Christopher Hein - SMSP Fall Seminar Series 2023

Abstract: Among the most dynamic coastal landforms, barrier islands play central roles in ecosystem processes, habitat resilience, and mainland storm protection. Though over centennial and longer periods barriers simply migrate landward in near-direct proportion to the rate of sea-level rise (SLR), mesoscale barrier morphodynamics are far more complex. Focusing on the 13 largely undeveloped barrier islands of Virginia’s Eastern Shore (USA), we combine new insights from the late Holocene geologic record, historical data, and numerical modeling to explore multidecadal to centennial barrier-system behavior. Specifically, we document shifts between phases of island erosion, migration, and progradation in response to changing rates of SLR, storminess, interactions with antecedent substrate, and long- and cross-shore sand exchanges. Overall, we find that the Virginia Barrier Islands have modestly (<5%) decreased in volume over historical time, with average shoreline transgression rates (due to island narrowing or landward migration) accelerating by ~45% in the last ~100 years. Given that the local SLR rate has increased by ≥200% over this same period, we infer multidecadal lag dynamics in barrier behavior with respect to SLR. Using a novel coastal evolution model, we reveal that observed shoreline retreat at the barrier-chain scale is not controlled by SLR over the last decades, but rather by the baseline SLR rate of the past centuries. As such, we predict a “committed” coastal response: the rate of retreat of undeveloped barriers will increase by ~50% within the next century, even if SLR remains at present rates. At the island scale, these dynamics play out in a far more complex manner: the potential for retreat is first realized through removal of ‘geomorphic capital’ (sand reservoirs associated with island stratigraphy, dune systems, tidal deltas, etc.), which can accelerate sand delivery to downdrift islands. Initiation of net migration once the eroding island becomes overwash-dominated, leads to an acceleration in shoreline migration and in the erosion of the downdrift islands, which are now experiencing a decreased (or net negative) longshore sand flux. Together, these findings help elucidate and better predict barrier behaviors, including those active through inter-island sand exchanges, over long coastal reaches, and with significant lag dynamics.