Comparative modelling of retrogressive landslide runout: 2D and 3D random large-deformation analyses using coupled Eulerian-Lagrangian method

• 3D CEL random analyses capture complex failure modes (lateral retrogression, asynchronous block mobilization) that 2D models miss. • 3D models predict longer runout distances (13.76 m vs 11.92 m), wider influence zones (11.35 m vs 8.73 m), and higher velocities (4.66 m/s vs 3.94 m/s) than 2D. • 3D analyses yield lower variability (COV 0.10 for runout) due to spatial averaging across slope width. • 2D models significantly underpredict near-field failure probabilities (48.8% vs 89.9% at 12 m from toe), underscoring the need for 3D probabilistic hazard assessment.