Maintenance dredging as a driver of morphodynamic evolution in a macrotidal estuary: insights from the Seine Estuary

: In estuarine environments, navigational channels require recurrent maintenance dredging (MD), involving large volumes of sediment. Despite its importance, the medium-term (∼10 years) impact of MD on hydro-morpho-sedimentary dynamics in sandy-muddy estuaries remains poorly documented. This study addresses this issue using the Seine Estuary (France) as a well-documented case. We used a 3D hydro-morpho-dynamic model, coupled with a wave model, to simulate sand-mud dynamics, accounting for MD and dumping as active processes. The simulated disposal-site stability is consistent with observations, and numerical sediment tracing clearly illustrates the contrasting behaviour of sand and mud on disposal sites. To assess the effects of MD on turbidity, sediment fluxes, and morphological evolution, three scenarios were simulated over 9 years: the reference scenario, which involved dredging and disposal as currently practised; a dredging-without-disposal scenario, to isolate the effect of dumping; and a no-dredging scenario, to isolate the effect of dredging. The estuarine turbidity maximum (ETM) mass shows only a slight increase with dredging, despite an increasing contribution of previously dredged material to its total mass, which reaches about 14% after 9 years. This paradox is explained by the cumulative remobilisation of sediment, which differs only slightly between scenarios with and without dredging, with the remobilised mass greatly exceeding the ETM mass. Morphological evolution is similar across scenarios, except near dredging and disposal sites, where dredging reduces siltation and alters local morphological evolution. MD enhances sediment fluxes and, in particular, reverses the sediment budget, shifting the system from net infilling to net export. Overall, the results demonstrate the potential control exerted by sediment management strategies on long-term estuarine evolution and resilience to climate change.