Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Rehabilitation Boosts Resiliency of New York Subway Facilities

By Kevin Wilcox

A new phase of Sandy recovery focuses on bolstering the critical transit system against future storms and sea level rise. 

 

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Water ingress will be prevented by portable barriers that can be stored in stations and deployed quickly over openings, including ventilation grates. Courtesy of ARCADIS

December 16, 2014—More than two years after Hurricane Sandy inundated the New York City subway system with a massive storm surge, a new round of projects is under way to construct permanent repairs and add resiliency to ensure that the system is better able to withstand such a storm in the future.

The projects are being implemented by the Metropolitan Transportation Authority's (MTA) New York City Transit (NYCT), which has issued a series of task orders to six engineering and design firms to develop repairs and resiliency solutions for the system. The design criteria are based on the storm surge of a category 2 hurricane, with a 3 ft freeboard to accommodate sea level rise projections.

"Sandy really created havoc in New York and New Jersey, and particularly the subway system in New York City. Almost 47 miles of the tracks were under water. They were inundated with salt water," says Sufian Khondker, Ph.D., P.E., D.WRE, F.ASCE. Khondker is the national technology director for water management at ARCADIS, a global engineering firm that is one of the companies working on designs for the project.

Nine of the 14 tunnels under the East River and Hudson River were flooded and damaged by Sandy, which struck at high tide, delivering a storm surge of more than 13 ft. Accompanying power outages delayed pumping activities for 10 days, leaving critical cables, switching equipment, and pumps submerged in salt water, which ten resulted in corrosion.

This new round of projects is projected to be completed within five years, the design work fast-tracked to be completed in as little as six months from the release of each task order, Khondker says.

 "Many stations were damaged, but not to the extent of lower Manhattan," Khondker explains. "Now that NYCT has a little breathing time, before another Sandy hits, they would like to make those structures hardened and take the necessary precautions so similar damage does not occur again."

ARCADIS is recommending a three-tier approach to the MTA. Tier one is preventative measures designed to better seal the 540 entry points that allow water ingress into the subway system. These access points include not only passenger entrances, but also ventilation grates, cable conduit openings, and elevator shafts openings.

"Every station has four to six passenger entrance/exit openings. You cannot close those up for an extended period," Khondker says. "Your solution has to be a removable system, so that you can install it in two to three hours the moment you've got a warning of a hurricane or strong storm, [to] close off entrances so that water cannot enter."

Such a system will likely take the form of a permanently installed frame around openings coupled with removable, aluminum shields assembled from components weighing no more than 70 lb and made watertight by employing a system of gaskets. Ideally the removable components would be stored in each station for rapid deployment.

For such larger facilities as rail yards and power substations, ARCADIS is examining the use of conventional reinforced-concrete walls, though this is not necessarily a preferred solution. Such vertical structures as walls can magnify the wave forces onto adjacent properties, so the engineering team is considering innovative solutions to minimize that impact. "The maximum damage is done when the waves hit the vertical wall and are reflected with as much as twice the energy," Khondker says. "That causes problems to the neighboring properties. We can design those walls such that there is not going to be any reflection of the waves. It will absorb the wave energy. That can be done with inclined walls, or fluted walls, or some other concept such as a gabion wall that absorbs wave energy and will have absolutely minimum impact to the neighboring properties."

The second tier is a mitigation approach that acknowledges that it might be neither feasible nor cost-effective to seal all the entry points. In this case, hardening such critical assets as signal systems, power substations, pumping stations, and other critical components will be necessary. Some of these crucial assets might be elevated, placed behind an enclosure with marine-safe doors, or replaced with components designed to be submerged in saltwater.

The third tier, recommended for noncritical assets, will be to allow floodwaters to enter but to pump them dry as soon as possible. "There cannot be a blanket solution. Each site is going to have a customized solution, and how we address that is where our global experience and expertise comes into play," Khondker says. "The design criteria have been established. Now we have to go to the individual sites and see how to incorporate the design."

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