Odds of storm waters overflowing Manhattan seawall up 20-fold

A press release from the American Geophysical Union: Maximum water levels in New York harbor during major storms have risen by nearly two and a half feet since the mid-1800s, making the chances of water overtopping the Manhattan seawall now at least 20 times greater than they were 170 years ago, according to a new study. Whereas sea-level rise, which is occurring globally, has raised water levels along New York harbor by nearly a foot and a half since the mid-19th century, the research shows that the maximum height of the city’s “once-in-10-years” storm tide has grown additionally by almost a foot in that same period.

The newly recognized storm-tide increase means that New York is at risk of more frequent and extensive flooding than was expected due to sea-level rise alone, said Stefan Talke, an assistant professor of civil and environmental engineering at Portland State University in Portland, Ore. He is lead author of the new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union. The research also confirms that the New York harbor storm tide produced by Hurricane Sandy was the largest since at least 1821.

Tide gauge data analyzed in the study show that a major, “10-year” storm hitting New York City today causes bigger storm tides and potentially more damage than the identical storm would have in the mid-1800s. Specifically, Talke explained, there’s a 10 percent chance today that, in any given year, a storm tide in New York harbor will reach a maximum height of nearly two meters (about six and a half feet), the so-called “10-year storm.” In the mid-19th century, however, that maximum height was about 1.7 meters (about 5.6 feet), or nearly a foot lower than it is today, according to tide gauge data going back to 1844, he noted.

“What we are finding is that the 10-year storm tide of your great-, great-grandparents is not the same as the 10-year storm tide of today,” Talke said.

... The study’s findings may indicate that “storm surges’ interaction with New York harbor has gotten larger so that in addition to sea level rise, the storm surges may have been enhanced,” said Chris Zervas, a scientist at NOAA’s Center for Operational Oceanographic Products and Services in Silver Spring, Md., who was not involved in the study. “For the latter part of the 1900s, [it shows] that the possibility of overtopping the seawall has increased quite a bit in addition” to sea-level rise, he added...

The Hugh L. Carey Tunnel (formerly known as the Brooklyn Battery Tunnel) on Oct. 30, 2012, during Hurricane Sandy, shot by Metropolitan Transportation Authority of the State of New York, Wikimedia Commons via Flickr, under the Creative Commons Attribution 2.0 Generic license