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From the Catskill Mountains to New York City faucets

New York City has been able to reap an impressive list of environmental, financial, economic, and aesthetic benefits from its improvements to the city’s water infrastructure.

Mott MacDonald's partnership with New York City agencies encompasses every stage of water delivery.


Each day, New York City provides more than a billion gallons (3.8 billion liters) of drinking water to more than nine million people. Each year, construction and maintenance of the city’s water infrastructure demands about 130,000 cubic yards (100,000 cubic meters) of concrete and the testing of 16,000 specimens.

Much of the city’s water infrastructure is more than a century old, and it must be kept working without disrupting transport and communications in a densely populated metropolis, and without interrupting trade in the country’s third largest port.


Since the year 2000, Mott MacDonald has worked with various New York City agencies in an extraordinary partnership to protect, maintain, and upgrade the infrastructure of the city’s water supply.

That partnership encompasses every stage of water delivery, from the aqueducts that bring drinking water from the Catskill Mountains, to the siphons that carry it under the East River from Brooklyn to Staten Island, to a sustainable new chlorination facility to ensure it remains safe to drink. We even help ensure the quality of the concrete and asphalt the city uses to build and maintain its water infrastructure.

Catskill-Delaware aqueduct interconnection

Together, the Catskill and Delaware Aqueducts deliver drinking water to New York City from six reservoirs located in the Catskill Mountains.

Constructed between 1907 and 1916, the Catskill Aqueduct runs 92 miles, beginning at the Ashokan Reservoir in Ulster County. Constructed between 1939 and 1945, the 85-mile (137-kilometer) Delaware Aqueduct is reportedly the longest continuous underground tunnel in the world.

Because heavy rainstorms cause turbidity in the water supply from the Catskill Aqueduct, we were retained in 2009 as part of a joint venture team to create an interconnection between the two aqueducts at Shaft 4 in Ulster County, where they cross.

The Mott MacDonald team performed the geotechnical investigations and the structural, architectural, and landscape designs for the Catskill-Delaware Interconnection. We also provided program management services for the overall joint venture contract, and technical reviews of the entire interconnection design.

The Interconnection can transfer up to 365 million gallons of water per day from the Delaware Aqueduct, reducing its pressure from as much as 100 pounds per square inch to open-channel flow conditions. We used Building Information Modeling (BIM) and physical scale models to optimize the hydraulic design of the Interconnection.

In May 2013, Department of Environmental Protection Commissioner Carter Strickland said, “Ensuring the delivery of more than 1 billion gallons of high-quality water to more than 9 million New Yorkers every day requires long-term planning, and the interconnect at Shaft 4 is an important project for the future of New York City’s water supply system.”

New York Harbor Water Siphon replacement

In 1917 and 1925, two water mains (known as siphons) were built to carry drinking water from Brooklyn to Staten Island. Located at depths of 56 and 60 feet (17 and 18 meters), they were laid into trenches at the bottom of the harbor in 12-foot (3.6-meter) lengths connected by flexible joints.

The Port of New York and New Jersey is the third largest port in the country, handling almost 40% of the East Coast shipping trade. But the port’s Anchorage Channel is 45 feet (13.7 meters) deep — not deep enough to accommodate the next generation of cargo mega-ships. Dredging operations are under way to deepen the channel to 50 feet (15 meters) below the mean low-water level over a 19,000-foot (5,800-meter) stretch from the Verrazano Bridge to the Port Jersey Channel.

Because the deeper channel may compromise the two aging water siphons, we were retained in 2005 as a joint venture partner to evaluate alternatives for replacing the existing siphons with one new 72-inch-diameter (1.8-meter) steel pipeline at a sufficient depth.

We recommended the use of a 72-inch-diameter steel siphon inside a tunnel with an overall diameter of 12 feet. The outer tunnel would be bored through soft ground at a depth of 100 feet (30 meters) and constructed with precast segmental lining.

Our recommendation was accepted, and we were retained to provide overall project management for the Joint Venture. Mott MacDonald is responsible for the design of the tunnel and siphon, the structural design of all valve chambers, and the abandonment of the existing siphons and their associated chambers. In addition to the water crossing, the project includes several thousand feet of water transmission mains on land, to connect the new siphon to existing pipelines.

