Parameterisation and evolution of non-breaking wave nonlinearity over flexible vegetation

Abstract

Wave nonlinearity significantly influences wave attenuation and can cause sediment transport imbalances in vegetated zones, impacting shoreline stability and ecosystem-based coastal defences. Despite its importance, the parameterisation and mechanisms of wave nonlinearity evolution over flexible vegetation remain unclear. This study examines the mechanisms behind non-breaking wave nonlinearity evolution over flexible vegetation and provides parameterisation of wave nonlinearity, based on experimental data collected by Anderson and Smith (2014). The study derives a new set of empirical formulae for predicting wave nonlinearity in Spartina alterniflora, which considers Ursell number, relative vegetation width, vegetation submergence, and vegetation density. The predictions made using these formulae are in good agreement with the measurements. The results indicate a decrease in wave skewness across the span of vegetation, whereas wave asymmetry increases until it reaches a maximum before decreasing. The findings suggest a preferential dissipation of high harmonics compared to low harmonics in the vegetation zone. Additionally, bispectrum analysis reveals that vegetation enhances the difference interaction while suppressing the sum interaction between wave components. The findings enable improvements in our understanding of wave nonlinearity in vegetation, providing better predictive capabilities for wave attenuation and sediment transport, which are crucial for the design and optimization of coastal defence systems.