Modelling the Rhine River Plume

When a river discharges into the sea it forms a river plume. The main property of these plumes is their buoyancy; fresh water discharged by rivers is less dense than the receiving, saline waters. Due to the rotation of the Earth, these plumes tend to turn to the right in the Northern Hemisphere after entering the sea. Previous studies investigated the advection of buoyant river plumes due to wind forcing and ambient currents. However, most studies have neglected the influence of tidal motions. Consequently in this study attention is paid to the spring-neap tidal cycle and tidal straining, in order to investigate the effects of tidally generated turbulence to the stratification. The objective of this study was to investigate the role of tidal forcing on the evolution of a river plume in a shallow frictional sea. The numerical experiments presented employ a three-dimensional, idealised coastal basin set-up. The basic physical properties of the basin are an approximation to the typical conditions associated with the Rhine River Plume. In addition the effects of wind stress and a high discharge are brieflydiscussed. The model set-up enabled the performance of a large number of simulations concerning the sensitivity to open boundaries, forcing conditions and numerical properties. The most apparent effect of tidal forcing appeared to be the on- and offshore movement of the plume and the recurrent mixed and stratified conditions associated with the spring-neap tidal cycle. A pulsed discharge and instabilities (see figure) in the coastal current are also identified. Despite a number of simplifications, the results of the study exhibit a fair resemblance with the behaviour of the Rhine plume. This indicates the significance of tidal forcing on a buoyant plume in a shallow basin; it appeared to be the most important process controlling the stratification.Hydraulic EngineeringCivil Engineering and Geoscience