United States Army Corps of Engineers (USACE) personnel were close to finishing their New York dewatering mission this week after Hurricane Sandy’s 4m high tidal surge flooded the city’s tunnel system.
The Corps had pumped most of the water from the last of the flooded tunnels - the Downtown Hudson River Tubes connecting New Jersey and Lower Manhattan as NCE went to press.
“I am reluctant to give time scales but it should be cleared very soon,” said USACE design branch chief Roger Less.
The Federal Emergency Management Agency (Fema) had assigned the Corps 11 key dewatering projects after Hurricane Sandy inundated tunnels and underpasses last month, (News last week).
“After Hurricane Katrina we gained a reputation for being able to remove water,” added Less.
One of the hardest hit was the twin bore 1.72km long Downtown Hudson River Tubes, which were filled with water from the storm surge.
The tunnels carry Port Authority Trans Hudson (Path) commuter trains from Hoboken station in New Jersey under the Hudson River to Ground Zero in Lower Manhattan.
The location of one of the portals, in the Ground Zero construction site added an extra complication to the dewatering exercise, according to Less. Ground Zero had been flooded with over 370M.l of seawater.
“We had to get the amount of water out [of the Ground Zero site] while concurrently starting on the Jersey side,” said Less.
“On the Jersey side it was just the logistics - pumping the water up through the terminal building,” he added.
The tunnel had been flooded from both sides - through Ground Zero and through Hoboken station, which was full of water following Hurricane Sandy.
While efforts on the Manhattan side of the tunnel focused on clearing the construction site of water, Less and his team began dewatering the tunnel from the New Jersey portal.
Using naval contractor Donjon - which has a background in clearing out submarines - USACE began dewatering through the ticketing hall level at Exchange Place.
“First off we had to pump out the terminals [on the Jersey side] and platform before we could start removing water from the tunnel,” explained Less.
Using submersible pumps the team cleared out the station before pumping water from the tunnel tubes.
Safety considerations limited the number of pumps which could be used to clear the tunnel.
“We’re constricted on how much hose line we can get down there while still maintaining the safety for our workers,” said Less.
Major safety considerations include air quality, as well as having a rapid worker evacuation strategy.
“We cannot have a rapid evacuation if the stairwells are full of hoses,” he added.
To dewater the tunnel the Corps used a 452mm, 3.75M.m3 per minute pump to get water from the tunnel into a sump close to the entrance.
They then used a 305mm wide centrifugal pump at 2.19M.m3 per minute to pump the water from trackbed level to the surface.
“The issue was the head height,” explained Less.
“It is going up 128 steps - it’s a rather long run.” The pumps needed to transfer the water up about 38m.
The Corps’ work has mainly been focused on the northern tunnel bore.
Engineers from the New York Fire Department and tunnel owner the Port Authority of New York and New Jersey are helping to dewater the southern tunnel using a 152mm wide hose with a 1.4M.m3 per minute pump and a 100mm wide hose and 702M.m3 per minute pump.
Once the water is completely removed, Port Authority engineers will fully inspect it before reopening timescales can be given.
New York governor Andrew Cuomo said initial reports suggest that the tunnel will not reopen for several weeks as it has suffered significant damage.