Investigating Seagrass As a Nature-Based Solution for Coastal Protection: Toward a Digital Twin Modelling Framework

The large presence of human infrastructure, protected natural environments, cultural heritage and major economic activities makes the coast one of the most vulnerable areas to climate change-related problems such as sea level rise and an increasing number of extreme events. Storm surges are the main cause of flooding and coastal erosion due to the combined effects of waves and currents that resuspend and transport sediments. In addition to the classic "gray" solutions, including engineered structures such as seawalls and groins, nature-based solutions have emerged in the last decade, based on the idea of finding solutions that are both effective and environmentally sustainable. It is well known that seagrass meadows can provide critical ecosystem services. Among them, coastal protection is one of the most important. The aim of this work is to investigate the effects of seagrass meadows on physical ocean variables relevant for sediment transport and sea level using a digital twin of the ocean. The seagrass is implemented in a three-dimensional unstructured grid ocean model (SHYFEM-MPI) as a form drag in the momentum equations, considering the flexibility of the plants. The implementation was verified with idealized test cases and three focus areas were selected along the Italian coast. The three areas are the Venice lagoon, the Emilia-Romagna coast (northern Adriatic Sea) and the Civitavecchia coast (Tyrrhenian Sea) and differ in morphology and seagrass species (Zostera marina, Posidonia oceanica). These areas are representative of a wide range of coastal environments. Interestingly, the results show different behavior depending on the geomorphology of the area. The lagoon environment, when exposed to extreme storm surge events, shows an alternating pattern of sea level variation with a reduction that can reach 5-10%, while in the other focus area the effect of seagrass on sea level is negligible. In all three areas, seagrass is very effective in reducing bottom current velocity by up to 50-60%, suggesting a possible important role against coastal erosion.