Development of a 3D Hydrodynamic Model for Coastal Ecosystem Modelling. Application to the Plume of the Seine River (France)

A 3D (three dimensional) numerical hydrodynamical model has been developed as the basis for ecological modelling and long time simulations. A compromise between computation time reduction and the model capacity to simulate fine-scale physical processes has been looked for. The model is based on a separation between external and internal modes and the coupling between the two modes can be locally optional. The horizontal grid is rectangular with irregular meshes and cross sections of tributaries are respected. The vertical grid uses real depth coordinates and the vertical axis is split into several layers bounded by fixed horizontal levels irregularly distributed. Turbulence closure is based on a mixing length model that accounts for turbulence damping by density gradients. This model is applied to the ‘ Baie de Seine ’ and the Seine River plume is studied. The model is validated by numerous comparisons with measurements (tide, currents, salinity fields). The three-dimensional behaviour of the Seine plume has been studied and compared to theoretical study cases. In particular the importance of topography is enlightened as well as the diffusive role of tides and the small influence of the earth rotation in the near field of the plume. On the other hand winds are shown to control the plume structure.