Gilboa Dam Repairs and Rehabilitation
Client: New York City Department of Environmental Protection
Constructed between 1920 and 1927, the 1,800-foot long Gilboa Dam impounds the Schoharie Reservoir in upstate New York. The 19.5-billion-gallon reservoir is a vital component of New York City’s Catskill Water Supply System, which provides about 40 percent of the City’s water supply. In the autumn of 2005, the New York City Department of Environmental Protection retained Hazen and Sawyer (in joint venture with another firm) to conduct a comprehensive evaluation of the dam and design improvements that would ensure the dam’s continued reliability, both short- and long-term.
Project Outcomes and Benefits
- Design of short-term improvements was completed within a 90-day period.
- Work was implemented in a phased manner that enabled the reservoir to remain in service throughout construction.
- Short-term repairs stabilized the dam until longer-term repairs could be implemented.
- Dam was brought into full compliance with current NYS Dam Safety stability standards, maintaining vital water supply storage and conveyance infrastructure.
- Gilboa Dam, Gilboa, NY – Shown with water spilling over the entire dam spillway, during emergency repairs.
- Gilboa Dam, Gilboa, NY – Shown undergoing emergency repairs with construction of the notch and emergency siphons for flow diversion complete.
- A Debris Boom across Schoharie Reservoir was installed during emergency contract to prevent debris accumulation on the Gilboa Dam face.
- Through the use of a turbidity curtain system, dredging around the Shandaken Tunnel Intake was conducted without spreading sediment to the rest of the Schoharie Reservoir.
- Future rehabilitation of the Shandaken Tunnel Intake will include repair and maintenance of intake gates, refurbishment of the building, and upgrading of the entrance road and parking area.
- Before reconstruction of the dam, the site is being prepared to ease future construction activities. Preparations include installation of several new and improved access roads, a site staging area, field engineer personnel facilities, and a temporary bridge crossing Schoharie Creek. Several areas of the site have been set aside for natural animal and plant habitat preservation.
- On-site road was constructed with dam and staging area access, and erosion control measures and protection of natural habitat in mind.
An inspection in 2005 indicated that the facing on the dam had largely been lost due to freeze-thaw cycles and its mechanical systems were in need of repair, and identified some rehabilitation work needed to meet modern dam safety standards and improve dam performance. Working closely with NYCDEP, Hazen and Sawyer inspected the facilities in joint venture and identified a number of immediate steps that would improve dam stability. These included installing:
- A debris boom upstream of the dam, to limit debris accumulation along the crest of the spillway.
- A 500-mgd temporary siphon system along the crest of the dam, to lower the level in the reservoir and reduce structural loading on the dam to allow for notch installation.
- A 220-feet wide temporary spillway notch with a capacity of about 8,500 cfs that served as the primary means of water level control during the anchor installation
- Permanent rock anchors in the spillway section, to improve dam stability under all service conditions including the probable maximum flood.
Following an expedited 90-day design period, the dam rehabilitation project was issued for bids as a series of four separate contracts. Hazen and Sawyer also performed an Environmental Assessment of the areas that would be affected by the project, and included natural resource surveys, sediment and erosion control plan, and streambank inventory. Construction commenced in January of 2006, and was completed in the fall of 2006, with installation of the rock anchor system.
Hazen and Sawyer is currently assisting the DEP in designing and implementing long-term rehabilitation of the dam, the Schoharie Reservoir, and the Shandaken Tunnel, an 11-foot diameter, 18-mile long aqueduct that transfers water from Schoharie Reservoir to Esopus Creek, the main tributary to the Ashokan Reservoir, another component in the Catskill water supply system. Improvements include dredging at the Shandaken Tunnel intake and improvements to the Tunnel’s electrical and mechanical systems, modifications to the spillway and gate structures, and additional dam rehabilitation work. A new Low-Level outlet consisting of a new submerged intake and a major tunnel operation around the right-side of the dam will be added to the facility. This new tunnel is expected to be 108-inch diameter and will be the largest diameter microtunneling operation constructed in the United States. The work effort included a comprehensive review of the environmental and socio-economic impacts of the planned improvements and will ultimately include design of wetland mitigation measures and habitat restoration at the site when the construction effort is completed.
In addition to work at the dam and intake, Hazen and Sawyer also conducted a turbidity control study for both the Schoharie and Ashokan Reservoirs, which periodically experience elevated turbidities, especially following major storm events.