Role of the Initial Topobathymetry in Washover Deposit Formation During Extreme Events

Abstract Low‐lying beaches, important ecological and socio‐economic assets, are vulnerable to storm erosion and flooding impacts, which are expected to increase due to climate change. This conceptual study uses the 2DH XBeach model to simulate the influence of the initial topobathymetric shape on the washover deposit formation during a beach overwash caused by an extreme storm, including a quantification of the wave chronology uncertainty. The employed initial alongshore‐uniform (AU) topobathymetries have different cross‐shore profiles, representing the real variability of Mediterranean beaches. The analysis also incorporates the presence of common alongshore‐rhythmic morphological patterns, such as megacusps, rhythmic dunes, and crescentic bars, with varying wavelengths. The results in the AU scenarios show that washover deposits are also periodic alongshore, forming through a 2D self‐organization process that establishes the natural wavelength of the modeled dry beach. The presence of megacusps in the initial topobathymetry can enhance washover deposit volumes up to a factor 4, especially for large initial wavelengths. Rhythmic dunes also influence the spatial distribution of the deposits, while crescentic bars generally have little direct effect on the deposit formation. These findings highlight the importance of using real and up‐to‐date topobathymetric data, including the presence of common alongshore‐rhythmic morphological patterns, in modeling storm impacts on beaches. Furthermore, modeling studies under future climate change scenarios, must consider the uncertainty associated with the unknown shape of the topobathymetry and wave group chronology. The results could also have implications for developing effective coastal management strategies.