GEOLOGIC VARIABLES

Figure 3. A) Northern Assateague Island, (A) shows an area that is low and overwashed; B, C, and D) show dunes along Assateague Island. Click on figure for larger image.

The geomorphology variable expresses the relative erodibility of different landform types (Table 1). These data were derived from 1-meter resolution digital orthophotos of Assateague Island (Table 2). In addition, field visits were made within the park to ground-truth the geomorphologic classification. The geomorphology of Assateague Island varies from high vulnerability stable barrier island with dunes to very high vulnerability washover-dominated barrier shoreline (Figure 3 A-D).

Figure 4. Historic shoreline positions for northern, south-central, and southern Assateague Island. Click on figure for larger image.

Shoreline erosion and accretion rates for Assateague were calculated from existing shoreline data provided by USGS in Virginia and the Maryland Geological Survey in Maryland (Table 2). Shoreline rates of change (m/yr) were calculated at 20 m intervals (transects) along the coast using Digital Shoreline Analysis System (DSAS) software 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 indicating accretion and negative numbers indicating erosion. Shoreline change rates on Assateague Island range from greater than 2 m/yr of accretion (very low vulnerability) to greater than 2 m/yr of erosion (very high vulnerability) (Figure 4 A-C).

Figure 2. Shoreline grid for Assateague Island National Seashore. Click on figure for larger image.

The determination of regional coastal slope identifies the relative vulnerability of 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 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. These data were resampled to 1-minute resolution (Figure 2). Regional coastal slopes for Assateague Island fall within the high vulnerability category.

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