Modeling hydrosedimentary transport in the eastern English Channel under extreme weather conditions

The eastern English Channel (EEC) experiences a dynamic interplay of extreme events, including powerful winds, tides, and complex bottom relief features, collectively shaping the region's hydrosedimentary transport dynamics. Extreme winds, frequently observed in the EEC, play a pivotal role in influencing surface currents and wave patterns. Coupled with strong tidal forcing, these events lead to intricate interactions with the seafloor topography, creating a complex hydrodynamic environment. The resulting effects on sediment transport are significant, with the potential for altered erosion and deposition patterns along the coastal areas of the EEC. Understanding this multiscale interaction is crucial for predicting and managing the impact of extreme events on hydrosedimentary transport, contributing to effective management of coastal ocean environment. In order to study all these processes, we used a 3D hydrodynamic model (MARS3D, IFREMER) coupled with a sediment transport model (MUSTANG, IFREMER) and calibrated it for the English Channel. This model employs nested grids of different extents and resolutions: (i) the northwest European continental shelf area with a 5 km resolution, (ii) the English Channel area with a 1 km resolution, and (iii) local zones along the eastern coast of the English Channel with a 100m resolution. The larger model transfers boundary conditions to the higher-resolution model. Such a cascade of resolutions allows for the consideration of both large-scale hydrodynamic processes and the replication of smaller-scale processes (eddies, turbulent current oscillations). Four fractions of suspended matter, including two size classes of mud and two size classes of fine sand, were specifically chosen for modeling based on in-situ data. The model then has been validated using the data collected in 2020-2021. The model can be employed to replicate the dynamics and sedimentary processes over a multi-year period. This enabled the estimation of the quantity and flux of suspended matter, as well as potential changes in the sediment transport regime induced by extreme weather conditions. The modeling outcomes highlighted tides as the primary driving force behind hydrosedimentary transport, surpassing the influence of wind. This effect is evident in the formation of eddies near capes, emerging for a few hours after the onset of the tide phase. These eddies create a counterflow of matter along the French coasts, opposing the general current direction that moves toward the North Sea. In the regions characterized by the highest velocities in offshore areas, larger particles (sands) prevail within the eddies, while the slower sections closer to the coast are predominantly composed of finer mud particles. Additionally, the wind introduces instabilities in the current structure, leading to an increased resuspension of fine sediments.