Geology and Sediment Availability - Coastal System Change at Fire Island, New York

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Sediment supply is a critical control on barrier island vulnerability and resilience over a variety of time scales (e.g., storms to sea level rise). Past work at Fire Island and elsewhere has demonstrated a link between inner shelf sediment availability and barrier island evolution over geologic and historical time scales. However, there have been few opportunities to explore nearshore sediment availability and its influence on post-storm beach recovery and interannual beach behavior.

As part of Hurricane Sandy Supplemental project GS2-2B and with collaborators at the National Park Service (NPS) and the U.S. Army Corps of Engineers (USACE), the USGS led large field campaigns to assess nearshore morphology and sediment availability along Fire Island, NY. The results of this effort fill a significant scientific knowledge gap and, when integrated with existing datasets, will improve model forecasts of coastal behavior by enhancing our understanding of the relationship between the geology and geomorphology of coastal systems.

Exploring nearshore geology and sediment availability requires specialized vessels and data collection techniques. First, shallow water depths require smaller boats that may not be equipped to handle heavy acoustic geophysical instruments typically used to map sub-seafloor geology. Furthermore, waves break in the nearshore causing acoustic instrument signals to weaken or dissipate completely. In order to map nearshore geology at Fire Island, specialized methods were used. In collaboration with the USACE, we used an amphibious vessel to acquire high-resolution sub-seafloor profiles across the nearshore. Additionally, the instrument was deployed just below the water surface on a catamaran sled to ensure that data would be able to be collected right up to the shoreline. The data generate images of the sub-seafloor structure of the nearshore that, after analysis, reveal significant variability in sediment availability just offshore of Fire Island which may influence post-storm beach recovery rates and future barrier island resilience to sea level rise. 

Photo of a larger research vessel towing a smaller one at the beach

The U.S. Army Corps of Engineers’ Lighter Amphibious Resupply Cargo (LARC) survey vessel towed an Edgetech chirp 512i subbottom profiler mounted on a catamaran sled. Credit: Cheryl Hapke, USGS

Photo from offshore of Fire Island showing methods of data collection using sled, personal watercraft, and beach-based personnel

Nearshore geophysical mapping with sub-bottom sled (foreground), personal watercraft equipped with echosounders (background, water) and beach-based personnel (background, beach). Credit: Jen Miselis, USGS

Two images of chirp profile data with interpretation labels

An example of a seismic profile collected from Fire Island showing the sub-seafloor geology of the shoreface. The lower image shows the geologic interpretation of the upper image. Credit: Noreen Buster, USGS

 

Two graphic maps contrasting disturbed and undisturbed Holocene sediment thickness

Maps of the thicknesses of the nearshore sediment units are created by combining geologic interpretations across all seismic profiles. Credit: Noreen Buster, USGS