Gilboa Dam Reconstruction Site Case Study: Protecting Natural Resources

Last Modified: Dec 15, 2009


  • Jennifer Cass, P.E. - Hazen and Sawyer

The New York City Department of Environmental Protection (NYCDEP) is undertaking a major rehabilitation of the Gilboa Dam utilizing innovative yet proven effective measures to control erosion and sediment during construction. This case study will discuss the extraordinary measures that will be taken to protect the sensitive natural resources surrounding the Gilboa Dam, located in the rural community of Gilboa, NY. Local natural resources include the Schoharie Reservoir, a 17.6 billion gallon reservoir that is a major component of the NYCDEP drinking water supply system; Schoharie Creek, which is fed by Schoharie Reservoir and flows downstream to the Blenheim Gilboa Reservoir; as well as several amphibian-breeding wetlands and small streams. The work on the Dam will be completed over six years in four phases, each with a customized set of erosion, sediment, and stormwater control measures protecting the adjacent surface water bodies.

During an initial contract, site improvements include repaving of a roadway and installation of utilities within the Dam Embankment. Since this work will be adjacent to the Reservoir, perimeter erosion and sediment controls will be implemented to prevent migration of sediment into the Reservoir. In addition, stormwater quality improvement measures will be constructed, including bioretention cells and a sand filter.

The second construction phase will prepare the 56-acre site surrounding the Dam for the main Dam Reconstruction project. This phase will involve the installation of temporary and permanent roadways, staging area, a field office complex, an on-site permanent disposal area, and perhaps most importantly, best management practices (BMP’s) to protect both natural resources on and off site. Comprehensive erosion and sediment control practices include nine sediment traps/basins, six pipe slope drains, 18,000 feet of silt fence, 17,000 feet of diversion and on-site swales, truck washes, stabilized construction entrances, Reservoir containment booms, seeding and mulching, erosion control matting, and weir tanks equipped with polyacrylamide dosing. In addition, several stormwater wetlands will be constructed to reduce runoff flow rates and enhance water quality from permanent access roads.

Under the third phase of work, the actual Dam Reconstruction will be performed and will rely heavily on the BMPs installed in the prior contract for protection of surface water bodies. However, maintenance of BMPs will be a very important part of this phase. The fourth phase of work will restore and improve the site from its preconstruction condition by removing temporary impervious surfaces, constructing additional stormwater quality wetlands, and planting native grasses, shrubs, and trees throughout the site. At each step of construction, the project will strive to protect sensitive natural resources with the use of innovative and tailored erosion and sediment control practices and permanent stormwater quality features that best match the respective application.

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