Characteristics of tropical and extratropical storms in the Metropolitan region

The height and reach of storm surges and flooding along low-lying coastlines are influenced by a variety of factors, including offshore morphology, coastline geometry, astronomical tides and both the regional and local wind and pressure fields. Tropical (for example, hurricanes) and extra-tropical (for example, nor'easters) storm systems are associated with different wind and pressure fields, and these produce characteristically different storm surges. Extra-tropical storms cover a larger geographical extent and often elevate water level across the entire shelf, whereas tropical storms are geographically smaller. However, their strong winds can drive large local surges that propagate with the eye of the storm.

The waterways surrounding New York City are particularly prone to flooding because of the gentle topography, indented coastline and shallow bathymetry of the region (both inside the New York-New Jersey (NY-NJ) harbor estuary and on the inner continental shelf). The orientation of the axis of Long Island Sound positions it as a natural funnel for strong northeasterly winds driving storm surges down to the western Sound, through the East River and into New York harbor. Northeasterly winds blowing parallel to the southern coast of Long Island also drive surges against the south shore of Long Island that then penetrate into the harbor's Upper Bay through the Verrazano Narrows, the main entrance to the Port of New York. This is explained by the Ekman effect due to the rotation of the Earth, where in the Northern Hemisphere, surface waters veer to the right of the wind direction.

The onset and duration of storm surges differ significantly between hurricanes and winter nor'easters. As illustrated in Figure 9.2, the oscillating surge from a hurricane typically lasts only a few hours, but rises and falls very rapidly. The extent of flooding, therefore, depends critically upon the state of the astronomical

Figure 9.2 Storm surge of the 21-22 September 1938 hurricane

Note: Typically, a hurricane storm surge oscillates rapidly but lasts only a few hours. Dates are shown at noon EST.

Source: Pore and Barrientos, 1976.





Figure 9.3 Storm surge of the 5-8 March 1962 nor'easter

Note: Typically, the storm surge of a nor'easter rises slowly but lasts several days. Dates are shown at noon EST.

Source: Pore and Barrientos, 1976.

tide at the time of landfall. For nor'easters, by contrast, the surge typically rises more slowly but lasts a few days (see Figure 9.3), running the risk of flooding with each high tide (Pore and Barrientos, 1976).

Previous investigations have shown the flooding susceptibility of Metropolitan New York associated with hurricanes of varying intensity. Figure 9.4 (Plate 12) is a digital terrain map of Metropolitan New York with elevation contours shown (in feet) in various shades of green and earth tones. Superimposed in shades of blue are estimates of inundation zones as calculated by the SLOSH storm surge model (see that would be caused by a direct hit by hurricanes of categories 1 to 4 on the Saffir-Simpson scale (Simpson and Riehl, 1981).

The SLOSH model is based on the characterization of the effects of a synthetic storm core vortex, defined by location, translation speed, radius and intensity of maximum winds. It is a useful planning tool for emergency managers, but its predictions are necessarily approximate (accurate to within ± 20 per cent) and are highly dependent on how the computational grid is set up. However, the possibility of a major flooding catastrophe is obvious even for a category 2 hurricane.

1962 Nor Easter

Figure 9.3 Storm surge of the 5-8 March 1962 nor'easter

Note: Typically, the storm surge of a nor'easter rises slowly but lasts several days. Dates are shown at noon EST.

Source: Pore and Barrientos, 1976.



On teat)



Slosh Model New York
Figure 9.4 Metropolitan New York elevations, landforms and SLOSH model inundation zones for category 1 to 4 hurricanes (see Plate 12 for color version)

Source: courtesy of C. Gersmehl.

Significant sections of the lower west and east sides of Manhattan Island, Queens and Brooklyn boroughs and the east coast of Staten Island are vulnerable. Jamaica Bay and environs (cell C5) are clearly at risk, as is JFK airport on the eastern shores of the Bay. Twenty-five subway stations in Brooklyn alone have entrances at or below 10 m above mean sea level (MSL). Not shown in Figure 9.4 are the flooding predictions for northern New Jersey, which would be considerable, including the Hackensack Meadows, Port Elizabeth and Newark Airport on the left side of the figure, as well as the ocean coast of New Jersey. Figure 9.5 (Plate 13) illustrates how all of the south shore of Long Island, plus the two eastern forks, are at risk of inundation from storm surges. Protecting Long Island poses a second major challenge, but beyond the scope of this contribution. Coastal New Jersey faces a similar predicament.

