Wave attenuation by rigid emergent vegetation under combined wave and current flows

Abstract

Coastal vegetations play crucial ecosystem functions including coastal protection. Considerable attention has been focused on the effects of vegetation and wave conditions on wave dissipation induced by vegetation (WDV), whereas the effects of tidal current are often ignored. A series of physical experiments were performed to explore the effects of following (current direction in the same direction as waves) and opposing currents on WDV inside the rigid emergent vegetation canopy. The experimental results indicate that following currents can either enhance or suppress WDV depending on the ratio between the current velocity magnitude and the maximum horizontal wave orbital velocity, while opposing currents may enhance WDV more. In addition, to better understand small-scale mechanisms responsible for WDV, a quasi three-dimensional (3-D) numerical model using periodic boundary conditions was employed to extend the physical experiments to provide velocity profiles and force evolutions along the canopy. A direct force measurement approach was applied to quantify drag coefficients under both pure wave and combined wave and current conditions. Finally, considering the coupling effect of wave and current, generic relations between drag coefficients and Re/KC were provided with relatively high correlation, which are useful for future studies with respect to interaction among wave, current and vegetation.