Flow structure in canopy models dominated by progressive waves

Abstract

Summary Laboratory experiments were carried out to study the flow structure both inside and above different canopy models which were dominated by progressive waves. A set of experimental conditions were considered in a laboratory flume: three vegetation models (submerged rigid, submerged flexible and emergent rigid), three plant densities (128, 640 and 1280 stems m −2 ) and three wave frequencies ( f = 0.8, 1 and 1.4 Hz). The progressive waves followed the second-order Stokes theory in the intermediate depth range. The observations revealed that submerged and emergent rigid vegetation models modified the wave-induced velocities i.e., both mean current and wave velocity. The submerged rigid vegetation model for plant densities higher than 640 plants m −2 and wave frequency higher than 1 Hz acted akin to a false floor, confining the mean current to above the plant bed. A penetration depth around 2 cm below the top of submerged rigid vegetation was found. For the other runs, the vertical profile of mean current did not present changes with respect to runs without plants. The emergent rigid vegetation model reversed the direction of the induced mean current, with the highest velocity corresponding to the highest plant density. In contrast, the submerged flexible vegetation model had a weak effect on the mean current, with the vertical velocity profile similar to that found in experiments without vegetation. The wave velocities inside the vegetation for the densest submerged rigid vegetation were found to be reduced by 20%, when compared to the wave velocities without vegetation, while in emergent rigid vegetation this reduction was of 45%.