Exploring Thresholds of Mesoscale Barrier Island State Change in a Cross-Shore Morphodynamic Model

Abstract:

Over the mesoscale (decades to centuries), barrier island evolution primarily reflects the balance of variations in sediment availability, relative sea-level rise (RSLR), and accommodation. Though conceptual models broadly capture changes in barrier appearance and behavior in response to these drivers of morphologic evolution, they are not equipped to define quantitative thresholds between barrier states. To fill that gap, we use a reduced-complexity, cross-shore numerical framework, the Subaerial Barrier Sediment Partitioning (SBSP) model, to explore the sensitivity of barrier island morphology to changes in sediment flux partitioning, RSLR, and vertical accommodation inspired from an assortment of barriers around the world. Our model results, which include quasi-stratigraphic barrier profiles, demonstrate that a diverse spectrum of barrier behaviors and coincident morphologies can be explained by the ratio of sediment fluxes delivered to the beach versus the dune/backbarrier. Modeled system responses include widely recognized states, such as progradation, aggradation, and transgression. However, we also capture less commonly studied states, such as cross-shore amalgamation and dune-dominated modes of transgression, where the barrier can continue to accumulate subaerial volume even while retreating. Importantly, model runs show that transitions between these states are controlled by a relatively small range of sediment input and RSLR rates, suggesting that minor changes in external forcing may affect large (meter to kilometer scale) changes in barrier morphology. Comparisons between modeled and observed morphological evolution over decades to centuries demonstrate that the model captures magnitudes of change seen in real-world barriers. Finally, the idealized stratigraphy produced by the model could be used to infer pre-historic state shifts in modern barrier islands that may otherwise not be observable, as well as predict sensitivity to future increases in rate of sea-level rise.