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Superstorm Sandy: Fix&Fortify Efforts Continue

 

Lower Manhattan



Readying the Subway for Future Storms
ANTONIO CABRERA, P.E., Track Engineering Officer, Engineering, Department of Subways, MTA-NYCT

On Sunday October 28, 2012, I was onboard one of the last Metro-North trains to arrive at Grand Central before the railroad ceased operation and the Terminal was closed.  I took the Shuttle over to Times Square and then rode the 8th Avenue Local uptown to 50th St.  Even for a Sunday in the City, the number of riders was much lighter than usual.  Customers were heeding the advisories.  I reported to the Incident Command Center (ICC) situation room, located in NYC Transit’s Rail Control Center, at 3:00 p.m. – not knowing at the time that I would spend the next four days there.

Digitally-animated rendering of deployable subway station entrance cover, designed to seal station to protect against flooding.

During my 29 years working for NYC Transit I had never experienced – except for the December 1992 Nor’easter which was not even close in terms of magnitude and extent -- the effects of the storm surge flooding generated by Sandy.

That Nor’easter, with a modest peak surge of 4.3 feet, raised the high tide to about eight feet, which resulted in flooding of subway tunnels and area roadways including the Canarsie and Greenpoint Tubes, the FDR Drive, LaGuardia Airport and a Con Edison facility which shut power to parts of the subway system.  Hurricane Irene made landfall at Coney Island on August 28, 2011 but did not produce a storm surge even equal to that of the 1992 Nor’easter.  Nevertheless, we were greatly concerned as officials watched the storm surge rise at high tide at the Battery.  It had been much too close a call.

Determining how bad could it get
Following Hurricane Irene, I contacted OEM officials and received a revised SLOSH Model (2010 NY3 Basin Model) data set.  Based on this model, I re-evaluated the height of a potential Category 1 storm surge at NYCT’s facilities and recomputed the extent of possible subway tunnel flooding.  Using the track alignment drawings which also show the rail elevations in the tracks, I prepared Tunnel Flooding Maps for a Category 1 Hurricane.

These maps show the length of flooding that would occur if the surge flooded the subway to the same elevation as the flood waters’ surge outside.  They were given to Operations Planning and RTO to plan for the safe underground storage of the subway fleet. 

However, after developing and reviewing the revised subway flooding maps, it was apparent that much more of the subway was vulnerable to flooding than previously believed.  Furthermore, the flooding was projected to be more severe than originally thought, meaning that more sections could be flooded above platform level, and even into station mezzanine areas.  This had dire implications for numerous equipment rooms that are either at platform or mezzanine levels, including numerous Signal Relay Rooms and Control Towers and Communications Equipment Rooms.

The picture had changed so drastically that the approach taken prior to Hurricane Irene, to power down critical and remove equipment from areas that would flood was no longer a viable option.   The risk of damaging large quantities of critical operational systems was now too great to accept since the potential damage if realized, was deemed to be crippling to the system.  It was decided that the surge must be kept out – in other words, water needed to be stopped from entering the subway system

Plans to stem the tide
A plan was developed and implemented to close the openings to the subway lines that were predicted by the SLOSH Model to be impacted by a Category 1 Hurricane storm surge.  The process began several days before the storm’s anticipated landfall, beginning with the closing of subway vent openings, and partial closings of stairs by closing three sides but keeping the stair and entrance functional until the planned system shutdown was in progress.  For under river tubes, only one of the emergency ventilation fan vents was secured, leaving the closure of the second plant until the shutdown.

During the days before the system was shut down, the Division of Infrastructure had already started installing plywood sheeting directly over the vent gratings at sidewalk vents, and fastened to the concrete perimeters, using the locations listed on the NYC Transit Critical Facilities list that I helped to update.  Spray foam insulation was applied to gaps and seams.  For street stairways, in most cases plywood sheeting was installed around three sides of the stairway and fastened directly to the installed railing systems and then framing and plywood sheeting was applied across the face of the stairway once passenger service was suspended.  Typically the finished elevation was approximately four feet above the sidewalk.

