Landslide debris inundation risk and uncertainty from a process-based hazard model for the West Coast region of New Zealand

Abstract. Regional landslide debris inundation risk assessments are commonly constrained by scenario-based formulations that require explicit enumeration of landslide sources and runout events, limiting scalability and introducing sensitivity to subjective modelling choices. This study presents a process-based framework for regional landslide debris inundation risk assessment that employs a flux-based hazard metric to quantify probability of inundation and mean annual property loss risk.

The approach builds on the Landslide Source-to-Inundation Process Model by coupling spatially distributed landslide sediment production with mass-conservative downslope routing, and converting resulting debris flux into probabilistic estimates of building damage. Risk is evaluated as a temporally integrated quantity arising from all feasible landslide sources, volumes, triggers, and return intervals, without reliance on discrete runout scenarios. We apply the method to the ~25,000 km² West Coast region of New Zealand. Results indicate the aggregate rainfall-induced property loss corresponds to approximately 1.6 equivalent complete residential-building losses per year under present-day conditions. Under the RCP6 2081–2100 climate scenario, rainfall-induced residential APLR increases by approximately 50 %, assuming landslide sediment production increases in proportion to rainfall-induced susceptibility.

Uncertainty is quantified through parametric sensitivity analysis, Monte Carlo simulation, and evaluation of topographic resolution effects. Combined uncertainty is estimated at approximately ±1.0 OOM at the 1-sigma-equivalent level for the regional 25 m model; a conservative ±1.25 OOM uncertainty is adopted for interpretation. This reduces substantially when higher-accuracy topography is employed. The framework provides a scalable and transparent basis for regional landslide risk analysis and supports risk-informed planning in landslide-prone regions.