Monitoring the medium-term retreat of a gravel spit barrier and management strategies, Sillon de Talbert (North Brittany, France)
Résumé
The Sillon de Talbert is a large swash-aligned gravel barrier spit of 3.5 km length, situated on the Northern coast of Brittany. Over the last decades, the spit experienced landward migration by rollover reaching 1.1 m.yr-1 at least since 1930, with maximum retreat rates of 1.35 m.yr-1 affecting the proximal section. This evolution has led to the construction of coastal defense structures (rip-rap and groin) during the 70’s and 80’s to stabilize the spit. In 2001, when the Sillon de Talbert became the property of the French public office called “Conservatoire du Littoral”, a different management strategy led to the removal of these hard coastal defence structures. At the same time, a topo-morphological survey was undertaken to analyze and quantify both cross-shore and longshore morphosedimentary processes of the spit. This monitoring began in 2002 and is still ongoing. The results show that the spit landward displacement has increased during the last fifteen years with rates of retreat almost twice as great as prior to 2002 when the monitoring began (2 m.yr-1 vs 1.2 m.yr-1). The most efficient migration process occurs when a high tide level coincides with storm waves inducing sluicing overwash and/or inundation regime. In that context, the spit barrier retreat reaches several tens of meters through rollover processes. However, following such episodes of overwashing the crest of the spit may rise rapidly during fair meteorological periods. Longshore sediment transfer through cannibalization processes is also driving the evolution of the Sillon de Talbert. Due to both of these dynamics, the spit is actually threatening to break in its proximal section in a zone called “wasp waist” where the landward retreat has been the most important since the survey began in 2002. Two strategies in terms of coastal erosion management are drawn according to the policy of the “Conservatoire du Littoral”, as owner of the Sillon de Talbert area, and the duty of the municipality of Pleubian to manage the coastal risks on its communal land. The first option is to remove the existing hard coastal defence structures in order to allow the spit to recover its natural morphodynamic. This option would imply the relocation of several buildings to prevent coastal erosion/flooding risk due to the withdrawal of coastal defence structures. The second option consists of sediment replenishment in the threatened zone with pebbles extracted from available sediment sources. The topo-morphological survey provided relevant scientific expertise in terms of volumetric requirements and existing sources to support this option.