Considering uncertainty in the Quantitative Risk Analysis process to inform decision-making for landslide risk mitigation strategies

The adoption of quantitative risk assessments (QRA) for land-slide management decision-making has increased over the last few decades, particularly when projects threaten sensitive built environments and heritage sites. The QRA process provides a quantitative estimate of the level of risk that can then be evaluated against adopted criteria for decision-making purposes regarding the need for prevention and mitigation. Although the QRA process provides for considerations of uncertainty in landslide hazard (occurrence probability, volumes, velocities, runout distances, etc.) and consequence (e.g. quantity and vulnerability of exposed population and infrastructure); The uncertainty associated with quantification in the QRA process is seldom understood or quantified.  

This presentation shares the outcome of a research project where the uncertainties associated with the QRA process were quantified in order to gain an understanding of the reliability in landslide QRA. The results are evaluated in terms of typical ranges within common risk tolerability criteria. The knowledge gained on this project was used to develop a simplified approach to consider uncertainty in QRA for practical purposes, which is illustrated for a section of highway exposed to rock fall hazards in Canmore, Alberta, Canada. The QRA was selected to inform decision-making for the selection of rock fall protection strategies at a location where environmental concerns, tourism activities, and economic activities are of significant value for the public. This significantly increased the complexity of the decision-making process, and therefore required a robust, clear approach for balancing public socio-economic expectations and safety. In the QRA process, uncertainty was associated with hazard and consequence quantification. The work elicited the plausible ranges for the input variables for risk calculation. The expected and the range in risk were calculated for the current conditions and considering the implementation of the mitigation option. The individual risk to highway users was considered low because of the limited exposure of any particular individual. The calculated current total risk (probability of fatality) was 2.9 × 10⁻⁴ with a plausible range between 2.0 × 10⁻⁵ and 5.5 × 10⁻³. The residual total risk considering implementation of the slope protection was calculated between 9.0 × 10⁻⁴ and 2.9 × 10⁻⁶, with an expected value of 4.5 × 10⁻⁵.The risk levels considering implementation of the mitigation options were evaluated against criteria previously used in Canada. These were considered an adequate balance between project costs, public safety, environmental concerns, tourism, and economic activities.