Manhattan Flood Risk Up 20-Fold Since 19th Century

762 Manhattan Flood Risk Up 20-Fold Since 19th Century
Tide data from Fort Hamilton, New York, August 18–24, 1893. Notice the 11.1-foot (3.4-meter) high water record at 6:00 a.m. on the 24th / Stefan Talke
The chances of storm waters overtopping Manhattan’s seawall are 20 times higher now than they were 170 years ago, according to an upcoming study. Since the mid-1800s, water levels during major “once in 10 years” storms have risen over a quarter of a meter in the New York harbor region -- and that’s in addition to the half meter increase due to sea-level rise. 
A storm tide is the amount that the water level rises in a storm -- it includes abnormal rises as well as predicted (astronomical) tidal changes. This newly recognized storm-tide increase means that New York is at risk of more frequent and extensive flooding than what’s expected due to just sea-level rise alone.
Three of the nine highest recorded water levels in the harbor region occurred since 2010, and eight of the largest 20 occurred since 1990. To see if that cluster is random or indicative of intensified storm tides, a team led by Stefan Talke from Portland State University analyzed hundreds of pages of handwritten, hourly and daily tide gauge data from the U.S. National Archives dating back to 1844. The gauge is “basically a pipe with holes at the bottom," Talke explains to Mashable, and it removes the effects of wind and waves. The team used newspaper accounts of big storms to fill in gaps in the record to create a long, continuous dataset.
“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 says in a news release. Water can be expected to overtop the lower Manhattan seawall -- which is 1.75 meters high -- once every four to five years. In the 19th century, when both sea levels and storm tides were lower, water overflowed the seawall just once every 100 to 400 years. 
Their results show that the 10-year storm tide has increased by 0.28 meters, and combined with a 0.44 meter increase in local sea levels, that’s a 0.72 meter increase in flood level. Here’s a NOAA graphic to help keep water level terms straight. 
The same storm hitting now would cause much more damage, but it’s not just because of climate warming. Other factors affecting variations in storm tides range from the decade-scale, intercontinental phenomenon called the North Atlantic Oscillation to local changes such as loss of wetlands and deepening of shipping channels around the city. 
“If it turns out to be a local reason, as has been suggested in some cases, there could be local solutions as well,” Talke says. “In some cases, we may be able to turn back the clock on that a bit.” The research also confirms that Hurricane Sandy produced the largest New York harbor storm tide since at least 1821.
The work was accepted for publication by Geophysical Research Letters this week. 
Images: Stefan Talke (top) & NOAA (middle)


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