Let it flow: how does an underlying current affect wave propagation over a natural seagrass meadow?

Temperate seagrass beds can be found within intertidal and tidal areas of variable hydrodynamic forcing. To investigate the interaction between hydrodynamics and seagrass plants, Zostera noltii meadows were exposed to a range of combinations of waves and flow in a flume. Velocity profiles were obtained to observe the evolution of hydrodynamic forcing due to the presence of a seagrass canopy. Observations under flow without waves agreed with those of previous studies and showed that turbulent kinetic energy (TKE) at the top of the canopy increased with increasing flow velocity as well as with distance from the leading edge. The highest TKE values were achieved for the highest flow velocity without waves, and the presence of waves generally led to reduced TKE values. Profiles of time-averaged downstream velocity were not affected by the presence of waves. However, flow reduction near the bed increased with higher seagrass density, resulting in an S-shaped profile for the highest density and flow velocity. Underlying currents affected wave orbital velocities, causing a reduction of orbital diameters in the free-stream region. In the canopy region, this process was superimposed by the damping effect of the seagrass meadow, but only positive flow components were reduced by seagrass presence. Streamlining and oscillating motion of seagrass leaves were observed under flow and waves, respectively. These processes did not affect the wave/flow attenuation capacity of the meadow, which indicates that the ecosystem-engineering traits of seagrasses are not compromised by increased flow velocity or in the presence of waves