Coastal Change Processes- Fire Island, NY

Coastal Change Processes

Coastal Change Processes

The primary objective of this project is to increase our understanding of the physical processes that cause coastal change, and ultimately improve our capability to predict the processes and their impacts. 

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Fire Island, a 50-km long barrier-island system between Fire Island Inlet and Moriches Inlet, attracts significant tourism, includes federal, state, and county parks, contains a number of coastal communities, provides storm damage protection to the adjacent heavily populated mainland, and supports a distinct barrier island ecosystem, all of which are affected by coastal change.  Mitigating the impacts of coastal erosion has been an important management objective for decades.

This effort focuses on coastal processes along the Fire Island segment of coastline along southern Long Island, NY and is associated with other previous and ongoing efforts to monitor coastal change along segments of the Fire Island National Seashore (FIIS) in cooperation with the National Park Service (NPS) and U.S. Army Corps of Engineers (USACE).

A primary scientific issue is the lack of understanding of basic coastal processes in the region, especially related to establishment of an accurate coastal sediment budget, a critical element used in modeling coastal evolution, and in the design and implementation of a feasible coastal-erosion mitigation strategy.


Geologic Framework

In 2011, we mapped the inner-continental shelf and lower shoreface offshore of Fire Island using Chirp high-resolution subbottom and interferometric sonar technology.  These data have been processed and interpreted to examine sediment distribution patterns, shelf physiography, antecedent stratigraphy and the interaction of this geospatial/sedimentologic framework with coastal change.

Preliminary findings from this mapping activity were published in FY2013 showing that the Holocene evolution and decadal- to century-timescale behavior of Fire Island is linked directly to the geologic framework of the inner-continental shelf.  The eastern segment of the island is migrating landward, while the central and western segments are relatively stable or accreting.  This behavior is likely related to the presence of a relatively larger, mobile Holocene sediment supply on the inner-continental shelf offshore of the central and western segments of the island which are organized into a series of shorface-attached sand ridges.  This larger sediment supply was created by erosion of a Pleistocene outwash lobe off central Fire Island by oceanographic processes associated with ongoing marine transgression.  Final interpretation of these geophysical data is scheduled for early fy2014 and will include maps of the regional coastal plain unconformity, the Holocene marine transgressive surface, and the modern sediment thickness and distribution. Comparison of these mapping products with marine geophysical data collected in the late ‘90s shows that the modern sand deposit is mobile and that ravinement processes are continuing to modify the inner shelf and lower shoreface 

Map showing Holocene (modern) sediment thickness in meters

Map showing Holocene (modern) sediment thickness in meters overlain on bathymetry. Regional bathymetric contours are in meters. Note the shoreface-attached sand ridges west of Watch Hill.

Oceanographic Observations

We have conducted two field deployments offshore of Fire Island to increase our understanding of the oceanographic and sediment transport processes and identify connections to the underlying geologic framework in the region. The first deployment was from February 21-April 18, 2012 to deploy equipment at seven sites. The main intent was to measure oceanographic conditions and sediment transport patterns to identify processes that formed and maintain the shore-face connected ridges. The equipment sites were chosen to locate sensors along ridge crests and in the troughs, with two main sites, one and two, (locations indicated in top right study area figure) to measure bottom stress, suspended-sediment concentrations, sea floor ripples, currents, waves, salinity, and temperature. Ancillary sites will provide data to determine wave refraction and ocean current variations in the ridge field. The experiment is described in Martini and others (2012).

Preliminary analysis of the data identifies that the along shelf flows have a strong tidal component and an equally significant storm driven component. The along-shelf tidally-averaged flows are highly correlated with along shelf winds. In the cross shelf direction the flows are more complex, but in general the surface currents appear to have a more off-shore directed component and the bottom currents appear to have a more onshore component. Preliminary analysis of the suspended sediment profiles identify that there were only a few storm events during the deployment that resuspened sediment at the depths of the instruments. These events are being correlated with storm types, and during the deployment there were several strong cold fronts that tended to provide a net offshore directed flux of sediment, and a low pressure type system that produced a net onshore flux. These different types of storms may help further understand the net sediment convergence processes that maintain the shore face connected ridge system.

The second deployment was from February - April 2014, with equipment deployed at 9 locations (see lower right study area figure). The main intent for this deployment was to measure the alongshore distribution of waves and the cross-shore variability of sediment fluxes. This information will be used to assess the alongshore variability in coastal response to storms. It is anticipated the large-scale offshore shore face connected ridge features modify the approaching waves and can alter the alongshore variability of sediment movement. Details are also described in

As part of this second experiment, we also deployed a specialized buoy that measures surface waves which telemeters the data back to the Coastal Data Information Program (CDIP) where it is available on-line ( station #207.  It is anticipated that local mariners, other Federal Agencies, and recreational users will utilize the data in real-time. This buoy is expected to remain on site for several years.

Numerical Modeling

Numerical modeling simulations are being conducted to simulate ocean currents, waves, and sediment movement on the inner shelf using both idealized settings and realistic conditions. A series of numerical experiments are being performed to examine the oceanographic processes that maintain the shore face connected ridge system. In an idealized experiment the numerical model simulates flow over a series of repeated submerged ridges. Theory identifies that the flow should be deflected offshore over the ridge crests and onshore in the troughs. Numerical results shown are consistent with the theory, showing flow being deflected offshore over the crests (orange colors) and back onshore in the troughs (blue color). Numerical results shown are consistent with the theory, showing flow being deflected offshore over the crests (blue colors) and back onshore in the troughs (orange color). Magnitude of the cross shore component is only a few percent of the total flow, and was scaled in the figure to make the directions identifiable. However, these small variations in the flow are the significant process that maintains the features. Additional simulations with sediment fluxes and on realistic bathymetry are currently being performed.

Numerical model grids

Numerical model grids showing downscaling from larger regional grid (far right) to more refined coastal grid (left). Colors are bathymetry (m).