Pressurizing the Catskill Aqueduct to Increase New York City’s Water System Reliability

Last Modified: Nov 18, 2009


  • Roopesh Joshi and Michael Broder, P.E., Hazen and Sawyer
  • David Tanzi, P.E., CDM
  • Swapan Gangopadhyay P.E. and Paul D. Smith, P.E., New York City Department of Environmental Protection (NYCDEP)
  • Leszek Glodkowski, Jenny Engineering Corporation

The NYCDEP is constructing an Ultraviolet Light (UV) Disinfection Facility at the Eastview site in Westchester County, NY, owned by the City, to treat the City’s Catskill and Delaware (Cat-Del) water supplies, which provide approximately 90 percent of the City’s water supply. Only the Delaware Aqueduct, which runs beneath the eastern portion of the site, has sufficient pressure to supply the 2-BGD water to the UV Facility. To use the 90-year old Catskill Aqueduct as a source of gravity supply requires a 2.5 mile segment, from the Kensico reservoir to the UV Facility, to be pressurized by over 40 feet of head to enable it to convey raw water to the Facility. There are several advantages to maintaining the service of this segment, one of which is to continue to use the aqueduct without changing to a pumping system. Pressurization would increase the hydraulic capacity of this aqueduct segment an additional 300 MGD and allow for reservoir bypass when needed, which provides advantages in system flexibility and operations. To establish the optimum approach for continuing to utilize this segment of the aqueduct a study was performed to determine the measures needed for pressurization, which included seven separate inspections of the aqueduct segment. Because the aqueduct is an active water supply to NYC and many other communities, each shutdown was limited to 24 hours, using procedures to yield the most information in the shortest possible time.

This project, one of a few of its kind, sets a precedent: it represents one of the largest aqueduct pressurization projects in the world for water supply. The conceptual design was developed taking into consideration several rehabilitation options for each section of the aqueduct segment, while carefully balancing considerations of costs, schedule and allowable leakage goals. The complexity of the design was increased because the aqueduct’s configuration is not homogenous. It is circular in some sections and horse-shoe shaped in others; some sections were constructed by cut-and-cover techniques, while others traverse rock. Because of these structural variations, each section requires different rehabilitation measures for pressurization. Therefore, a variety of materials for strengthening and waterproofing the aqueduct, and an extensive grouting program was developed.

This paper provides an overview of the approach taken for the inspections, the study of alternatives, and the conceptual design of rehabilitation for the Catskill Aqueduct, its existing structures and the new structures needed for pressurization of the 2.5 mile segment.

For a copy of the full paper, please contact the author at

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