Coastal hazards from slope mass movements: Analysis and management approach on the Barlavento Coast, Algarve, Portugal
Abstract The Barlavento Coast, Algarve, Portugal, is dominated by rocky sea-cliffs, cut on Miocene calcarenites which evolves through intermittent and discontinuous events of slope mass movements, along a 46 km cliff front. Here, the main coastal geologic hazards result from the conflict between human occupation and sea-cliff recession. Most of the research on the dynamics of the cliffs has been directed to the risk with the aim of defining long term set-back lines, for a preventive planning of the cliff top occupation. Little attention has been given to the hazard associated with mass movements on bathing beaches backed by sea-cliffs. This article presents the results of a field inventory of 244 slope mass movements single events, collected in a rocky shore with tens of touristic pocket beaches, covering an nineteen year time span (1995–2014). Results show that landslides have seasonal pattern with higher incidence in the period between winter and early spring. More than 15% of movements occur during the Easter holidays (April) and 4% of landslides occur during the official bathing season (June–September). The spatial distribution of landslides shows that only 22% of the mass movements occur in capes and headlands, while 78% occur on the beaches, which demonstrates that the beaches are real hot spots of risk. Based on the size distribution of slope mass movements runout ratio (the ratio between the radius of the base of the cone of and the height of the movement) a table of levels of security and hazard on beaches was built. Security levels enable the definition of cartographic hazard areas on beaches which can be provided to the beach users on information boards at the beach entrance.