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The Next Steps for New York City Flood Protection

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Could New York City have foreseen and protected itself from Superstorm Sandy's tidal surge? The question has been asked many times. Some, including my colleagues and I, believe natural barriers in the form of artificial reefs, sturdy dunes and barrier islands can help do the job. But while these can reduce the damaging effects of waves—the cold temperatures during Sandy kept the winds at a high altitude, thus producing fewer waves—if we are to protect against the storm surge itself, a broader system of specific and engineered protection is required.

NORDENSON
Looking back to Sandy, we see how easily we can be caught off guard. Thanks to Hurricane Irene in 2011, transit officials knew to shut down the system in advance and move the trains to safety, but they did not seal the road and rail tunnels against flooding. Irene caused the first evacuation in memory, but the lack of severe flooding led some residents to ignore evacuation orders during Sandy, with terrible consequences. The unexpected flooding of an electrical substation downtown caused an extensive and lasting blackout. Many buildings were made uninhabitable when basement flooding knocked out their utilities. It seems that building-code provisions for flood-resistant design and construction had been, at best, inconsistently applied.

So what can be done? The answer can be found in the nation's experience mitigating the hazards of earthquakes and extreme winds. Individuals and professional organizations, most notably the Structural Engineers Association of California and the Earthquake Engineering Research Institute, decades ago began pressing local and federal governments for change while shaping a theoretical basis for earthquake-resistant design. The National Earthquake Hazards Reduction Program (NEHRP), initiated by Congress in 1977, formalized the government's role and steadily funded basic research. Spurred by the 1989 and 1994 California earthquakes, these initiatives led to the creation of rigorous, probability-based seismic-hazard maps and thoroughly vetted building and bridge code provisions.

Steady Advances

There is no national wind-hazard-reduction program. But the field has advanced steadily since the 1960s, thanks to the contribution of key wind-tunnel facilities, notably in Ontario, Canada, and the tornado-related research at the National Oceanic & Atmospheric Administration's National Severe Storms Laboratory in Oklahoma. Despite great advances in computational fluid dynamics, the physical wind-tunnel testing of building models and field measurements of extreme storms are still necessary. In 2001, the geographer Gilbert White (1911–2006), considered the father of floodplain management, wrote, "A full range of floodplain management tools should be used to address flooding problems, and assessing the effectiveness of these tools should be done on individual buildings and reaches for floods of up to 500-year frequency." He succinctly points to what is lacking now in flood-resistant design. For one thing, we don't understand well enough what happens to individual buildings when water and debris flow rapidly around them in a storm surge. We need physical testing for this just as much as we need to design for the turbulent flow of wind around buildings. YouTube videos of the 2011 tsunami flooding towns in Japan show the challenge and importance of improving our understanding of such complex flows.

Even more critical is the lack of consistently reliable, probability-based flood-hazard maps for the built environment executed at the same standards as our seismic- and wind-hazard maps. We need a sustained national hazard-reduction program for floods that includes these new kinds of maps and incorporates the best estimates of the effects of climate change. We also need improved research on what happens to structures in floods and extreme winds, with both numerical simulations and physical testing in large wave tanks.

The average funding for the NEHRP since 2005 has been $125 million a year. It has accomplished a great deal of good. Considering that the losses associated with recent storms have been far greater, it is clear what needs to be done. 

Structural engineer Guy Nordenson is a partner at New York City-based Guy Nordenson and Associates and a professor of architecture and structural engineering at Princeton University. He can be reached at gjpn@nordenson.com.

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