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Levees Could Protect Lower Manhattan From Future Floods

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If another Sandy-like storm hits New York City, the city government wants southern Manhattan to be much better protected than it was during the devastating 2012 hurricane, from which recovery is still incomplete. So, the New York City Economic Development Corporation and the Mayor's Office of Recovery and Resiliency have an idea—they could not stress enough that this is just an idea—to extend Manhattan with a 1.3-mile-long living barrier made up of a multi-purpose levee system. Manhattan would be extended by fill, on top of which parkland, residential buildings, and office buildings would be built. This proposal was previously called Seaport City, but the moniker caused confusion about the plan—people thought, naturally, it would be a new neighborhood, like Battery Park City—and there is no mention of the name in the new study.

First: what is a multi-purpose levee? The report says:

An MPL is a high and wide standard river embankment, or a wall or bank of earth or stone, that is built to prevent flooding and designed to withstand flood overtopping [ed. note: this is when water rises over the top of a barrier]. In traditional designs, MPLs are passive, earthen levees that are about 30 times as wide as they are high. MPLs are designed to have an extended, gradual slope behind what would otherwise be a traditional levee on the edge of a water body. The extended slope essentially raises the land behind the traditional levee, providing structural reinforcement and a stable region that can be developed. This design allows for overtopping without complete failure because flowing water does not breach the levee but rather flows slowly across its top surface.The levee system would run 1.3 miles from Whitehall Street at the Battery up the East River to Montgomery Street on the Lower East Side. Dan Zarrilli, Mayor Bill de Blasio's director for ORR, told a small group of reporters on Thursday that these levees would be more practical and less intrusive than the idea of long system of very tall vertical barriers. Zarrilli said the city government wants us to remain a coastal harbor city.

Essentially, the island would be extended by fill, on top of which would be parkland, residential buildings, office buildings, and possibly more—that's what is meant by "multi-purpose." The basic land reclamation idea would mean a 250-foot extension at points, and further expansion could bring the extension to 500 feet if a platform were added. The report includes photos of MPLs in cities in the Netherlands and Japan, showing that the land closest to the water is used as public space, and the new buildings are on the opposite side of the slope.

It's hoped that this system would protect southern Manhattan from a flood of 19 feet, which would be possible if a Sandy-like storm hit again in 100 years. Zarrilli said they can't make the city "flood proof," but "we can protect ourselves."

The three biggest questions EDC and ORR wanted answered before proceeding to even propose this idea were 1: Would it work? 2: Was there a viable legal and environmental framework for it? and 3: Would there be a way to finance it?

Zarrilli said the answer to all of those was yes. They determined that not only would this system protect Manhattan, it would also not produce additional flooding across the East River. On the environment and legal issues, it would be a complex process, but not impossible. He said that financing would most likely be accomplished by a public/private partnership.

What's next for this idea? There is quite a lot of environmental study still to be done and a lot has been done for the lengthy feasibility study (which you can read for yourself here). But first the city will be reaching out to the public. Zarrilli appeared before Community Board 1's Planning Committee on Thursday evening and the idea will be presented to Community Board 3 in the near future. Plus other public hearings will no doubt be planned.
—Evan Bindelglass is a local freelance journalist, photographer, cinephile, and foodie. You can e-mail him, follow him on Twitter @evabin, or check out his personal blog.
· Southern Manhattan Coastal Protection Study [NYCEDC]
· Seaport City coverage [Curbed]
· Hurricane Sandy coverage [Curbed]