The Science Behind NYC Flood Protection

Mayor Bloomberg recently released a report detailing plans to make NYC more resilient in the face of rising sea levels and climate change. Dr. Philip Orton, a research scientist at Stevens Institute of Technology who studies physical oceanography and storm surges, consulted on the report. Here he shares his perspective on the science behind the protection and adaptation strategy.

Published June 23, 2013

The Science Behind NYC Flood Protection

Mayor Bloomberg recently released a plan to make NYC better prepared for, and adaptive to, rising sea level and extreme weather threats. The report, titled "A Stronger, More Resilient New York," is based on input from scientists with diverse areas of expertise as well as community organizations. Dr. Philip Orton, a research scientist at Stevens Institute of Technology who studies physical oceanography and storm surges, describes the report as solidly grounded in science and quantitative methods.

The report takes climate change as an unequivocal given that demands acknowledgement and adaptive action. At a press conference, Mayor Bloomberg explained,

"Our city will be much more vulnerable to flooding in the decades ahead...We expect that by mid-century up to one-quarter of all of New York City's land area, where 800,000 residents live today, will be in the floodplain. If we do nothing, more than 40 miles of our waterfront could see flooding on a regular basis, just during normal high tides...[Hurricane] Sandy cost our City $19 billion in damages and lost economic activity. And we now forecast that a storm like Sandy could cost nearly five times that much by mid-century—around $90 billion."

"We wish that everyone had agreed that there was this threat from storm surges before Sandy," says Dr. Orton, "but putting up the protections that we should have put up before is a huge step in the right direction. Just having that attitude that yes, we do get hit by hurricanes, is impactful. Considering sea level rise on top of that, as the Mayor's plan does, can help protect us from future storm surges."

The Coastal Protection chapter of the report outlines the strategies to fortify and defend NYC's diverse coast areas. The recommendations focus mainly on local projects (such as restoring and widening sand dune systems, cultivating oyster reefs and wetland areas, and installing tide gates) rather than harbor-wide, in-sea barrier structures. Such large-scale projects, it is estimated, would cost between $20 and $25 billion and take decades to construct. Harbor barrier structures would also have hydrodynamical consequences that would deflect damaging storm surges to vulnerable areas. Dr. Orton explains,

"The fluid dynamics is a definite reason for not building barriers in the harbor. Any barrier raises the flood level somewhere else some amount. At Stevens Institute, we've run models and quantified the storm surge increase off of hypothetical barriers due to the reflection of the storm surge back out to sea. Since we're right next to the open ocean, the extra water radiates out to sea and the storm surge increase is not actually that large. But even a decimeter is still too much for some high-risk, low-lying neighborhoods to find it palatable."

Will localized measures prove adequate for the task of protecting coastal neighborhoods? According to Dr. Orton, sand dune expansion "is a proven method of effectively reducing flooding. It had a huge positive influence on areas along open shores during Sandy." Other recommended measures, such as wetland expansion and oyster bed growth, require more research before the degree of their efficacy can be fully understood. Current models indicate that, in larger areas such as Jamaica Bay and New York Harbor, such systems are indeed likely to be vast enough to reduce flooding as well as waves. More quantitative research on natural shorelines and their influence on flooding remains necessary. "The operative word being 'quantitative,'" Dr. Orton emphasizes.

What does such research entail? Dr. Orton elaborates, 

"It's hugely interesting that we have the tools to study this now. The same models that are used for storm surge forecasting can be used for testing the effects of adaptation strategies, and the modeling techniques have improved dramatically over recent years, especially as computing power increases. This has been the same with climate modeling. We apply conservation principals for water velocity, momentum, mass, heat, and other variables. Storm surge doesn't involve biology, carbon cycles, or chemistry, so it's a simpler problem than climate and these are easier predictions to make. It's just about the physics of the movement of water and it's very reliable, given good weather forecasts (though those have a lot of uncertainty). In the fluid dynamics models, you input values for factors like water depths and land elevation, and you add in obstructions to test your adaptation strategies. You can put in barriers or frictional elements in places to simplistically represent wetlands or oysters or whatever, and observe the outcomes."

Reeva Dua at Columbia's Center for Climate Change Law Blog and Eric Goldstein at the Natural Resource Defense Council Blog point out that there is some controversy over climate change response strategies based on adaptation versus retreat. Some predictions place certain low-lying areas underneath six feet of water by the end of the century. Governor Cuomo has proposed an alternative floodplain buy-out program to acquire property within flood zones and convert it to natural buffers. "With the accelerated sea level rise that we expect to kick in really rapidly in the next century, eventually there will need to be more plans," says Dr. Orton.  A 500-year coastal flood (a flood with a height expected with a 0.2 percent probability of occurring at a given location in a given year) in the next century could overwhelm the protections being devised now.

There's likely to be ongoing debate on this point. In the meantime, refusing to cede ground "gives a really strong message of support for all low-lying, high risk neighborhoods," says Dr. Orton. "Some of those areas are very high-value, like lower Manhattan, and some of them are sparsely populated or have lower property values, like the shore of Staten Island. Insisting on defending all these areas sends a strong message of solidarity."


Disclaimer: The views and opinions expressed in the articles on nyas.org are those of the author(s) and do not necessarily reflect the views or opinions of the New York Academy of Sciences.