A Wave Tank Study on Turbulent Transport of a Passive Tracer Above Gravelly Sand Beds

Exchange processes at the seabed interface have a non‐negligible impact on biogeochemical processes of the sea. Acknowledging this impact on the input of nutrients from the seabed into the water column further entails properly assessing the transport and mixing of these nutrients within the water column. Here, the role of turbulence in the bottom boundary layer is crucial but simultaneously challenging to assess. A wave tank study is presented where seven different seabeds (one flat sand bed, six gravelly sand beds) are investigated in three wave scenarios , resulting in a total of 21 different configurations. The roughness of the gravelly sand beds is systematically varied in height, shape, and arrangement. Velocity fields are measured simultaneously with the concentration fields of a passive tracer fluid released from the seabed. The experiments revealed a strong imbalance between the production and dissipation of turbulence above the gravelly beds, as a result of flow separation. A one‐dimensional simulation of the water column could not fully capture this imbalance and the implications for other turbulent quantities. Exchange processes at the seabed varied strongly across configurations but responded differently to the alteration of roughness properties than the water column dynamics. These findings suggest a separate parametrization of waves interacting with roughness for turbulent boundary layers and for seabed exchange processes. We suggest three distinct aspects to consider for the parametrization of seabed exchange, related to the height, shape, and arrangement of the seabed.