The assessment of quantitative risk to road users from debris flow

Abstract Background 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 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. Results The 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. Conclusions The 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.