The influence of wind on the evolution of freshwater fronts in the Rhine ROFI

The Rhine River discharges freshwater into the North Sea, forming one of the largest Regions of Freshwater Influence (ROFI) in Europe. Every tidal cycle, a freshwater lens is released. These fronts were captured by the STRAINS (STRAtification Impacts Near-shore Sediment) field campaign of 2014. The data consists of current velocity, temperature and salinity at a fixed location 10 km northeast of the river mouth. Here, we explore the effect of the wind on the evolution of the freshwater lenses using a high-resolution 3D model, which is validated against the field data. We find a stratified river plume that consists of multiple freshwater fronts. On every ebb tide, a new freshwater lens is formed, which is subsequently advected by the tidal flow. Remaining lenses from previous tidal cycles are still present when the next one is formed. The properties and evolution of the lenses strongly depends on the wind magnitude and direction. Under upwelling winds, they evolve separately and the downstream plume is detached from the coast. The thickest lenses are found under downwelling winds, when their propagation speed is maximum and the downstream river plume is pushed against the coast. During storm conditions, when the wind speed exceeds 15 m/s, the river plume becomes well-mixed and no separate lenses are found. The model shows a detailed picture of the formation and evolution of the freshwater lenses in the Rhine ROFI and the vertical structure of the water column. We find a multiple front system, where lenses interact under the influence of tidal flow and prevailing winds; diverging flows causes the lenses to separate, while they seem to merge under converging flows.