Debris-flow risk-to-life: Preliminary screening

In many active landscapes debris-flow hazards are largely unrecognised and, as a result, developments can be sited in locations where they are unknowingly exposed to debris-flow impact, with correspondingly unrecognised risks to lives. While modelling can represent debris-flow occurrence and behaviour, it is unsuited to the routine use at local office level that is required to identify potentially unacceptable debris-flow risks to existing and proposed developments. To address this urgent and widespread need, we develop an empirical procedure for estimating the maximum probability of debris-flow occurrence at a specified location, involving (i) calculation of catchment Melton ratio R from topographic data; (ii) a well-defined linear upper-bound envelope of field relationships between catchment R and the probability of evidence for debris-flow occurrence; and (iii) an upper bound from field data relating R to the annual probability of debris-flow occurrence. We test this procedure against a model-based risk-to-life analysis for Matata, New Zealand and find satisfactory agreement. The procedure is intended to enable local officials to routinely identify locations where debris flow risk to life might exceed acceptable levels so that expert risk modelling and analysis can be utilised where required. Our data suggest that any development that can be impacted by debris flows is likely to require detailed risk assessment to ensure that risk-to-life is reduced to acceptable levels.