Spatial analysis of cliff-top retreat velocity: methodological contribution of strip graphs

The present study proposes a methodology for the visualization of cliff-top retreat velocity, using maps. This method is applied to the cliffs of La Flotte (Ré Island, France). These cliffs are composed of sedimentary rocks, alternating carbonate, bioclastic limestone, and marl. The retreat at the cliff-top, a key indicator of the erosion of these geomorphological objects, is typically measured through time series analysis employing orthophotographs. However, these linear approaches are unable to adequately represent at once the spatial and temporal variability of the phenomenon. To address this issue, we have developed a methodology for generating strip graphs from multiscalar sources, encompassing both annual (UAV orthophotographs, 2021-2024) and decadal (IGN orthophotographs, 1950-2021) data. These strip graphs are derived from statistical results obtained with DSAS software, which compare cliff-top positions at varying dates. These strip graphs are then segmented to identify recurring areas of active erosion, particularly in the central and western parts of the cliffs studied. Uncertainties (georeferencing, digitizing, and resolution) are taken into account to eliminate unreliable values. This process, however, limits the exploitation of the results, as many transects are located within the periodic error margins. To compensate for this limitation, transects are cross-referenced with external indicators (landslides and scree cones inventories) to overcome uncertainty. A comparison of strip graphs and French regulatory mapping methods reveals that the former provides a more precise and localized assessment of erosive dynamics. However, using strip graphs requires additional information (geological, climatic, and marine characteristics) to explain the underlying mechanisms. This could be accomplished in the future. In conclusion, although subject to methodological constraints, strip graphs analysis is a relevant tool for spatializing medium-term cliff-top retreat velocity hazards, for coastal risk management.