A new probabilistic rockfall trajectory model incorporating fragmentation, rock mass quality and block shape variability

Rockfall fragmentation significantly influences the dynamics and hazard footprint of rockfall events, yet it remains poorly represented in most trajectory models. This study presents RockFRAG, a novel probabilistic lumped-mass trajectory model that explicitly incorporates fragmentation processes, block shape variability and rock–mass condition within an experimentally calibrated framework. The fragmentation module accounts for the influence of impact energy, block geometry and structural features, using simplified but physically informed representations of discontinuity orientation, rock–mass quality and block shape to describe their effect on fragmentation likelihood and outcomes. Fragmentation onset is governed by a probabilistic energy-based threshold corrected for shape and discontinuity orientation, while fragment-size distributions and post-impact kinematics are derived from controlled laboratory tests. As a first assessment, the model is compared against laboratory data, large-scale fragment travel distances, and shape-dependent runout from the Vallirana and Authume field experiments. These preliminary comparisons show that the model captures fragmentation probability, fragment-size statistics and the combined influence of shape and structural quality on runout, offering a robust and computationally efficient tool for simulating fragmentation-driven rockfall dynamics and rockfall hazard analysis.