Numerical simulation of tide-induced transport of heterogeneous sediments in the English Channel

The three-dimensional numerical model COHERENS (COupled Hydrodynamical-Ecological model for REgioNal and Shelf seas) has been adapted to compute the rates of transport as bedload and suspended load of heterogeneous bottom sediments induced by the dominant M2 tide in the English Channel. A pre-processing of an extensive surficial sediments dataset has been performed to determine the seabed composition (grain-size distribution or presence of rocks) at the computational grid nodes. Maximum bedload and suspended load transport rates over the tidal cycle as well as the contributions of the 10 different sedimentary classes to the mean transports are computed. Highest sediment transport rates occur in fine sediments areas located in the surroundings of high shear stresses areas. Medium sand ( d 4 = 350 μ m ) is found to be predominant in bedload, while suspension load implies mainly silts ( d 1 = 25 μ m ) in the inner shoreface and both fine and medium sands ( d 3 = 150 μ m , d 4 = 350 μ m ) in the outer shoreface. The offshore residual bedload transport pathways are orientated westerly in the western part of the Channel and easterly in the eastern part defining a “parting” zone which runs from the Isle of Wight to the Cotentin Peninsula. An offshore “bedload convergence” occurs in the southwest of the Dover Strait; a narrow transport pathway bypassing it along the French coastline. These features reproduce those predicted by Grochowski et al. (1993a) and provide higher resolution features like inshore headland-induced gyres, particularly along the English coastline. The new predicted general pattern of residual suspended load transport is very similar to the bedload pattern. Differences arise in the central “divergence” zone which exhibits a “Y” shape with two branches ending on both sides of the Isle of Wight, in the Baie de Seine characterized by a central “convergence” and along the English coastline studded with many headland-induced recirculations.