Partial Delaware Aqueduct drawdown to begin Oct. 16

The New York City Department of Environmental Protection will partially dewater the Delaware Aqueduct for three weeks starting on Oct. 16 in a trial run of a complete shutdown lasting months beginning next October to make repairs to the leaky tunnel.

The temporary shutdown will help the DEP assess what challenges it might face during the prolonged shutdown in 2024, Jennifer Garigliano, the department’s director of water resources management, explained at an Upper Delaware Council meeting on Thursday night.

And those challenges might be considerable.

A trial shutdown in March revealed that water was infiltrating the pipeline at a “much higher rate” than anticipated, she said.

DEP officials had expected 20 million gallons of water per day would need to be pumped out. Instead, they discovered it would be as much as 35 million gallons.

That discovery is what prompted the DEP on June 28 to announce that it was again postponing the project to repair the leaks. (The DEP had expected to shut down the aqueduct in the fall of 2022, but in June of that year it announced it was pushing it back a year.)

Officials have said the latest delay will allow extra time to install additional pumping, power and backup electrical equipment to ensure the safety of workers, who will be 700 feet underground during the complex capital repair project.

What happens during the temporary shutdown?

The dewatering later this month will take place in stages and therefore will extend the trial run by a week compared to the temporary dewatering effort in March.

Water will be drawn down and then progress incrementally from there to a predetermined level, though the pipeline will not be completely emptied.

The inflow, which is largely coming from groundwater, will be pumped from the tunnel and directed into the Hudson River.

Once the trial drawdown is completed, engineers will compile a report. “We have new math and we’re going to redo that math one more time,” Garigliano said.

What could go wrong?

There have been issues raised about a potential catastrophic collapse of the tunnel after it’s dewatered because of the lack of hydrostatic pressure.

Asked about the possibility of a collapse, Garigliano said there was a “very small chance” of that happening.

“It’s in the back of everybody’s mind the whole time,” she said, adding, “The chances of that happening are not great.”

There have also been worries about flooding if water levels in the reservoirs get too high.

If so much water is regularly draining off the reservoirs, and that flow gets shut off for five to eight months, where will all that water go?

For the trial shutdown, the city reservoirs will not need the large void, or reduction in volume, that would be sought when the full shutdown happens, Garigliano said.

Ahead of the October 2024 shutdown, the DEP will draw down on the reservoirs but releases will be based on hydrologic conditions at the time.

“It really depends on what Mother Nature throws at us,” she said.

Why are the repairs needed?

The Delaware Aqueduct, which runs under the Hudson River, delivers about half of New York City’s water supply — typically about 600 million gallons a day — using only gravity to carry the water from four Catskill Mountain region reservoirs.

For decades, the DEP has known about leaks in the Delaware Aqueduct that are leading to the loss of as much as 30 million gallons of water per day.

In 2010, the DEP announced plans to repair the aqueduct by connecting a 2.5-mile-long bypass tunnel around the area of the largest leak in the Town of Newburgh in Orange County, N.Y.

The other significant leak, in the Town of Wawarsing in Ulster County, will be repaired by grouting the cracks.

At a cost of $1 billion, fixing the 85-mile-long tunnel (the longest in the world) represents the largest and most complex repair project in the 180-year history of New York City’s municipal water supply system.

The last time the pipeline was completely dewatered was in 1957-58, Garigliano said. In her presentation, she displayed a black-and-white photo of a military jeep that fit in the pipeline at the time.