Interaction of waves and currents with kelp forests (Macrocystis pyrifera): Insights from a dynamically scaled laboratory model

The interaction of surface waves and currents with kelp forests was examined under controlled conditions using a dynamically matched 1/25-scale physical model in a laboratory flume. In experiments with kelp mimics, waves increased the time-averaged drag by a factor of 2 and altered the shape of current profiles. Relative motion between model kelp and water under waves increased wake generation of turbulence, resulting in turbulent kinetic energies 2-5 times larger, and eddy viscosities 20-50% larger, than for experiments without waves. Because mixing lengths were reduced to wake-scales in the model kelp forest, eddy viscosities were 25-50% smaller than when kelp was absent. In the model kelp-forest surface canopy where solid obstacles were most densely spaced, wave orbital velocities were reduced by similar to 10% from linear wave theory predictions. This decrease in wave orbital velocities is thought to result primarily from inertial forces exerted on water by model kelp. Stokes drift was reduced by, 20% as a result of the change in wave orbital velocities. Although hydrodynamics within kelp forests are more complex than in the laboratory experiments and a wide range of flow conditions can occur, laboratory results suggest that (1) kelp forest drag is increased by waves; (2) wave properties can be altered by drag and inertial forces; and (3) wake production of turbulence caused by waves may be the main source of turbulence in dense kelp stands. Interactions between kelp and waves must therefore be taken into account when developing models for drag and mixing in these systems.