Coastal bluffs, dunes and the future of New York’s ocean shoreline

The ocean coast of New York is protected naturally by dunes, bluffs and beaches. The distribution and combination of these natural protect features (NPFs) control the response of the shoreline to both extreme storms and rising sea level. Hurricane Sandy recently impacted this coast in October 2012, and sea level may rise over a meter by 2100. Geometries of NPFs were analyzed at 750 cross-shore transects along 36 km of the New York’s easternmost ocean shoreline. Profile data was compiled from NOAA Light Detection and Ranging (LiDAR) surveys between November, 2011 and April, 2012 by NOAA. Low relief beaches are only found as the seawardmost NPF only in front of coastal ponds covering less than one percent of the shoreline. Steep bluffs of unconsolidated, or semi consolidated, sediment, without dunes, comprised 33% of the shoreline. Dunes, often multiple dune lines, constitute the most seaward NPF in 66% of the study area, but were often found in combination with bluffs. In some places a dune has formed on top of a bluff (8%), in others a dune is found in front of a bluff (10%). The average elevation of bluff crest was 11.6 m; dunes formed in front of the bluff face reaching average dune-crest elevations of 6.8 m. Where the overlap exists, the dune would provide the first line of protection against erosion and flooding during storms. Numerical modeling with CSHORE showed that the storm of record (Hurricane Sandy) excavated about 71m³/m of the dune. With rising sea level, dunes that have formed in front of the bluff at the present time would be displaced onto the bluff crest. Whereas the bluff crest above sea level would necessarily decrease as sea level rises, the elevation of the dune above sea should be expected to remain the same, being controlled by the site-specific nature of aeolian transport. The excavation of the bluff is likely to be the controlling factor in the face of a long-term rise in sea level. Further recession of the shoreline would then require the excavation or drowning of the bluff face.