La thèse est avant tout une expérience humaine très enrichissante, qui m'a permis au cours de ces trois années de rencontrer de nombreuses personnes, de partager de précieux moments avec elles que ce soit au laboratoire, sur le terrain, en congrès ou en dehors. Cette thèse m'a permis de fouler le bel archipel de Saint-Pierre-et-Miquelon, un endroit que je n'aurai sans doute jamais connu autrement, de voir sa nature et de rencontrer ses habitants. Je remercie avant tout Nicolas Robin et Raphael Certain qui m'ont fait confiance dès le début et sans qui cette expérience n'aurait pas pu se produire. Ces trois années ont été bien remplies et nous ont permis, une fois nos marques prises, d'évoluer ensemble. C'est d'ailleurs avec beaucoup d'émotion que je tournerai cette page. Je remercie l'ensemble des financeurs qui ont permis la réalisation du projet EGIML, le Conseil

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The vast majority of coastal dunes in Europe have been stabilized by increasing vegetation cover since the mid‐20th century. However, some systems may experience a remobilization phase, generally occurring locally and further propagating alongshore, the drivers of which remain poorly documented. This study investigates the evolutionary paths (stabilization/destabilization/remobilization) from 1945 to 2020 of a 2 km‐long modern coastal transgressive dunefield located in southwest France with a holistic approach (GPR profiles, aerial photographs and LiDAR topographic data). Results show a landward migration of the transgressive dune by approximately 233 ± 7.5 m, through two distinct stages of rapid landward migration from 10 to 23 m/yr (Stage I: 1949–1959 and Stage III: 2000–2021) separated by an approximately 40‐year stage of slow to no migration, but with substantial windward slope deflation (Stage II). The onset of Stage II is due to the fixation of vegetation by human action between 1950 and 1959. The onset of Stage III is hypothesized to be driven by long and sustained upper backshore/dune toe erosion beginning in 1968 due to a massive shoal welding that locally disturbed the longshore drift. It induced a destabilization of the dune and erosion of the vegetation cover over some decades. A non‐synchronization is therefore observed between the start of the perturbation (1968), then the migration (2000), in line with the hysteresis concept of Tsoar (2005). This study shows that almost all of the sedimentary volume of the 1945 dune has been remobilized by translation to shape the dune system in its current form. The 2.2 km dunefield has grown by approximately 673 000 ± 190 000 m3 during the 2005–2020 period. Among this volume, there is a new foredune that was built from 2005 between the upper beach and the transgressive dune (volume in 2020 of about 394 000 ± 68 000 m3).

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