Why the City’s Flood Maps Got It Wrong

When Hurricane Sandy’s storm surge ripped through New York City on the evening of October 29, it exposed seaside houses to devastating waves, basement electrical systems to the corroding menace of salt water and subway tunnels to unprecedented flooding. But Sandy also exposed flaws in the maps New York City uses to order evacuations, and in the models scientists employ to predict the impact of tropical storms.

Using modeling data provided by the National Oceanic and Atmospheric Administration, New York City’s Office of Emergency Management (OEM) divides the city’s low-lying and coastal areas up into three zones according to their risk of flooding. Zone A, demarcated in red on the above map, would be at risk of flooding from any potential hurricane. Zone B, demarcated in orange, could flood during category 2 or higher hurricanes, while Zone C, outlined in yellow, warned of flooding from any Category 3 or 4 hurricane that struck just south of of New York City.

On the Sunday before the storm hit, Mayor Bloomberg signed an executive order mandating that people living in Zone A leave their homes ahead of the storm. “This is a serious and dangerous storm. For those in Zone A, evacuation is mandatory,” the mayor said at a press conference. He didn’t mention Zone B at all.

2003 maps for a 2012 storm

But when the storm hit, Zone B areas like Canarsie, Gravesend, Gerritsen Beach, Bergen Beach, portions of Bath Beach, Mill Basin, Marine Park, Lindenwood, areas of Howard Beach, Springfield Gardens, Brookville, Rosewood, North Woodmere and Woodmere all flooded to varying degrees. In Manhattan, portions of the Upper East Side between the 70s and the 90s near the East River flooded despite their designation as Zone B areas.

According to Christopher Miller, the OEM press secretary, the NOAA Sea Lake Overland Surge from Hurricanes (SLOSH) data used to create the hurricane zones “took the worst case model for each hurricane category and built zones that were even more conservative by incorporating entire streets for any block that experience surge in the model.”

However, the SLOSH data was from 2003, and was not updated except for the addition of the Rockaways, City Island and Hamilton Beach into Zone A following Hurricane Irene in 2011.

Meanwhile, Special Flood Hazard Area maps prepared by Federal Emergency Management Administration—and updated in 2009—list significant sections of South Brooklyn and Queens that are not part of the city’s ZONE A as at moderate or high risk of flooding in the event of a serious storm.

Areas categorized as Zone B by city authorities but highlighted as flood risks by FEMA by the Special Flood Hazard Area maps include Gerritsen Beach, waterfront areas of Canarsie, East New York, Starrett City, Mill Basin, Howard Beach, portions of Bath Beach and almost all of Bergen Beach.

The FEMA maps are based on the locale’s annual chance of experiencing flooding during normal rainfall or precipitation. These federal maps also determine the 100-year floodplain for a region, or the areas that will flood with a once-in-a-century storm.

By contrast, the SLOSH maps prepared by NOAA that underpinned NYC OEM’s evacuation zone map are based on predictions of hurricane-driven storm surge and rainfall.

FEMA’s maps don’t specifically address hurricane storm surges, but do note that,”Hurricane storm surge areas overlap many areas that are designated as the 100-year floodplain, but the hurricane storm surge areas are considerably larger and represent a different hazard.”

“Behind any map you see, there’s some science, there’s some policy, but there’s also interpretation.” says Hunter College geography Professor William Solecki.

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Scientists also missed the mark

According to storm surge data collected by Haydee Salmun, a professor of Geography at CUNY-Hunter, predictions and models about the extent of the storm surge were significantly off. According to Salmun, the highest predicted storm surge for the Battery in Lower Manhattan was for 11.16 feet, made by the Laboratory for Marine and Atmospheric Research at CUNY-Hunter, and higher than the projections by the National Oceans and Atmospheric Administration or the Stevens Institute of Technology in Hoboken, N.J.

The actual observed level of water at the battery on October 29 was 13.87 feet.

“Sometimes we go beyond what we can expect of science,” Solecki says.” How much can we expect the modeling to give us a crystal-clear prediction?”

OEM says the models and forecast used during the run-up for Hurricane Sandy did not provide enough information to warrant an evacuation of Zone B areas. However, the damage incurred during Hurricane Sandy and the divergence between the city’s evacuation zones and the extent of flooding will prompt OEM to take a harder look at flood zones.

“The National Hurricane Center has started to look at larger and slower moving storms with new 2013 SLOSH models. OEM will take the new data into consideration when determining whether evacuation zones for 2013 hurricane season need to be modified,” says OEM Press Secretary Christopher Miller.

Other city agencies already have more conservative approaches to determining which areas of the city are at risk of impact from climate change. In March, the Department of City Planning put out a draft revision of its Waterfront Revitalization Plan, a document which guides development and planning efforts in the coastal areas of the city. The DCP revisions designate major areas of the city as “coastal areas” deemed at risk of the impact of climate change—though the map doesn’t make clear when or how those impacts will be felt.

Solecki notes that Sandy’s impact, while devastating, is less than the potential storm damage that a stronger Hurricane could wreak. What’s more, in 80 years, he speculated, even another storm of Sandy’s limited strength could creep significantly inland, as New York could see up to a four-foot increase in overall sea level.

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