New York City's Bypass Surgery

The rig in Shaft 5B drills a series of holes for explosive charges. Each blast moves the floor of the shaft down another 10 feet.

*Photo Credit: Courtesy of the N.Y. Dept. of Environmental Protection*

The submarine-like remote operated vehicle was used to identify the leaks in the Delaware Aqueduct near Wawarsing.

*Photo Credit: Courtesy of the N.Y. Dept. of Environmental Protection*

An autonomous underwater vehicle took 360° photographs every eight feet of the leaking section of the Delaware Aqueduct under the Hudson River.

*Photo Credit: Courtesy of the N.Y. Dept. of Environmental Protection*

When the TBM has done its work, a second phase of the project will begin, the drilling and blasting of the 100-ft connections at either end of the bypass tunnel. The bypass tunnel will be 1⁄2 in. larger in diameter than the existing tunnel in order to make up for the loss of water pressure that comes from diverting the flow of water to and from the bypass. The exact angle of the connection tunnels has not yet been determined but the greater the angle of the connection, the more pressure will be lost.

Aside from the engineering feat that the New York City water system represents in the construction of hundreds of miles of aqueducts and tunnels—the dams, reservoirs and tunnels of the original Croton section of the system began in the mid-18th Century— the other remarkable aspect of the system is that it works on gravity without the aid of mechanical pumps except for buildings taller than five or six stories.

Generally speaking 100 ft of elevation yields about 50 psi. The Roundout Reservoir where the Delaware Aqueduct starts its run to New York City is 840 ft above sea level. The highest point in Manhattan in is a point in Bennett Park in Fort Washington that is 265 ft above sea level.

The highest point in the city is Todt Hill on Staten Island, which is 425 ft above sea level.

But, of course, the original engineering of the system was not as simple as letting water run down hill. The lowest point in the water system is where the Delaware Aqueduct crosses to the east side of the Hudson River. From there a siphon junction raises the water and re-establishes the water pressure for the 50-mile trip to the New York City border.

The same pressure that moves the water from the Catskills and under the Hudson also presents one of the biggest challenges of the bypass project.

Current plans call for the water in the Delaware Aqueduct to be turned off in the fall of 2022 and to remain off for five to eight months. The last time the aqueduct was shut down was in 1957.

The city has been pondering how to make up for that loss for decades. Some of the shortfall requirement has been mitigated by the city’s ongoing efforts to reduce consumption. Those efforts have reduced peak demand from 1.6 billion gallons per day in 1979 to the current 1 billion gallons per day.

The city has also brought the Croton system, which can supply up to 25% of the city’s needs, back into operation following the completion early this year of a $3.2-billion filtration system under Van Cortland Park in the Bronx.

Having the confidence that the city would not run short of water while the Delaware Aqueduct was turned off was a critical factor that enabled the bypass project to move forward.