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Challenges in the Catskills

Reconstructing the NYC Gilboa Dam

Rodney E. Holderbaum, P.E.
Paul Costa, P.E.
Alon Dominitz, P.E.

The 84-year-old Gilboa Dam that impounds Schoharie Reservoir is a key component of the New York City Department of Environmental Protection's (NYCDEP) water supply system. This system serves approximately 8 million people in New York City and 1 million people upstate, delivers approximately 1.2 billion gallons of water per day, and is arguably the largest gravity fed, unfiltered, water supply system in the world. The Schoharie Reservoir accounts for approximately 15% of the water delivered by this system. In 2004, the NYCDEP embarked on a program to completely renovate and update Gilboa Dam. The NYCDEP, along with its design team, a joint-venture of Gannett Fleming and Hazen and Sawyer, has navigated through a maze of technical, operational, environmental, public involvement, and regulatory permitting issues, and is now in the early stages of construction. This article describes the evolution of the project and the measures taken to renew this facility for the next 80 to 100 years of operation.

Project Overview

The Catskill Water Supply system is one of three surface water systems operated by NYCDEP. The northern-most component of New York City's raw water system, the Catskill system provides over 40 percent of the city's water.

Two reservoirs, Schoharie and Ashokan, form the Catskill system's water supply source and represent major components of the overall system. The two reservoirs, situated in New York's Catskill Mountain region, are located about 75-120 miles north-northwest of Manhattan. Schoharie Reservoir, which can store up to 19.5 billion gallons, is impounded by the Gilboa Dam, constructed in 1919-1927. The Gilboa Dam and Schoharie Reservoir mainly serve as a diversion system for transferring water from the 314-square-mile Schoharie Creek watershed to the 256-square-mile Esopus Creek watershed via an intake chamber located about 3 miles upstream of Gilboa Dam on the western shore of the 5-mile-long Schoharie Reservoir. From the Shandaken Tunnel intake chamber, diverted water travels about 18 miles through the Shandaken Tunnel, where it discharges into Esopus Creek and then travels approximately another 5 miles to Ashokan Reservoir.

Ashokan Reservoir, which can store up to 127.8 billion gallons, is impounded by Olive Bridge Dam and a system of seven large earthfill dikes constructed in 1909-1915. Water is transferred from the Ashokan Reservoir to the city via the 92-mile-long Catskill Aqueduct. Figure 1 provides an illustration of New York City's water supply system, including the Catskill System.

Historical Perspective / Project Timeline

Gilboa Dam consists of a 700-foot-long earth embankment with a concrete corewall, and a 160-foot-high, 1,324-foot-long cyclopean concrete spillway (Figures 2 and 3). Design of the dam was completed in 1917, but start of construction was delayed until 1919. Original design records indicate that the spillway design flood was estimated to result in 5 feet of spillway crest overtopping. However, the spillway was conservatively designed with 20 feet of freeboard between the spillway crest and the dam crest. Throughout the 84-year service life of the structure, several floods resulted in spillway heads greater than 5 feet, with the January 1996 record flood resulting in a maximum spillway head of 6.7 feet.

The downstream face of the stepped, spillway control section was constructed with a veneer of dressed stone. Deterioration and loss of stone was observed as early as the 1930s, although periodic repairs by NYCDEP maintained the face in relatively good condition until the 1990s. The spillway side-channel slab was originally surfaced with hand-placed cut stone. In the 1950s, the stone paving was surfaced with a thin concrete overlay. A major flood in January 1996 resulted in dislodging of several side-channel concrete overlay segments, resulting in significant areas of scour at the downstream toe of the control section. These areas were repaired by removing loose materials and filling with concrete.

Soils within the watershed are highly prone to erosion and, as a result, significant accumulation of silt has occurred in the reservoir over its 84-year operation. The depth of siltation at locations near the dam is estimated to be nearly 20 feet. The original design of the intake for the low-level outlet works did not accommodate significant siltation; therefore, the outlet works system has been rendered non-functional.

