South Reservoir Dam – High Hazard Dam Rehabilitation

For nearly a decade, Weston & Sampson has conducted safety inspections of the Town of Winchester’s six dams and dikes, on an as-needed basis. In 2017, Weston & Sampson worked with the town to rehabilitate the inoperable low-level outlet (LLO) of its largest, South Reservoir Dam. In 2022, Weston & Sampson started working with the town to rehabilitate the dam, which is currently in Poor condition.

South Reservoir Dam is a 1,000-foot-long, 45-foot tall HIGH hazard potential earth embankment dam with a masonry core wall. The dam, circa 1875, was constructed for public water supply purposes, and South Reservoir still serves as the town’s water supply.

As part of the LLO rehabilitation Weston & Sampson performed dive inspections, test excavations, video line inspections, and hydraulic evaluations of the LLO. Our team also designed and permitted repairs including a new electrical-actuated knife gate for the LLO, pipe improvements including sliplining and spray-applied epoxy lining, a new stilling basin and downstream channel improvements, new slide gates in the gatehouse and primary spillway, and a new operations floor in the gatehouse. The work was largely completed using a dive team.

In 2022, Weston & Sampson performed a Phase II evaluation of South Reservoir Dam. The evaluation included hydraulic and hydrologic analyses, subsurface explorations consisting of soil borings, monitoring well installation, structural assessment of the gatehouse and bridge, and geotechnical analyses and assessment of embankment stability. The study showed that the embankment had inadequate factors of safety against slope and seepage instability and, in sections, consisted of loose sands susceptible to liquefaction and lateral spreading, presenting a significant safety risk in the event of an earthquake. Using a team of in-house multi-disciplinary engineers and scientists, Weston & Sampson is providing rehabilitation design and permitting services to bring South Reservoir Dam into compliance with current dam safety regulations and preserve this critical piece of the town’s drinking water supply infrastructure.

Due to its large volume, the town requested that the reservoir not be lowered. Weston & Sampson is preparing a design that can be implemented in the wet, and includes flattening, buttressing, and armoring the slopes with considerations for liquefaction, constructing a two-stage graded seepage filter, rehabilitating the bridge over the spillway and to the gatehouse, wetland replication. Weston & Sampson’s environmental scientists are currently working with regulators as part of the design permitting process. Construction is anticipated for the summer of 2024.

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