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Fire Island Coastal Change

Morphologic Change - Morphologic Change and Shoreline Dynamics

Morphologic Change

An assessment of morphologic change for the beaches and dunes at Fire Island over a 40 year period (from 1969 to 2009) shows changes are linked with human activity, as well as documented variations in island morphology, and offshore bathymetry which influences the wave energy reaching the coast.

The western reach of the island has an ample sediment supply, demonstrated through rapid growth of Democrat Point over the last several centuries. The western and central portions of the island are linked with offshore geologic features, and home to a number of communities where replenishment projects have occurred over the years.

The profiles along western and central Fire Island show minimal change and in some cases the dune system is growing seaward.   Alongshore plots of different morphologic features show that movement of the shoreline and dune crest, and change in the dune elevation are minimal (near zero) in the western and central portions of Fire Island.
Above left: The profiles along western and central Fire Island show minimal change and in some cases the dune system is growing seaward (Profiles a-d). In contrast, on the eastern side of the island where there is documented historical and recent inlet formation and extensive bayside marsh development, there is substantial landward translation of the dune–beach profile over time (Profile e). [larger version]

Above right: Alongshore plots of different morphologic features show that movement of the shoreline and dune crest, and change in the dune elevation are minimal (near zero) in the western and central portions of Fire Island. Along the eastern segment of the island, there is measurable landward movement of the shoreline and dune crest (below zero), as well as dune crest elevation loss. Changes in the width of the beach and the volume from the dune crest to the shoreline are largely positive over the entire length of the island, suggesting that there is conservation of sediment; there is landward transport of material in the eastern portion of the island but not in the western and central portions. [larger version]

Shoreline Dynamics

A study of alongshore variation of shoreline change shows distinct differences in the movement and position of this dynamic feature. The shoreline assessment was conducted using a database of over 35 shorelines spanning nearly 80 years (1933-2012) and is regularly updated as new data are available.

The shoreline change pattern appears to be largely controlled by the regional geology. The 80-year shoreline change record demonstrates three distinct zones of behavior. To the east, the overall pattern is long-term, persistent erosion and corresponds to the portion of the island that has been shown to be moving landward over much longer time scales (centuries and longer).

Shoreline behavior along the western portion of the island is more variable than other portions of the island, and the average net change is near zero (although there are zones of erosion and accretion). The patterns of change are pervasive and repeating and have a length scaling similar to the field of sand ridges mapped on the adjacent seabed.

The central portion of the barrier is exhibiting seaward progradation over the time-scale of this analysis. This area of gain corresponds to the offshore location of a lobe of glaciofluvial sediment that is is linked to a bathymetric high on the seabed. The island is also on average higher in elevation in this area making it less susceptible to erosion.


Plot showing rates of shoreline erosion (red) and accretion (green) for 79 years at Fire Island.
Plot showing rates of shoreline erosion (red) and accretion (green) for 79 years at Fire Island. There are three distinct zone of long-term behavior that mimic the overall shape of the island suggesting the 79-year rates of change are representative of the longer-term island evolution. The different zones correspond to mapped variation in the geology offshore of Fire Island. [larger version]

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