Following the 1996 flood, NYCDEP embarked on an extensive investigation program to evaluate conditions at the dam and to determine compliance with current industry design criteria. This program, completed in 2003, revealed the need for interim safety measures followed by a comprehensive rehabilitation and upgrade of the dam. In the spring of 2004, the NYCDEP commissioned the Gannett Fleming/Hazen and Sawyer Joint Venture (Design Team) to provide engineering services related to the reconstruction of the two Catskill System dams and their appurtenant facilities.

Interim Improvements

In the fall of 2005, during the preliminary design phase for reconstruction of Gilboa Dam, the Design Team advised NYCDEP that preliminary stability analyses indicated marginal stability for the record flood conditions ( January 1996 flood) and potential failure for an event approaching the one-half probable maximum flood (1/2 PMF). Discontinuities within shale layers in the dam foundation provided potential sliding surfaces that resulted in safety factors that did not meet acceptable minimum requirements. Given the critical nature of this reservoir, both in terms of public safety to approximately 8,000 residents living downstream and dependability of New York City's water supply, NYCDEP elected to proceed with an interim stability improvement project with a targeted completion date of December 2006. This would provide an additional margin of safety during design and construction of the other improvements, which at that time were envisioned to be completed in 2013. At the request of the New York Dam Safety program, NYCDEP also implemented a formal, multi-faceted monitoring program designed to establish baseline performance data and detect any acceleration in the dam's deterioration.

To implement the interim improvements, the Design Team prepared five construction contracts within a three-month period. The first four contracts involved constructing features to help regulate the reservoir level and/or protect the work site. They included a 2,100-ft long debris barrier in the reservoir, four 4-ft-diameter steel siphons, a 5.5-ft-high by 220-ft-long spillway crest notch for flow control, and a 2.5 to 4-ft-high by 630-ft-long berm system-all of which were constructed within 45 days during the 2005-2006 winter season. (Figure 4 depicts the first three interim improvements.)

The fifth contract involved installation of 80 high-capacity post-tensioned rock anchors, the largest consisting of 58 strands and over a 1,000 ton design service load. The anchors required drilling both vertical and inclined 15-inch-diameter holes with a total linear footage of over 13,000 feet. The anchors were installed within 9 months and the overall project was completed in early December 2006, despite a large flood event on June 28, 2006 that flooded the work area and affected much of New York and surrounding states.

A fast-track approach allowed design, permitting, and construction to proceed much more efficiently than routine construction projects. NYCDEP's normal construction contracting procurement process usually requires 9 to 12 months; however, NYCDEP procedures allow for a substantially more expedited process if the project has been declared an "emergency." Under an emergency declaration, contract award and "order to commence" can be issued within one or two days following bid opening. This emergency contract procurement process was a major contributor in getting all five construction contracts underway in accordance with the desired construction schedule.

Similarly, permitting for a major dam construction project can typically take several years, but in this case the project had begun under an Emergency Authorization from the New York Department of Environmental Conservation (NYSDEC). The NYSDEC retained the services of a private consultant to assist in the technical review of the dam safety aspects of the emergency anchoring project, facilitating an expedited review. This additional technical expertise also provided a broader base of engineering experience with concrete dam stability and anchoring, giving extra assurance to the downstream community that the project would meet appropriate safety objectives. The goal of the anchoring project was to increase the dam's factors of safety to acceptable values for the ½ PMF, which is New York's Spillway Design Flood for an existing High Hazard dam. The project was recognized by New York Construction magazine as one of the best completed projects in 2007.

Implementing the Reconstruction Project

Engineering studies leading up to the interim improvements revealed several other deficiencies at the dam, including:

  • Marginal flow conditions over the stepped spillway control section
  • Deterioration of the stone-masonry spillway fact
  • Vulnerability of the spillway side-channel slab to erosion during significant floods
  • Lack of an operating reservoir drain system
  • Distress and movement in some of the spillway training walls
  • Erosion at the downstream end of the spillway plunge pool
Figure 5: Overview of Planned Improvements

In addition, other key project features, although not necessarily showing signs of distress, did not meet current criteria.

