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Preserved beach and foredune ridges may serve as proxies for coastal change, reflecting alterations in sea level, wave energy, or past sediment fluxes. In particular, time‐varying shoreface sediment budgets have been inferred from the relative size of foredune ridges through application of radiocarbon and optically stimulated luminescence dating to these systems over the last decades. However, geochronological control requires extensive field investigation and analysis. Purely field‐based studies might also overlook relationships between the mechanics of sediment delivery to the shoreface and foredune ridges, missing insights about sensitivity to changes in sediment budget. We therefore propose a simple geomorphic model of beach/foredune‐ridge and swale morphology to quantify the magnitude of changes in cross‐shore sediment budget, employing field measurements of ridge volume, ridge spacing, elevation, and shoreline progradation. Model behaviors are constrained by the partitioning of sediment fluxes to the shoreface and foredune ridge and can be used to reproduce several cross‐shore patterns observed in nature. These include regularly spaced ridges (“washboards”), large singular ridges, and wide swales with poorly developed ridges. We evaluate our model against well‐preserved ridge and swale systems at two sites along the Virginia Eastern Shore (USA): Fishing Point, for which historical records provide a detailed history of shoreline progradation and ridge growth, and Parramore Island, for which a relatively more complex morphology developed over a poorly constrained period of prehistoric growth. Our results suggest this new model could be used to infer the sensitivity of field sites across the globe to variations in sediment delivery.

The processes driving barrier-island state changes between erosion/migration and growth/progradation are poorly understood. Stratigraphic and chronologic data are used to infer past state changes of Cedar Island, VA, USA. These data indicate that Cedar Island formed seaward of its present position approximately 5000-6000 years ago. Following a period of net landward migration, the north-central section of the island was breached prior to ca. 450 years ago. During this time, an extensive flood tidal delta was deposited in the backbarrier. After inlet closure, the island migrated landward atop these flood delta deposits, and aggraded, eventually entering a phase of net progradation. From 1852 to present the island has eroded and migrated landward ~1 km. The modern barrier is located stratigraphically above a shallow antecedent high (-5.5 m MSL). The state changes and antecedent geology observed here are discussed and used to infer potential future sediment sources to the island.

The response of barrier islands to sea-level rise is modulated by combinations of coastal processes, eco-geomorphic feedbacks and structural controls, such as antecedent topography. Interactions among these drivers can lead to complex and non-linear changes in island morphology and transitions between migrational, erosional or progradational states. This study seeks to constrain the morphological consequences of barrier islands migrating across complex antecedent topography in response to rising sea level. The stratigraphy of four barrier-backbarrier systems along the United States Mid-Atlantic coast informs idealized geometries of diverse antecedent substrate. These outcomes are integrated into a cross-shore morphodynamic model of barrierisland migration to quantify the influence of this antecedent geology on barrier-retreat behaviour. Additionally, this study explores the future response of specific barrier islands to various rates of sea-level rise over multi-decadal to millennial timescales. The results show that antecedent substrate slope plays a central role in barrier morphodynamic behaviour. In particular, migration across a subaqueous backbarrier ridge (for example, coastal barrier or dune deposits from earlier sea-level highstands) can cause a succession of phase changes in a modern island. For example, the case studies illustrate that the steep slopes and decreased backbarrier accommodation associated with antecedent highs greater than 3 m in profile can greatly reduce island migration rates, effectively 'pinning' the island in place, even with sea-level rise rates up to 6 mm yr −1 . However, once the island migrates over the high, backbarrier accommodation increases, leading to enhanced overwash fluxes, more rapid landward migration, and possible drowning. Additionally, the results indicate that antecedent substrate may slow barrier-island migration by providing sediment through both shoreface and inlet processes. The field and modelling insights from this study are presented as a conceptual model of the relative influence of various antecedent features on barrier-island dynamics along sandy, siliciclastic coasts.

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