Debris-flow hazard assessments: a practitioner's view

Abstract

Substantial advances have been made in various aspects of debris-flow hazard and risk assessments over the past decade. These include sophisticated ways to date previous events, runout models including multi-phase flows and debris entrainment options, and applications of extreme value statistics to assemble frequency-magnitude analyses. Finally, quantitative risk management (QRM) has emerged as the most rational and defensible method to assess debris- flow risk and optimize mitigation efforts. Pertinent questions, of course, have remained the same: How often, how big, how fast, how deep, how intense, how far and how bad? Similarly, while major life loss attributable to debris flows can often, but not always, be avoided in developed nations, debris flows remain one of the principal geophysical killers in mountainous terrains. Substantial differences persist between nations in hazard or risk management. Some rely on a design magnitude associated with a specific return period, others use relationships between intensity and frequency, and some allow for, but do not mandate, in-depth quantitative risk assessments. The range in return periods considered in hazard and risk assessments varies over two orders of magnitude from 1:100 to 1:10,000. In many nations, access to funding and lack of at least regional prioritization provides the biggest obstacles to widespread safeguarding against debris flows. Two factors conspire to challenge future generations of debris flow researchers, practitioners and decision makers: Population growth and climate change. The former will invariably invite continued development in debris-flow prone areas, especially fans, floodplains and terraces subject to lahars or landslide/moraine dam/glacial outburst floods which, at times, assume debris-flow characteristics. As far as debris flows are concerned, climate change is manifesting itself increasingly by augmenting hydroclimatic extremes, especially a several-fold increase in the frequency of short-duration high-intensity rainfall that may soon exceed historical precedents. While researchers will undoubtedly finesse future remote sensing, dating and runout techniques and models and bring some of those to a degree of maturity, the practitioners will need to focus on translating those advances into practical cost- efficient tools and closely collaborate with clients to integrate those tools into meaningful long-term debris-flow risk management. Future debris flow disasters will not occur due to a lack of quantitative methods, but likely due to the lack of recognition, wilful ignorance of debris-flow hazards, lack of enforcement of risk management policies, or simply a lack of means to mitigate against known debris flow risks.