BARRIER ISLAND MIGRATION, SALT MARSH LOSS, AND ECOGEOMORPHIC FEEDBACKS ON THE EASTERN SHORE OF VIRGINIA

The coupling of barrier islands and their associated backbarrier environments (salt marshes and tidal flats) leads to complex ecogeomorphic feedbacks that will control barrier system response to sea level rise. This study seeks to better understand these feedbacks through investigation of the barrier islands fronting Virginia's Eastern Shore (VES). These largely undeveloped islands are long recognized to be migrational and/or rotational over historic time. However, long-term changes in migration rates and the relationship between barrier migration and associated changes in backbarrier configurations (marsh and open-water area) are unknown. Using a set of aerial photographs, historical maps, and NOS t-sheets dating from 1850 to 2010, we determine that migration rates for individual islands within the VES (Wallops Island south to Smith Island) range from ca. 1 to 6 m yr^-1. This rapid migration has not only significantly reduced backbarrier area along the barrier chain, but has also driven substantial reductions in the extent of backbarrier marsh due to burial, eventually leading to exposure on the ocean side of the barriers as they overrun the marsh. Since 1870, the total marsh area on the VES has decreased by a net 55 km², or nearly 18% of the 1870 extent. This is equivalent to a rate of 0.4 km² yr^-1, a substantial loss of long-term carbon sequestration potential. Barrier migration was responsible for 57% of the marsh loss, with the remainder due to backbarrier processes such as drowning and marsh-edge erosion. Comparisons of barrier migration rates to changes in backbarrier marsh and open-water areas revealed no evidence to support the hypothesis that tidal prism or changes therein control barrier retreat rate. These results indicate that while marsh-barrier couplings are important, especially in terms of marsh survival, external factors such as sediment delivery by longshore transport may play a driving role in determining the response of mixed-energy barrier systems to sea level rise.