DYNAMIC EVOLUTION OF BARRIER ISLAND MORPHOLOGY AND ECOLOGY FROM 1996 TO 2002 DOCUMENTED USING HIGH-RESOLUTION GPS-GIS TOPOGRAPHIC MAPPING SURVEYS, VIRGINIA COAST RESERVE

Dramatic topographic and ecological changes have been documented on the barrier islands of the Virginia Coast Reserve between 1996 and 2002. Long-term study sites included in the VCR-LTER (Virginia Coast Reserve - Long Term Ecological Research) site range from the macro-scale encompassing Myrtle Island to meso-scale washover sites on Parramore and Hog Islands. The barrier environments are so dynamic that conventional surveying methods were abandoned in favor of using GPS. Hi-resolution GPS mapping with cm-scale accuracy is an effective method for detailed geomorphic mapping, illustrating the evolution and destruction of dunes, washover flats, and beach environments. Importing field GPS data into ArcView GIS permits aerial and volumetric calculations to be made of changes within elevational slices of the islands between surveys. During the past 7 years, Myrtle Island has been transformed from a high-profile barrier with dunes over 5 m above MSL (most of the island was between 2-3 m above MSL) to a washover flat with most of the island less than 1 m above MSL. Since 1996, the subaerial extent of Myrtle Island has gone from 268,895 m2 to 48,787 m2 (less than 18% of the surface area in 1996). This represents a total net volume loss of 329,359 m3 of sand, leaving Myrtle with only 8% of the subaerial volume it had in 1996. The majority of these changes have occurred in the absence of major coastal storms, probably influenced by alterations in the longshore drift related to the dynamics of Ship Shoal Inlet and the effects of moderate extratropical storms. More than 400 m of westward retreat of the shoreline has occurred since 1996, averaging about 80 m/yr. Wholesale subaerial island ecologies have been lost as the dunes which hosted freshwater woody vegetation have eroded. Less dramatic, but significant shoreline retreat of more than 125m has occurred on south Parramore Island during this time. In sharp contrast, the Hog Island site has experienced negligible shoreline retreat and significant subaerial volume increase by dune building. Vertical growth at the Hog Island site has been documented by conventional surveys and later GPS surveys since the major overwash event during the Halloween storm of 1991. These sites illustrate the remarkable spatial variation in the geomorphic response of barrier islands on the Virginia coast, even when major storms have been absent.