Recent events have demonstrated that regardless of one’s point of view on the topic of global warming, we are experiencing more frequent and destructive storms. In 2011 Irene caused massive flooding as a result of rainfall and in 2012 Sandy demonstrated the vulnerability of our shorelines to storm surge and high tides. The future of sea level rise, though uncertain in terms of its rapidity, will further threaten our coastlines. New York City must begin to make some significant changes to become more resilient to these forces of nature.
Solutions to flood mitigation will take enormous political will and civic support. Indeed there is not a “one-size-fits-all” solution. Barrier islands created by natural sand deposits such as the Rockaways are very different from the bedrock of Manhattan. However, much of Manhattan’s shoreline once contained mud flats, tidal marshes and sandy beaches. Decades of landfill have now artificially elevated thousands of acres using seawalls to retain their new contours. Given the density of people, buildings and infrastructure that surround the edges of Manhattan, multiple strategies will be required to recalibrate our past and future relationship to our estuarine reality.
 |
| Hunts Point Landing two days after Superstorm Sandy hit. Photo courtesy of Signe Nielsen. |
Solutions to flood mitigation will take enormous political will and civic support. Indeed there is not a “one-size-fits-all” solution. Barrier islands created by natural sand deposits such as the Rockaways are very different from the bedrock of Manhattan. However, much of Manhattan’s shoreline once contained mud flats, tidal marshes and sandy beaches. Decades of landfill have now artificially elevated thousands of acres using seawalls to retain their new contours. Given the density of people, buildings and infrastructure that surround the edges of Manhattan, multiple strategies will be required to recalibrate our past and future relationship to our estuarine reality.
Many people, including designers, have begun to explore both theoretical and built examples of ways to mitigate the damaging effects of water. Premised on the notion that resilient and ecologically productive natural edges are formed as a gradual gradient from river bottom to land surface, several new public waterfronts exemplify this “soft edge” approach. These newly contoured shorelines are intended to reduce the damaging impact of wave energy creating a gentle transition from land to water which is then planted in combination with boulders or similar wave attenuating materials. Soft edges, however, presume that water will flow in and out and that the plants and rocks can withstand the effects of salinity, tidal action and even forceful waves. They are not designed to stop flooding but rather buffer the damaging effects of storm surges.
 |
Reef balls at Hunts Point Landing. A reef ball is made from concrete and imitates the habitat conditions preferred by mussels when set within the intertidal zone. Photo courtesy of Signe Nielsen.
|
Under current regulations promulgated by an alphabet soup of federal, state and local agencies, soft edges cannot project out into the rivers; rather, they need to be created by carving into the land. The extent to which a soft edge penetrates landward is a function of the bathymetry, or topography of the river bed. The steeper the shoreline drops off, the greater the soft edge must cut landward. At the present time, this approach is only viable where the real estate is otherwise unoccupied by residents, critical infrastructure, or water-dependent industry.
As yet unbuilt ideas include creating artificial reefs, wave-attenuating bridges and filling in parts of the rivers to create new salt marshes. While there is hard scientific evidence to support these concepts, there is also concern that they may do permanent harm to existing habitat, jeopardize the shipping industry, and fail to provide adequate flood protection against future, more violent and frequent storms.
As yet unbuilt ideas include creating artificial reefs, wave-attenuating bridges and filling in parts of the rivers to create new salt marshes. While there is hard scientific evidence to support these concepts, there is also concern that they may do permanent harm to existing habitat, jeopardize the shipping industry, and fail to provide adequate flood protection against future, more violent and frequent storms.
Some of the soft edge public shorelines that have been built recently are Brooklyn Bridge Park, Hunts Point Landing and Riverside South Park. Others that have existed for a long time include the gently sloping stone-lined edge along Fort Washington Park and Manhattan’s only remaining natural wetland in Inwood Hill Park. I visited each of these sites within a week after Superstorm Sandy and was surprised to see how unscathed they were. In the case of Hunts Point Landing in the South Bronx, a park I designed and completed just two months before the storm, the only damage was to the fixed furnishings which were either knocked off their footings or dislodged by floatables carried by the rushing tide. The grasses in the intertidal wetland remained firmly rooted and the boulder wave attenuation edge did not move. Even more surprising was that the oyster “reef balls” which we placed at the base of the tidal wetland remained unharmed. We have also conducted soil tests within the planted areas that we had not expected to receive saline flood water and the results did not show elevated salt levels. I will only feel confident that our plant palette was successfully resilient until this spring when I can observe how well they reemerge from winter dormancy. While I was not the designer of the other parks mentioned, my observation is that they too withstood record high saline water inundation with minimal damage, most of which was as a result of water-borne debris.
Signe Nielsen is a Principal with Mathews Nielsen Landscape Architects, and has been practicing as a landscape architect and urban designer in New York since 1978. She is the president of the NYC Public Design Commission.
No comments:
Post a Comment