Geologic Variables

Figure 3. Index map for geomorphology photos. Click on figure for larger image.

The geomorphology variable expresses the relative erodibility of different landform types (Table 1). These data were derived from USGS 1-meter resolution digital orthophotos of Dry Tortugas (Table 2). In addition, field visits were made within the park to ground-truth the geomorphologic classification. All areas of the Dry Tortugas are considered very high vulnerability with the exception of Loggerhead Key, which was classified as high vulnerability because of the presence of substantial stretches of beach rock that has helped protect the island and what remains of the Carnegie Institute Lab (Ginsburg, 1953) (Figure 3 A). Fort Jefferson and the moat wall surrounding it are classified as very high vulnerability, even though the fort is 15 meters higher than anything else in the Dry Tortugas. This classification was made because the Fort is built upon carbonate sand, and the human resources required to maintain the Fort over time are less frequent than the replenishment of carbonate sand through biologic and physical processes (Figure 3A-E).

Figure 4. Index map for historic shoreline change figures. Click on figure for larger image.

Shoreline erosion and accretion rates for Dry Tortugas were calculated using digitized shorelines from NOAA T-sheets, Nautical Charts, and USGS aerial photography (Table 2). Shoreline rates of change (m/yr) were calculated at 200 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 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 on Dry Tortugas range from 2 m/yr of accretion (low vulnerability) to almost greater than 2 m/yr of erosion (very high vulnerability) (Figure 4 A-C).

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 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 3 arc-second (~90 m) grid cells. Regional coastal slopes for Dry Tortugas all fall within the high vulnerability category (0.3 - 0.6 % slope).

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