The Mill River Reservoir Dam is a 100-ft high earthen embankment structure located in Pound Ridge, New York and impounds the 2,180 acre-ft Mill River Reservoir. Nearby, the Trinity Reservoir Dam is a smaller earthen embankment dam. Both impoundments are used for water supply by Aquarion Water. In 2009, the State of New York promulgated new regulations requiring Engineering Assessments (EAs) of all Class C (High Hazard) Dams, including these two structures. This is particularly important because Mill River Reservoir Dam had a historic and persistent seepage issue.
In 2008, Aquarion Water Company (Aquarion) retained GZA to conduct field explorations and geotechnical evaluations to further understand the observed seepage. A number of subsurface explorations were performed, piezometers and monitoring wells were installed, and a series of V-notch weirs were installed to monitor surface flows. GZA developed a conceptual repair design consisting of seepage controls (e.g., a toe drain and filter). GZA went on to perform an EA of the dam and also Aquarion’s nearby Trinity Reservoir Dam to fulfill New York State Department of Environmental Conservation Part 673 Dam Safety Regulations in 2011. The EAs included stability and seepage analysis, hydrologic and hydraulic analysis, and visual inspection. In 2016-2017, GZA was asked by Aquarion to advance our seepage collection system design concept to final design, assist in the bid process, complete required state and local permits, and provide construction-phase engineering services. The final dam improvements designed also included a surface swale on the left and right side of the embankment to control surface flows that were resulting in minor erosion at the downstream slope.
The EAs satisfied New York State’s regulatory requirements. A focused effort to address seepage was undertaken as a result of the EAs and previous analyses. Permits for the work were successfully procured in Fall, 2017. Subsequent construction of the seepage collection and surface drainage improvements was completed in Summer 2018. The project finished within Aquarion’s budget, and the improvements are functioning as anticipated. The completed work allows this important water supply dam to function in good condition for future years.