Quantifying coastal cliff retreat, sediment volume, and dynamics using photogrammetry and geospatial analysis in the Pénestin Peninsula, France

Recently, rocky cliffs globally have garnered immense attention due to their susceptibility to erosion, collapse, and instability, similar to the frequent monitoring of low-lying coastal zones for risks associated with climate change-related sea-level rise and flooding. Historically, sediment volumes have been extensively studied and constrained; however, recent advancements in remote sensing and UAV technology now facilitate more precise quantitative assessments of the volumes of sediment loads involved in these transfers. In this study, we analyse erosion and sediment transport in two sectors of the Pénestin Peninsula in South Brittany using high-resolution and high-frequency drone imagery. We integrate an array of geospatial datasets to perform a detailed photogrammetric analysis of approximately 900 m of the Mine d'Or cliff. The resulting topographic and morphological differentials at the land-sea interface reveal significant erosional asymmetry between the southern and northern sectors of the beach, primarily influenced by gravitational forces and urban development at beach access points. Quantitatively, since 1952, the southern sector has experienced a higher average annual retreat compared to the northern slope. Our mapping of the dynamics of coarse and medium sand stocks along the northern coast of the peninsula, spanning the period from 2010 to 2020, indicates that approximately 32,000 m ³ of material from cliff erosion has contributed to the sediment accumulation on the shore, particularly within the Branzais Marsh. Furthermore, while 10,000 m ³ were removed from the beach due to littoral drift transport, approximately 12,000 m ³ of sand and gravel were revealed to be in transit to the northern coast. This quantitative analysis underscores the substantial impact of rocky coastal cliff erosion as a major sedimentary source in a context of limited external sediment supply. The sediment stock, made available for transit and deposition due to local hydrodynamic factors, is notably influenced by a northward longshore drift.