Abstract
A stretch of 48 km of sea cliffs up to 120 m high in the Algarve area of Portugal exposed heavily folded Palaeozoic shales. A study has been undertaken to assess the retreat of these sea cliffs. Nine sets of aerial photographs taken between 1947 and 1991 have been assessed, particularly to determine the retreat at the top of the cliffs. The time distribution of landslides shows some relationship with the average annual rainfall, and the space distribution provides a perspective of rates and patterns of cliff retreat in the different geographic locations. The landslides were predominantly planar, the larger ones having failure surfaces with a low dip consistent with long-term displacements at near residual strength. Examples are described. Back analysis indicates an average friction angle at the time of failure of between 23 and 25°, assuming negligible cohesion.
Résumé
Des schistes argileux paléozoïques, intensément plissés, affleurent sur 48 km de falaises côtières, de 120 m de hauteur au plus, dans la région de l'Algarve au Portugal. Une étude a été engagée pour évaluer le recul de ces falaises. Neuf séries de photographies aériennes, prises entre 1947 et 1991, ont été analysées pour déterminer en particulier le recul des sommets de falaise. La répartition dans le temps des glissements de terrain montre une corrélation avec le module pluviométrique. La répartition dans l'espace permet d'envisager des vitesses et des schémas de recul des falaises dans les différents secteurs concernés. Les glissements sont principalement des glissements plans, les plus grands présentant des surfaces de rupture de faible pendage en accord avec des processus de rupture à long terme ne mobilisant pratiquement que la résistance résiduelle au cisaillement. Des exemples sont décrits. Une rétro-analyse donne un angle moyen de frottement entre 23 et 25° en supposant une cohésion négligeable.
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Acknowledgements
The author would like to thank to Dr. Michael H. de Freitas for helpful discussions in the early stages of the work and Prof. Isabel M. Almeida for the critical revision of the manuscript. Funding was partially provided by Fundação para a Ciência e Tecnologia, POCT program, co-financed by FEDER.
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Marques, F.M.S.F. Landslide activity in Upper Palaeozoic shale sea cliffs: a case study along the western coast of the Algarve (Portugal). Bull Eng Geol Environ 62, 299–313 (2003). https://doi.org/10.1007/s10064-003-0204-7
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DOI: https://doi.org/10.1007/s10064-003-0204-7