Numerical modelling of indirect landslide–infrastructure interaction under debris-flow scenarios and parametric uncertainty

Rapid debris flows represent a critical hazard for bridges and viaducts in mountainous environments, where indirect interaction mechanisms may generate significant dynamic actions on structural elements. This study investigates the indirect interaction between rapid debris-flow phenomena and an existing highway viaduct located in a landslide-prone area of northern Italy, adopting a large-deformation numerical approach. Landslide runout is simulated using the Smoothed Particle Hydrodynamics (SPH) method, explicitly accounting for rheological uncertainty through a Monte Carlo framework. Two alternative initiation scenarios, consistent with geomorphological evidence, landslide inventories and orthoimage analysis, are considered to explore different configurations of potential source areas under extreme triggering conditions. The results are analysed in terms of final deposit thicknesses, velocity fields at the instant of maximum interaction with the infrastructure, and estimated impact forces. While peak values of velocity and impact force remain of comparable magnitude between the two scenarios, the multiple-source configuration leads to a marked increase in the spatial extent of the interaction zone along the viaduct. Impact forces of the order of several hundreds of kilonewtons are obtained even for single-source scenarios, confirming the relevance of rapid debris flows for existing bridge infrastructure. The proposed framework provides physically based information to support susceptibility-oriented assessments of landslide–infrastructure interaction in data-scarce contexts and highlights the importance of accounting for spatially distributed interaction effects in the management of bridge networks exposed to rapid mass movements.