Modelisation morphodynamique de l'embouchure de la seine

The mouth of the Seine has been undergoing intense morphological changes for decades, because of containment work to improve access to the ports of Rouen and Le Havre. A significant trend is the progradation of the downstream estuary. Simultaneously, the surface area of the intertidal mudflats tends to decrease. The estuary's sediments are composed of silt and fine sand (respectively transported from the river Seine and the open sea). Their distribution is linked to the hydrodynamic characteristics of the estuary and may display layered vertical structures. To understand the estuary's morphological changes, a morphodynamic ditigal model is developed from a validated (3D) model of silty sediment transport. A model of suspended fine sand transport as well as a bathymetric readjustment module are developed. The sediment's erodability takes into account the proportions of sand and silt; two erosion regimes are recognized (respectively non-cohesive and cohesive). The sediment is discretised into fine layers (~ mm) and the consolidation of the silty layers is simulated. Thus, the erodability of the superficial layers depends upon the relative concentration of silt (i.e., between the grains of sand). Simulations performed for an arbitrary initial condition of the sediment bed show that the model reproduces a realistic distribution of the silt and sand. The main morphodynamic trends, such as progradation and the rotating of banks at the river mouth, are qualitatively reproduced. The results show that sand and silt contribute in a specific way to the morphological changes.