VARIABILITY IN FOREDUNE RIDGE ELEVATIONS WITHIN THE SOUTHEASTERN U.S. NATIONAL SEASHORES

Elevations of foredune ridges within the National Seashores of the Southeast U.S. have been determined from airborne scanning laser altimetry using a GIS-based analysis program. The spatially dense altimetry data allows estimates of ridge elevations about every meter over longshore distances of hundreds of kilometers. Two parameters define the geomorphology of the foredune ridge: 1) the elevation of the first line of defense (D_HIGH), either the dune crest or, in the absence of dunes, the beach berm, and, 2) the base of the foredune ridge (D_LOW). Comparing the elevation of D_HIGH and D_LOW to the elevation of runup determines the vulnerability of coastal regions to severe storms. For example, when storm runup exceeds D_LOW, dune erosion takes place, and when runup exceeds D_HIGH, the dunes are overwashed. These parameters vary considerably within and between the four National Seashores in this region. Dune heights on Cape Hatteras National Seashore display an alongshore, rhythmic spatial pattern with D_HIGH values of 6.0 +/- 1.6 m (relative to NAVD88) while the adjacent Cape Lookout National Seashore contains relatively low dunes with less spatial variability (D_HIGH=3.2 +/- 1.0 m). Cumberland Island and Canaveral National Seashores contain areas of very high dunes (12-m and 8-m dunes, respectively) with adjacent low-lying, overwash-prone regions. Previous altimetry studies show that similar patterns in longshore variability on Assateague Island National Seashore resulted in varied storm impacts along the island, such as net dune erosion, berm overwash, and inundation of the barrier system. Thus, the type and magnitude of coastal change caused by a severe storm will differ within each National Park.