Washout Versus Washover: Distinct Trajectories of Barrier Reshaping

Abstract

Barrier islands are dynamic coastal landforms that can migrate landward from the press of sea-level rise and the pulse of storms. Previous work on barriers largely focuses on landward sediment mobilization, particularly through overwash, while the role of outwash—where sediment is transported seaward—remains underexamined. There exists a lack of direct comparisons between the processes that restore sediment volume and the timescales of recovery following outwash and overwash events. Here, we used high-resolution mapping and in situ and modeled water levels to quantify morphologic change and its relation to inundation at three contrasting sites. Our results demonstrate that outwash can remain a net erosive scar for years after formation, while overwash magnitude, frequency, and thus persistence vary largely depending on the width and elevational resistance of the barrier. When elevational resistance to overtopping is low, we show that intermediate high-water events can contribute as much sediment to island overwash as larger named storms and that these processes are key for outwash recovery. We find that modeled total water level correlates positively with volume change, while discrepancies between modeled and observed water levels implicate runup overwash as the dominant mode of transport. Together, we use these data to suggest a differentiation between overwash and outwash processes and their resulting morphologies in studies that aim to predict the impact of storms on barrier island transgression rates and broader ecological function.