Flow-topography interaction over gravelly sand beds and the implications for transport of submarine groundwater discharge

Submarine groundwater discharge is an important pathway of various nutrients and solutes from the land to the ocean. The groundwater discharging across the seabed interface is exposed to the highly dynamic conditions of the coastal ocean. The flow-topography interaction in the benthic boundary layer defines these dynamics and affects how the discharged groundwater is transported and mixed within the water column. Additionally, the flow-topography interaction can also drive convection in the seabed that can enhance fluxes across the seabed interface. 
To investigate these effects, laboratory experiments are conducted, where waves are generated over gravelly sand beds. In these types of beds, gravel protrudes from the seabed. The protruding part is altered to change its size and effective slope, to quantify their respective impact on the flow conditions. Additionally, water with a fluorescent dye seeps through the seabed, resembling groundwater discharge. A coupled PIV-LIF approach (Particle Image Velocimetry, Laser Induced Fluorescence) is used to measure the velocity field and the concentration of discharged water in the water column simultaneously. Both quantities are then correlated, which gives the turbulent Reynolds flux. This approach grants insights on the distribution, transport, and mixing of discharged water within the water column. 
Both, the seabed geometry and the wave scenario, significantly influence the turbulent flux. The results show different processes, such as wave pumping and separated vortex pumping, which drive convection in the seabed and alter the discharge rates. This leads to different concentrations of discharged water being measured in the water column, depending on the boundary conditions. While the outcome of these processes can be visualized and quantified in the water column from experimental data, complimentary surface-subsurface modeling holds the potential of additionally resolving the flow field within the seabed and expanding the investigated two-dimensional region to a three-dimensional domain.