Modelling the Turbidity Maximum in the Seine Estuary (France): Identification of Formation Processes

A two-dimensional depth-averaged model and a three-dimensional model for tide propagation and fine sediment transport are used to determine the respective role of several processes on the turbidity maximum behaviour in the Seine estuary. Consolidation processes have been taken into account in the 2DH model. Despite simplified sedimentological formulations and even without any gravitational effect, a turbidity maximum, close to the observations, is reproduced during spring tides. This result confirms the large contribution of tidal pumping on the turbidity maximum formation. The sediment behaviour (settling, erodability) acts in modifying the concentration and location of the tidal-induced turbidity maximum. The bathymetry influences the tidal propagation, but also the location of deposited sediment patterns and consequently, modifies the location of the turbidity maximum. The three-dimensional model allows accounting for all the forcing while the 2DH model does not include the salinity gradients and thus the density circulation, nor the vertical profile of velocity and concentration, which change the deposited sediment distribution. Density gradients act in maintaining the fine sediment in the estuary and in stretching the dynamical accumulation towards the upper limit of the salinity gradients. The turbidity maximum results from the superposition of both processes: (a) the dynamical accumulation of suspended sediment, related to tidal pumping and river flow, modelled by bathymetry and dependent on sediment behaviour, (b) salinity gradients, only related to river flow and bathymetry, which modify its structure.