Following completion of the interim improvements in December 2006, investigations and design were resumed to address these deficiencies. Remediation within the spillway, which is dictated by a combination of geotechnical, hydraulic, and structural criteria, represents the largest capital outlay. The spillway control section, which has a maximum height of 160 feet and length of 1,324 feet, is being reconstructed with a new downstream stepped profile that will improve flow conditions and energy dissipation. Physical modeling, performed by Utah State University, was utilized to predict hydraulic conditions for a range of flood events, ranging from a 2-year event to the probable maximum flood. The improvements will also increase the safety factors against sliding to a minimum of 2.0 for the worst static condition (probable maximum flood) and 1.5 for seismic loading. The planned improvements to the spillway control section are shown on Figure 6.

Other spillway improvements include installing 5-foot-high by 220-foot-long spillway crest gates, removing and reconstructing the spillway channel slab, lining the plunge pool, reconstructing the plunge pool end sill, installing foundation drainage beneath the spillway control section and side channel slab, and modifying training walls to improve stability. Figures 5 and 6 provide an overview of the planned improvements and schematic of the proposed spillway reconstruction.

New Low -Level Outlet

As previously indicated, the existing outlet facility is non-functional. A new low-level outlet consisting of a 9-foot diameter tunnel will be constructed from the reservoir, around the right abutment of the dam to the downstream channel, as depicted by the red line in Figure 7. A 180-foot-deep vertical shaft to be constructed at the dam's right abutment will house a limited number of control gates. It is envisioned that tunneling for the upstream leg that originates at the gate shaft and passes underneath the reservoir will be performed with a micro-tunneling boring machine with wet retrieval in the reservoir. Primary control of flows will be accomplished at a control/outlet structure located at the downstream end of the tunnel. The low level outlet will facilitate water releases required for the snowpack offset program (discussed later), lowering the reservoir for periodic maintenance, and emergency drawdown in the unlikely event of a dam safety incident.

Construction work is being accomplished via six separate construction contracts:

(1) Installation of spillway crest gates and operating equipment,

(2) Site Preparation,

(3) Dam Reconstruction,

(4) Low-Level Outlet Construction,

(5) Shandaken Tunnel Intake Chamber Improvements,

(6) Environmental Site Restoration.

These contracts were selected to accelerate key parts of the work, segment specialty work, and limit the size of individual contracts to encourage greater contractor participation. The first three contracts have been awarded and are currently underway. The dam reconstruction and low-level outlet are currently targeted for completion in 2016. The estimated construction cost of all six contracts is approximately $350 million.

Sustaining the Built Environment with Smart Operations

Although Schoharie Reservoir's primary function is to supply drinking water to residents of New York City, the NYCDEP has been sensitive to the concerns of the citizens living near the dam. Flooding in the Schoharie Valley has long been a significant problem often resulting from a combination of snowmelt and heavy rainfall. The largest known flood occurred in October 1903, prior to the construction of the dam, when 10 inches of rain fell in 24 hours.

Although the reservoir does not include a flood control component, some attenuation of peak flows occurs as a result of temporary storage during storm events. The attenuation can be significant during storms that begin when the reservoir is substantially below the spillway crest. To further mitigate flooding in the Schoharie Valley, the NYCDEP has initiated a "Snowpack Offset Program." The program involves controlled lowering of the reservoir during the winter and early spring months as snow accumulates in the watershed, eventually melts, and replenishes the water supply storage lost by the drawdown. The snowpack is determined based on measurements made by the NYCDEP at monitoring stations in the watershed and then converted to equivalent water content. This program has minimal impact on water supply operations but provides a substantial benefit to the local population by capturing snowmelt which historically has contributed to many of the worst floods recorded. Drawdown is currently accomplished by the four 48-inch-diameter siphons installed in the spillway. Eventually, the spillway crest gates and new low-level outlet will be utilized for this function. When these facilities become functional, a wintertime reservoir void equivalent to one-half of the water content of the accumulated snowpack in the watershed will be maintained.

Maintaining Dam Safety through Monitoring Improvements

Events leading up to the interim improvements revealed a need for enhanced monitoring and surveillance of the dam. Recognizing that a complete reconstruction project would take several years, the state dam safety office requested immediate initiation of a program to visually inspect and document conditions at the site on a periodic basis, acquire and evaluate instrumentation data, measure and record structural changes, and monitor onsite activities. Results are reported to the state dam safety office, and reviewed by NYCDEP operations personnel. Following is a brief description of each activity:

Inspections: The current site inspection program is a formalized, three-tiered inspection program developed by NYCDEP specifically for Gilboa Dam. Each successive tier of the inspection program is intended to provide a higher level of scrutiny at the site. The three inspection tiers are:

(1) periodic (approximately weekly), performed by maintenance personnel;

(2) monthly, performed by a supervisor; and

(3) semi-annual, performed by NYCDEP engineers, usually the Western Operations Dam Safety Engineer.

