This study explores the influence of the wave characteristics on the attenuation process of waves through coastal mangroves, which are threatened by the coastal mangrove squeeze phenomenon. Coastal mangrove squeeze is the phenomenon where coastal regions, even when sediment availability is sufficient, are eroding due to a lack of accommodation space caused by the land use on the landside and by sea level rise on the waterside. Along the Mekong Delta Coast, only a narrow strip of mangroves of less than 140 m is left at the locations where a strong erosion of up to 100myr¡1 is observed. Furthermore, observations at the south eastern and the eastern coasts of the Mekong Delta are, that a mangrove width ranging from approximately 30m to260m and 140m on average, appears to be stable. Therefore, a hypothesis regarding coastal mangrove squeeze is proposed based on the empirical relationship between mangrove forest width and coastline evolution. The hypothesis is proposed, that a minimum space of coastal mangroves is required for a sustainable development of the mangrove forest...@en

A new wave-vegetation model is implemented in an open-source code, SWASH (Simulating WAves till SHore). The governing equations are the nonlinear shallow water equations, including non-hydrostatic pressure. Besides the commonly considered drag force induced by vertical vegetation cylinders, drag force induced by horizontal vegetation cylinders in complex mangrove root systems, as well as porosity and inertia effects, are included in the vegetation model, providing a logical supplement to the existing models. The vegetation model is tested against lab measurements and existing models. Good model performance is found in simulating wave height distribution and maximum water level in vegetation fields. The relevance of including the additional effects is demonstrated by illustrative model runs. We show that the difference between vertical and horizontal vegetation cylinders in wave dissipation is larger when exposed to shorter waves, because in these wave conditions the vertical component of orbital velocity is more prominent. Both porosity and inertia effects are more pronounced with higher vegetation density. Porosity effects can cause wave reflection and lead to reduced wave height in and behind vegetation fields, while inertia force leads to negative energy dissipation that reduces the wave-damping capacity of vegetation. Overall, the inclusion of both effects leads to greater wave reduction compared to common modeling practice that ignores these effects, but the maximum water level is increased due to porosity. With good model performance and extended functions, the new vegetation model in SWASH code is a solid advancement toward refined simulation of wave propagation over vegetation fields.@en

Wave attenuation through mangrove forests has received more and more attention, especially in the context of increasing coastal erosion and sea-level-rise. Numerous studies have focused on studying the reduction of wave height in a mangrove forest. However, the understanding of this attenuation process is still in its infancy. In order to obtain more insight, a laboratory experiment, mimicking the processes of wave attenuation by coastal mangroves in the Mekong Delta, Vietnam was conducted. The reduction of wave height for different scenarios of mangrove densities and wave conditions was investigated. A new method to quantify vegetation attenuation induced by vegetation is presented. The wave height reduction is presented over a relative length scale (viz. the number of wavelengths), instead of an absolute length scale of the forest (e.g per meter or per 100 m). The effects of wave non-linearity on the wave height attenuation over the mangrove forest were investigated using the Ursell number. It is suggested that the non-linear character of waves has a strong influence on the attenuation of the waves inside the mangrove forest. A numerical model, mimicking the experiment was constructed in SWASH and validated using the experimental data. Finally, the data set was extended through numerical modelling so that a larger ranging relationship between wave attenuation per wave length and the Ursell number could be formulated.@en

The authors regret that there was a typo in Equation (14b). @en