LONDON (TIP): It is now confirmed – Manhattan should start bracing for never before seen flooding from the New York harbour. Scientists have for the first time confirmed that the odds of storm waters overflowing Manhattan seawall is up 20-folds.

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. Whereas sea-level rise, which is occurring globally, has raised water levels along New York harbour 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.

Combining the newly calculated rise in storm tide with the rise in sea level that has taken place since the mid-1800s, researchers found that today, waters can be expected to overtop the lower Manhattan seawall – 1.75 meters (5.74 feet) high, once every four to five years. In the 19th century, when both sea levels and storm tides were lower, water was expected to overtop the Manhattan seawall only once every 100 to 400 years. Tide gauge data analysed 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.

There is a 10% chance today that, in any given year, a storm tide in New York harbour will reach a maximum height of nearly two meters (about six and a half feet), the socalled “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. 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.

The research also confirms that the New York harbour storm tide produced by Hurricane Sandy was the largest since at least 1821. “What we are finding is that the 10-year storm tide of your great grandparents is not the same as the 10-year storm tide of today,” Talke said. Talke photographed hundreds of pages of handwritten hourly and daily tide gauge data going back to 1844 that is stored at the US National Archives. The researchers then analyzed the data to calculate storm tide levels and look for trends, and compared the information with climate data.