Statistically based sea cliff instability hazard assessment of Burgau-Lagos coastal section (Algarve, Portugal)

Mass movements of different types and sizes are the main processes of sea cliff evolution, being a considerable source of natural hazard. To address the problem of the spatial component of sea cliff hazard assessment, i.e. the hazard susceptibility, a study was made to objectively predict the possibility of occurrence of future failures affecting areas located along the top of the sea cliffs. The study was based on the application of the Information Value Method, a bi-variate statistical method, using a set of predisposing factors for cliff failures, mainly related with geology (lithology, structure, faults) and geomorphology (maximum and mean slope, height, aspect, curvature, toe protection) which were correlated with a photogrammetry based inventory of cliff failures occurred in a 60 years period (1947-2007). The susceptibility model was validated against the inventory data using standard Receiver Operator Curves, and provided encouraging results, indicating that the proposed approaches are effective for objective and quantitative hazard assessment. The results obtained also stress the need for improvement of the predisposing factors to be used in this type of studies.