Risk-informed design of debris-flow mitigation at Cheekeye Fan

The Cheekeye Fan, located within the District of Squamish, is prone to debris-flow hazards that pose unacceptable risk to development. This article describes how a debris-flow barrier that would protect existing and proposed development was designed to achieve locally adopted risk tolerance criteria. A risk assessment showed that the barrier should manage debris flows with volumes up to 2.8 Mm 3 (1:10 000-year return-period events) to achieve tolerable risk, and that debris flows with volumes below 0.2 Mm 3 (10–30-year events) can pass the barrier through an outlet without exceeding risk tolerance thresholds. The local government specifies that tolerable debris-flow risks be reduced “as low as reasonably practicable” (ALARP), defined in this project as the point where the cost of additional mitigation measures is grossly disproportionate to the benefits gained. By estimating the disproportionality ratio for potential auxiliary measures, this study shows that the barrier reduces risk ALARP without additional measures in place. The authors believe that new development approval on Cheekeye fan would not be possible without the risk-informed decision-making process described in this article.