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.