Supplementary inspections are performed following major spill events, earthquakes, and other unusual events or reports.

Instrumentation Data: Instruments at the site include piezometers, observation wells, sentinel anchors, inclinometers, extensometers, and survey control points. With the exception of the extensometers, data from the instruments is currently acquired manually. Data from all instruments are analyzed and reported on a quarterly basis.

Structural Monitoring: In addition to the instrumentation data, monitoring to assess loss of stone from the spillway face is performed and reported on a quarterly basis. The monitoring consists of surveys to record rate and extent of stone loss and photographic documentation of conditions at the time of the inspection.

As part of the planned dam reconstruction, an automated data acquisition system will be installed, which will permit real-time access to all instrumentation data. In addition, reservoir level, and spillway and low-level outlet discharge data will be made available to the New York Power Authority to facilitate operations at their Blenheim-Gilboa hydroelectric facility located downstream of Gilboa Dam.

Technical Review and Permitting

Due to changes in the state's contracts, the state dam safety office was not able to hire its own consultants to assist in the technical review of the Dam Reconstruction and Low Level Outlet Tunnel projects. The state dam safety office had recently participated in the technical review using a board of consultants for the rehabilitation of the FERC-regulated Swinging Bridge Dam. Based on that experience, the state requested, and NYCDEP agreed to, an Independent Technical Review Team (ITRT) to review and comment on the dam rehabilitation's design. The team consisted of five members, recommended by a NYSDEC/NYCDEP selection team. The ITRT members were chosen for their expertise in hydrology and hydraulics, concrete structures and stability, geology and geotechnical, and tunnel design and construction.

NYCDEP held several meetings, at approximately 30%, 60%, and 90% of design. The meetings were attended by core NYCDEP and NYSDEC staff responsible for design and technical review of dam safety aspects, but also NYCDEP operations staff, NYSDEC regional staff who inspect dams, NYSDEC environmental quality staff, and engineers representing the downstream communities. Several other important issues related to the dam emerged, including the opportunity to incorporate low-flow release works into the low-level outlet's design to provide the potential for a conservation release into the Schoharie Creek downstream of the dam. The ITRT meetings provided an opportunity for state regulators, local municipalities represented by their own engineers, and the NYCDEP to discuss the various issues and get subject-matter experts to provide their opinions when needed.

Key Factors for Project Success

Although the project is still in the early stages of construction, numerous milestones have been achieved and all parties believe that project success is just around the corner. There have been several keys to the successes to date:

  • A major funding commitment by NYCDEP assured implementation of the project on a reasonable timeline.
  • The state dam safety office requested, and NYCDEP provided, a significant commitment to improved inspection and surveillance of the dam.
  • Open discussions with the public and local officials have communicated progress on a regular basis. Tours of the project for public officials and public briefings have aided in explaining complex technical issues and created an environment of mutual respect and trust.
  • The owner, NYCDEP, solicited public input and incorporated solutions, when possible, to address residents' concerns.
  • Initiating early and regular communication with all regulatory agencies expedited the permitting process. Bringing regulators into the design process at an early stage can substantially reduce the potential for impacts to the schedule caused by the permitting process.
  • Implementation of an Independent Technical Review Team provided the owner and dam safety regulator with a second engineering opinion.
  • The Design Team conducted technical workshops to convey key engineering aspects of the project to all stakeholders. Representatives at these workshops have included the owner, regulatory agencies, local community engineering representatives, the Corps of Engineers, New York Power Authority (downstream dam owner) the Federal Energy Regulatory Commission, Independent Technical Review Team representatives, and the Construction Management Team.
  • Building flexibility into the design process and construction contracts has allowed for adjustments in response to actual conditions encountered.
  • Evaluating project risks in advance and developing mitigation plans where possible has prevented adverse cost and schedule impacts.


