QUANTITATIVE DEBRIS FLOW RISK ANALYSIS

Debris flow is one of the most intensively studied mass movement phenomenon, which can be attributed to abundant worldwide occurrence and high risk potential. Debris flows are often the second- or third order downstream effect of a continuum of landslide types that are transient in time and space. Due to their remote source and long travel distance (up to 105 m) they are often not recognized until disaster strikes. Once the hazard is recognized and elements at risk identified, debris flow hazard assessments are carried out by geotechnical or geological engineers, geoscientists or engineering geologists. There are a number of tool available to the practitioner in deciphering frequency-magnitude relationships, and modeling or back-calculating intensities exemplified by velocity, peak discharge or flow depth. The quality of work depends on budget constraints, the methods known and applied by the professional, as well as the professional's experience and judgment. Once the debris flow hazard is quantified, a decision needs to be made as to the type and scale of mitigation. The gap between quantifying the hazard and choosing the most appropriate mitigation strategy can be closed by quantifying and evaluating risk. However, such an approach requires that an acceptable level of risk be defined by the local approving authority or client. This paper provides case studies from Washington State, British Columbia and Argentina and proposes a standard for geo risk evaluation that could be adopted for geohazard management by most western societies. Risk should be quantified for individuals and for groups. Standards developed elsewhere suggest limits for existing development of 10-5 annual probability and 10-6 for proposed development. Group risk can be defined by F-N curves that plot the annual probability of the event against the expected number of death adjusted for temporal and spatial vulnerabilities. The proposed standardization would provide jurisdictions with a defensible tool to plan and execute debris flow mitigation without the problem of over-or under design. Following an accepted risk acceptance standard would also protect practitioners from the omnipresent danger of litigation should an event lead to excessive damage or fatalities.