by Rachel Gruzen, M.E.M., LEED AP
In the aftermath of Hurricane Sandy, there has been extensive discussion on how New York City will protect itself from the next storm. Of great concern is how a city at sea level with more than 500 miles of shoreline can protect itself from storm surge and flooding waters, which are only anticipated to increase in frequency with climate change and sea level rise in the coming decades.
One of the most cited flood management proposals in the news during the last weeks has been a sea wall: a structure of steel gates and concrete designed to barricade the City against the surging Atlantic that flooded the City up to nine feet in some places during Hurricane Sandy, and could be as high as 25 feet in storms of the next century . Other popular proposals have been ecological in nature, centered on repopulating the City’s bays with marshes and oyster reefs, indicative of the growing popular awareness that the health of humans and our economy is tied to ecological restoration.
But the conversation has just begun. Hardening structures, such as sea walls, can be effective barriers, but they have a myriad of negative environmental and cost impacts. Healthy ecosystems provide effective natural defenses against storm damage, but these habitats must be well established and widespread to provide adequate protection against a hurricane like Sandy. To move this discussion forward, and make wise investments in the City’s resiliency, we need to understand the opportunities and limitations of the options and embrace a more informed understanding of coastal systems and littoral processes.
Hardening structures, such as sea walls, have high costs.
Hardening structures function to deflect wave energy and volumes of water. While marine habitats are designed by nature to absorb energy, hardening structures shift the energy, resulting in erosion, scouring and habitat loss, usually in the areas in front of and adjacent to the hardening structure. Hardening structures tend to transfer the damages from one landowner to his neighbor, or in the case of the New York sea wall proposal, from one borough to another, which has been raised as a social justice issue by some.  A sea wall can also alter the ecology of the water behind it, impacting water exchange, salinity, temperature, and species diversity in an estuary. In addition, the sea wall could restrict shipping traffic in the New York-New Jersey Port, affecting the City’s economy. These costs are in addition to the high capital investment costs. A New York sea wall is estimated to cost $8 to 15 billion to construct and $75 million to maintain annually. Many question whether such a large investment in the City’s resiliency would yield a better return if spread across a larger portfolio of protections.
Coastal habitats provide diverse services—and have their limitations.
Wetlands and oyster reefs filter and clean an estuary’s waters by binding pollutants and consuming nitrogen-containing compounds. Wetland vegetation is a high-friction surface that is effective at slowing down surface water. Dense and porous oyster reefs serve as underwater obstacles to surging waters. But we must understand the limitations of these habitats. It is unlikely that marshes and oyster reefs could have blocked Sandy’s floods from moving up the bays. They may have slowed down the flooding waters and reduced the damage in some locations, but they probably would have been most effective after the storm, processing the high nitrogen loads in the rivers and helping to cycle the pollutants out of our system faster. Other habitat typologies, such as barrier islands and dunes, provide the height and bulk to potentially repel storm surge, but for them to function in this way, we must allow for their natural accretion and limit the development of structures on them.
Coastlines change, constantly.
The ocean is a high-energy mass that is constantly re-sculpting shorelines. Coastal systems change, migrate, and even depend upon movement to sustain life. The history of habitat loss in New York City in the 20th century is one of overharvesting resources and diminishing water quality, and largely one of developing up to the water’s edge and leaving no room for coasts to shift. In the last thirty years we have improved water quality in New York, and our biodiversity is returning, but we also need to provide the space for these habitats to expand, especially given climate change. As sea level rises, the intertidal zone will move inland. Marshes will either migrate into upland areas, or if they cannot, the marshes will drown. The sand of our barrier islands is carried alongshore by the waves and creates a scallop formation—the beach in one location is narrow today and years later it is wide. These sands ultimately feed our barrier islands and dunes, but not if the sands are blocked by groins and waterfront development.
We must reduce our efforts to make the coastal environment permanent, because it will not remain so. Without this understanding, we will undermine the City’s ecological restoration and our waterfront investments. The ocean with its great energy will teach us this lesson over and over.
New York City’s resilience to future storms and climate change will be a multi-strategy approach. It may involve hardening structures at small scales—to protect our most critical infrastructure and vulnerable neighborhoods—and ecological restoration of wetlands, oyster reefs and barrier islands—in locations with supportive conditions. As the City’s planners, scientists, designers, and policymakers have stated during the last few weeks, a diverse portfolio of solutions, a strong interdisciplinary collaboration, and a deep understanding of coastal processes will be key to managing risk and vulnerability against future storms. Because after all, no matter how developed New York City, it is still a collection of islands of marsh, rock, and dune in an estuary where ecological processes will never cease.
Make sure to check out NOAA’s Aerial Photos of Hurricane Sandy Damage.
 New York Panel on Climate Change, “Climate Change Adaptation in New York City: Building a Risk Management Response” (2010)
Robert Puentes, “Worth the Investment”. The New York Times, November 1, 2012.
Editors, “Should New York Build Sea Gates?”. The New York Times. November 1, 2012.