Fifteen years of hydrodynamic forcing and morphological changes leading to breaching of a gravel spit, Sillon de Talbert (Brittany)

The Sillon de Talbert is situated on the Northern coast of Brittany; it is a large 3.5 km-long swash-aligned gravel spit barrier comprising a volume of sediment of 1.23x106 m³. Since 2002 a morphodynamic survey was carried out. It is based on annual DEMs and monthly beach profile measurements. Waves and water levels are also surveyed using modelling data and field measurements. The 15-years (2002-2017) sediment budget calculation shows that cross-shore transfers are dominant and represent a total volume of 370,000 m³, while the longshore sediment transfer is estimated at about 50,000 m³. However, considering the volume of sediment eroded from the beachface (as much as ‑411,000 m³), and the cross-shore and longshore sediment accumulation (reaching +420,000 m³), the global sediment budget is balanced for the whole survey period. The landward displacement of the spit due to rollover processes has increased almost doubled during the last 15 years with rates of retreat of 2 m.yr‑¹between 2002 and 2017, vs 1.2 m.yr‑¹between 1930 and 2010. Storm events control more than 95% of this retreat due to catastrophic overwash/inundation processes. The spit retreat has led to the opening of a breach in the proximal section at the beginning of March 2018, which has experienced a rapid enlargement (35 vs 15 m) and deepening (3.4 vs 1.25 m) over the last seven months. The morphological evolution of the Sillon de Talbert is driven by anthropogenic forcing (i.e., impact of coastal defence structures, cutting off of longshore sediment transport), and natural forcing such as the depletion of the supply of sediment from the platform, and the increase of extreme storm events over the last few years.