An expert judgement approach to determining the physical vulnerability of roads to debris flow

The physical vulnerability of roads to debris flow may be expressed through fragility functions that relate flow volume to damage probabilities. Fragility relationships are essential components of quantitative risk assessments as they allow for the estimation of risk within a consequence-based framework. To the best of the authors’ knowledge, this is the first time that fragility curves have been produced in order to provide the conditional probability for a road to be in, or to exceed, a certain damage state for a given debris flow volume. Preliminary assessments were undertaken by means of a detailed questionnaire. A total of 47 returns were received from experts in 17 countries: 32 % academia, 51 % the commercial sector and 17 % governments. Fragility curves have been defined for three damage states (limited damage, serious damage and destroyed) for each of low-speed and high-speed roads in order to cover the typical characteristics of roads vulnerable to debris flow. The probability of any given damage state being reached or exceeded by a debris flow of a given volume (10–100,000 m³) was derived from the mean of the responses received. Inevitably there was a degree of scatter in the results, and the treatment of such variation, or ‘experimental errors’, was crucial to understanding the data and developing the fragility curves. Fragility curves are quantitative expressions of vulnerability. The method adopted is based upon qualitative, expert judgment of quantitative probabilities. In addition to an assessment of the probabilities of given damage states being exceeded, respondents to the questionnaire were polled as to their level of experience and confidence in their ability to provide a valid and coherent set of answers to the questions posed. The development of the fragility curves and their validation are described in the paper.