A TEST OF THE THEORETICAL BARRIER FRAGMENTATION MODEL USING A GEOGRAPHIC INFORMATION SYSTEMS ANALYSIS OF THE MIXED-ENERGY VIRGINIA BARRIER ISLANDS

Assessing the dynamics of coastal systems during a period of accelerated sea-level rise has implications for land use planning, coastal hazard predictions, and ecosystem management as well as for determining sediment budgets and the preservation potential of coastal litho- and biofacies in Coastal Plain settings. This study used a Geographic Information Systems (GIS) analysis of georectified, orthophotographic data sets spanning 15 yr (1994 and 2009) to assess the effects of accelerated sea-level rise (nearly double those of the eustatic rates) on one of the largest undeveloped, mixed-energy barrier island systems in the world (Virginia barrier islands ≈ 800 km²). In particular, we test the barrier fragmentation model which predicts that accelerated sea-level rise causes drowning of backbarrier marshes (conversion to subtidal environments) and increases the backbarrier drainage basin (bay) area and tidal prism. The increased tidal flow, in turn, widens tidal inlets, removes sediment from the littoral transport system by sequestering sediment on ebb-tidal deltas, and narrows and fragments (by breaching) the barriers. The results from our analysis do not entirely support the model hypothesis. In support of the model, inlet widths increased by 37.9%f (3.0 km) and island areas decreased by 5.0% (1.8 km²). However, contrary to model expectations, the bay area decreased by 0.6% (4.4 km²) and islands became less fragmented over a 15 year period. The results also indicate that the measured parameters varied by coastal compartment and among individual islands. Consequently, other shorter-term processes (e.g., variation in relative contributions of storms, waves and tides), regional phenomena (e.g., a stable mainland shoreline, bay hypsometry and island migration rate variability), and/or local controls/feedbacks (e.g., presence or absence of backbarrier fringing marsh, nearshore antecedent geology, breaching, overwash) influence island morphodynamics more than accelerated sea-level rise during this time period. Analyses currently underway include a study of the same parameters using a 1950s photographic dataset to expand the temporal record to nearly 60 yr, an assessment of marsh loss, and application of additional quantitative models.