A statistical approach for sea cliff hazard and setback lines mapping in the northeast of Peniche, Portugal

Abstract

The considerable increase in the occupation of coastal areas requires strategies to conceive and apply hazard prevention and mitigation measures, especially in coastlines with sea cliffs where mass movements are more likely to occur. To predict the potential of sea cliff failures and identify the more unstable cliff sections, analytical, physical and/or statistical tools can enable the assessment of cliff failure susceptibility, which is a progress towards a better understanding of sea cliff failure hazard assessment and mapping.In this study, statistical approaches for the assessment of sea cliff failure susceptibility and definition of safety set-back lines located landwards of the cliff top crest were adopted, in the coast located to northeast of Peniche, between Baleal and the Óbidos lagoon, in the western coast of Portugal. For the susceptibility assessment, the logistic regression was used to correlate a set of predictive factors, obtained from thematic maps of geology (lithology, structure, faults), geomorphology (cliff height, slope angle, exposure, curvature, toe protection) and wave action, with an inventory of cliff failures that caused cliff top retreat, compiled from aerial photographs from 1947 to 2010. This method enabled the production of cliff failure susceptibility models using terrain units with different sizes, encompassing cliff crest length segments of 50 m, 20 m, and 10 m. The ordinal logistic regression was applied for the setback lines definition.The modeling results were analyzed using diagnostic measures and external validation, with each model being validated against the cliff instability inventory which indicated very good statistical performance and allowed the production of reliable susceptibility maps.The application of such statistical techniques enables estimation of the degree of susceptibility for cliff failures at regional scales of analysis, and assists the definition of cliff setback lines, which provide answers to planning and hazard prevention problems of coastal areas with sea cliffs.