Individual risk evaluation for landslides: key details

Abstract

Risk-taking is an essential part of life. As individuals, we evaluate risks intuitively and often subconsciously by comparing the perceived risks with expected benefits. We do this so commonly that it passes unnoticed, like when we decide to speed home from work or go for a swim. The comparison changes, however, when one entity (such as a government) imposes a risk evaluation on another person. For example, in a quantitative risk management framework, the estimated risk is compared with a tolerable risk threshold to decide if the person is 'safe enough'. Landslide risk management methods are well established and there is consensus on tolerable life-loss risk thresholds. However, beneath this consensus lie several key details that are explored by this article, along with suggestions for refinement. Specifically, we suggest using the risk unit, micromort (one micromort equals a life loss risk of 1 in 1 million), in describing risk estimates and thresholds, to improve risk communication. For risk estimation, we provide guidance for defining and combining landslide scenarios and for recognizing where unquantified risk from low-probability/high-consequence scenarios ought to inform risk management decisions. For risk tolerance thresholds, we highlight the pitfalls of selecting unachievably low thresholds and suggest that there is no single universal threshold. Additionally, we argue that gross disproportion between costs and benefits of further risk reduction, which is integral to the As Low As Reasonably Practicable (ALARP) principle, is a commonly unachievable and counter-productive condition for risk tolerance, and other conditions centered on proportionality often apply. Finally, we provide several figures that can be used as risk communication tools, to provide context for risk estimates and risk tolerance thresholds when these values are reported to decision makers and the public.

Debris‐Flood Hazard Assessments in Steep Streams

“…While the authors believe that it is the best representation of true hazard (i.e., intensity and probability), this method may not be applicable in every jurisdiction or policy makers may also choose to rely on specific return period maps. A truncation by return period (e.g., 300 years) may underestimate hazard and risk (Strouth & McDougall, 2022) even though most life loss and economic risk is concentrated at the highest frequency at which substantial damage occurs. Contrasting this is argument is the tendency of many hydroclimatic events to increase in frequency in a changing climate which may necessitate an upward adjustment in the return period range considered for hazard and risk assessments (i.e., Jakob, 2022).…”

“…Instilling further elements of conservativism (such as using the upper error bounds of the F-M relations) does not appear to be warranted as it may result in overly conservative debris flood sediment volume estimates, hence overly conservative risk assessment results and, ultimately, possibly overly conservative (and thus expensive) mitigation design. Instead, we prefer to use best estimates as suggested, for example, by Strouth and McDougall (2022).…”

Exploratory analysis of the annual risk to life from debris flows

“…A more complex formulation would account for the reality that debris flows of different 170 magnitudes/intensities may come from the same catchment, with larger, more intense events having lower frequencies. For example, Strouth and McDougall (2022) estimate separate model parameter values for each landslide frequency-magnitude scenario, then integrate these to estimate an overall risk to life.…”

“…The lack of a quantified ARI makes accurately calculating risk difficult. In this case, "…unquantified (or ignored) risks can lead to incomplete or irrational risk management" decisions (Strouth and McDougall, 2022).…”

Risk tolerance as a complementary concept to risk perception of natural hazards: A conceptual review and application

“…However, like RP, the concept of RT has been explored within the psychological tradition of risk research (Renn, 1998a; Slovic et al., 1985) and has been found to vary substantially among individuals—often contradicting “revealed” societal thresholds (i.e., risk of death per person per year; Fell, 1993; Strouth & McDougall, 2022). This strand of RT research focuses on individual RPs with stated preferences (i.e., asking study participants), and how differing degrees of tolerance may determine mitigative behavior (Fischhoff et al., 1978; Gough, 1990; Sjöberg, 1999).…”