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Catskill Watershed Dams Reservoirs and Associated Facilities Reconstruction

Gilboa Dam reconstruction ensures New York City water taps continue to flow.
  • Gilboa Dam spillway crumbling  Gannett Fleming

    The Gilboa Dam spillway was crumbling from decades of freeze-thaw and overflow.

  • Removing stone facade and deteriorated concrete  Gannett Fleming

    Workers removed the stone facade and deteriorated concrete from the downstream face of Gilboa Dam.

  • Gated notch and twin siphons  Gannett Fleming

    A gated notch in the spillway and twin siphons controlled the reservoir level during construction.

  • Reconstructed Gilboa Dam  Gannett Fleming

    The reconstructed Gilboa Dam can withstand deluges more than double that of Tropical Storm Irene.

  • Gilboa Dam dedication  Gannett Fleming

    Officials pour water from buckets onto the dam’s new concrete spillway at the dedication ceremony.

New York City Department of Environmental Protection

Joint venture with Hazen and Sawyer

Schoharie County, New York

Our Role
Design Engineer, Hydrology, Hydraulics, Hydrogeology, Permitting, Construction Management.

180-foot-high, 1,324-foot-long concrete spillway section, 700-foot-long earthfill section
Construction Cost
$600 million
In Progress
In Progress
  • Preliminary analysis reveals immediate repair work to Gilboa Dam is necessary 
  • Long-term reconstruction ensures dam meets safety and stability standards
  • Improvements to other watershed facilities protect vital water supply.

After record flooding occurred in 1996, New York City’s 1,324-foot-long Gilboa Dam spillway was found to be a safety hazard. Completed in 1927, the dam’s masonry was deteriorating from decades of freeze-thaw and spillway overflow. It holds approximately 20-billion-gallons of water and provides 16 percent of New York City's water supply. The New York City Department of Environmental Protection (NYDEP) launched a $192 million reconstruction project to enhance the Gilboa Dam’s stability, repair deteriorating materials, and make improvements to other Catskill watershed system facilities, many of which were nearly 100 years old. Gannett Fleming provided permitting, design, and construction services for the remediation of the dam and improvements to the other NYDEP facilities.

Our 2005 investigation determined that immediate repair work to the Gilboa Dam was necessary to meet current safety standards and ensure its stability. We performed emergency engineering design services for the dam’s short-term stabilization and design services for its long-term reconstruction. The resulting repairs assured the safety of downstream residents, and maintained vital water supply storage and conveyance infrastructure.

What We Did

Gannett Fleming performed emergency engineering design services for the remediation of the cyclopean concrete gravity structure as part of a joint venture with Hazen and Sawyer. The emergency construction contracts featured installation of a debris boom, four spillway siphons, a 220-foot long temporary spillway notch, and 80 stabilizing post-tensioned anchors. Concurrently, the team designed the major, long-term reconstruction of the dam to improve its hydraulic performance and accommodate the probable maximum flood. We performed field investigations; structural monitoring and quarterly reporting to state agencies; civil, structural, geotechnical and electrical-instrumentation design services that included preparation of drawings, specifications, and cost estimates; and design services during construction.

A five-member independent technical review team provided consultation on the design to state regulators. This spirit of cooperation continued during construction via a formal partnering program between the owner, designer, construction manager, and contractor, which was key to navigating the project through a devastating mid-construction hurricane and successfully completing the project 14 months ahead of schedule. In addition, Gannett Fleming performed improvements to other system facilities, including the Ashokan Reservoir, the Hillview Reservoir, the New Croton Dam, and the West of Hudson and East of Hudson reservoirs. Throughout the entire project, Gannett Fleming performed risk assessments to identify design, permitting, and construction risks and assess the impacts on cost and schedule. After 10 years of safety improvements and upgrades, the Gilboa Dam was dedicated in October 2014.

Key Features

  • Field investigations included surveying, drilling, sampling, testing, and geophysical exploration
  • Preliminary and final design of dam rehabilitation
  • Demolition and refacing of hydraulically efficient stair-step spillway 
  • Reconstruction of support structures, including spillway channel, plunge pool, siphons, and access roads
  • Partial reconstruction, anchor stabilization and extension of west training wall, and repairs, stabilization, and cleaning of other training walls and spillway masonry
  • Site security, health and safety, and accessibility improvements 
  • Emergency response, design, and support during hurricane and flood damage recovery
  • Hosted a series of technical advisory group meetings with state and federal regulatory agencies to expedite their review and endorsement of the emergency repairs.

Sustainability Features & Outcomes

  • Removed approximately 5,000 cubic yards of sediment material from the Shandaken Tunnel intake channel to improve water quality and restore intake capacity
  • Specified the re-use of masonry façade stones during reconstruction to the extent practicable, which preserved the aesthetic appearance of the dam and reduced the amount of newly quarried materials needed
  • Performed natural resource studies of small mammals, reptile, avian, fish, aquatic insect, wetlands, and vegetation in compliance with the National Environmental Protection Act.

Awards & Recognition

  • National Rehabilitation Project of the Year Award, 2015, Association of State Dam Safety Officials
  • Best Project Award, Water/Environment, 2015, ENR New York
  • Diamond Award, 2007, New York American Council of Engineering Companies Engineering Excellence Awards
  • Project of the Year, 2007, New York Construction magazine.

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