Outflow and Three-Dimensional Spreading of Rhine River Water in the Netherlands Coastal Zone

Abstract

The water quality of the Netherlands coastal zone depends largely on the input of pollutants by the river Rhine and the interaction between river and seawater after its debouchment into the North Sea through the Rotterdam Waterway and Haringvliet. Current knowledge about scales, effects and relative importance of the hydrodynamic processes involved in this interaction is insufficient for an appropriate coastal zone management. Therefore, an observational program is carried out in an area extending from the river mouth to about 50 km in the direction of the residual flow (i.e. to the northeast). This chapter presents first results and tries to interpret the observations in terms of physical processes. Characteristic scale factors are derived from the data. These are used to establish the main dynamical balances for various subregions of the coastal zone.

CTD sections, made near the mouth of the Rotterdam Waterway, the main outlet of the Rhine, show that, depending on the discharge, the buoyant outflow is mainly confined to the upper 4 to 10 m. Its offshore extent ranges between 5 and 20 km. Continuous surface salinity, temperature and turbidity observations show a frontal boundary between river and coastal waters, most clearly during the flood phase of the tide. Outflow dynamics in the immediate vicinity of the mouth (in the order of 1 km) are mainly controlled by inertia and the action of gravity. Large variations appear within the tidal cycle.

On a larger scale (in the order of the Rossby deformation radius, 10 km in this case) a crude tidally averaged description is that of a buoyant plume, its spreading strongly affected by earth rotation and the along-shore residual current. The latter is highly susceptible to varying wind conditions. Discharge and current variations within the tidal period lead, among other things, to flow convergence with associated frontogenesis.

Cross-shore CTD sections taken 10, 30 and 55 km from the main outlet show a gradual weakening of vertical stratification. Under quiescent meteorological conditions this is thought to mainly result from tidally generated vertical mixing. The scale over which the vertical density gradient disappears is related to river discharge, among other things. At average discharges (2000 m³ s⁻¹) stratification mainly occurs over a distance of approximately 30 km from the Rotterdam Waterway.