtuckaway Lake.*

While the low lake level at present is an inconvenience to anglers who wish to cast their lines in the shallow areas of the lake, the delay in raising the lake to normal summer level should ultimately be of long-term environmental benefit for all stakeholders.

Removal at the Burnhams Marsh Dike/Dam is proceeding smoothly. The removal effort is anticipated to finish up by the end of this week. That same crew has some work to perform on a couple of other dams within the State Park next week, but they should be out of there after that.

At Drowns Dam, the gate is open a few inches to allow the required minimum flow for the stream, but the lake level is still 1.5-2 ft below the spillway elevation associated with normal summer lake level.

At Dolloff, the lake is holding steady at the level of the stop logs and spilling over to provide the minimum required discharge as well. Once the crew has finished with their work within the State Park next week, the plan is to continue bringing the lake up to normal summer level. This will involve adding more stop logs at Dolloff. NHDES continues to monitor the Dolloff Dam and the sinkhole on the crest to ensure the dam remains safe as the water levels rise. As Corey Clark previously shared, if conditions at Dolloff Dam change in such a way as to generate concern, then it may prove necessary to lower the lake level back down to reduce the loading on the dam.

* And moving water out through Drowns Dam is another way to get rid of the nutrient-rich effluent leaving Burnhams Marsh – Ed.

 

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.

    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.)

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.

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 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.

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!