Alongshore Varying Dune Retreat at a Barrier Island

Abstract. Barrier islands often exhibit spatially uneven morphological change due to alongshore variations in wave exposure, sediment supply, and dune morphology, which are influenced by both long-term evolution and short-term storm impacts. We investigate the alongshore variability of shoreline and dune evolution on Culatra Island (Ria Formosa, S. Portugal) over a two-year period (Nov 2009–Nov 2011) encompassing 31 storm events. A coupled modeling framework was developed, combining the one-line ShorelineS coastline model with an impact-based dune erosion and recovery module. The model was forced with hourly offshore wave data from the ERA5 reanalysis (corrected for biases in wave height, direction and period using Faro buoy observations) and transformed to the nearshore with the unstructured-grid SnapWave model. In situ LiDAR surveys and satellite-derived shorelines and dune vegetation lines (using Normalized Difference Vegetation Index (NDVI)) were used for calibration and validation. Model results show that longshore sediment transport gradients dominated shoreline change across most of the island, with net erosion in the sediment-starved western sector and net accretion in the eastern sector. Notably, an erosion "hotspot" in the central-west (transects T42–77) experienced intense storm-induced dune retreat, which supplied sand to the adjacent beach and caused local shoreline advance counter to the regional trend. Overwash at the eastern end contributed to the accretion aligned with the regional trend. The dune module's performance was sensitive to its calibration: a higher wave impact coefficient Cs led to greater dune erosion, while coarse sand d50 and low winds markedly reduced modeled post-storm dune recovery. Model validation using satellite-derived shoreline and dune positions employed R² and Pearson correlation metrics, revealing moderate shoreline performance and weaker dune agreement – though general retreat trends were captured across several transects. A correlation analysis indicates that cumulative cross-shore sediment flux (corr = 0.73) and sturm duration (corr = 0.48) exert the strongest control on dune retreat whereas peak storm wave direction and initial berm width had minimal influence. Overall, the coupled model captured the observed pattern of alongshore-varying coastal response – including the emergence of a dune erosion hotspot – highlighting the importance of cross-shore sediment exchanges in barrier island evolution and providing useful insights for coastal management of dune systems.