Two-Way 1D–2D Numerical Coupling for Process-Based Simulation of Landslide Dam Breach, Flood Inundation and Sediment Hazards

Landslide dam failures pose risks to downstream areas. However, models often overlook backwater and sediment hazards. This study created a two-way coupled 1D–2D hydro-morphodynamic framework to analyze breach evolution, flooding, and sediment movement, which was validated using the 2018 Baige failures (Jinsha River, China). Near-field breach processes use the 1D Saint-Venant and Exner equations, considering erodibility, armoring, and fractional sediment transport, whereas far-field routing uses a 2D shallow-water model on an unstructured mesh (500,000 cells). The master controller exchanges the discharge, sediment, and water levels every second. The calibration of the first event (NSE=0.92) and validation of the second showed similar peak outflow (30,176 vs. 31,000 m³/s; −2.7%) and breach geometry (top width −0.5%). Two-way coupling simulated a 15 m backwater at peak, reducing the gradient by ~20%, peak discharge by 3%, and final breach width by 9% compared with one-way coupling. Downstream routing matched peaks (e.g., 23,150 vs. 22,800 m³/s; +1.5%) and inundation (87.4% fit). The model captures scour (up to 12 m), aggradation (3–6 m), and net deposition (41 Mm³). The coupled workflow runs in 11 h on a workstation (3.3× faster than the full-2D model). The 1D–2D framework provides a standard for hazard assessments in confined valleys.