Kathryn Cox, Emily Jennings, Daniel Schwartz, Sylvia Zaki

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Kathryn Cox, Emily Jennings, Daniel Schwartz, Sylvia Zaki

Professor Bird

Seminar 3 Final Report

December 21, 2012

Combatting Storm Surge Flooding and its Effects on the NYC Subway

Introduction:

According to the National Weather Service’s “Introduction to Storm Surge” report, storm surge is “an abnormal rise of water generated by a storm, over and above the predicted astronomical tide” (NWS 2008). In other words, this is an increase in the usual water level that is caused specifically by a storm. There is no specific benchmark for what height of water constitutes storm surge, as each body of water has its own usual water level. Additionally, storm surge is the difference between the height of the storm-influenced tide and the level the tide usually reaches; combining the two numbers would produce the storm tide level. Storm surge has become an increasingly dire problem in New York City, particularly in light of the Massachusetts Institute of Technology study “Physically based assessment of hurricane surge threat under climate change”, in which the researchers predict that due to increasing sea levels, by the end of the century, the current 100-year surge floods will occur every 3-20 years and 500-year surges will occur every 25-240 years (Lin et al 2012).

Just a few months after the publishing of that study, it has been estimated that on October 29, 2012 the water level surged to 14 feet over the average low tide mark and 9 feet over the average high because of Hurricane Sandy. As one might assume, surge heights of that magnitude caused flooding throughout the lower part of Manhattan, and with that came the flooding of the subway lines with East River tunnels. According to Klaus Jacob of Columbia University, the minimum surge height that would have caused subway flooding was 5.9 feet, and the surges caused by Sandy greatly surpassed that mark (Jacob et al 2011). Moreover, this severe flooding was not without warning—Jacob calculated that if the storm surge caused by Tropical Storm Irene in August 2011 swelled just another foot, the East River subway tunnels would have flooded then as well.

Although the majority of subway lines flooded by Sandy have been restored, city dwellers had to make do for several days riding on compromised lines that maneuvered around the flooded areas of lower Manhattan. Even though a week or so seems like a lifetime to many residents of New York City, the damage to the subway system could have potentially been much more severe. Jacob originally estimated that top-to-bottom flooding of the East River tunnels would take at least 29 days to fully recover from. What saved the city from damage to that extent was the fact that the city’s mass transit system was completely shut down at 7 p.m. on October 28, the day before the storm was due to hit.

In addition to causing the inconvenience of the subways being out of commission for weeks, storm surge damage also costs a significant amount of money to repair. In the Metropolitan Transportation Authority’s November 2011 budget report, it was announced that $49 million would be needed to repair the damage caused by Tropical Storm Irene in August 2011 (MTA 2011). No numbers have been released yet as to what the total amount of repairs after Sandy will cost, but considering the East River tunnels did not experience significant flooding during Irene, the cost will most probably be more than $49 million. This is why it is unacceptable for the East River subway tunnels to flood as a result of storm surge, and therefore why the goal is to keep storm surge waters out of the tunnels and prevent any flooding of these tunnels by the year 2082.

Strategy: Elevated Subway Station Entrances

Currently, most of New York City’s subway stations that lie along the East River and south of Canal Street have lowest critical elevations (LCE) greater than or equal to 5.9 feet above NGVD29 (Jacob et al., 2001). A station’s LCE is determined by the lowest station opening that provides access to its tunnel, and can be derived from the elevation of a station entrance, emergency exit, ventilation shaft or street level grate (Jacob et al., 2011). These elevations are measured in reference to NGVD29, the National Geodetic Vertical Datum of 1929, a set of control data used to measure position above or below mean sea level (FEMA, 2007).

More specifically, the A and C lines have a lowest critical elevation of 7.0 feet, the R line of 7.5 feet, the 2 and 3 lines of 9.1 feet, the 4 and 5 lines of 9.9 feet and the F line of 10.0 feet (Jacob et al., 2001). According to a 2011 report titled “Transportation,” without accounting for sea-level rise and without prior preparations, 100-year flooding will cause complete flooding of the tunnels leading into Brooklyn of the aforementioned subway lines (Figure 1). Low lying subway entrances, especially those situated in proximity to the shore, are at high risk for incurring floodwater during an intense storm. The potential for subway tunnel flooding can be met by flood heights associated with 1-in-100 year flood events, storm surge flooding associated with hurricanes of all intensities, and even 1-in-10 year flood events by the end of the century (Horton et al. 2010).

