The impact of wave-topography interaction on submarine groundwater discharge – Transport, turbulence, and mixing over rough seabeds

In the coastal zone, submarine groundwater discharge (SGD) is a vital pathway for terrestrial-derived substances to the sea. Wave-topography interaction can drive porewater exchange that influences the fluxes of SGD into the water column and affect its distribution, transport, and mixing within the water column. To study this interaction, we conduct a laboratory experiment, altering both the wave scenario and the seabed topography. The selected seabeds comprise a flat bed and three gravelly beds, whose gravel elements vary in size. SGD is emulated by pumping a tracer fluid into the seabed and its concentration in the water column is measured. Coupled with measurements of the velocity field, the turbulent fluxes of the tracer are thus determined, providing information on the SGD fluxes into the water column, as well as the transport and mixing behavior within the water column. The results show a strong variation of the turbulent flux and patterns of the tracer concentration based on both wave scenario and seabed topography. In combination, they suggest different drivers of porewater exchange being present and enhancing the tracer fluxes. Wave pumping and separated vortices contributed to varying degrees for the different seabeds. Unexpectedly, the increased roughness of the gravelly beds did not explicitly result in increased porewater exchange compared to the flat bed, as wave pumping was blocked over the gravelly beds. Nonetheless, the impact of separated vortices above gravelly beds grew with size and ultimately reached comparable dimension to wave pumping over the flat bed.