MODELING TRANSPORT PROCESSES IN THE RIBBLE ESTUARY

Field measurements of velocity and salinity at three stations along a reach of the Ribble estuary were used to evaluate the temporal variations of the vertical shear induced component of the longitudinal dispersion coefficient. The results yield clear temporal trends which lead to the conclusion that vertical shear-induced dispersion, resulting from differential advection and density gradients, is the most important component of the longitudinal mixing on the ebb tide. Turbulent diffusion, due to the high velocities, dominates the mixing mechanisms on the flood tide. The sensitivity to the dispersion coefficient in a one-dimensional model based on a schematised form of the channel geometry of the Ribble estuary revealed the need for a time dependent dispersion coefficient. Measurements of suspended solids revealed low concentrations compared to previous records. It is proposed that this is due to a pumping effect in the estuary, induced by a combination of the tidal asymmetry and the ebb tide vertical density gradients leading to upstream storage during low river flow spring tide periods. Modelling of the turbidity maximum is shown to be possible by adjustment of critical erosion and deposition shear stress terms.