A SYNTHESIS OF FIELD AND GEOGRAPHIC INFORMATION SYSTEMS DATA FROM THE VIRGINIA BARRIER ISLANDS USED TO TEST THE CONCEPTUAL RUNAWAY BARRIER ISLAND TRANSGRESSION MODEL

Field and remote sensing data from the mixed-energy Virginia barrier islands (VBIs) were used collectively to test the conceptual “Runaway Barrier Island Transgression” (RBIT) model. This model predicts the morphodynamic changes that would occur to a barrier island system during a regime of accelerated sea-level rise. In particular, accelerated sea-level rise would convert salt marsh environments to subtidal environments and increase the backbarrier drainage basin area and tidal prism. The increased tidal flow will, in turn, widen tidal inlets, remove sediment from the littoral transport system by sequestering sediment on ebb-tidal deltas, and narrow and fragment the barriers(by breaching). Studies conducted during the past five years along one of the largest undeveloped, mixed-energy barrier island systems in the world (VBIs ≈ 800 km²) and in one of the highest seal-level rise rates on the U.S. east coast (≈4mm/yr) were used to test the RBIT hypothesis. The database included a quantitative analysis of sediment textural and tidal inlet dynamics data with emphasis on tidal prism changes, seismic reflection profiling surveys, an updated shoreline change analysis, and a Geographic Information Systems (GIS) analysis of orthophotographs (spanning 60 yr). The results from the synthesis of these data are building a case that supports the RBIT hypothesis. In particular, island areas have decreased (by 5.6% in 60 yr), tidal widths have increased (by 17.2% in 60 yr), tidal prisms have increased (12-25% over a 38 year period) with an attendant decrease in bay area (by 5.0% in 60 yr) and sediment textural data and shoreline change data point toward the potential sediment sequestration by Wachapreague Inlet. However, other results, such as a decrease in barrier fragmentation (although highly variable) and change of Wachapreague Inlet from ebb- to flood dominance call the main hypothesis into question. These results suggest that this barrier island system also responds to shorter-term, “noisier” processes over the decadal to centennial time scales used in this study which tend to complicate trend detection.