Wave‐Driven Mean Flow Dynamics in Submerged Canopies

Abstract

The physical roughness (canopies) formed by organisms within aquatic ecosystems (e.g., seagrass, kelp, and mangroves) modifies the local wave‐driven hydrodynamics within coastal and estuarine regions. In wave‐dominated environments, an understanding of the mean wave‐driven flows generated within and above canopies is important, as it governs material transport (e.g., of nutrients, sediment, and biota). However, until recently the effect of submerged canopies on wave‐current interactions and the resulting mean (wave‐averaged) flow dynamics has received relatively little attention. In this study, a combination of wave flume experiments and numerical modeling is used to investigate the wave‐induced mean flow profiles in the presence of a submerged canopy. The measured velocities and vegetation forces were used to derive bulk drag and inertia coefficients, and to validate a nonhydrostatic 2DV wave‐flow model. The numerical model results were used to conduct an in‐depth analysis of the mean horizontal momentum terms responsible for driving the mean (horizontal) flow within and above the submerged canopies. We show that the mean canopy hydrodynamics are driven by vertical gradients in wave and turbulent Reynolds stresses, balanced by the mean canopy drag forces. The wave Reynolds stress gradient is the dominant force driving the in‐canopy mean flow and is directly related to the vorticity that is generated when the wave orbital motions become rotational near the canopy interface. This study provides new insight in the mechanisms responsible for wave‐driven mean flows within submerged canopies and guidance for how these hydrodynamics can be predicted in coastal wave‐circulation models.

Material Residence Time in Marine Canopies Under Wave-Driven Flows

“…While Equation (4) has been developed using linear wave theory (for simplicity), its applicability has been tested and validated over a wide range of experimental and field conditions (including shallow, intermediate and deep water waves, as well as in both rigid and flexible canopies) (Abdolahpour, 2017). This model is also in excellent agreement with numerical simulations (van Rooijen et al, 2020). Thus, we expect the advection model to be valid in a variety of submerged canopies exposed to a wide range of water waves (in the absence of ambient mean currents).…”

Generation of attached Langmuir circulations by a suspended macroalgal farm

“…2010, 2013; Abdolahpour, Hambleton & Ghisalberti 2017; Chen, Liu & Zou 2019; van Rooijen et al. 2020). This wave-induced current is caused mainly by the reduction of the wave orbital velocity within the canopy, and inclusion in our model would require explicitly resolving the surface waves.…”

A Numerical Study of Wave‐Driven Mean Flows and Setup Dynamics at a Coral Reef‐Lagoon System

“…The mean bottom stress term in Equation 9 integrates the total contributions of mean currents (nonlinear) wave velocities, and interactions between the two. To differentiate between these contributions, we separated the bottom stress from Equation 6 into A mean flow, wave, and a wave-current interaction contribution (e.g., Rooijen et al, 2020). For this purpose, we decomposed the velocity signals in a mean and oscillatory component (e.g.,      U U U).…”