Breaching of the Sillon de Talbert gravel spit (North Brittany, France) and coastal flooding risk assessment

Coastal barriers are a natural defence against the marine submersion of low-lying back-barrier areas. In March 2018, a breach opened up in the proximal section of the Sillon de Talbert spit barrier (North Brittany, France), causing great concern among local residents and elected representatives. A study to assess the risk of submersion of the low-lying areas of the Laneros Peninsula, located behind the barrier, was carried out in 2020. Hydrodynamic conditions (waves, currents, and water levels) were modeled using Telemac-2D coupled with the phase-averaged wave model TOMAWAC. Simulations were compiled for the storm of February 1, 2014, corresponding to the most morphogenic event to occur in the last two decades. Five topomorphological configurations were considered for the numerical simulations: configurations #1 and #2 correspond to the ante-breach and post-breach morphological settings, while configurations #3, #4, and #5 correspond to hypothetical scenarios, from the enlargement of the breach (from 300 to 500 m width), to complete crest lowering of the barrier after a complete removing of the crest due to catastrophic overwash events. The results show that the breach accelerates the draining of the back-barrier sandflat, thereby reducing the height of extreme water levels at the coastline. This is mainly due to different tidal impacts on either side of the barrier, with a higher water level and the tidal peak occurring 15 min later and the on the back-barrier than on the ocean-side zone. As a result, ebb currents begin to flow before the high tide level is reached, inducing a significant draining with a lowering of the extreme water level at the back-barrier zone. However, these simulations do not take into account the effects of climate change over the coming years, in particular the future rise in sea level and/or the acceleration of erosion processes already underway. [Display omitted] •The breach opening on gravel spit barrier through cannibalization and rollover processes is described.•A method of coastal flooding risk assessment is proposed through modelling of hydrodynamic forcings.•An example of nature-based coastal management is presented as a model for a new regional strategy.