Computational Modelling of the Coastal Protection Function of Salt Marshes with Flexible Vegetation Cover

Salt marshes are intertidal coastal wetlands that are typically found in sheltered locations such as estuaries. They exhibit a diverse vegetation cover with exible grasses and rigid shrubs. This vegetation provides coastal protection by attenuat-ing currents and waves. Unlike traditional hard defences, they oer co-benets by stabilising shorelines and enhancing natural habitats. However, it has remained unclear how salt marshes with a exible vegetation cover contribute to coastal protection under storms with surge and wave components.In this thesis, I have developed a new coupled current-wave-vegetation model which includes the eect of vegetation exibility on wave attenuation. The wave-vegetation model builds on novel laboratory experiments using articial vegeta-tion in the Swansea University Wave Flume, where wave damping, water velocity elds, and plant motion were measured simultaneously for the rst time. A new work factor is introduced to explicitly account for vegetation exibility in compu-tational models. Furthermore, a momentum sink term parameterisation is found to best resemble current-vegetation interactions. The advanced coupled model is successfully applied to simulate ood risk in the Taf Estuary under six contrast-ing vegetation scenarios.My results highlight how the vegetation cover aects the coastal protection pro-vided by salt marshes. All modelled vegetation species constrain ood currents to the main estuary channel and damp incoming waves. Although exible grasses are 50% less eective in wave damping than rigid shrubs in the Taf Estuary. The wave conditions, wind conditions and local topography further aect the protec-tion provided. Additionally, rigid species can amplify orbital velocities above the canopy by inducing wave-averaged currents, but exible species do not.It is recommended that the biomechanical properties of vegetation, including the exibility, are included when modelling the coastal protection by salt marshes. My new computational modelling framework provides evidence to support the continuing uptake of salt marshes as sustainable coastal defences.