Wave control on the rhythmic development of a wide estuary mouth sandbank: A process-based modelling study

•40-year morphodynamic-stratigraphic modelling of an estuary mouth sandbank•Morphological predictions are greatly improved when waves are added to tidal forcing•Short-term sediment dynamics are consistent with long-term evolution of the sandbank•Wave climate seasonality potentially explains the observed rhythmic lateral accretion•Modelling results suggest that seismic reflectors are related to grain-size changes

This study presents the numerical hindcast of the morphological and stratigraphic evolution of a wide estuary mouth sandbank located along the French Atlantic coast (Marennes-Oléron Bay), and primarily investigates the respective contribution of tide and waves to its pluri-decadal evolution. Firstly, the morphodynamic hindcast appears strongly improved when tide and waves are considered in the forcing rather than only tide. Secondly, the modelling results suggest that the strong seasonal variation of the wave climate in this area explains the rhythmic lateral accretion of the bank (i.e. normal to the main currents direction) observed in seismic reflection profiles. The grain size variation in the area of sediment accretion is also characterized by a seasonal cycle. Finally, repetitive bathymetric surveys and seismic profiles revealed that the lateral accretion of the bank is associated with about one seismic reflector per year, which is in agreement with our modelling results when assuming that these seismic reflectors are due to the seasonal variation of the grain size. These new results may be considered when studying the morphological evolution and the stratigraphy of other tidal sandbanks potentially affected by waves.