In times of rapid urbanization, engineering interventions in coastal systems have become more common. It is important to understand the interplay of these engineered solutions with their natural environment to minimize hazardous side effects to our ecology, economy and coastal safety. In a numerical study by Rijnsburger (2021), a flow recirculation with a diameter of 10 to 20 km - similar to an eddy - was identified in the North Sea. It remained unclear what causes the recirculation, but its location in the Rhine region of freshwater influence and near the artificial Maasvlakte headland, suggest that the recirculation could both be baroclinic or barotropic. This numerical model study is set out to understand what processes drive the recirculation. Identifying these processes is important to understand what is required to properly model the hydrodynamics along the Dutch coast, and accordingly, the spreading of sediment, pollutants and phytoplankton.

A realistic 3D hydrodynamic model was used to assess the recirculation during a validated spring-neap cycle. Our analysis shows that the recirculation has a maximum diameter of roughly 15 km during neap tide and 5 km during spring tide. The recirculation occurs in the buoyant top layer of the water column and grows offshore from its onset north of the Maasmond around HW+3 until it is overtaken by ebb tidal velocities around HW+5. This study shows that the onshore advection of negative (clockwise) vorticity water results in most water flowing back to the river mouth into an expanding clockwise recirculation, the vorticity and strength of which are strongly influenced by buoyant river outflow.
Our analysis shows that the headland is not a prerequisite for its onset. Nevertheless, two mechanisms are identified of how the headland influences the recirculation. First, during spring tide, the headland is found to ‘shelter’ the recirculation from strong ambient currents in the midfield which would otherwise hamper the recirculation. Secondly, the geometry of the river mouth influences the vorticity input in the North Sea associated with the buoyant river outflow.

To further investigate what processes influenced by the river outflow could contribute to the recirculation, a scale analysis and particle tracking study were deployed. Although, the study is not yet conclusive on what drives the onshore advection of the spinning water that flows into the recirculation, several potential processes have been identified.

For future studies that want to model the spreading of freshwater, pollutants or suspended sediment near the river mouth of the Rhine-Meuse system, this study underlines the importance of using a three-dimensional numerical model that accounts for density differences and the influence of the studied recirculation. The latter is especially important when the boundaries of the numerical domain are located close to the river mouth, due to which the influence of the recirculation is not resolved unless explicitly accounted for in the boundary conditions. This study therefore contributes to the improvement of future modelling studies of the Rhine ROFI and to our understanding of the processes that govern the hydrodynamics along the Dutch coast.