Sediment mobility over a dispersive inner shelf from combined wave and tide bed shear stress

The sediment transfers that take place between the beach and the depth of closure on the inner continental shelf play a major role in the evolution of sandy shores at different time scales. The depth of closure of the foreshore sedimentary prism, which is generally dependent on wave conditions, remains poorly constrained in the context of an internal macrotidal platform. This work aims to define and evaluate this theoretical depth, which delimits the extension of sedimentary exchanges, in particular by including the constraints applied on the sea bottom by the tidal circulation, which are very strong on the inner continental shelf of western Brittany. The use of wave (WAVEWATCH III®), tidal (MARS3D), and bottom sediment (EMODnet) databases allows us to follow an original cartographic approach to study closure depths, based on the formulations of Hallermeier (1978 and 1981) and Soulsby (1997). This cartographic approach is adapted to a spatial analysis of sediment mobility on a regional scale. First, the depth of closure shows a regional spatialisation of the seaward extension of the sediment mobility zone according to exposure to different wave climates and tidal ranges. Secondly, the method for calculating the depth of closure with the combined shear stresses (wave and tide), applied at the scale of the internal platform of western Brittany, allows the identification of three areas of movement of sedimentary particles according to critical mobility thresholds: 1) Area of no motion; 2) Area of transport and deposition; 3) Area of transport without deposition. The main contribution of this work is to propose at distinction between 2 offshore sediment motion limits, the transition to the upper plane bed (DoTupb), and the incipient motion of particle (DoTmotion). In addition, this study confirms the potential of transport and deposition of sand particles at depths >100 m in the most hydrodynamically intense areas, which implies the possibility of interconnections between hydro-sedimentary cells, through bypassing of headlands and crossing over rocky outcrops.