Figure 1:

Figure 1: 100-year flooding without sea level rise of lower Manhattan subways and adjacent East River tunnels crossing into Brooklyn; the heavy blue lines indicate fully flooded tunnels, and broken lines show overflow into tunnels located in areas that are not flooded above-ground; background colors show topographic surface elevations (yellow≥30ft). Sea level rise is expected to only worsen subway-flooding events (Jacob et al., 2011).

Category 2 hurricanes are generally associated with storm surge-induced flooding of up to 16.6 feet and SLOSH maps indicated that lower Manhattan would experience 12.0 feet of flooding under such hurricanes (Figure 2). In order to reduce the risk of subway tunnel flooding, subway tunnel entrances that precede East River tunnels should be reconstructed so as to increase their respective LCE to 20 feet. A similar suggestion was proposed during a recent conference titled “What is the State of the Art in Preparing for Extreme Weather Events?” by research scientist Klaus Jacob. Dubbed the “Taipei Solution,” Jacob’s proposition suggested that the New York City government look to the organization and construction of Taipei’s Mass Rapid Transit (MRT) system to improve infrastructure resilience. Suggested improvements include elevating subway station entrances, installing gates and installing higher capacity water pumps (Chiang & Huang, 2012). Similarly, Bangkok’s MRT stations, through a combination of elevation and floodgate protection, are able to withstand up to 9 feet of water (Fernquest, 2011).

Figure 2:

Figure 2: SLOSH Map flood levels associated with category 2 hurricanes. As indicated above, lower Manhattan would experience 12 ft. of flooding during such a hurricane (The City of New York, 2009).

Commuters in Taipei and Bangkok must first go up a flight of stairs or ascend via escalator into the station before descending towards the train platform. Furthermore, some subway stations in these cities are outfitted with computer-controlled floodgates that are activated when floodwater levels reach a critical point; they also further increase the station’s LCE. Construction of similar elevated structures throughout lower Manhattan will ensure the impermeability of subway station entrances to floodwaters resulting from a Category 2 hurricane or lower.

Discussion: Elevated Subway Station Entrances

Elevating the LCE of lower Manhattan subway station entrances might prove to be an intuitive, efficient strategy for combatting East River tunnel flooding, however, there are major holes in the information available about the feasibility and cost of this project. In a metropolis such as New York City, the concept of a major overhaul of downtown topography created by constructing elevated station will be met with opposition. However, should this strategy prove effective, it will provide an impenetrable barrier to floodwaters entering through subway entrances and into tunnels. By protecting the tunnels and the stations, this strategy will aid in preventing the $23 million lost everyday due to transportation shutdown (Jacob et al., 2011). Therefore, it is necessary that, through evaluation, land and station surveys and research, the efficacy and cost of such a project be determined. The following, however, is an evaluation of the feasibility of implementing this strategy based on available research.

One of the most obvious obstacles implementation of this strategy faces is the shortage of available space for subway entrance construction. Lower Manhattan, especially, is incredibly dense in terms of pedestrian thoroughfare and infrastructure (buildings, roads, traffic control equipment, etc.). Many of the subway stations that precede stretches of under-river tunnel track are located in areas that do not have the space required for the construction of above-grade structures.

Possible alternatives exist, however. Subway station entrances can be integrated into buildings that lie above or around the below-grade portion of the station (Teo & Woo, 2011). However, close coordination between architects, engineers and the owners of the buildings in question must be achieved in order to seamlessly integrate commuter thoroughfare into the lobby or basement of such buildings. Furthermore, safety regulations must be followed to ensure evacuation routes and proper ventilation is available for the station.

Subway station entrances can be consolidated into one central, elevated entrance if a sizable area of space is allocated for a certain subway station. Subway entrances of this sort might encounter problems with congestion—long lines and large crowds—if metro card machines malfunctioned, for example. Therefore, station designs of this sort would have to allow for seamless access to station platforms.

