Turbulent transport and mixing of discharged groundwater on structured surfaces at the coastal benthic seafloor

In recent years, the discharge of groundwater has increasingly been regarded as a notable influence on the terrestrial-marine relation in the coastal zone with respect to the flux of nutrients, carbon and metals into coastal waters. Assessing the net amount of discharging groundwater is a challenge in itself, but the transport and mixing within the water column is an important topic that has been unrevealed so far.

To investigate the transport and mixing processes and their dependence on wave scenario and bottom topography, we performed synchronized particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) experiments using a passive tracer in a wave channel. At the bottom of the channel, a permeable seabed model with defined roughness features, such as ripples and coarser sediment, is mounted and perfused with a fluorescent tracer fluid that resembles the discharging groundwater. Different oscillating flows with variable wave amplitude and period were investigated over each seabed model. The correlation of the measured concentration and velocity fields gives the turbulent transport quantities of the tracer fluid within the water column. Additionally, Prandtl mixing lengths can be determined from the coupled PIV and PLIF results.

Results show a great influence of both, the bottom topography and the wave scenario, as well as their strong coupling through wave-seabed-interaction. Furthermore, different types of bottom roughness showed a great variation in turbulent transport, indicating that the roughness features should be treated in a more complex fashion than being modelled as scalar quantities.