runoutSIM – An R package for regional debris-flow runout simulation and source-area connectivity

Regional-scale runout modelling is a critical component of landslide hazard assessment. The spatial prediction of debris-flow hazards over large regions requires the integration of source-area susceptibility with robust runout simulation. While various empirical and process-based models exist, there remains a need for flexible, cross-platform tools that integrate seamlessly with modern statistical and machine-learning workflows. We present runoutSIM, an open-source R package designed to facilitate data-driven regional runout modelling and source-area connectivity analysis.

By leveraging the R environment, commonly used for geoscientific computing and visualization, runoutSIM streamlines the transition from susceptibility mapping to runout distribution. The package implements a random-walk spreading algorithm to simulate potential runout paths, offering a statistical–physical framework to assess debris-flow spatial extent, velocity, and connectivity. Key features include the ability to estimate connectivity probability – the likelihood that a specific source area will impact downslope features of interest – and to adjust runout spatial probabilities and connectivity using spatial likelihoods from statistical and machine-learning predictions of source areas. This ensures that runout spatial and connectivity probabilities reflect the inherent variability in source-area initiation.

We demonstrate the application of runoutSIM through a case study in the central Andes of Chile, a region characterized by high-frequency debris-flow activity. The example couples machine-learning source-area prediction with optimization approaches, such as random grid search, to calibrate the runout model. The model is used to identify river-channel exposure and potential risks to water quality, highlighting the package’s utility for both spatial planning and local hazard mitigation. Overall, as a tool for applied landslide research, method development and teaching, runoutSIM aims to lower the barrier to accessing process-based models, enabling more comprehensive, source-to-impact hazard assessments. We anticipate that this open-source framework will support advances in quantitative geomorphic modelling and contribute to more reliable regional-scale debris flow risk management.