Geologic Variables

Figure 3-10. Photos taken along Cape Hatteras National Seashore in August 2003. Click on figure for larger image.

The geomorphology variable expresses the relative erodibility of different landform types (Table 1). These data were derived from 1998 1-meter resolution digital aerial orthophotographs of Cape Hatteras (Table 2). In addition, field visits were made within the park to ground-truth the geomorphologic classifications. The geomorphology of Cape Hatteras varies from high vulnerability barrier island with dunes to very high vulnerability washover-dominated barrier shoreline (Figures 3-10).

Figure 11. Photo from Hurricane Isabel impact study (USGS, 2003). Click on figure for larger image.

A Hurricane Isabel impact study shows the barrier breach that formed between Frisco and Hatteras Village on September 18, 2003 (Figure 11) (USGS, 2003). This breach occurred in a washover-dominated very high vulnerability area. The U.S. Army Corps of Engineers and the NCDOT closed the inlet and reconstructed Highway 12 during October and November of 2003.

Figure 12 A-D. Historic shoreline positions. Click on figure for larger image.

Shoreline erosion and accretion rates for Cape Hatteras were calculated from existing shoreline data provided by USGS (Table 2). Shoreline rates of change (m/yr) were calculated at 200 m intervals (transects) along the coast using Digital Shoreline Analysis System software to derive the rate of shoreline change over time (Thieler and others, 2003). 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 Cape Hatteras range from greater than 2 m/yr of accretion (very low vulnerability ) to greater than 2 m/yr of erosion (very high vulnerability) (Figure 12 A-D).

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 10 km landward and seaward of the present-day 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 (Table 2). Regional coastal slopes for Cape Hatteras fall within the high vulnerability (4) to very high vulnerability (5) category (Table 1).

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