The chances of New York City flooding during a storm are even greater than scientists previously thought, a new study suggests.
Maximum water levels in New York Harbor during storms have risen by nearly two and a half feet (about 0.7 meters) since the mid-1800s, making the chances of water overtopping the Manhattan seawall at least 20 times greater than they were 170 years ago, scientists report in a new paper accepted for publication in Geophysical Research Letters.
The cause, they say, is not just global sea-level rise: That accounts for less than a foot and a half (around 0.4 meters) of the increase. Surprisingly, the maximum height of the storm tide in New York Harbor caused by a "once-in-ten-years" storm has grown an additional foot (0.3 meters) since the mid-19th century. (Read "Rising Seas" in National Geographic magazine.)
"Knowing that there has been an increase in storm tides and figuring out why the increase occurred could help scientists better predict what will happen in the coming decades and help cities mitigate future problems," says Stefan Talke, the lead author of the new study and a professor in the civil and environmental engineering department of Portland State University. (A storm tide is the combination of normal high tide and storm surge—the extra volume of water driven onshore by the storm's winds.)
In response to the new study, Daniel Zarrilli, director of the newly launched Office of Recovery and Resiliency for New York City Mayor Bill de Blasio, says, "Historic sea-level rise has clearly increased the odds of flooding in New York Harbor. Climate change will only accelerate that impact."
Zarrilli says the administration is taking steps to reduce risk and "build a stronger, more resilient New York." (See "A Superstorm in 2100.")
He points to a report released by Mayor de Blasio on April 22, which included a progress update on efforts started by former Mayor Michael Bloomberg to protect the city's 520 miles (840 kilometers) of coastline. Work to date includes adding sand to beaches, reforming flood insurance, strengthening building codes in low-lying areas, and rebuilding areas damaged by Hurricane Sandy in October 2012. (See "Hurricane Sandy Pictures: Floods, Fire, Snow in the Aftermath.")
Mining the Historic Record
To better understand the changing impact of storm tides, Talke and colleagues from Portland State and the Stevens Institute of Technology in Hoboken, New Jersey, went to the National Archives in College Park, Maryland, where they combed through data collected since 1844 from tide gauges in New York Harbor. They scanned the written records and transferred the numbers to spreadsheets. To fill occasional gaps in the tide records, they searched the New York Times archive for reports of storms.
"In those days they were more connected to the sea and they would talk a lot about how high the water was, so I'm pretty confident that I didn't miss any big storms," says Talke.
After analyzing the data, Talke and team concluded that New York City is at risk of more frequent and extensive flooding than had been expected due to sea-level rise alone. A major, "ten-year" storm today would cause bigger storm tides and more damage than a ten-year storm in the mid-1800s.
A ten-year storm is one that has a 10 percent chance of occurring in any given year. Today, the surge from such a storm in New York Harbor would reach a maximum height of nearly two meters (about six and a half feet). In the mid-19th century, the maximum height was only 1.7 meters (about 5.6 feet).
Searching for Causes
Global climate change and local changes in the harbor itself are two possible causes of the increased height of storm surges in New York, Talke says. "If I had to wager a guess, I would say it is probably a combination, but that remains to be seen," he says.
Sea level is rising as the planet warms because ice on land is melting, adding water to the ocean, and because seawater itself is expanding as its temperature increases. The latest projections from the Intergovernmental Panel on Climate Change suggest that by 2100 we'll see a further sea-level rise of 0.26 to 0.55 meters (10 to 22 inches) if we manage to restrain our carbon emissions, and 0.52 to 0.98 meters (20 to 39 inches) if we don't.
Since the 19th century, the warming might have increased the intensity of ten-year storms striking the city. Some of the increase in storm surges might also be attributable to a natural decadal climate cycle called the North Atlantic Oscillation, which affects the severity of storms hitting the northeastern U.S. or western Europe. Before Hurricane Sandy, the index of that atmospheric pressure seesaw was very low, which allowed the storm to hook inland and do more damage, says Talke. "If we know ahead of time the long-term climate pattern, then we have a better idea whether we have above- or below-average risk of storm surges," he explains.
But the topography of New York Harbor also affects storm surges. Since the 19th century, storm-buffering wetlands and shoals have been removed, and the connection with Long Island Sound was widened. In addition, the shipping channel was deepened from about 23 feet (7 meters) to around 50 feet (15 meters). "If you double the depth, you basically decrease friction on the tide wave by half," Talke says. "Our hypothesis is that if there is less friction, the storm surge could go higher."
The next step is computer modeling of New York Harbor during the 1800s, during historic storm events like the devastating hurricane of 1821, and in the present day.
"My hope is that if we can figure out the local contribution, then maybe there is a chance for local solutions," Talke says. "It's not practical to go back to a 23-foot-deep [7 meters] shipping channel, but there are some things we could do."
For instance, wetlands could be restored and certain bays could be allowed to fill in with sediment to their previous depths, which might help dampen surges.
Talke says the new results lend "more urgency" to existing long-term plans to raise the height of the city's seawalls. "We show there is a long-term trend: Storm surge and tides are getting larger," he says. "So that should be taken into the calculus when devising the design height of surge-protection barriers."