Debris-Flow Risk Assessment

Risk assessments can inform a wide variety of decisions related to debris flows, such as: Is the community safe enough? Where is mitigation needed? How much should be invested? However, risk assessments tend to be complicated. They can be structured in innumerable ways, requiring practitioners to navigate through many options, nuances, and pitfalls. This chapter is a guide for that journey, intended for both new and experienced debris-flow risk management practitioners. In it, we describe the three components (hazard, exposure, and vulnerability) that are combined to create debris-flow risk, highlighting key points and case examples. We then detail the three phases of a risk assessment: scope definition, risk analysis, and risk evaluation. In the scope definition section, we describe how to identify the most effective way to inform and communicate a decision, including identifying where risk assessments are likely to add unnecessary complexity and uncertainty rather than clarity. In the risk analysis section, we describe various methods for estimating and describing debris-flow risk, including the typical uses, advantages, and disadvantages of each method. In the risk evaluation section, we describe how risk estimates can be compared with uncertainty, available resources, and perceptions of tolerable risk to decide what actions, if any, are needed. The chapter concludes with a summary of key lessons and a list of research that is needed to improve future risk assessments.