The physical vulnerability of roads to debris flow

The physical vulnerability of roads to debris flow is expressed through fragility functions that relate flow volume to damage probabilities. Fragility relations are essential components of quantitative risk assessments (QRA) and allow for the estimation of risk within a consequence-based framework. This paper describes fragility curves 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. Fragility curves have been defined for three damage states for high speed (reported herein) and for low speed roads (to be reported later) in order to cover the typical characteristics of roads vulnerable to debris flow. The probability of any given damage state being met or exceeded by a debris flow of a given volume (10m3 to 100, 000m3) was derived from the mean of the responses received. The development of the fragility curves is described and data issues discussed.