Long-term morphodynamic evolution of the Sillon de Talbert gravel barrier (Brittany, France)

The Sillon de Talbert is the largest gravel barrier spit in Brittany and forms a swash-aligned formation exposed to swell that enters the English Channel from the west. It partially protects the islands of the Bréhat Archipelago against coastal erosion and marine flooding of low-lying land. The main morphological changes to the Sillon de Talbert have been studied since the 17th century, and were accurately quantified between 1930 and 2010, based on early marine maps, aerial photographs and topographic surveys recently conducted by DGPS. The examination of early maps shows that this barrier was attached to the Olone Islands until the end of the 17th century. Towards the mid-18th century, a breach that had formed in the north of the barrier resulted in its transformation into a trailing spit. A gradual change in its general direction towards a more drift-aligned position promoted longshore sediment drift towards the distal end. A slow cannibalisation processes began, dividing the spit into a source area at the proximal section, a transit zone in the median section and an accumulation zone at the distal section of the spit. Long-term analysis of shoreline changes over the past decades highlights the mobility of the Sillon de Talbert, which is characterized by rapid landward retreat by rollover. This trend is mainly due to a sediment supply deficit, which can be explained above all by sediment depletion on the continental shelf and, to a lesser extent, by anthropogenic activities particularly gravel extraction. The results obtained show that for the entire period (1930-2010), the mean migration rate was 1.1 m/yr. Upon closer investigation, the rate of retreat and the main morphological changes proves to differ between the morphosedimentary units of the spit and illustrates cannibalisation processes. Thus, the proximal section shows very high retreat rates (1.35 m/yr) due to a sediment budget deficit, which can be explained by gravel migration towards the north-east section of the spit. For these sections, this evolution results in greater sensitivity to erosion and breaching during severe storms, as was the case in April 1962 and during winter 1989-90. Its median section corresponding to the transit zone, shows lower retreat rates than the proximal section (1.05 m/yr) and a relatively balanced sediment budget, as illustrated by the high resilience of this section of the spit submitted to sluicing overwash. Finally, the distal section has undergone retreat by rollover of its exposed outer face and progradation of its inner face which benefited from sediment supply from proximal section. From the 1970s, several human interventions based on coastal defense strategy attempted to slow the spit's retreat due to rollover. The failure of these interventions led to the definition of a new coastal management policy in 2004.