The impact of flexibility on flow, turbulence, and vertical mixing 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.