Relative influence of antecedent topography and sea-level rise on barrier-island migration

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 barrier-island 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.