Long–term variability of supratidal coastal boulder activation in Brittany (France)

High–energy supratidal coastal boulder deposit (SCBD) dynamics were investigated on Vierge Island and Pors Carn Point, north and south of western Brittany, France, respectively. Morphological changes induced by boulder transport and quarrying were quantified using high–resolution topographic survey data taken between 2012 and 2017. Additional in–situ wave parameters and water levels were also recorded over this period (2014–2017) in order to compute the maximum water levels and assess the relationship between SCBD morphological changes and specific hydrodynamic conditions. During extreme water levels (for maximum water levels exceeding a one in ten year event), SCBDs were broadly reworked (up to 40% of the total volume). During lower intensity events, for which maximum water levels were still very high, morphological changes represented 1% to 5% of the total volume. These morphological and hydrodynamic observations were then used to calibrate a chronology of SCBD activation events based on 70years of hindcast winter maximum water levels. These long–term time–series showed great interannual variability in SCBD activation but no significant long–term trend. Winter–frequency SCBD activation was better correlated to the WEPA index (r=0.46) than the NAO index (r=0.1). Therefore, the WEPA index can be considered to be a more significant climate proxy for assessing storm–related geomorphic changes in the temperate latitudes of the N–E Atlantic basin (36°–52° N), including the Brittany coast. The potential of SCBDs as a morphological storm proxy for macrotidal high–energy rocky coasts is addressed. •Supratidal boulder dynamics were quantified using kite aerial photographs taken from 2012 to 2017.•Supratidal boulders can be activated by low intensity storms (Hs≤1year return period).•The long–term supratidal boulder activation frequency shows a strong inter–annual variability.•Inter–annual variability of supratidal boulder activation can be related to the WEPA index.