Coupled limit equilibrium/depth-averaged approach for debris flow hazard assessment: the 2012 Te Maari avalanche

Mass flow models are widely used for hazard assessment and risk mitigation, yet their predictive reliability is constrained by uncertainties in key input parameters. This study examines how incorporating failure characteristics influences runout predictions by coupling limit-equilibrium-derived source geometries with depth-averaged, two-phase D-Claw simulations. Thirty candidate source geometries were selected based on observations from the 2012 Te Maari debris avalanche (New Zealand). The coupled approach demonstrated a good ability to back-calculate complex, channelised mass flows, with the best-performing simulations achieving a critical success index (CSI) greater than 0·6, comparable to values reported in other studies. However, the ensemble of scenarios revealed substantial variability in runout extent and model performance, largely driven by differences in source geometry, volume and alignment. The potential of the method for predicting inundation extent in small-volume scenarios was evaluated using an exceedance probability map of flow depth. An unweighted version was compared with a factor of safety (FOS)-weighted version, in which the FOS served as a proxy for relative failure probability. The FOS-weighted map provided a closer match to the observed debris avalanche extent and demonstrated the potential of this approach to incorporate epistemic uncertainty, offering a more robust framework for probabilistic hazard assessment of future debris flows.