Flood risk reduction interventions for the New York City subway system

Flood risk reduction interventions for the New York City subway system: A research on the impact of storm surge and sea level rise on the safety against flooding in urban delta's

Vermeij, D.A.A.N.

Jonkman, S.N. (mentor)
Molenaar, W.F. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2016-09-19

On 29 October 2012 hurricane Sandy hit the coast of New York City. This category 1 hurricane had a devastating effect: large parts of the city were flooded and the city was out of business for several days. An estimated total damage of $41.9 billion had occurred of which $5 billion in account of the metro system. The largest damage was to the subway (underground) on Manhattan. The main challenge of this research was to design a protection for the subway of Manhattan. The main sources of flood water are the rivers that enter the city via overflow of the river bank. Flood water enters the subway system mainly via the ventilation grates and the commuter entrances. There are not many tubes that start in a flood prone area. The tube most vulnerable for flooding is the Steinway tube of the Flushing line, running from Queens under the East River to Grand Central Station on Manhattan. The Steinway tube has 200,000 daily users and was flooded during Sandy via the ventilation grates on 50th Avenue. The future flood levels in this report take the 90th percentile sea level rise (SLR) projection into account as calculated by the NPCC. The safety level is currently 1/60-years, but decreases to 1/5-years is SLR projections in 2080 are reached. Long Island City is modelled as a storage basin where water enters via overflow of the East River Bank. The inflow of the 100 to 10,000 year floods is modelled as a harmonic tidal wave. The results show that the water level in the basin is equal to the storm surge flood level in most cases. Combined with surface elevation data, the damage to the three subsystems Long Island City, the Steinway tube and the subway system of Manhattan shows that the risk is significant. At the considered flood levels, the Flushing line will flood via the start of the Steinway tube. The Steinway tube inflow model shows whether the flood water reaches Grand Central Station on Manhattan. A 1/500-years flood in 2080 results in a total damage of $2,571 million to all three systems. Compared to protection of only the Steinway tube, the multifunctional bank protection at the East River, called the Greenline, adds the most value to Long Island City. Considering the political and economic feasibility, the Greenline is not designed into detail. The Steinway Flood Gate scores the highest on benefit/costs-ratio, while the protection of Manhattan only is economic not feasible: the net present value is negative for each design level. The Steinway tube is protected by closing the ventilation grates and commuter entrances with covers that are designed by a third party. Directly in front of the Steinway tube, the Steinway Flood Gate is designed. This vertical lifting gate has a door of 6.00 meters high and 7.70 meters wide. Two towers of 12 meters support this flood door. The door is able to move in horizontal direction to enable the sealing of the tube. The Steinway Flood Gate is a perfect long-term flood protection that can be realized in the near future. It is an economically attractive investment that improves the safety of the New York City Subway system. However, in the faraway future the Greenline gives much more value per invested dollar.

Flood risk
New York City
Hydraulic structures
metro
subway
protection

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Student theses

master thesis

(c) 2016 Vermeij, D.A.A.N.