Slightly battered, but still emitting coded chirps, a small camera-studded robotic submarine was raised from the Delaware Aqueduct this morning after a 15-hour trip seeking leaks in the 58-year-old upstate water tunnel, vital to New York City's water supply.
In coming days, city engineers and consultants will determine if the torpedo-shaped device succeeded in its mission: to photograph every foot of a 45-mile stretch of the aqueduct where water is escaping and -- most important -- help determine if the seepage threatens the tunnel's integrity.
The aqueduct, hewn over six years from solid rock, is one of the most important links in the city's far-flung system of 19 reservoirs, with the tunnel typically carrying more than half of the water used each day by 9 million people in the city and some of its suburbs.
Leakage of up to 36 million gallons a day was detected starting in 1991. The leaking stretch lies somewhere between the Rondout Reservoir in the Catskills and the West Branch Reservoir, a way station for city-bound water here in Putnam County.
The escaping water is just a small percentage of the 1.3 billion gallons supplied by the system each day, but still equals the daily consumption in Rochester. Water percolating upward hundreds of feet from tunnel leaks has created wetlands and damp areas in Ulster and Orange counties that endure even in the region's worst droughts. But the main impetus for the $2.2-million inspection project, city officials said, was the need to determine if the leaks could erode the aqueduct walls and overlying rock.
''Risk analysis using existing data shows we're O.K.,'' said William Meakin, the chief of facilities improvement for the city Department of Environmental Protection. But, he added, ''as with most things, anything engineers build or mankind makes, it deteriorates.''
The leaks increased from 1991 to 1997, but in a news release today, the department said they had not grown since then. The department said that ''independent engineering analyses have confirmed that the aqueduct is not in danger of collapse.'' The statement made it clear, though, that finding and repairing the faulty spots was essential. ''If not fixed,'' the department said, ''over many years the leaks could develop into threats to the aqueduct.''
Using data and images gathered by the probe, the city will update its risk calculations in coming months. Over all, Mr. Meakin said, ''We're confident that something isn't suddenly going to happen.''
The project that unfolded over the last 24 hours took three years of planning, tinkering, experimentation and trial runs. In 1999, city engineers concluded that a new survey was needed. The last inspection took place in 1958, when the 13-foot-diameter tunnel was drained and engineers examined it by driving through its length in a Jeep. But the risks of draining the tunnel again without knowing the extent of any damage were too great; without the supporting pressure of the water, a wall weakened by the seepage might crumble. So some means had to be found to survey the conduit while it was still filled with water. With existing technology four years ago, this was impossible.
There was no underwater remote-control robot with a tether longer than six miles. Although one company offered to drop a piloted mini-sub into the aqueduct, city officials turned it down, saying the risk was too great to send a person on a voyage in a flooded tunnel cutting deep beneath the Shawangunk Mountains and 550 feet beneath the bed of the Hudson River.
The challenge was tackled by marine engineers from the Woods Hole Oceanographic Institution, on Cape Cod. Led by Ben G. Allen, the team there had built more than three dozen self-piloting submersibles, mainly for missions like finding mines or charting the sea floor.
Working with ASI Group, a tunnel-inspection company from St. Catharines, Ontario, the Woods Hole engineers devised a specialized device for the project and the means to get it down narrow shafts, one of which was 1,000 feet deep, and up again.
After 16 hours of listening for the probe from shafts along the aqueduct's length, the team now huddled in a cavernous chamber by the shore of the West Branch Reservoir as the 800-pound craft, rose dripping and chirping from a 300-foot-deep, water-filled access shaft. There was scattered applause. This was the first time anyone had sent an untethered probe through such a long tunnel.
The device's two names reflect the extreme nature of its assignment. City engineers call it Uliisys (pronounced like Homer's hero), for Underwater Linear Infrastructure Investigation System, while its inventors, from Woods Hole, call it Persephone, after the Greek goddess of the underworld.
But there was no time for celebration. The trip through the tunnel had taken two hours longer than the 13 hours they had estimated, and everyone had a theory about what had gone wrong. Jaws were tight.
Immediately, Mr. Allen noticed that one of the eight whisker-like titanium wires radiating from the 9-foot long, 16-inch diameter craft's nose to soften collisions was broken off. A closer look revealed that it had been worn away as the probe apparently scraped along one wall of the tunnel.
The craft had been carefully designed and programmed, using navigational gear and acoustic beacons, to stay dead center in the tunnel so the five cameras ringing its nose could capture a 360-degree view of its walls.
Had the systems failed before, or after, the probe passed the two regions where leaks were already known to exist?
Team members quickly plugged in a yellow cord to start loading the craft's enormous cache of data, including 160,000 digital photographs, into computers. The cameras alone held 600 gigabytes of information -- as much as 60 fairly capacious desktop computers.
Roger Stokey, a Woods Hole engineer sitting in a control room in front of a five-foot array of computer screens, began replaying the probe's trip. Animated graphics showed that the probe held its position in the center of the tunnel until the point, six miles from the end, where it encountered a short left and right jog -- about where it passed deep beneath Route 9 in Fishkill, Dutchess County.
There it collided with one wall and, perhaps through a programming glitch, its rudder stayed to one side, forcing the nose to push against that wall.
It scraped along the final miles, not only shaving off one of its whiskers, but also cutting its speed sharply.
''Well, it needs to be a little smarter next time,'' Mr. Stokey said. ''It's a form of insanity to keep doing the same thing and expect a different result.''
Luckily, Mr. Stokey noted, it appeared that the earlier part of the mission went fine, including the spots west of the Hudson where the worst leaks were known to lie.
It would take nearly two days using a high-speed connection to transfer all the data from the submersible and many weeks to begin sifting the photographs, he said.
Many people are eager to see the results. Riverkeeper, a private environmental group that, among other things, monitors the city water supply, had been pressing the city for years to press ahead with the tunnel inspection and repairs.
''It's great that they've taken this important first step,'' said Marc A. Yaggi, the lawyer tracking water-supply issues for the group. ''It's very critical that they keep on top of this. If the aqueduct were to collapse, the city could run out of water within 80 days.''
At the shaft house near the West Branch Reservoir, the team worked into the night, watching bars bump up on the computer screens as data steadily flowed from the little submarine.
|Author:||Andrew C. Revkin|