Influence of shoreface morphological changes since the 19th century on nearshore hydrodynamics and shoreline evolution in Wissant Bay (northern France)

摘要

Located at the southern approaches to the North Sea, the Wissant Bay has had the most rapidly eroding shoreline in mainland France in recent decades. Using historical bathymetries spanning the second half of the 19th century to the present, an analysis of the long-term coastal morphological changes and evolution of a prominent sand bank (Line bank), extending across the bay, was carried out. A period of overall seabed erosion with a width-reduction of the Line bank; a deepening of the channel between the Line bank and the coast; and a lowering of the foreshore have been ongoing since the middle of the 20th century. This phase followed a period of channel infill in the western part of Wissant Bay in the early 20th century. Numerical modelling of wave propagation and tidal circulation was performed using the TELEMAC suite of models to get some insights into the influence of these bathymetry changes on coastal hydrodynamics. Two seabed configurations were identified revealing a contrasting hydrodynamic circulation. An erosional one induces an acceleration of tidal currents and an increase in wave height. Conversely, accumulation leads to an hydrodynamic regime weakening with a decrease in current velocity and to more wave energy dissipation in the nearshore. This study highlights the role of morphological feedbacks between nearshore morphology and coastal hydrodynamics. The identification of these feedbacks mechanisms at a historical time scale is important to assess potential drivers of coastal changes.