Wave-induced velocities inside a model seagrass bed

Abstract

Laboratory measurements reveal the flow structure within and above a model seagrass meadow (dynamically similar to Zostera marina) forced by progressive waves. Despite being driven by purely oscillatory flow, a mean current in the direction of wave propagation is generated within the meadow. This mean current is forced by a nonzero wave stress, similar to the streaming observed in wave boundary layers. The measured mean current is roughly four times that predicted by laminar boundary layer theory, with magnitudes as high as 38% of the near-bed orbital velocity. A simple theoretical model is developed to predict the magnitude of this mean current based on the energy dissipated within the meadow. Unlike unidirectional flow, which can be significantly damped within a meadow, the in-canopy orbital velocity is not significantly damped. Consistent with previous studies, the reduction of in-canopy velocity is a function of the ratio of orbital excursion and blade spacing.