CoPraNet: Coastal erosion related to a sediment deficit in the nearshore zone: The case of Wissant Bay, Northern France
The coast of the Bay of Wissant, northern France, mainly consists of a wide macrotidal beach backed by coastal dunes located on either side of the small coastal resort of Wissant. A major part of the coast of the Bay of Wissant has been affected by very severe erosion since at least the middle of the 20th century, especially in the central part of the bay where coastal retreat rates reached up to 4 to 5 m/yr between 1949 and 2000. The analysis of bathymetry changes from 1911 to 2002 showed that coastline evolution is strongly related to nearshore morphological variations. There is actually a very strong correspondence between areas of significant nearshore erosion and sectors of coastline which experienced rapid retreat in the central and western parts of the Bay. Conversely, east of the resort of Wissant, where the shoreline advanced seaward several tens of meters, the seabed in the nearshore zone was either stable or characterized by sediment accumulation. Computation of sediment volume change showed that the net sediment budget in the Bay of Wissant was strongly negative during the 20th century, the sediment deficit being in excess of 100 000 m3/yr on average.
This study shows that shoreline evolution in this area strongly depends on erosion and sedimentation processes that affect the whole littoral profile from the coastal dunes down to the shoreface several meters below sea level. Seabed erosion in the nearshore zone caused a lowering of the level of the intertidal beach, which favoured coastal retreat since lower beach levels result in higher frequencies of water levels that may reach and erode coastal dunes. This example of coastal evolution underlines the need to take into account the nearshore sediment budget prior to the implementation of protection measures.
|Beach, Beach morphology and evolution, Erosion, Sediment transport
Université du Littoral-Côte d'Opale, Laboratoire GéoDAL
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