Contribution of wave-driven sediment transport to the infilling of a hypertidal embayment
Résumé
Sedimentary infilling of estuaries and embayment is the result of complex interactions between tide asymmetry and circulation patterns, wave climate and autocyclic channel dynamics. Their morphological evolution is dependent on a sensitive balance between sediment fluxes and available space, which can be altered by climate change and subsequent rising sea level. Predicting the future evolution of such environments is crucial to anticipate the modification of ecosystems at the land-sea interface, and to evaluate the potential impacts on human activities (navigation, exploitation of marine resources, tourism and recreation, etc.).
In this context, sediment transport in a hypertidal sandy bay (bay of Somme, NW France) is studied using numerical modeling in combination with in-situ data at the bay scale and for a 2-week period in order to understand the processes involved in filling the bay. A 3D fully coupled hydrodynamics-wave-sediment model, combining a hydrodynamic model (CROCO) to a spectral wave model (WAVEWATCH-III), both feeding a sediment transport model developed by USGS, is implemented. After a validation step involving the realistic simulations of nearshore hydrodynamics under tide, wave and wind forcings, based on in-situ data from two field campaigns, the coupled model is considered reliable for sediment transport assessment. Then, bay-scale simulations are carried out for a broad range of metocean conditions, from calm to storm seas and for spring and neap tide events. Our main conclusions are as follows: i) with a maximum tidal range of 10 m, semi-diurnal tide is unsurprisingly the main driver of the bay hydrodynamics, ii) the bay fills with seawater during the flood and empties during the ebb, iii) the wave-induced circulation appears to have a significant impact on the bay sand filling during the storm and iv) ocean waves generate a longshore current at the bay entrance, which is intense and unstable during the storm, showing significant cross-shore velocities that influence sediment transport. Next step is to evaluate the consequences in terms of morphodynamics.