Modelling wave attenuation by quasi-flexible coastal vegetation

Coastal vegetation such as seagrass fields, salt marshes, and mangroves, contributes to coastal defence by damping incoming waves. Yet, plant species differ in flexibility due to which they interact differently with incoming waves and damp waves to a variable degree. Current wave damping models struggle to balance accuracy against computational costs when accounting for wave-vegetation interactions. Instead, they often rely on a plant-specific calibration of the drag coefficient, which limits their application across plant species. Here we show, using novel simultaneous experimental data of wave damping, water velocities and stem motion, that wave damping by quasi-flexible cylindrical vegetation is controlled by the relative velocity between water and vegetation at the upright bottom section of a stem. For the quasi-flexible vegetation conditions considered in this manuscript (L>1.4 and Ca<700), our experimental evidence justifies the application of a model based on the Euler-Bernoulli beam theory to estimate plant motion. Building on the solution of plant motion, we simulate wave damping over flexible vegetation fields through a new work factor. Our model successfully predicts damping of regular waves by rigid and flexible artificial vegetation, and real S. Anglica, P. Maritima and E. Athericus plants in the right order of magnitude under medium and high energy wave conditions. The simulated wave damping is directly linked to vegetation and wave conditions and does not require a plant-specific calibration of the drag coefficient. It is anticipated that the model will be of wide practical use in simulating wave damping by quasi-flexible cylindrical coastal vegetation across large areas with diverse plant species and wave conditions.