No abstract

The mouth of the Seine River estuary (France) has undergone marked morphological evolution over several decades mainly due to engineering works aimed at improving access to Rouen and Le Havre harbours. The intertidal areas are decreasing in size and the lower estuary is accumulating sediment and prograding. In order to understand and better describe the major morphological behaviours of the estuary, a morphodynamic numerical model was developed within the Seine-Aval program. At the end of the 1st part of the research program, a validated fine sediment transport model (3D) was available (Le Hir et al., 2001b). As the present morphological study addresses medium-term issues (a few decades), and because of the need to investigate impacts of local structures or events, we chose to use the so-called ''process-based approach'' starting from the existing model. First, the existing model was upgraded to account for (suspended) sand transport, and to achieve coupling between morphological changes and sediment transport. Erodability of the sediment accounts for the respective proportions of mud and sand. Simulations starting from an arbitrary surficial sediment cover show that the model is able to reproduce realistic sediment patterns. For example, it is able to change the sediment nature on the intertidal flat near Le Havre from sand to mud. Observed structures of suspended sediment are also reproduced: fine particles mainly follow the turbidity maximum whereas significant concentrations of sand grains in suspension are found where the hydrodynamic stresses are intense. Concerning morphodynamics, simulations with real forcing over one year are discussed. The effect of waves on the bathymetric evolution of the mouth is shown and the sensitivity of morphodynamics to the coupling procedure is tested.

No abstract

International audienceThe results of a seismo-stratigraphic study performed to study the Holocene sedimentary infilling at the mouth of the Seine River are presented. Combined data comprise very high resolution seismic reflection profiles, vibrocores, up to 4.5 m long, radiocarbon dating and old bathymetric charts. The infill is divided into two main stages: the transgressive systems tract, relative to the early Holocene high rate of sea-level rise, from 9000 to 7000 cal B.P., made of fine-grained, organic-rich, tidal flat to swamp deposits; the highstand systems tract, relative to the middle to late Holocene low rate of sea-level rise, from 7000 to 3000 cal B.P., characterized by the development of the high energy estuarine body above the main axis of the incised valley. Simultaneously, on the southern edge of the valley, a coastal barrier constructed above a bedrock plateau under the dominant action of waves. At 3000 cal B.P., probably in relation to a major climatic deterioration, the destruction of the barrier occurred and the tidal estuarine body expanded finally over the whole incised valley area. The rate of sea-level rise and the bedrock morphology appear to be the two main key-factors that control the sedimentary infilling architecture of this macrotidal, tide-dominated estuarine setting at the pluri-millenium Holocene time scale. Also, rapid climate changes determine part of the infill pattern, especially during the late Holocene sea-level highstand context