Dune Dynamics Drive Discontinuous Barrier Retreat

Barrier islands and spits tend to migrate landward in response to sea‐level rise through the storm‐driven process of overwash, but overwash flux depends on the height of the frontal dunes. Here, we explore this fundamental linkage between dune dynamics and barrier migration using the new model Barrier3D. Our experiments demonstrate that discontinuous barrier retreat is a prevalent behavior that can arise directly from the bistability of foredune height, occurring most likely when the storm return period and characteristic time scale of dune growth are of similar magnitudes. Under conditions of greater storm intensity, discontinuous retreat becomes the dominant behavior of barriers that were previously stable. Alternatively, higher rates of sea‐level rise decrease the overall likelihood of discontinuous retreat in favor of continuous transgression. We find that internal dune dynamics, while previously neglected in exploratory barrier modeling, are an essential component of barrier evolution on time scales relevant to coastal management. Plain Language Summary Barrier islands and spits tend to move landward in response to sea‐level rise and storms when sediment from the beach is washed into the barrier interior and beyond, but tall dunes at the front of a barrier can prevent this process from happening for all but the largest storms. Here, we explore the interactions between dunes, storms, and barrier migration with a new model (Barrier3D). Our model shows that barriers migrate when dunes are low but are stationary when dunes are tall. Over decades to centuries, repeated cycles of dune loss and regrowth lead to sporadic (or “discontinuous”) barrier retreat. Additionally, we find that sporadic behavior may become less common in the future in response to rising sea levels, while increasing storm intensity will change which environments are most likely to experience discontinuous retreat. These findings emphasize the importance of dune dynamics in controlling barrier evolution. Barrier migration over decadal timescales ‐ timescales relevant to coastal communities and managers ‐ is commonly modeled as a constant background process, but our results suggest that is not always the case. Key Points Dune height bistability drives alternating periods of barrier immobility and transgression over decades to centuries Simulations suggest discontinuous (continuous) retreat will become less (more) common with greater sea‐level rise rates and storm intensity Dune dynamics a