Elevated station designs also face accessibility issues. Commuters with disabilities would only be able to access elevated station platforms via elevator. However, the elevator shaft provides another entrance for floodwater during an intense storm. If the integrity of the station were to be kept intact, two sets of elevators would have to be installed—one to provide access to the peak of the station, and one to descend toward the platform. Further alternatives would have to be conceived and tested to ensure an effective method of entrance for commuters with disabilities while providing no further entry to floodwaters.

In light of the issues implementation faces, close cooperation and coordination between the MTA, New York City and New York State government must maintained in order to assess the feasibility of construction at the sites of many station proximal to Manhattan’s east bank. A massive survey of the topography of lower Manhattan will be conducted between 2014 and 2017 to address integration and accessibility issues and the changes that will have to be made in order to accommodate elevated entrances.

In regards to an order of implementation, Subway stations with the highest risk of incurring floodwaters (closest to the shore, lowest critical elevation) would be elevated first. This method of implementation would allow officials to evaluate the effectiveness of this strategy by testing the impermeability of the first elevated station before moving construction forward onto further stations. However, an in-depth survey of every subway station lying below Canal Street and east of Broadway must be conducted to document their respective LCEs, proximity to the East River, and overall susceptibility to flooding to determine the order of implementation. As proposed by this report, starting June 2014, a survey of this sort would commence.

Similarly, the cost of construction per subway line would have to be determined within the three-year period (2014-2017) set aside for information gathering. This way, funds can be allocated per subway line and would ensure that under river tunnels are protected one and at a time. A possible source of funding could be the federal government (The Department of Transportation). The daily $23 million lost due to transportation shutdown reflects the economic lag resulting from the loss of transportation options, especially in lower Manhattan where Wall Street, one of the nation’s, if not the world’s, largest economic hub resides (Jacob et al., 2011). Funds allotted from the federal government could take time to process; therefore, roughly 15 years will be given per subway line for construction so that by 2080, the R, 2, 3, 4, 5, and F line stations will be elevated.

Strategy: Elevated Subway Grates

Because the New York City’s public transit system in lower Manhattan is an underground infrastructure, it is extremely vulnerable to flooding, especially flooding caused by storm surge. In August 2007, 3.5 inches fell within two hours during the morning rush. More than 30 sections of the subway network were flooded, practically crippling the system (Metropolitan Transportation Authority 2007). In 2012, damages from Tropical Storm Sandy shut down subway service beneath 34th Street for one week, and even longer for select lines.

One cause for this vulnerability is the floodwater that flows from the street into subway ventilation grates, and inevitably onto the subway tracks, which lay 30 feet below (Metropolitan Transportation Authority 2007). The MTA’s drainage system can only handle 1.5 inches of rain per hour, meaning that any rainfall exceeding this amount could potentially paralyze the system (Chan 2007).

Unfortunately for their poor water capacity, subway grates often prove to be necessary components to our underground transportation system. According to Tunnels and Tunneling International, underground trains’ auxiliary systems and breaks cause a great deal of heat, which would be contained within subway tunnels if it were not for subway grates (SOURCE?). Additionally, subway grates serve to combat the “piston effect,” which is an increase in pressure caused “when air is trapped in a shaft and forced through by a moving object.” (SOURCE?) Lastly, subway grates help to regulate temperature and humidity within subway tunnels by allowing fresh air to enter the tunnels and smoke to exit through the grate in the event of a fire.

Because subway grates prove to be critical components of the mass transit system, the MTA searched for methods to make necessary subway grates more resistant to flood water. In 2008, under then-CEO and Executive Director Elliot Sander, a new proposal was made to modify subway ventilation grates by increasing their elevation 6 to 18 inches above the sidewalk. Rogers Marvel Architects and di Domenico + Partners proposed that these ventilation grates be elevated in an effort to divert some of the water and rubbish from entering the subway system. This same year, the MTA began examining the 1,212 grates located in Hillside, Queens to determine which of these were necessary. Over 200 were planned to be elevated, and another 300 were deemed un-necessary and were to be permanently sealed (Metropolitan Transportation Authority 2007). These grates have also been introduced in Astoria, Queens and along West Broadway in Manhattan. To combat flooding in low elevation areas surrounding East River subway tunnels, these elevation grates can be implemented in the areas surrounding subway stations in these areas. (Duap, David W. 2008).