On the Prediction of Swash Excursion and the Role of Seagrass Beach-cast Litter: Modelling and Observations

Criticality in coastal management and environmental protection is intensified by coastal hazards and climate changes which nowadays should not be ignored. Due to ongoing sea level rise and the increase in the severity of wave storms, knowing in advance the effects of storms and human impacts at the coasts becomes fundamental for management and environmental protection purposes. This research deals with the wave runup and swash on the beach which, representing the final expression of wave action at the coast, are often included into hazards management, prediction and mitigation frameworks. Existing swash parametrizations are tested against worldwide-collected field datasets showing that still large errors in wave runup prediction could be made. The original contribution of this work consists of coastal processes knowledge enhancement, providing four new swash predictors: 2 for total and 2 for infragravity swash. Moreover, the innovative machine learning approach adopted (genetic programming) can replicate the functionality and dependencies of previously published formulas, improving predictability (compared to well-established parametrizations derived by classical regression approaches) and providing physical insight into coastal processes. The beach slope emerged as an important parameter when predicting infragravity swash, contributing to clarify the existing discussion in the literature. Novelty in this research also consists of field measurements of wave swash and runup with the presence of seagrass (\textit{Posidonia oceanica}) litter on the beach face and berm. Measurements are performed through an ad-hoc installed easily-reproducible coastal video monitoring station, from which the interaction between wave swash and seagrass berm is studied. Results show that runup process is shifted offshore due to seagrass wrack accumulation and that the swash energy is shifted towards higher frequencies, with the swash peak frequency being located within the incident band, in contrast with a nearby seagrass-free profile under the same wave forcing condition. This behaviour is probably due to the beach slope made steeper by the seagrass beach-cast accumulations. The findings of this research involve implications for coastal protection and management (e.g. seagrass deposits role on coastal protection), hazards mitigation and prediction (e.g. inundation).