Modelling the wind-driven motions in the Rhine ROFI

Abstract

This Master Thesis presents a numerical model analysis of the response of the Rhine ROFI on wind direction and speed. The aim of this study is to gain a further insight in the physical processes of the Rhine ROFI. Previous work on the Rhine ROFI focused mostly on tidal effects (straining and mixing) and inertial motions (earth rotation). Other studies have focussed on the response of freshwater plumes to wind forcing alone. Recently, the combined forcing has been studied in an idealised model of the Rhine ROFI. The current study extends this research, however with the incorporation of the actual bathymetry and coastline of the Netherlands in a new, state-of-the-art three-dimensional hydrodynamic model. A sensitivity analysis of the number and distribution of vertical layers in the model found that 48 vertical layers is recommended under strongly stratified conditions The numerical results show that the effect of earths rotation on the wind-drive flow plays an important role in the stratification of the Rhine ROFI. Under neap tide conditions the Rhine ROFI exhibits periods of stratification. Due to a reduced eddy viscosity at the pycnocline in between, the surface layer responds more independently and is less affected by bottom friction. It implies that Coriolis force becomes relatively more important in the dynamics of the surface layer. Differential advection and depth mean advection make the size and shape of the freshwater plume highly variable. If the system is forced by an upwelling-favourable wind the freshwater plume is transported offshore, resulting in an increase of stratification. Downwelling-favourable winds transport the freshwater landwards, which results in a vertically well-mixed water column. An increased magnitude of the wind causes a more stratified water column for the upwelling-favourable winds and a more mixed state for the downwelling-favourable winds. Direct wind-induced vertical mixing of the plume is observed for an increased magnitude of the wind. The integrated potential energy anomaly is presented. It is found to be a powerful tool in analysing the interaction and competition of the wind and the ebb-flood tidal cycle in mixing and straining of the Rhine ROFI. The mixing and straining by the ebb-flood tidal cycle is found to be dominant over the mixing and straining by the wind. The influence of the wind on the Rhine ROFI is however clearly observed. The spatial changes in the potential energy anomaly show that for upwelling-favourable winds an increase of stratification occurs in the far-field. In the near-field a decrease is observed. For downwelling-favourable winds the opposite happens. The influence of the Ekman dynamics is observed in the results. For increased downwelling-favourable winds a reduction of the potential energy anomaly is found in both the near- and far-field of the Rhine ROFI.