NYC has one of the most intricate mass transit systems in the world, supporting around 8 million people each day, but its dependence on decades-old pumps and an ill-prepared city drainage system makes it difficult to recover from disasters.
On a normal day, sans Sandy, the NYC subway system can expect to pump anywhere from 13 to 15 million gallons of water out from the subways. Last night, however, Hurricane Sandy wreaked havoc on a vital part of the city's infrastructure — at the South Ferry hub in lower Manhattan, MTA Chairman Joseph Lhota said that water levels were "ceiling high." Meanwhile, there are seven subway tunnels under the East River that are flooded. So even with working pump systems, extra pump trains and portable pumps, there isn't an exact time frame for how long before all the water is cleared out.
The NYC system has over 700 pumps, some built as early as 1914 (according to a Gizmodo story that is currently down) that work to move water up and out of subway tunnels. The current pump system can handle 1.5 inches of rain per hour and winds of 39 mph or less, drastically less than what Sandy brought last night. When pressure sensors inside the sump pumps detect high water pressure, they activate an impeller — similar to a propeller, but instead of moving something forward through water, it sucks water in — which is then pumped through pipes back up to the ground to travel through normal city drainage.
Seems simple enough, but problems arise when the drainage system itself becomes inundated, sending that pumped-out water right back down through grates and subway stairwells, overwhelming the pumping system. New York City's outdated sewage system is often criticized, urged to make improvements in storage tanks for processing large amounts of rainwater runoff as well as more updated computer systems that can better monitor the flow (or overflow) of water in the pipes.
A bigger issue with the pump system, however, is that it runs on electricity—and during severe flooding, like last night, must be turned off to prevent further disaster. In a New York Times article from 2007, the author explains, "As water seeps onto subway tracks, it is electrified by the 600 volts running through the third rail, causing the water to boil and setting floating debris on fire."
That's why subways were shut down on Sunday, and why it's going to take even longer to get things back up and running in the flooded stations — the pumps installed throughout the subway system to remove water run on electricity and "will not function if electric power to the system is interrupted," read an MTA statement. At these stations, the MTA will deploy one of the three "pump trains" that can manually pump about 300 gallons of water every minute, along with a fleet of portable pumps that can pump 600 gallons per minute.
A Columbia University case study for a "100-year storm" in New York City put the pumping time frame closer to 21 days if all 14 tunnels had flooded — based on Chairman Lhota's sentiments, the study's predictions don't seem as far-fetched.
Since New York City officials closed the entire subway system on Sunday in advance of Hurricane Sandy, the five boroughs have felt more like an archipelago than one massive, hyper-connected place. The faster it comes back together, the faster this city will start feeling like one big city again.