A process-based framework for regional landslide debris inundation hazard assessment: application to the West Coast region of New Zealand

Kerry Leith , Chris Massey, Biljana Lukovic, Volker Wichmann

Regional landslide debris inundation hazard assessments commonly rely on scenario-based formulations that require explicit enumeration of discrete landslide events. While effective for local studies, these approaches are computationally intensive, sensitive to scenario design choices, and difficult to apply consistently at regional scales where landslide catalogues are incomplete and long-term behaviour is poorly constrained. In parallel, geomorphological models provide insight into landslide-driven sediment production over long timescales, but are rarely formulated to generate spatially explicit hazard metrics relevant to runout and inundation.
Here we present the Landslide Source-to-Inundation Process Model (LS-IPM), a reduced-complexity, process-based framework that reframes landslide debris inundation hazard as a flux-based intensity problem. Rather than simulating discrete landslide scenarios, LS-IPM couples spatially distributed landslide sediment production to mass-conservative, topography-controlled debris routing, producing continuous fields of landslide-derived debris flux. Landslide magnitude and frequency are implicitly linked through a constraint on sediment production, allowing hazard to be evaluated everywhere in the landscape without explicit specification of individual failure events.
We apply the LS-IPM to the ~25,000 km² West Coast region of New Zealand at 25 m resolution, generating hazard estimates for present-day conditions, future climate scenarios (RCP6, 2081–2100), and a scenario Alpine Fault Mw 8.0 earthquake. Results show highest debris flux on steep slopes, at the base of high-relief landforms, and along convergent topography, consistent with geomorphological expectations. Approximately 80% of the region is affected by some degree of landslide debris inundation hazard, with future climate conditions expected to increase mean annual debris flux by ~50%.
By integrating geomorphological constraints with reduced-complexity runout modelling, the LS-IPM provides a scalable framework for regional landslide debris inundation hazard assessment that is compatible with subsequent probabilistic risk analysis.