Breaker-generated turbulence in and above a seagrass meadow

In shallow water the turbulence generated by wave breaking can reach the bed, so that the interaction between breaker-generated turbulence and submerged aquatic vegetation (SAV) can be important. The aim of this study is to understand the effect of the submerged aquatic vegetation on the production and dissipation of turbulent kinetic energy associated with the passage of a breaking wave. A model SAV meadow was constructed based on dynamic and geometric similarity to natural plants. The model meadow was subject to an isolated breaker generated within a wave packet, which mimics transient wave breaking in the ocean. The evolution of the turbulent flow field was measured by an Acoustic Doppler Velocimeter. The observations suggest that near the water surface, i.e. above the meadow, the turbulent kinetic energy (TKE) without SAV remains elevated longer than with SAV, and this is attributed to the damping of breaker-generated turbulence by the SAV. In contrast, the TKE measured near the bed is always higher and more persistent with SAV having roughly twice the value observed without SAV. This is attributed to the generation of stem-wake turbulence by the elevated near-bed orbital velocity during the passage of the wave packet. The generation of stem-wake turbulence is associated with a greater loss of wave energy. Specific to the wave and plant conditions considered in this study, in the presence of the SAV a total of 32% wave energy loss was observed; ¾ of this loss (24% loss) can be associated with the breaking event, and ¼ of this loss (8% loss) can be associated with dissipation by meadow drag.

► Breaker turbulence generated above a meadow is damped by the meadow drag.
► Turbulence within the meadow is stronger and more persistent than above bare bed.
► Wave energy is transferred into the near-bed turbulence by stem-wake production.
► 3/4 of wave energy loss is associated with the breaking event and 1/4 by meadow drag.