Sea cliff instability susceptibility at regional scale: a statistically based assessment in the southern Algarve, Portugal

Abstract

Abstract. Sea cliff evolution is dominated by the occurrence of slope mass movements of different types and sizes, which are a considerable source of natural hazard, making their assessment a relevant issue in terms of human loss prevention and land use regulations. To address the assessment of the spatial component of sea cliff hazards, i.e. the susceptibility, a statistically based study was made to assess the capacity of a set of conditioning factors to express the occurrence of sea cliff failures affecting areas located along their top.The study was based on the application of the bivariate information value and multivariate logistic regression statistical methods, using a set of predisposing factors for cliff failures, mainly related to geology (lithology, bedding dip, faults) and geomorphology (maximum and mean slope, height, aspect, plan curvature, toe protection), which were correlated with a photogrammetry-based inventory of cliff failures that occurred in a 60 yr period . The susceptibility models were validated against the inventory data using standard success rate and ROC curves, and provided encouraging results, indicating that the proposed approaches are effective for susceptibility assessment. The results obtained also stress the need for improvement of the predisposing factors to be used in this type of study and the need for detailed and systematic cliff failure inventories.

Susceptibility assessment to different types of landslides in the coastal cliffs of Lourinhã (Central Portugal)

“…Terrain units were drawn along the cliff top, following 50 m divisions of the cliff top contour (smoothed with a small radius of tolerance in mixed geomorphologic controls), as suggested by Marques et al (2011Marques et al ( , 2013, with the upper and lower limits matching, respectively, the cliff top and the cliff toe; the lateral limits are approximately perpendicular to the contour lines of the cliff face (Fig. 5).…”

“…Recently, Marques et al (2011Marques et al ( , 2013 mentioned that all factors influencing the stability of coastal cliffs should be considered because they may contribute to future instability. This approach led to the adoption of methodologies that divide cliffs into terrain units.…”

Regional Scale Sea Cliff Hazard Assessment at Sintra and Cascais Counties, Western Coast of Portugal

“…Sea cliff and rock coasts have received much lower research efforts than the fast evolution sandy shorelines [11], with soft cliff being the most commonly studied, and the lower retreat rate and strong and intermediate strength cliffs receiving much less attention [12]. This is probably due to a combination of factors which include the difficulties in monitoring an episodic, comparatively low space and time frequency event-based process, located mostly in highly irregular and frequently inaccessible locations, which is poorly represented in aerial photos and maps.…”

“…In this context, it is assumed that a separation between soft cliffs, with mean cliff retreat rates typically higher than 0.1 m/year, and hard cliffs, with mean retreat rates lower than 0.1 m/year [12][13][14][15], mainly composed by rock masses where wave attack is not directly related or followed by cliff failure, and the toe protecting role of sand or shingle beaches, or abrasion platforms, may only be effective at intermediate or large time scales. In the latter, the cliff top line remains essentially unchanged at time scales in excess of half a century in aerial photographs based evolution monitoring, being only affected by localized cliff failures.…”

“…In this context, the assessment of the spatial component of sea cliff failure hazard of low retreat rate cliffs, i.e., the susceptibility of a given sector of cliff being affected by failures which cause cliff top retreat, based on the spatial predisposing factors and without implications on magnitude or time of occurrence, was performed by a statistically based study, following previous work on this topic [12,37], and applied to cliffs cut in rock masses with less contrasting strength, with the inclusion of more morphometric predisposing factors (profile curvature and standard variation of slope angle) and the mean annual wave power at the near offshore of the cliffs. In this context it is assumed that cliff evolution in the long term is the parallel retreat of the cliff face, implying that the cliff top failure is a stage of the cycle of its evolution [1,2].…”