Abstract
BackgroundA methodology for the quantitative risk assessment (QRA) of the impact of debris flow in a road environment has been developed and applied to two sites that are typically subject to high frequency-low magnitude and low frequency-high magnitude events. The methodology considers the probability of an event of a typical size, and the conditional probabilities of a vehicle being affected, given an event, and of damage (fatality) occurring given that the vehicle is affected. Scenarios covering a vehicle being hit by a debris flow and of a vehicle hitting a debris flow are considered. The computed Personal Individual Risk (PIR) is used to calculate worst case fatality probabilities for commuters and logistics truck drivers. The overall risk to society is expressed both by the annual probability of fatality amongst all road users, the Potential Loss of Life (PLL), and using the F-N diagram and is used to demonstrate the effect of a programme of management and mitigation works on the societal risk at one of the sites. The authors believe that this is the first full, formal quantitative risk assessment for debris flow risk to road users.ResultsThe PIR for a single trip through the sites ranges between 1.147E-10 for the low frequency-high magnitude site and 1.583E-09 for the high frequency-low magnitude site. These figures increase to 1.248E-07 and 1.922E-06, respectively, when more frequent travellers are considered. The PLL for the two sites ranges between 2.616E-04 for the low frequency-high magnitude site and 4.083E-03 for the high frequency-low magnitude site. The F-N diagrams illustrate the Broadly Acceptable level of risk at the low frequency-high magnitude site and the partially Unacceptable level of risk at the high frequency-low magnitude site. The risk at the high frequency-low magnitude site is reduced to ALARP levels when management and mitigation measures extant as of October 2014 are considered.ConclusionsThe QRA proves an effective technique for understanding, comparing and articulating the differences in levels of risk and the temporal changes in risk at a given site as a result of landslide risk reduction activities.
Highlights
The Scottish Road Network Landslides Study (Winter et al 2005, 2006, 2009) evaluated the hazards and risks associated with landslides at a regional/national scale and identified sites most at risk, within a qualitative/semi-quantitative framework.Typically, not exclusively, as assessments move from small-scale to mediumscale to large-scale (e.g. site or Winter and Wong Geoenvironmental DisastersIn contrast with qualitative risk analysis, which yields results in terms of weighted indices, relative ranks or numerical classification, Quantitative Risk Assessment (QRA) quantifies the probability of a given level of loss and the associated uncertainties (Corominas et al 2014).while the qualitative risk assessment allowed the effective targeting of funds for management and mitigation activities, quantitative risk assessment (QRA) at the highest risk sites is intended to provide greater focus on the risks and consequent management and mitigation actions
Personal individual risk (PIR) Given the mobility of the elements at risk it is clear that the concepts of ‘Individual Risk’ (ERM 1998) and of ‘Location-specific individual risk’ (Lee and Jones 2014), representing the risk for a theoretical individual exposed to a hazard for 100% of the time (i.e. 24 h per day, 365 days per year), are not relevant to this study
As a road user on a single trip in the study area would be subjected to both risk scenarios A and B, the total Personal Individual Risk (PIR) is the sum of the PIR under both scenarios
Summary
The Scottish Road Network Landslides Study (Winter et al 2005, 2006, 2009) evaluated the hazards and risks associated with landslides at a regional/national (panScotland) scale and identified sites most at risk, within a qualitative/semi-quantitative framework.Typically, not exclusively, as assessments move from small-scale (e.g. global or continental) to mediumscale (e.g. national or regional) to large-scale (e.g. site or Winter and Wong Geoenvironmental DisastersIn contrast with qualitative risk analysis, which yields results in terms of weighted indices, relative ranks (e.g. low, moderate and high) or numerical classification, Quantitative Risk Assessment (QRA) quantifies the probability of a given level of loss and the associated uncertainties (Corominas et al 2014).while the qualitative risk assessment allowed the effective targeting of funds for management and mitigation activities, QRA at the highest risk sites is intended to provide greater focus on the risks and consequent management and mitigation actions. The Scottish Road Network Landslides Study (Winter et al 2005, 2006, 2009) evaluated the hazards and risks associated with landslides at a regional/national (panScotland) scale and identified sites most at risk, within a qualitative/semi-quantitative framework. Because QRA is an objective and, in principle, repeatable process, comparisons can be made from one location (site, region, etc.) to another (Corominas et al 2014) and costbenefit analyses can be performed to prioritise investments between risk reduction activities for different hazards (e.g. landslide, flood, road traffic collision) (Winter 2019). A methodology for the quantitative risk assessment (QRA) of the impact of debris flow in a road environment has been developed and applied to two sites that are typically subject to high frequency-low magnitude and low frequency-high magnitude events. The authors believe that this is the first full, formal quantitative risk assessment for debris flow risk to road users
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