Catastrophic overwash and rapid retreat of a gravel barrier spit during storm events (Sillon de Talbert, North Brittany, France)

Abstract

AbstractThe morphodynamic functioning of the Sillon de Talbert gravel barrier spit is analysed using a high‐frequency survey carried out between September 2012 and December 2019. It is based on beach profile measurements along two transects, modelling offshore wave data (WW3), tide gauge records, and shallow waves and water levels recorded in the intertidal zone. A barrier retreat of −23 to −30 m over the 7‐year survey (i.e. −3.3 to −4.3 m yr−1) is measured. This retreat is not related to long‐term sea level rise (macroscale of 102–103 yr), but to mesoscale (100–102 yr) morphogenic events combining storm wave and high spring tide. Over 87–90% of the barrier retreat is due to three significant events (1–2 February 2014, 9 February 2016, and 3 January 2018). The storm impact scale model of Orford and Carter is tested. The estimation of the wave runup for the calculation of extreme water levels [i.e. peak overflow elevation (Oe) component] is based on the calibration of an equation performed from in‐situ measurements of the swash elevation. The flow depth (Od,q) overtopping the crest of the barrier (Bh) is thresholded by taking into account the morphological response of the barrier in order to define regimes corresponding to overtopping, discrete overwash, and sluicing overwash. While the Orford and Carter model is generally successful in reproducing the morphodynamic evolution of Sillon de Talbert, the wave energy flux (F) must be considered as an additional parameter in order to improve the fit of the model, so far as it contributes in some cases to change the morphodynamic regime. Thus, the wave energy flux constitutes a key component in the quantification of the water flow across the barrier (Od,q) corresponding to the hydrodynamic forcing of the model, which becomes (Od,F).