FROM HAZARD TO RISK - THE MORBID ART OF QUANTIFYING DEBRIS-FLOW RISK TO LOSS OF LIFE

Debris flows destroy property, infrastructure and kill people. Since human life is generally valued higher than property, it is arguably the primary metric in quantifying potential losses due to debris flows. Worldwide, debris flows are a curse due to their abundance, long runout, destructiveness and difficulty in their timely prediction. Moreover, the hazard is still often not well recognized or poorly quantified which results in continued heavy life loss, particularly in poorly developed nations. The perceived short-term economic benefits of urban development, combined with the capitalistic "must-grow" mantra, continue to trump prudent zoning with sterelization of those lands subject to debris flows, although expensive mitigation measures are erected where society can afford. In locations where a debris-flow hazard has been recognized and sufficiently characterized through frequency-magnitude analysis and modeling, the central question remains: "How safe is safe enough?" for those who live or wish to live in the area potentially affected by debris flows. Quantitative risk assessments can provide such answer in their evaluation stage where quantified debris flow risk to individuals or groups is compared to tolerance thresholds established in some jurisdictions. Since the hazard analysis, nor the consequence estimates, can even remotely be classified as precise, errors tend to compound unless broad and credible ranges are considered and carried through to the risk analysis. In lieu of a better, yet to be established, method in responding to the question how safe is safe, quantitative risk assessments, despite their apparent morbidity and flaws, appears to provide a reasonable framework to reduce fatalities to levels deemed tolerable by western society. Furthermore, once a local government has agreed to this approach, they are forced to decide on tolerable risk, a decision that many governments have been reluctant to make to date.