From the inside out: Analyzing the pressure tunnel that carries New York City water

Transporting materials in Rondout pressure tunnel.

Transporting materials in Rondout pressure tunnel.

For more nearly a week in October, a remote-operated vehicle worked to pinpoint suspected leaks in a portion of the Catskill Aqueduct that runs several hundred feet below the Rondout Creek in Ulster County. The vehicle used high-definition video cameras, acoustic equipment and other instruments to examine this portion of the aqueduct, known as the Rondout Pressure Tunnel. Data gathered through that analysis will be used by the New York City Department of Environmental Protection (DEP) to design necessary repairs. DEP anticipates those repairs will happen some time after it completes work for the Delaware Aqueduct bypass tunnel under the Hudson River, scheduled to be finished in 2023.

The Rondout Pressure Tunnel is a 14.5-foot-diameter pipe that stretches four and a half miles (23,608 feet) from a wooded area north of Stone Ridge to a site within the Mohonk Preserve. Water within the Catskill Aqueduct travels at ground level before it enters the pressure tunnel, plunges approximately 500 feet below the surface to convey that water under the Rondout Valley, and then rises back to surface level on the eastern side of the valley, where the water continues its journey south to Kensico Reservoir in Westchester County.

The Rondout Pressure Tunnel is the longest of seven pressure tunnels that allow the Catskill Aqueduct to carry drinking water beneath broad valleys that include creeks or rivers. Other pressure tunnels of the Catskill Aqueduct carry water below the Wallkill River, Moodna Creek, Hudson River and New Croton Reservoir.

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Pressure tunnels comprise about 15 percent of the total length of the 92-mile Catskill Aqueduct. About 55 miles of the aqueduct are built through cut-and-cover methods, where a trench was excavated and the aqueduct was built at the surface. About 14 miles of the aqueduct are grade tunnels cut through hills or mountains. Through the cut-and-cover and grade-tunnel sections, water inside the Catskill Aqueduct is not under pressure. Rather, water inside the aqueduct flows like an enclosed river.

The remaining 23 miles of aqueduct is comprised of pressure tunnels and steel-pipe siphons that plunge downward into the earth and then return to surface level. In these sections, the aqueduct is under great pressure from water pushing down on itself and outward on the aqueduct walls.

 

 Rondout Pressure Tunnel leaks 

DEP believes the Rondout Pressure Tunnel has developed leaks in at least three locations. The location of only one leak — believed to be the largest — is known for certain. That leak is located below a DEP facility in High Falls known as the Rondout Drainage Chamber, built in the 1910s at the same time as the aqueduct. The drainage chamber, located at the surface about 475 feet above the Catskill Aqueduct, was designed to empty this portion of the aqueduct by opening a valve, allowing the aqueduct water to rise to the surface and flow into the nearby Rondout Creek. The chamber’s original design also included provisions for pumping the last of the water from the tunnel into the creek.

The leak at this site is coming from around the valve itself, which is also 475 feet below ground. DEP believes that stress over time has caused some elements associated with the valve to deteriorate or lose function, allowing leakage through or around the valve. As a result, water at this site is steadily coming to the surface, through a culvert connected to the chamber, and into the Rondout Creek. While the exact amount of leakage is unknown, DEP estimates it is roughly three million gallons per day. (For perspective, gauge data shows the mean flow of the Rondout Creek at Rosendale is more than 175 million gallons per day.)

DEP plans to provide a temporary repair for this leak when the Catskill Aqueduct is shut down for ten weeks in the fall of each year from 2018 through 2020. Those shutdowns were already planned as part of the Catskill Aqueduct repair and rehabilitation project, which will remove biofilm (a thin, slimy but harmless film of bacteria that adheres to a surface) from inside the aqueduct and replace other century-old valves connected to the aqueduct. The biofilm creates friction inside the aqueduct, slowing and reducing the water flow. By removing it, DEP expects to regain 40 million gallons of capacity in the aqueduct.

During one of those years, the valve below the Rondout drainage chamber will be removed and the pipe it’s connected to will be sealed with a specially manufactured plug. Because the Rondout Pressure Tunnel cannot be drained, this work will be performed by a trained diver in an atmospheric diving suit.

In addition to the drainage chamber, DEP believes the Rondout Pressure Tunnel has developed leaks in at least two other locations. These much smaller leaks have created surface expressions on vacant portions of privately-owned land in the vicinity of the tunnel.

 

Designing repairs

DEP sent the remote-operated vehicle through the tunnel to pinpoint the other leaks. The operation marked the first time anyone has seen the inside of the Rondout Pressure Tunnel since construction was completed in 1913. The vehicle was connected to a long wire that transmitted high-definition video, acoustic information and other data back to a work trailer.

Cameras and lights positioned around the entire circumference of the vehicle provided a 360-degree view of the tunnel in areas where leaks were suspected. Food dye was injected to help engineers see whether it leaves the tunnel through a crack, indicating a leak location. Data gathered by the remote-operated vehicle will take many months to analyze before engineers can draw accurate conclusions about leak locations or quantities.

The U.S. Geological Survey (USGS) will also begin work to monitor the leaks and what impact they might have on local groundwater levels. This may be done through the installation of monitoring wells in the vicinity of the Rondout Pressure Tunnel.

Data gathered by the remote operated vehicle and USGS will inform the design of a permanent repair for the leaks. The repair work will likely be complex because the deep-rock tunnel always remains filled with water, always under pressure, and no mechanism currently exists to pump it dry.  A method for pumping the tunnel dry and entering it safely will likely need to be incorporated in plans to fix the leaks.

 

Pressure tunnel history

Workers encountered especially difficult conditions and problematic geology when they constructed the Rondout Pressure Tunnel from 1910 to 1913. Generally, pressure tunnels were driven through dense bedrock and lined with thick concrete. But the underlying geology of the valley required workers to drive the Rondout Pressure Tunnel through poor-quality rock. Field notes from that time indicate “badly folded and crushed strata of High Falls shale and Binnewater sandstone” that allowed nearly two million gallons per day of groundwater to pour in on the workers as they pushed ahead. Historic records indicate that the inflow of water made it difficult for workers to lay concrete for the tunnel lining.

In fact, the original Rondout Pressure Tunnel failed a hydrostatic test in 1912, before the Catskill Aqueduct was put into service. Cracks in the tunnel that year allowed groundwater to flow in. Repairs were made by welding steel rings encased in concrete into the weak sections of the tunnel, providing additional support and reducing the amount of leakage. The current-day surface expressions are located in the vicinity of the problem areas recorded in historic records when New York City built the tunnel.

The Catskill Aqueduct carries drinking water 92 miles from Ashokan Reservoir in Ulster County to Hillview Reservoir in Yonkers, on the northern edge of the Bronx. The aqueduct conveys about 40 percent of New York City’s drinking water on an average day, and it can deliver a maximum of 590 million gallons per day. By fixing the leaks, removing the biofilm, fixing the minor cracks in the walls and performing other repairs, DEP expects to increase that maximum. Historic records show the Catskill Aqueduct once had a maximum capacity of 660 million gallons per day.

The Catskill Aqueduct first delivered water to New York City on Decmber 27, 1915, starting with water for the Bronx only. The water that came from the Catskills through the aqueduct was key to allowing New York City to grow through the industrial and population booms that followed World War I. The Catskill Aqueduct is also the primary water source for several upstate communities, including High Falls, New Paltz and New Windsor.

 

– Adam Bosch (courtesy of New York City Department of Environmental Protection)

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