GEOLOGIC VARIABLES

The geomorphology variable expresses the relative erodibility of different landform types (Table 1). These data were derived from 1-meter resolution digital orthophotos (Table 2), historic washover distribution (Leatherman and Joneja, 1980), and nearshore geology (Williams, 1976; Schwab and others, 1999). In addition, field visits were made within the park to ground-truth the geomorphologic classification. The geomorphology of Fire Island varies from high vulnerability stable barrier island to very high vulnerability washover-dominated barrier shoreline (Figure 3-13).

Shoreline erosion and accretion rates for Fire Island were calculated from existing shoreline data provided by the US Army Corps of Engineers and the USGS. Shoreline rates of change (m/yr) were calculated at 20 m intervals (transects) along the coast using Digital Shoreline Analysis System (DSAS) software (http://woodshole.er.usgs.gov/project-pages/dsas/) to derive the rate of shoreline change over time. The rates for each transect within a 1-minute grid cell were averaged to determine the shoreline change value used here, with positive numbers indication accretion and negative numbers indicating erosion. Shoreline change rates on Fire Island range from greater than 2 m/yr accretion (very low vulnerability) to greater than 2 m/yr erosion (very high vulnerability) (Figure 14 A-E).

The determination of regional coastal slope identifies the relative vulnerability of inundation and the potential rapidity (rate) of shoreline retreat because low-sloping coastal regions should retreat faster than steeper regions (Pilkey and Davis, 1987). The regional slope of the coastal zone was calculated from a grid of topographic and bathymetric elevations extending landward and seaward of the shoreline. Elevation data were obtained from the National Geophysical Data Center (NGDC) as gridded topographic and bathymetric elevations at 0.1 meter vertical resolution for 3 arc-second (~90 m) grid cells. Regional coastal slopes for Fire Island are high vulnerability between 0.30 - 0.60 percent.

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