Chapitre 4. Suspended sediment dynamics induced by the passage of a tidal bore in an upper estuary

Tidal bores are complex but popular phenomena that occur in estuarine rivers during rising tides. Until recently, few studies were devoted to the understanding of this process, although it should have a significant influence on sediment erosion and deposition in estuarine systems. Herein, we propose to reconstruct the evolution of sediment concentration close to the channel bed and in the water column during the passage of several tidal bores. Field data were collected in the Sée River channel, approximately 15?km upstream of the Mont-Saint-Michel outer estuary (NW France). An Acoustic Doppler Velocimeter (ADV) was used to measure longitudinal and vertical flow velocities. Suspended Sediment Concentration (SSC) evolution was monitored using an Argus Surface Meter (144 OBS sensors) as well as direct water sampling at different elevations above the channel bed. The highest sediment concentration was recorded at the channel bed a few seconds after the passage of the bore front. Values reached up to 53.5?g?L?1. Then, in the few minutes after the bore's passage, an upward advection of suspended sediment was observed which resulted in the homogenization of the water column concentration to approximately 10 to 15?g?L?1. These results demonstrate the high potential of this tidal process to induce very large upstream-directed sediment fluxes in upper estuaries.