Consideration of probability assessments relevant to hazard and risk for landslides

Probability of occurrence is the most important component of landslide hazard and risk and this paper outlines different approaches for its assessment. The reasons for the popularity of qualitative approaches are first outlined. Quantitative approaches can be best applied if the important influencing factors and issues are fully understood. Formal probabilistic approaches are often based on geotechnical models or on a combination of hydrological and geotechnical models. The paper also highlights the situations for which the performance function must be formulated in terms of lateral displacements rather than the conventional safety factor. Reference is then made to a procedure, based on observational data on lateral displacements, which may be used for quantitative assessment of probability and hazard of slow-moving landslides. Finally, the results of a quantitative probabilistic assessment for a landslide site (natural slope)are presented considering different scenarios and assumptions. The example highlights the changing magnitude of hazard before and after landsliding and the factors which influence this magnitude.