Keeping the Sea at Bay


Proposed Outer Harbor Gateway. Image: Halcrow Inc.

Hurricane Sandy made us painfully aware of the risks to New York City from ocean flooding. With the probability of more storms like this in the future, city leaders and engineers may need to plan for the 1,000-year storm event, not the 100-year event. To prevent the kind of devastation that Sandy delivered, some experts are calling for construction of sea walls and gates to hold back storm surges.

These systems have been installed in other flood-prone cities around the world such as London, St. Petersburg, and Venice. In 1982, London installed a movable flood barrier in the Thames River that, when raised, is five stories high and protects about 50 square miles of downtown London. St. Petersburg's $6-billion solution is a 15-mile-long perimeter barrier that consists of gates and locks that protect the city from storm surges in Neva Bay. Last year, when storms brought near-record flood waters, the system protected the city from flooding.

Then there is the $6.7-billion Modulo Sperimentale Elettromeccanico project in Venice, which becomes operational in 2014. This system consists of 78 inflatable gates that are filled with water and lay flat during normal sea conditions in concrete caissons attached to the seabed. When flooding is predicted, compressed air forces the water from the gates which then rise, rotating on their hinges to a vertical position to block the incoming seawater.

St Petersburg Flood Barrier - aerial view. Image:

Protecting the Big Apple

Movable sea barriers have been considered as a solution to New York City's flood risk for years. The city's office of Long-Term Planning and Sustainability has hired Jeroen Aerts, a risk-management consultant from the Institute for Environmental Studies in Amsterdam, to evaluate flood-management options. He estimates that a barrier system would cost about $15 billion and an additional $10 billion to $12 billion would be required for constructing levees, reinforcing beaches, and restoring wetlands.

One option that has been proposed is the New York-New Jersey Outer Harbor Gateway, a five-mile-long system of gates and causeways extending from Sandy Hook in New Jersey to the Rockaways in Queens that would deflect most of the destruction of a storm surge.

"The New York-New Jersey Outer Harbor Gateway would have very similar characteristics to the St. Petersburg barrier," says John Corsi, a spokesman for Halcrow, the engineering firm that directed the St. Petersburg project. "We think the barrier solution is applicable to the requirements of New York and therefore worth evaluating by the New York policy makers."

RSA Flood Wall System, base covers only (top), and with deployed removable walls (bottom). Image: RSA Protective Technologies, LLC.



Portable Systems

Another possible solution—portable flood walls that can be secured before the onset of the storm—is being studied by Stevens Davidson Laboratory at the Stevens Institute of Technology (SIT) in Hoboken, NJ.

Manufactured by RSA Protective Technologies, the flood-wall system consists of half-ton bases that are permanently installed into a city's infrastructure at low elevations. As a storm approaches, 20-foot by 4-foot panels are quickly installed into the bases using trained crews and heavy equipment. Rubber gaskets seal the panels to restrain water; other components can be used to fit the panels around curves and compensate for changes in grade and terrain.

The laboratory will evaluate the robustness, strength, and durability of the system by constructing a flood-wall model for its wave testing tank and conducting experiments to see how well the wall withstands various storm forces.

RSA estimates that the flood-wall invention system will cost about $15 million per mile of coastline to design and install. Other protections RSA recommends to complement the flood-wall system are pumps at strategic locations and sewage system blockages that would prevent flooding from underground.

"Cities are going to have to answer a critical question: 'What do we want to protect ourselves from? The 100-year storm or the 10,000 year storm?" says Alan Blumberg, professor of ocean engineering at SIT and director of the Stevens Davidson Laboratory. "In my opinion, storms of the future will be more frequent and more intense. So, when you compare the cost of the flood-wall system to the costs of remediation after a storm, or the costs of other storm protection alternatives, it no longer seems like such a large investment."

Mark Crawford is an independent writer.

When you compare the cost of the flood-wall system to the costs of remediation after a storm, or the costs of other storm protection alternatives, it no longer seems like such a large investment.

Alan Blumberg, professor of ocean engineering, SIT


May 2013

by Mark Crawford,