DEP seeks new craft to probe Delaware Aqueduct

DEP seeks new craft to probe Delaware Aqueduct
By ROGER WITHERSPOON
THE JOURNAL NEWS
(Original publication: December 5, 2004)

The Department of Environmental Protection is seeking a new remotely piloted submarine with a 16.5-mile tether — a length never before attempted — in an attempt to figure out why the Delaware Aqueduct continues to lose 1.2 billion gallons of drinking water a month.

The remotely operated vehicle, or ROV, would be used to provide detailed information about the extent of two sets of cracks in the concrete and steel water tunnel. The aqueduct supplies up to 70 percent of the drinking water for Westchester and Putnam counties and New York City.

The monthly water loss represents more than the combined daily water usage of Pelham Manor, New Rochelle, Eastchester, Port Chester, Dobbs Ferry, Greenburgh, Mount Pleasant, Pelham, Mamaroneck, Harrison, Rye, Rye Brook and Larchmont.

"It certainly is a challenge," said John McCarthy, vice president of the New Jersey-based Malcolm Pirnie engineering firm, which will oversee the project. "It's never been done at this length. There has been no need for it anywhere else in the world. It's quite rare to find someone who needs a 16-mile cable."

The remote-control crafts are usually used in the open ocean and are dropped a mile or two to explore such sites as the wreck of the Titanic in the North Atlantic. The longest tether, only six miles, was used in a water tunnel in Helsinki, Finland. DEP officials said the tether for the aqueduct will have to be unusually long to enable engineers to control the craft between the widely spaced access shafts.

The Delaware Aqueduct is the largest and most crucial of the three aqueducts that bring water from New York City's network of reservoirs and lakes to a system that serves 9 million residents. The aqueduct was built in three sections, which range from 13.5 feet to 19.5 feet in diameter, and was cut through bedrock that is between 300 feet and 2,400 feet deep. The section to be explored, under the towns of Wawarsing and Roseton on the western side of the Hudson River, starts at 800 feet below the surface and slopes downward to 1,200 feet.

The aqueduct crosses two seismic fault lines, originating in Wappingers Falls, which are believed to be responsible for the cracks that were first detected in the aqueduct around 1990. The faults became active in a 1974 earthquake triggered by extensive surface mining of a rock quarry in the town. Leonard Sykes, Higgins professor of earth and environmental science at the Lamont-Doherty Earth Observatory in Palisades, said most of the eastern U.S. is under tremendous horizontal pressure.
"When you remove a vertical load like a rock quarry," said Sykes, "you can move a fault towards failure."

The leaks in the aqueduct at Roseton and Wawarsing have pushed through the cracked bedrock to the surface in the Roseton area, creating a freshwater spring, a small lake and a 35-foot sinkhole. A horizontal tunnel drilled parallel to the aqueduct found that the water came through the fractured rock with such force that it eroded the limestone bedrock and created an unstable and heavy water-filled slurry.

For this reason, the DEP cannot simply drain the tunnel to examine it because only the pressure from the water is keeping the Delaware from being crushed by the slurry.

McCarthy said the water pressure in the tunnel is about 640 pounds per square inch, about 21 times the pressure in the average bathroom shower. The DEP's intention is to close off the aqueduct for about 10 days so it remains filled with water, but the water would not be moving and pulling on the tethered craft.

William Meekan, the DEP's project manager, said the agency wanted to use an ROV two years ago, but companies that make and operate the machines were not willing to try to make a vehicle with such a long tether. But with improvements in technology and materials, he said, several firms now believe the long exploration line is possible.

In June 2003, the DEP launched an AUV, or autonomous underwater vehicle, built by the Woods Hole Oceanographic Institution in Massachusetts, which floated through the center of the tunnel and took pictures of a 44-mile segment from the Rondout Reservoir in Wawarsing to the West Branch Reservoir in Kent.

"The AUV gave us a quick snapshot in a small amount of time," Meekan said. "The information was useful in helping us pinpoint areas we want to look at."

The ROV, however, will be able to stop at specific locations, maneuver to the wall and perform a variety of functions, including releasing dyes to test the water's flow into the cracks and drilling through the steel lining and concrete to take samples of those materials and the fractured rock beyond. McCarthy said the craft should be ready to operate in late 2006.

Reach Roger Witherspoon at rwithers@thejournalnews.com or 914-696-8566.Reach Roger Witherspoon at rwithers@thejournalnews.com or 914-696-8566.


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