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.