Assessing the Contribution of Maintenance Dredging to Sediment Dynamics in the Mouth of the Seine Estuary (France)
Type:
Presented during:
WODCON XXII - Enhance the Harmony between Dredging and Ecology
Authors:
J.P. Lemoine and P. Le Hir
Abstract
"The macrotidal Seine estuary shelters two of the major French harbours. Rouen harbour is located 80 km upstream from the mouth, its navigation channel is crossing a very morphodynamic area which needs to be maintained by continuous dredging operations. Le Havre harbour is opened to the Baie de Seine nearby the estuary mouth, and is subject to fine sediment settling especially after storms in the area and/or when the estuarine turbidity maximum is located downstream, during high river discharge. Maintenance dredging is operated continuously and yearly dredged volumes have the same order of magnitude as natural sediment fluxes at the mouth of the estuary. Prior to 2017, the main dumping site was in the vicinity of the estuarine turbidity maximum (ETM), therefore impacting sediment dynamics in the area. In this context, the aim of our research is to understand and model the contributions of maintenance dredging to the ETM characteristics and to sediment fluxes, and to compare natural fluxes to the anthropogenic ones related to these activities. Our approach relies on a 3D multi-class hydro-sedimentary model, in which dredging and dumping were implemented as processes following rules defined by port authorities of Rouen and Le Havre. Model results are first evaluated regarding the temporal and spatial variability of dredging in the Seine Estuary, and secondly regarding the stability of dumping sites. In order to follow the fate of dredged materials, the latter are numerically marked by assigning them to specific state variables of the model that respect the particle characteristics (gravel, sands or mud). The corresponding concentrations of these new variables can be computed either in the water column (suspensions) or in the layered sediment. Their contributions to the total suspended sediment mass of the estuary and to the siltation rate are evaluated. Results show that most of the dredged material remains in the proximity of the dumping site but the rest contributes to the turbidity and to the siltation pattern in the estuary. The similitude between the global siltation pattern and the distribution of re-deposited dredged material reveals a strong mixing of the latter within the whole estuary mouth."
Keywords: estuary, sediment transport, sand and mud mixture, numerical tracing, maintenance dredging.