Landslide dam breaching under combined hydraulic loads: Experimental insights into surge—overflow synergy

Landslide dam breaching poses severe threats to downstream communities and infrastructure, making the understanding of their failure mechanisms under complex hydraulic conditions crucial for disaster risk mitigation. This study systematically investigated the breaching hydraulics of a landslide dam under two scenarios: (1) overflow-only conditions and (2) combined surge wave and overflow conditions, through 12 controlled flume experiments (eight small-scale and four large-scale tests). Comparative analysis revealed critical differences in dam failure characteristics. Under overflow-only conditions, the breaching process followed conventional hydraulic erosion patterns, while the combined surge—overflow scenario exhibited significantly different behaviors. Experimental results showed that surge wave action increased total discharge by 2.3—4.9 times that in overflow-only conditions, and prolonged breach development. The interaction between surge waves and overflow produced complex hydrodynamics characterized by water level fluctuations, partial blockage, and reduced flow efficiency. Whether surge waves can trigger failure of a critically overtopped landslide dam depends on simultaneous satisfaction of three conditions: (1) ΔH > 0 (where ΔH is the difference between the highest surge wave level and effective dam height), (2) ξ_f/W_c > 0.092 (where ξ_f is the wave height of the first surge wave overtopping the dam and W_c is the dam crest width along the flow direction), and (3) N_E > 10.5 (where N_E is the number of effective secondary wave erosion events). The timing between surge arrival and overflow peak discharge predominantly controlled total outflow volume, with early surge-wave superposition causing the most severe discharge impacts. These findings provide a new quantitative framework for assessing landslide dam stability under complex loading conditions.