Sea level as recorded at the NOAA Battery primary tide station at the southern tip of Manhattan Island has risen inexorably over the past one and a half centuries at a rate of about 30 cm (1 ft) per century. In the metropolitan region as a whole, sea level increased by 23-38 cm (9-15 inches) during the 20th century. The statistical return periods of storm surge-related flooding events decrease with sea level rise, independent of any global warming effects on the weather itself, so getting a handle on the future rate of rise is very important.

Extrapolating the current trend, global sea level would be expected to rise by another 0.3 m (1 ft) by the 2090s. As a result, surge-related floods would be higher, cover a wider area and occur more often. Future storm surges would ride on this elevated base level, so the potential damage inflicted by a future 30-year event would be expected to be equivalent to that of a present-day 100 year storm.

However, the pace of global warming is expected to intensify unless very sharp limitations in emissions occur (see Figure 9.6), and the vulnerability and threats of inundation of the New York metropolitan region will correspondingly increase. Projections based on climate change simulations made in 2001 suggest that, excluding the contributions from dynamical ice flow of the Greenland and the West Antarctic ice sheets, sea level will rise by 10-30 cm (4-12 inches) in the next 20 years, 18-60 cm (7-24 inches) by the 2050s, and 25-110 cm (10-42

Figure 9.5 Digital terrain map of Long Island, NY (see Plate 13 for color version)

Note: This illustrates the extent of low-lying land and the inherent difficulty of protecting Long Island against major storm surge events and rising sea level. Approximate elevation scale: blue 0-8 m, green 8-50 m, orange 50-80 m and red 80-130 m. Source: Courtesy G. Hanson


Figure 9.6 Goddard Institute of Space Studies projected sea level rise for New York City to the year 2100 using IPCC scenarios A1B and B1

2000 2100

Figure 9.6 Goddard Institute of Space Studies projected sea level rise for New York City to the year 2100 using IPCC scenarios A1B and B1

Note: The current trend is the extrapolated secular rise in sea level as recorded at the Battery tide station over the last 150 years and is not primarily associated with global warming, but with isostatic adjustment of the continent following the last glaciation. See earthobservatory.*nasa*. gov/Newsroom/*NASA*News/2006/2006102523436.html inches) by the 2080s (Gornitz, 2001). More recent projections for these terms (i.e., other than from significant deterioration of the polar ice sheets) are slightly lower than those estimated in the Third Assessment Report of the IPCC. For example, recent analyses by C. Rosenzweig and V. Gornitz using the version of the Goddard Institute of Space Studies (GISS) Atmospheric-Ocean Model global climate model used to estimate the effects of warming on sea level rise for the newest IPCC assessment, project a sea level rise of 38—48 cm (15—19 inches) by the 2050s in New York City (see http://earthobservatory.*nasa*.gov/Newsroom/ *NASA*News/2006/2006102523436.html). However, there are early signs that the Greenland and West Antarctic ice sheets are starting to lose mass (see Chapters 5 and 6), so higher levels of sea level rise continue to be quite plausible. For the highest estimate of sea level rise by the 2090s, which is roughly 1.6 m (3.8 ft), the return period of a 100-year equivalent-damage storm may drop to every other year (Bowman et al, 2005).

In a worst-case scenario envisioned in the Metro New York Hurricane Transportation Study (US Army Corps of Engineers et al, 1995), hurricane winds striking New York's skyscrapers would result in debris falling onto the streets from broken windows and dislodged masonry. Pedestrians would seek shelter in the subways from severe winds, rain and the falling debris. Hurricane surge waters would quickly fill the subway tunnels, even if the elevation at the surface were above potential flood levels, drowning those underground. No estimate was made of the likely number of casualties.

Short-term fixes have already been undertaken, such as fitting moveable gates at the entrances to the PATH train station in Hoboken (which was inundated with seawater during the 1992 nor'easter). However, to protect the myriad individual structures with seawalls or, where feasible, by raising subway entrances and ventilation shafts above grade, would become increasingly difficult and would end up, presumably, with seawalls being constructed along the several hundred miles of shoreline in the metropolitan region.

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