Assessing the role of reed and seagrass wracks in coastal protection. An example with numerical modelling integrated with videomonitoring system data in a southern Sardinia beach

This work aims to assess the role played by a beach berm, reinforced by the accumulation of reed and seagrass wracks, on coastal protection against storm-induced floodings. A huge depositional event, mostly consisting of Arundo donax reeds from nearby river systems and Posidonia oceanica wracks, happened in December 2019 in Poetto beach (southern Sardinia, western Mediterranean). Following the concerns that this event raised among local tourism service providers, the municipal authorities decided to remove the reed wracks. In support of coastal management, a scientific inquiry was commissioned to the Coastal and Marine Geomorphology Group (CMGG), belonging to the Department of Chemical and Geological Sciences of the University of Cagliari (Italy) for the assessment of the berm processes before the removal of the reed wracks. Within the five months between the reed deposition and removal, a video monitoring system collected runup and flooding data during storm wave conditions. These data were integrated by five topographic, three bathymetric and two drone surveys. The presence of the reeds intertwined with seagrass wracks, seems to strengthen the berm, making it more resilient to wave action, eventually offering additional protection against overwash and flooding. These results are supported by a numerical model chain, with the nesting of a phase-resolving model into a phase-averaged model, providing a characterization of wave runup dynamics