Lexington Old Reservoir Dam – Significant Hazard Dam Rehabilitation

Weston & Sampson provided design, permitting, bidding, and construction services for the rehabilitation of significant hazard Old Reservoir Dam in Lexington, Massachusetts. Lexington Old Reservoir Dam is an approximately 600-foot-long, 10-foot-tall earthen embankment with a drop inlet primary spillway structure. Prior to rehabilitation the dam was in “poor” overall condition with deficiencies including significant tree growth with exposed tree roots and associated erosion on the crest, uncontrolled seepage at the dam toe, and a non-operable low-level outlet (LLO).

Weston & Sampson provided Phase I inspections and Phase II evaluation for the dam, as well as preparing the Emergency Action Plan (EAP) and the Operations and Maintenance Plan for the dam. As part of the rehabilitation design, Weston & Sampson provided site topographic survey, LLO investigations, hydrologic/ hydraulic analyses, wetland delineation, geotechnical explorations, geotechnical slope and seepage stability analyses, and structural evaluations prior to preparing rehabilitation design plans that included the removal of trees, flattening of the dam slopes, raising the dam crest, installation of a mineral filter at the downstream toe, installation of a new manhole for the existing LLO, protection of existing wetlands and wetland replication, installation of a pedestrian path on the dam crest, and planting of new trees nearby. For construction, Weston & Sampson prepared and applied for environmental and Office of Dam Safety permits, prepared full bid documents, and provided bidding and provided full-time on-site observation and engineering during construction, as well as long-term wetland monitoring to ensure realization of the project goals.

The rehabilitation was implemented in a tight schedule of July and August of 2018, during the summer break of the nearby school. Lexington Old Reservoir Dam was the second dam that Weston & Sampson rehabilitated for the Town of Lexington, with the first one having been rehabilitated a few years prior. Weston & Sampson continues to perform dam safety inspections for both dams owned and operated by the Town of Lexington.

Key Project Features

  • significant hazard dam rehabilitation
  • hydraulic and hydrologic analyses
  • LLO alterations
  • in-house, multi-disciplinary approach
  • permitting
  • bidding
  • construction administration


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