Barrier breaching versus overwash deposition: predicting the morphologic impact of storms on coastal barriers

Abstract

Storms can have large impacts on barrier islands. Overwashing flows and waves can move sediment across barrier islands and result in washover deposition (Figure 1a) or barrier island breaching (Figure 1b) (Pierce, 1970). These outcomes are strongly sensitive to barrier characteristics and storm intensity (Suter et al., 1982;Plomaritis et al., 2018). Hurricane (also called "superstorm") Sandy hit the US East Coast in 2012 and resulted in widespread overwashing and numerous breaches (Figure 1) (Sopkin et al., 2014). Breaching is likely to become more common as a result of sea-level rise and barrier island flooding (Nienhuis & Lorenzo-Trueba, 2019a;Passeri et al., 2020). At the same time, washover deposition is a critical landward-directed sediment flux that can support barrier aggradation and prevent barrier drowning. Reliable predictions of barrier breaching and washover deposition, whether for long-term models or short-term assessment before landfall, remain difficult.In this study, we propose that storms make barrier islands breach when the cumulative sediment flux of an overwashing flow exceeds the barrier subaerial volume. Conversely, a washover deposit will form when an overwashing flow does not erode the barrier down to sea level, with increasing washover volumes as overwashing flows approach the washover-to-breaching threshold.The objective of this study is to test this theory using Delft3D simulations complemented with observations from Hurricane Sandy. We systematically explore the effect of barrier island morphology, storm characteristics, and dune vegetation on overwashing flows and the morphologic response of barrier coasts.