Caractérisation spatiale et temporelle du recul du sommet de falaises basques : recherche méthodologique

Rocky and cliffy coasts represent more than half of the world’s coastline. Their retreat varies spatially and temporally and threatens the safety of people and property. Qualifying and quantifying this recession in a representative way remains complex, as the different existing approaches give variable results. The French Basque rocky cliff coast, particular for its geomorphological diversity, is subject to a significant heterogeneity of retreat rates. The purpose of this study, carried out as part of the FEDER Ezponda project, is to conduct methodological research on the quantification of the retreat of this coast. It involves: (i) in spatializing and quantify the recession rates from aerial photographs, between 1938 and 2018, (ii) in performing a comparative analysis of the results from different methods. The cliff top was used as a reference indicator and the approaches used are based on: (i) area and (ii) baseline. These approaches allowed the quantification of different average annual retreat rates, but also to reflect the heterogeneity of this retreat along the coastal line. Thus, the average annual rate of recession of the French Basque cliff top is 11.5 cm from 1938 to 2018 according to the area-based approach which appears, as mentioned in previous studies in Normandy, to be the most appropriate of the methods tested in this context.