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 29th, 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 Bill Solecki.

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. 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 4 foot increase in overall sea level.

Juan Osorio, a planner with the New York Environmental Justice Alliance, a non-profit that has advocated for improving infrastructure to protect both neighborhoods and industrial areas on the waterfront against flooding, says the harsh reality of rising sea levels is that, thanks to climate change, New York City will experience many more severe weather events in the years to come.

“It’s scary to say this, but with Irene and Sandy, we’ve experienced the first level of climate change’s impact, and there are stronger storms out there,” Osorio says. (Not that floods were uncommon to begin with: FEMA’s 2009 Flooding Hazard Analysis of the New York City region, which accompanied the public release of the FEMA maps, noted the city experienced 60 floods between 1993 and 2007, with an average of four floods a year.)

One of the ways OEM could compensate for the unpredictability of surges and unpredictable flooding in future storms, Solecki said, is to make residents of neighborhoods near coastal areas and waterways aware of the worst-case scenarios of remaining in their homes during such events. “One could have said in the evacuation advisory that ‘Your house could be swept away.’ It might present a clearer picture of the strength of the storm surge.”

Sandy could have a very direct impact on how OEM’s new evacuation maps look: the storm’s surge reshaped barrier islands, dunes and other national bulwarks against inundation. For communities that once enjoyed some measure of protection from those natural barriers, the boundaries of risk must be redrawn.

More are at risk now

Coastal geography, sea levels and weather patterns aren’t the only things that have changed the city’s hurricane risk profile. Increased development on the waterfront means more people would be at risk from coastal storms even if storm surges were not increasing in size.

At a November 26 townhall at P.S. in Carroll Gardens, Katia Kelly, a neighborhood resident who runs a local news blog, asked federal and state officials about the lack of sufficient drainage infrastructure in the Gowanus area to deal with storms, a problem which has been exacerbated by new housing developments.

“With serious rain, what is Zone B right now would be considered Zone A” for flooding, Kelly said.

Assemblywoman Joan Millman echoed Kelly’s remarks, noting that residential development along Fourth Avenue over the past decade was not accompanied by an expansion in the sewer system or drainage capacity, a dynamic which has serious impacts for downhill neighborhoods in Northeast Brooklyn.

“People in Gowanus are trapped in their cars, even during heavy rains, [let alone] serious storms,” says Millman.