Wave‐driven flows over canopies of aquatic vegetation (such as seagrass) are characterized by the generation of a strong, shoreward mean current near the top of the canopy. This shoreward drift, which is observed to be up to 75% of the RMS above‐canopy orbital velocity, can have a significant impact on residence times within coastal canopies. There have been limited observations of this current and an accurate formulation of its magnitude is still lacking. Accordingly, this study aims to develop a practical relationship to describe the strength of this current as a function of both wave and canopy characteristics. A simple model for the Lagrangian drift velocity indicates that the magnitude of the wave‐driven current increases with the above‐canopy oscillatory velocity, the vertical orbital excursion at the top of the canopy, and the canopy density. An extensive laboratory study, using both rigid and (dynamically scaled) flexible model vegetation, was carried out to evaluate the proposed model. Experimental results reveal a strong agreement between predicted and measured current velocities over a wide and realistic range of canopy and wave conditions. The validity of this model is also confirmed through available field measurements. Characterization of this wave‐induced mean current will allow an enhanced capacity for predicting residence time, and thus key ecological processes, in coastal canopies.

Physical modeling of canopy‐flow interactions has mostly employed rigid model vegetation, whereby the canopy geometry (i.e., its height and frontal area) is invariant and easily quantified. Here, we demonstrate that embedding realism in model vegetation, in the form of buoyancy and flexibility, can profoundly impact the structure of the flow and rates of vertical mixing in wave‐dominated conditions. A laboratory investigation was undertaken with two types of model canopy: (1) rigid canopies consisting of wooden dowels, and (2) flexible, buoyant model plants designed to mimic meadows of the seagrass Posidonia australis. To isolate the impact of flexibility, the maximum heights and frontal areas of the two types of canopy were matched. These canopies were subjected to oscillatory flows with a realistic range of wave heights and periods. Drag reduction caused by the reconfiguration of flexible canopies leads to a greatly diminished velocity attenuation in the canopy (by, on average, 65%). The reduced average height of flexible canopies shifts the canopy shear layer toward the bed, resulting in significantly enhanced levels of near‐bed turbulence. Finally, a decreased vertical diffusivity (by approximately 35%) was observed in the flexible model canopies, relative to the rigid analogues. Thus, while the use of dynamically scaled vegetation adds complexity to modeling efforts, it represents a step toward a more accurate quantitative understanding of flow and mixing in these environments.

The spatial extent over which meadows of submerged aquatic vegetation, such as seagrass, have an ecological and environmental influence is tightly limited by the exchange of water across canopy boundaries. In coastal environments, the process of vertical mixing can govern this material exchange, particularly when mean currents are weak. Despite a recently improved understanding of vertical mixing in steady canopy flows, a framework that can predict mixing in wave-dominated canopy flows is still lacking. Accordingly, an extensive laboratory investigation was conducted to characterize the rate of vertical mixing in wave-dominated flows through measurement of the vertical turbulent diffusivity (Dt,z) of an injected dye sheet. A simple model of coastal canopies, an array of wooden dowels of variable packing density, was subjected to waves with a wide and realistic range of height and period. Vertical mixing across the top of a submerged canopy is shown to be driven by both the shear layer that forms at the top of the canopy and wake turbulence generated by canopy stems. By allowing for an additive contribution from these two processes, we present a predictive formulation for the rate of vertical mixing in coastal canopies across a range of wave and canopy conditions. The rate of vertical mixing, and the dominant mixing mechanism, is highly dependent upon a Keulegan–Carpenter number (KC) that represents the ratio of the particle excursion length to the length scale that defines the canopy drag. This study enables a significantly enhanced predictive capability for the residence time of ecologically and environmentally significant species within coastal canopies