Potential role of beach wrack in reducing storm overtopping on a Mediterranean urban beach: observations from Poetto Beach, Sardinia

Beach wrack accumulations, composed of Posidonia oceanica remains and other vegetal debris, are common features along Mediterranean coasts but are frequently removed from beaches for aesthetic purposes. This study investigates the geomorphological and hydrodynamic role of beach wrack deposits in mitigating storm-wave overtopping and temporary backshore inundation events on a Mediterranean urban beach (Poetto beach, Sardinia, Italy). Two storm events characterised by comparable peak offshore wave heights, but different storm durations and backshore configurations, were investigated through an integrated approach combining coastal video monitoring, wave and wind reanalysis, hydrodynamic modelling, Total Water Level (TWL) analysis, DGPS-RTK surveys and infiltration measurements. Despite a higher estimated TWL used as a first-order empirical indicator of storm-wave conditions, no berm overtopping or temporary backshore inundation was observed during the analysed recording interval when extensive mixed beach wrack deposits were present. In contrast, repeated berm overtopping and temporary backshore inundation occurred during the event characterised by the absence of beach wrack. Infiltration measurements showed that mixed wrack deposits exhibited higher permeability than the sandy backshore, while successive DGPS-RTK surveys documented post-storm aggradation associated with storm-wave redistribution and subsequent aeolian sediment trapping. Together, these processes promoted sediment retention and the progressive integration of beach wrack into the beach system. These findings highlight the potential contribution of mixed beach wrack deposits to mitigating storm impacts and enhancing beach resilience. Rather than the natural deposits themselves, the potential Nature-Based Solution lies in adaptive management practices that preserve mixed wrack deposits in situ whenever compatible with local environmental and socio-economic conditions. Such management approaches may enhance the resilience of Mediterranean beaches to storm impacts and sea-level rise by supporting natural processes of wave energy dissipation, sediment retention and backshore accretion.