Fortnightly tidal asymmetry inversions and perspectives on sediment dynamics in a macrotidal estuary (Charente, France)

• We investigate tidal asymmetry inversions in the Charente estuary.
• Both M4 and MS4 overtides play a key role in duration and velocity asymmetries.
• Overtides are generated externally by interaction with the continental shelf.
• Interaction with the estuarine morphology also generates overtides internally.

Tidal asymmetry is a phenomenon that characterises estuarine hydrodynamics and has a strong impact on sediment dynamics. Extensive research has been dedicated to studying tidal dynamics in semidiurnal macrotidal estuaries, highlighting several general principles. The ratio of flood to ebb peak velocities and differences in ebb and flood durations are often used to characterise the asymmetry encountered in estuaries.In the Charente estuary (French Atlantic coast), water surface elevation data obtained using an ADCP (Acoustic Doppler Current Profiler) and a tide gauge show that the duration asymmetry undergoes inversions during the spring-neap tidal cycle. 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. Different constituents (M2, S2, M4 and MS4) are considered at the open boundary. The results show that M4 and MS4 play a key role in the occurrence of these inversions. The influence of the morphology is also discussed and modifications of the bathymetry are performed to evaluate its impact. In the Charente estuary, the existence of both externally and internally generated overtides thus results in a spatially and fortnightly variable tidal asymmetry.The modelled barotropic tidal currents are used to estimate the possible impact on sediment dynamics. The results suggest that asymmetry inversions tend to create sediment accumulation in an intermediate zone between the river mouth and Rochefort, located approximately 20 km upstream.