Dynamics in the near-field region of the Rhine ROFI: a multiple front system

The Rhine River forms one of the largest Regions of Freshwater Influence (ROFI) in Europe, discharging freshwater into a strong tidal cross flow on every ebb tide. The formation and evolution of the tidal plume fronts was captured by the STRAINS (STRAtification Impacts Near-shore Sediment) field campaign of 2014. Here, we use a high-resolution 3D model together with the field data to explore the interaction of the lenses in the near-field region, where the fronts dominate the dynamics. The freshwater lenses are sensitive to the wind magnitude and direction, and control the evolution of the stratification in the near-field plume. During calm periods, the tidal plume fronts are strongly modulated by the strong tidal cross flow. In the case of upwelling winds, the lenses evolve as separate lenses and the downstream plume widens and moves offshore, while during downwelling winds the lenses merge. In that case we observe thicker lenses, the frontal propagation speed is maximum and the downstream plume narrows against the coast. On every ebb tide, a new freshwater lens is released, while the ones from previous cycles are still present. The model reveals a detailed picture of the interaction between the multiple lenses and the resulting vertical structure of the water column in the near-field region. We find that the lenses interact under the influence of the tidal cross flow and prevailing winds; they either merge due to converging flows or separate due to diverging flows. Furthermore, we compare our results to the Connecticut and Columbia Rivers, highlighting the similarities and addressing the differences due to the dominant role of friction.