Geologic Variables

Figure 4. Coastal Geomorphology of Virgin Islands National Park. Click on figure for larger image.

The geomorphology variable expresses the relative erodibility of different landform types (Table 1). These data were derived from 1999 vertical aerial photography provided by NOAA's biogeography program. In addition, field visits were made within accessible locations of the park to verify the geomorphologic classification (Figure 5 A-H). Virgin Islands National Park contains several geomorphology types, including low vulnerability rock cliffs, moderate vulnerability alluvium and cliffs with fringing reefs, high vulnerability gravel beaches or cliff backed beaches, and very high vulnerability mangrove wetlands and sand beaches (Figure 4 and Figure 5 A-H).

Figure 5. Photos of different geomorphologic types within Virgin Islands National Park. Click on figure for larger image.

Shoreline erosion and accretion rates for Virgin Islands National Park were calculated using digitized shorelines from 1971 and 1983 NOAA Coastal Survey Charts and 1999 aerial photography (Table 2). Shoreline rates of change (m/yr) were calculated at 50 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. The

Figure 6. Shoreline change rates for Virgin Islands National Park. Click on figure for larger image.

change rates for each transect within each grid cell were averaged to determine the shoreline change value used here, with positive numbers indicating accretion and negative numbers indicating erosion. Shoreline change rates for St. John fall almost completely within the moderate vulnerability category, which is between -1 m/yr and +1 m/yr. Consolidated sections of the shoreline have not changed. There has been some erosion/accretion within bays and sandy areas such as Cinnamon Bay and Coral Bay. However, most of the rates still lie within +/- 1 m/yr (Figure 6).

Regional coastal slope is an indication of the relative vulnerability to inundation and the potential rapidity of shoreline retreat because low-sloping coastal regions should retreat faster than steeper

Figure 7. Regional coastal slope for Virgin Islands National Park. Click on figure for larger image.

regions (Pilkey and Davis, 1987). The regional slope of the coastal zone was calculated from a grid of topographic and bathymetric elevations extending 5 km 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 90-meter cells. Regional coastal slopes for Virgin Islands National Park fall within the very low to very high vulnerability category (< 4.55% - > 14.7%) (Figure 7).

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