When a storm, such as Hurricane Sandy, sets waters in New
York Harbor rising, those sloshing seas are now 20 times more likely to
overtop the Manhattan seawall than 170 years ago, a new study finds.
The increased risk comes from a combination of sea level rise —
which has raised water levels near New York City by nearly 1.5 feet
since the mid-1800s — and storm tide, or the amount that local seas rise
during a storm. Storm tide is itself a combination of storm surge (the
water that a hurricane pushes ahead of it) and the astronomical tide.
The rise in sea level and storm tide combined puts the odds of storm waters overtopping Manhattan’s defenses at
one in every 4 to 5 years, compared to only once in every 100 to 400
years in the 19th century, the study found. (Put another way, the annual
chance of a storm overtopping the seawall has gone from about 1 percent
to 20 to 25 percent.)
Allison Joyce/Getty Images
Water floods the Plaza Shops in the wake of Hurricane Sandy, on Oct. 30, 2012, in Manhattan, New York.
The storm tide at Battery Park, at the tip of Manhattan, during Hurricane Sandy reached a record 14.06 feet according to the National Hurricane Center’s report on Sandy.
That
high storm tide — more than 4 feet higher than the previous record set
in December 1992 and the largest since 1821 — was created by a 9.4-foot storm surge and
the evening high tide during a full moon, when tides are higher than
normal (though the evening high tide was not as large as the morning
one).
The extreme rise in water level sent the Harbor
flooding into the streets of the Financial District and other parts of
Manhattan, as well as other city boroughs at depths between 2 to 9 feet
above ground level. An estimated 305,000 houses in New York were
destroyed, mostly by storm surge, according to the NHC.
Total damage to the city was estimated at $19 billion, including $5 billion in damage to the city’s subway systems caused by flooding. At least 21 people were killed by the storm surge on Staten Island.
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