HALINE STRATIFICATION IN THE RHINE-MEUSE FRESH-WATER PLUME - A 3-DIMENSIONAL MODEL SENSITIVITY ANALYSIS

Results are presented of a three-dimensional model study of the tidally-averaged salinity field in the Rhine-Meuse plume. In conditions of low mixing (no wind, neap tide) freshwater emerges from the river as a jet, turns right under the influence of Coriolis force and attaches to the coast as a buoyancy current. Surface residual currents are essentially geostrophic. Within the plume the surface layer is strongly stratified and overlies a bottom mixed layer. Results are strongly sensitive to the parameterization of vertical mixing: models ranging from constant diffusion coefficients through simple algebraic Richardson number dependent formulations to turbulence closure with evolution equations for turbulent kinetic energy and length scale were tested. Turbulence closure with a single evolution equation for turbulent kinetic energy and an algebraic length scale formulation was found to provide a suitable balance between physical realism (assessed by theoretical considerations and the practical ability to represent a well-mixed bottom boundary layer and stratified surface layer) and computational efficiency. Simpler models, which Vary diffusion coefficients as function of Richardson number, may produce similar results, though require more careful calibration. Constant diffusion coefficients are clearly inadequate for the application considered. Even the preferred ''k'' model requires some calibration as a ''background'' or ''ambient'' mixing coefficient had to be introduced to avoid unrealistically strong stratification. The tidally-averaged salinity field was found to be qualitatively similar at neap and spring tide, though cross-shore penetration of the outflow jet was reduced, bottom-mixed layer thickness increased and overall stratification reduced at springs. In contrast, the salinity field was found to be strongly dependent on wind strength and direction, both through wind-induced surface mixing and advection by wind-driven surface currents.