Local hydrodynamics at edges of marine canopies under oscillatory flows.

Canopy fragmentation increases both spatial heterogeneity and patch edges which, in turn, is then likely to modify the local hydrodynamics in the canopy. The orientation of the edge versus the wave and current field is also expected to play an important role in determining wave attenuation and sheltering at the edge of a canopy. We investigated the effect a longitudinal edge (i.e. with its main axis aligned to wave direction) of a simulated canopy has on local edge hydrodynamics. The effect that both canopy density and flexibility have on the hydrodynamics was studied. Flexible plants reduced the wave velocity and the turbulent kinetic energy with distance into the canopy and this attenuation increased as the density of the canopy increased. Compared to flexible plants, an edge of rigid plants produced a higher wave velocity attenuation coupled with an increase in the turbulent kinetic energy with distance into the canopy despite having the same canopy density. This greater wave attenuation at the edge coincided with the shifting of the associated mean current that, in turn, produced an increase in the turbulent kinetic energy at the edge in the canopy. The effect was accentuated when the canopy density increased. The wave velocity attenuation was a linear function of the canopy cover. While flexible plants reduced the turbulent kinetic energy following a linear function of the canopy cover, rigid canopies increased the turbulent kinetic energy following a linear function of the canopy cover. In the case of the flexible vegetation, the lengths of both the inner and outer canopy boundary layers increased as the canopy cover increased.