At the 148th St Portal, which has experienced flooding from the Harlem River during past storms numerous times, a four-foot high wall had often been erected across the face of the Portal for expected high tide/heavy rain events.  During the preparations for Hurricane Sandy, Chief Infrastructure Officer Frank Jezycki called me at the Incident Command Center (ICC) on Sunday afternoon, and asked what height of the flood wall should be to keep the potential surge of Hurricane Sandy at bay.  Since the track elevation at the Portal is known from our Track Alignment Drawings, I used the SLOSH model to predict the potential height of the flood for a worst case Category 1 Hurricane scenario at that location, therefore establishing that at least a height of eight feet would be needed to prevent the flood of the subway. The Division of Infrastructure then built the wall to an elevation of eight feet above base of rail.  Due to their effort, flooding was averted at that location.

Surge would cause heavy level of damage
But just how realistic is the concern that storm surge could extensively flood the system?  Using the SLOSH maps and NYC Transit engineering documents, it was clear that the concern was very real.  The tunnel flooding maps show that if the water level in the subway reached equilibrium with the surge level, thousands of feet of the subway would flood as far north as 34th street on some Lines. 

Using the Whitehall St. Station as a case study, I computed the volume of water that would enter during a storm surge from a Category 1 Hurricane assuming that the stairs and vent gratings located south of Water St. were left unprotected.  According to the SLOSH model, a Category 1 Hurricane would result in a storm surge at various elevations over the unprotected openings as shown in the SLOSH time/history of surge below.  At least 270 sq. ft. of openings would be affected. 

SLOSH Time/History of Surge

As the computations showed, this scenario results in 36 million gallons of water entering the system, which would flood not only the Montague St. tunnel, but would also flood the BMT Broadway Line up to north of Whitehall Street and the Nassau Line up to north of Broad Street.  Within these limits alone, several critical equipment locations would be impacted which would leave signal and other systems out of service after the storm passed.  Recovery and restoration of service would be costly and prolonged.

Unfortunately, the above scenario was realized for Hurricane Sandy, which although downgraded to just below hurricane strength winds, resulted in a surge roughly equivalent to a maximum Category 1 Hurricane with widespread flooding and lasting impacts that have been well documented.  NYCT experienced moderate to severe flooding in eight subway tunnels.  The Montague Street tunnel connecting Court Street Station in Brooklyn with Whitehall Street Station in Manhattan was completely flooded to the ceiling throughout most of its length, requiring the pumping of an estimated 27 Million gallons of water.

My involvement with the Sandy recovery and mitigation efforts has continued almost non-stop since those four continuous days that I spent at the ICC. I walked the Rockaway Line the day after the storm to assess the destruction of its tracks and facilities, and Track Engineering teams inspected and documented the damage done to tracks and switches elsewhere in the system: the eight under river tubes and several yards.

Planning to stay ahead of the storm
I identified all NYCT facilities which would be impacted by either a Category 1 or Category 2 Hurricane flooding in the surface and prepared a Critical Facilities List showing localized details of each facility.

In December 2012 I prepared a presentation titled “Storm Surge Flooding in NYCT” which addressed the main issues regarding past studies of hurricane impacts on Metro NYC transportation facilities, the effects of future storms and how we can mitigate those effects.  I was privileged to give this presentation to Transportation Officials in Albany, an ASCE Seminar in NYC in April of 2013 regarding Hurricane Sandy’s Storm Surge in NYC, MTA Officials (including the Presidents of all MTA Agencies) and to others in NYCT.

I continue to be involved in Sandy mitigation and resiliency projects by providing elevation and mapping data for affected facilities as well as commenting in the adequacy of the proposed solutions, and to prepare for the next storm which sooner or later we will have to deal with – this time better prepared thanks to the lessons learned while dealing with the aftermaths of Hurricane Sandy.

 

 
 
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