The 84-year-old Gilboa Dam is a key component of the New York City water supply system. The NYCDEP, along with its Design Team, a joint-venture of Gannett Fleming and Hazen and Sawyer, has embarked on a program to completely renovate and update Gilboa Dam. This program included interim stability and monitoring improvements-most notably the addition of 80 post-tensioned anchors through the control section of the spillway completed in December 2006. The project team is using a phased design and construction approach divided into 6 construction contracts to address the significant deficiencies as rapidly as possible. The team's approach includes measures to foster public awareness, enable early regulatory involvement, account for risks in advance to mitigate adverse cost and schedule impacts, and several measures to instill confidence in the design and final project completion.

About the Authors

Rodney E. Holderbaum, PE
Vice President
Gannett Fleming, Inc.
P.O. Box 100
Harrisburg Pennsylvania

Mr. Holderbaum is vice president of Gannett Fleming, Inc. and manager of the firm's Dams and Hydraulics Section. His career includes more than 30 years' experience in the inspection, design, and construction of dams. He has participated in inspection, analysis, design, and/or construction management for over 200 dams. He has served as project manager and/or principal designer for several major dam rehabilitation projects, including New York City's Gilboa Dam Reconstruction Project. He is a registered professional engineer in seven states, author of numerous papers on the topic of dam design and construction, and a member of the American Concrete Institute, ACI Committee 207-"Mass Concrete", US Committee on Large Dams, and the Association of State Dam Safety Officials.

Paul Costa, PE
Design Accountable Manager NYC Environmental Protection
New York City Department of Environmental Protection
Bureau of Engineering Design and Construction
96-05 Horace Harding Expressway
Corona, NY 11368

Mr. Costa joined the NYC Department of Environmental Protection in 1993 and has been the Project Manager for NYCDEP's Dam Reconstruction Program since 1999. New York City's Dam Reconstruction Program has been ongoing since the mid-1980s. DEP is upgrading all of its dams and spillways to comply with state guidelines, starting with its oldest dams in the East-of-Hudson watersheds. Twelve dams have been upgraded over the last 25 years, at a cost of more than $248 million to date. DEP plans to commit another $417.5 million until 2021 to complete the dam reconstruction program. The city owns 32 "high hazard" dams based on the likelihood of serious economic damage, environmental harm, and loss of human life if they were to fail. In addition, DEP has purchased 69 small dams through the Land Acquisition Program since 1997. Mr. Costa is responsible for development and implementation of studies, designs and construction contracts for the implementation of upgrades, rehabilitation and repairs to NYC dams. Mr. Costa's division provides these efforts in support of the NYCDEP's Bureau of Water Supply, the DEP's operational Bureau that is responsible for the operation and maintenance of NYC Watershed Dams. Mr. Costa, a licensed professional engineer, received a Bachelor of Engineering from The City College of New York. Mr. Costa was born, raised and continues to live in NYC with his family.

Alon Dominitz, PE
Section Chief
NYS Department of Environmental Conservation
Division of Water / Dam Safety Section
625 Broadway, 4th Floor
Albany NY, 12233-3504

Mr. Dominitz joined the NYS Department of Environmental Conservation in 1993 and has been the section chief of the Dam Safety Section since 2005. Prior to becoming section chief, he was a senior engineer in the Dam Safety program for 6 years. Mr. Dominitz has represented New York State on dam safety issues to federal agencies and national associations. He has planned and provided technical assistance to state agencies related to dam emergencies. The New York State Dam Safety Program regulates more than 5,000 dams to protect lives, property, and the environment from the consequences of dam failure. The Section is responsible for enforcing New York's dam safety laws, including inspections, technical reviews, enforcement, and development and implementation of regulations and guidance documents.

Mr. Dominitz, a licensed professional engineer, received a Bachelor of Engineering and a Master of Science from The Cooper Union in Manhattan. After college and before joining the Dam Safety Section, he worked in New York City for the Department of Environmental Conservation on air quality implementation. Mr. Dominitz was born in Israel, and now lives near Albany, New York with his family.

Reprinted from the Journal of Dam Safety (2011), V9 n3, courtesy of the Association of State Dam Safety Officials (ASDSO),
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