Numerical Study of Sediment Transport in the Seine Estuary: Contribution of Dredging

In the Seine estuary, 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 validated by (Grasso et al. in J Geophys Res: Oceans 12(3):558–577, 2018), 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. After 3 years of marking, the contribution of dredged sediment to the ETM can be estimated around 1%.