An integrated study of wave attenuation by vegetation

Abstract

Coastal protection is an important issue. For many years, protection of coastal areas has been approached from an engineering perspective, creating hard defences to withstand wave forces. However, ‘soft engineering’ alternatives that blend in with the natural environment. Vegetation such as seagrass can dissipate wave energy, acting as a natural barrier. In contrast to hard defences, there are currently limited design guidelines for designing soft defences based on vegetation. Here, we present a mathematical model of wave attenuation by vegetation. Of interest is whether a relatively simple wave model is capable of capturing the attenuation of wave energy by vegetation. Specifically, whether the attenuation of short and long waves can be mimicked through the introduction of a linear drag or diffusion term in the momentum equation. The model is based on the shallow water equations. To accommodate the dispersive effect that appears in short waves, we modified the model further to include the hydrodynamic pressure. The equations are solved numerically using a free-damping-error method. Analytical solutions derived for the case where the seabed is flat are used to verify the computational solutions. In addition, results of laboratory experiments of wave attenuation by artificial seagrass provide additional testing of the computational model. Extremely good agreement between computations and analytical solutions was found. In comparisons with the laboratory experiments, the best results with the computational model were found using diffusion for long waves and using a simple drag term for short waves. Wave attenuation was also found to depend strongly on the dimension of the vegetated area.