GEOMORPHIC, TIME-SLICE MAPPING OF DYNAMIC BARRIER ISLANDS, NORTH CAROLINA’S OUTER BANKS: A BASIS FOR PRUDENT MANAGEMENT

Understanding the geomorphic evolution of the NC barrier islands over the past 15 decades is part of the ongoing USGS-ECU-NCGS cooperative coastal research program in partnership with the USNPS and USFWS. The evolutionary model is based on modern barrier island surveys between Cape Lookout and Kitty Hawk, NC and process-response studies utilizing time-slice analysis of georeferenced aerial photographs (1932-2003) and topographic surveys (1852-2003). This study includes the “more natural” Core Banks (Cape Lookout National Seashore) and the “severely human modified” northern Outer Banks (Cape Hatteras National Seashore). The modern data are integrated with historical data to develop the evolutionary responses of geomorphic-ecologic systems to sea-level rise, storms, and human modification.

Predicting the geomorphic succession of barrier islands, on time scales of several centuries and in response to ongoing sea-level rise, results from understanding the interaction of key destructive and constructive processes critical for long-term barrier island evolution. Coastal storms cause barrier island transgression through ocean-shoreline erosion and back-barrier construction by deposition of overwash fans and inlet flood-tide deltas that build island elevation and width. In contrast, human modification of these barrier island dynamics prevent island building by constructing barrier dune ridges, mining overwash sand, and artificially closing inlets. The consequence is island narrowing in response to increased rates of both ocean- and estuarine-shoreline erosion. If all processes were in balance, the barrier islands would maintain a dynamic equilibrium during their geomorphic evolution. However, with escalating economic development, the natural processes are increasingly being throttled, minimizing island building and maximizing island narrowing. With ongoing sea-level rise, this disequilibrium will cause sediment-poor barrier island segments to collapse. The model predicts the location and sequence of inlet formation and barrier island collapse over the next several decades to centuries.