Velocity and turbulence affected by submerged rigid vegetation under waves, currents and combined wave–current flows

Abstract

Vegetation can change flow structure and turbulence characteristics. However, few studies have focused on combined wave–current flows in canopy with vertically varying density. Therefore, laboratory experiments were carried out to investigate hydrodynamics affected by submerged rigid vegetation with different densities (i.e. sparse, dense and vertically varying density) under waves, currents and combined wave–current flows. In submerged canopy with vertically uniform density, large shear stress occurs around the canopy top. Thus in the vertical distributions, the greatest values of maximum wave velocity under pure waves, and the greatest values of turbulent kinetic energy (TKE) under unidirectional currents and combined wave–current flows appear around the canopy top. In canopy with vertically varying density, the shear stress at each canopy top is relatively small, resulting in the vertically smooth distributions of velocity and TKE under pure waves. While under unidirectional currents and combined wave–current flows, canopy-scale turbulence throughout the entire water column occurs owing to the influence of multiple canopy tops. An improved formula considering drag coefficient, vegetation density and diameter, was proposed to predict stem-scale turbulence using characteristic velocity. The formula is appropriate for vegetated flow in which stem-scale turbulence is dominant, and it can be further used as a simple method to assess TKE for sediment or substances transport affected by vegetation.