Exploring the Importance of Back-barrier Marsh Deposits in Barrier Island Response to Sea Level Rise, Virginia Coast Reserve, U.S.A

As sea level rises, many barrier islands respond by migrating landward via storm-driven overwash processes. Through landward migration, sediment is extracted from the shoreface to supply sand to the island while, at the same time, the island moves to higher elevations. In this way, landward migration may allow an island to persist even as sea level rise rates increase. Intricate geometric relationships dictate both the distance an island must migrate and the amount of sediment required to maintain a subaerial barrier. Our objective is to explore the late-Holocene and potential future response of Metompkin Island (one of the most rapidly migrating barrier island within the Virginia Coast Reserve (VCR)) to sea level rise in order to better understand the role of back-barrier marsh deposits in island evolution. We hypothesize that because the extent of subaerial marsh behind a barrier island partially determines how much sand is required to maintain subaerial exposure, the presence or absence of back-barrier marsh is critical in determining how an island system evolves. More specifically, we hypothesize that the absence of marsh behind southern Metompkin Island contributes to higher observed migration rates relative to the northern half of the island where a substantial marsh is present. We use GEOMBEST (Geomorphic Model of Barrier, Estuarine, and Shoreface Translations), a 2-d cross shore numerical morphological-behavior model, to simulate the evolution of Metompkin Island over the last 4600 years and into the future. We use recent NOAA bathymetry and USGS lidar data to develop a cross-shore profile representing average modern shoreface morphology and stratigraphy (extending 45km offshore) for the study area. Using modern geometric relationships as a guide, we then create a cross-shore profile representing a plausible initial morphology and stratigraphy for model simulations dating back to the oldest evidence of island existence (4600yrs BP). Additional model inputs, such as late-Holocene and future sea level rise rates, sediment supply/loss rates, shoreface depth, and lagoonal/marsh deposition rates come from both recent geologic literature and field observations. Preliminary results suggest that the fine-grained substrate of islands in the VCR and islands having limited back-barrier marsh deposits will require increasingly rapid rates of migration to maintain subaerial exposure. Initial work also indicates that some of the previously drawn conclusions regarding barrier island evolution in the VCR are incorrect. For example, in developing initial conditions based on geologic observations, we have found (geometrically) that the islands must have formed farther seaward than the 0.9 to 1.1 km (at approximately 4600 yrs BP), suggested by the work of Byrnes et al. (1988). Instead, the islands more likely formed closer to the 4 km offshore distance suggested by Finkelstein and Ferland (1987).