The project also requires two trenchless crossings of the Staten Island Railroad, with 60-inch (1.5-meter) steel pipe installed in 84-inch (2.1-meter) steel casings. Passive cathodic protection is provided for all land-side steel pipelines.

The tunnel, under construction as of 2013, is the first major underwater soft-ground water tunnel in New York City, and the first tunnel of any kind built under the harbor in many decades.

Speaking in April 2012, Mayor Mike Bloomberg said, “New York Harbor has been a critical part of our economy since the founding of our great city some 400 years ago. And if we want New York City’s economy to stay competitive, we must accommodate new mega-ships and their cargo. This investment in our infrastructure will spur economic activity all along our working waterfront.”

Staten Island Chlorination Station

New York City drinking water is unfiltered, but chlorine for disinfection is added at the exit tunnels of the Kensico and Hillview reservoirs. Because chlorine dissipates with distance, more is added at locations including the Richmond Chlorination Facility, built in Staten Island in 1974.

This facility is located at the end of the Richmond Tunnel, the primary conduit for supply of water to Staten Island. As part of the Water Siphon Replacement project discussed above, we were asked to upgrade the chlorination station, keeping it hidden from planned residential development, and designing it to function as an attractive and sustainable “living building.”

We incorporated sustainable design concepts into both the site and building designs. Mowed lawns were replaced with wildflower meadows, and green walls and roofs were incorporated into the above-ground structures.

NYCDEP concrete quality

The New York City Department of Environmental Protection (NYCDEP) has an annual capital improvement plan of almost $2 billion — and that requires a lot of concrete. The well-being of New York City’s water system, infrastructure, and ultimately the health of the city’s people depends in part on the quality of that concrete.

As part of a joint venture, we were contracted by the NYCDEP to undertake quality assurance testing for the concrete and asphalt used in all its capital improvement construction projects. The contract was renewed once and awarded twice to the same project team for a seven-year period spanning 2000 and 2006.

Over the life of the project, our team inspected over 900,000 cubic yards (688,000 cubic meters) of concrete at 10,000 placements, over 63 New York City and upstate locations. In addition the team also performed the corresponding batch plant inspections and pre-cast plant inspections. A total of over 110,000 concrete samples and cylinders were taken and tested, using both NYCDEP and subcontractor laboratories.

More than 3 million pieces of data on concrete quality were entered into Quadrel, a real-time cloud-based Data Information System that can generate a range of strength and control reports. We provided the NYCDEP with access to a Technical Advisory Committee made up of national-level concrete experts, for consultation on technical topics.


With our help, New York City has been able to reap an impressive list of environmental, financial, economic, and aesthetic benefits from its improvements to the city’s water infrastructure.

The Catskill-Delaware interconnection will reduce the turbidity of the city’s water supply, lessening the need for chemical water treatment while maintaining high-quality water without filtration. The Interconnection is designed to be below-ground, preserving the agricultural environment of the site and minimizing the creation of impervious surfaces that increase stormwater runoff. Our extensive use of BIM and visualization tools optimized hydraulic design, minimized aqueduct outages, and helped communicate the features and benefits of the Interconnection to stakeholders.

The New York Harbor Water Siphon will support the next generation of ocean shipping, one of the most economical and environmentally sustainable means of transporting freight. According to Staten Island Borough President James Molinaro, “The dredging of the Harbor will allow Staten Island’s shipping and receiving industry to remain competitive throughout our region, protecting existing jobs and encouraging the development of new ones. Additionally, the new water siphon will ensure that our Borough’s water needs are met, especially as our population continues to grow.”

The Staten Island chlorination project has increased the aesthetic appeal of Staten Island’s chlorination facility, reduced its environmental impact, cut its maintenance costs, and extended its useful life. Green design protects the roof from damaging UV rays and acid rain, and green walls extend the building’s life by minimizing temperature fluctuations that expand and contract the building envelope. The use of recycled water avoids additional water demand, and the use of wildflowers and other native, noninvasive species in landscaping eliminates the cost and environmental impact of fertilizers, pesticides, mowing, and trimming.

Finally, the NYCDEP concrete quality initiative not only ensured the quality of concrete over the period of the contract, but gave the NYCDEP tools to ensure quality into the future.

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