Fortnightly tidal asymmetry inversion and its impact on salinity distribution and sediment net transport in a shallow macrotidal estuary (Charente, France)

International audienceTidal asymmetry has a strong impact on estuarine hydrodynamics and sediment dynamics. Extensive research has been dedicated to studying tidal dynamics in semi-diurnal macrotidal estuaries, leading to the expression of several generic behaviors. Maximum current velocities and durations of the ebb and flood are often used to characterize the asymmetry encountered in the estuary. The Charente estuary (French Atlantic coast) presents the particularity of being connected to a basin where the oceanic tide is already modified by the interactions with the continental shelf. The interaction between these externally generated overtides and the estuarine dynamics results in a spatially and fortnightly variable tidal asymmetry. The studied estuary is alternatively dominated by the ebb during neap tides and by the flood during spring tides, whereas most macrotidal estuaries subject to a semi-diurnal tidal regime are flood-dominant, without asymmetry inversions. A two-dimensional hydrodynamics model is used to investigate the connection between spring-neap inversions of the tidal asymmetry and the harmonic composition of the tide. Resulting salinity distributions and barotropic tidal currents are compared for different configurations, and the possible impact on sediment dynamics is discussed. Results show that asymmetry inversions tend to create sediment accumulation and salinity concentration in an intermediate zone between the river mouth and Rochefort, approximately 20 km upstream