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!