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). In addition, field visits were made within the park to ground-truth the geomorphologic classification. The Gulf Islands consist almost entirely of low elevation barrier islands with numerous washovers (5- very high vulnerability) (Figures 3 and 4). However, there are locations on East Ship Island, Petit Bois Island and Santa Rosa Island where the dune ridge is mature, these locations are given a slightly lower vulnerability ranking (4- high vulnerability) (Figure 5 and 6).

Shoreline erosion and accretion rates for GUIS were calculated from existing shoreline data provided by the USGS. Four shorelines from 1855-2001 were digitized. 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 (Figure 7A-E). Shoreline change rates in Mississippi are much more variable (very low to very high) than in Florida (mostly moderate vulnerability).

The determination of regional coastal slope identifies the relative vulnerability of inundation and the potential rapidity of shoreline retreat because low-sloping coastal regions are thought to 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. For GUIS, elevation data were obtained from the National Geophysical Data Center (NGDC) as gridded topographic and bathymetric elevations at 0.1 meter resolution for 3 arc-second (~90 m) grid cells. Coastal slope was calculated for a 10 km (6.2 miles) radius (landward and seaward of the barrier) and then resampled to 1-minute resolution (Figure 2). The coastal slope for the Gulf Islands in Mississippi was shallower (0.14-0.27%) than the coastal slope in Florida (0.624-1.032%) (Figure 8A-B).

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