Impact of seepage on the breaching of non-cohesive landslide dams with different grain size distributions

Landslide dams often undergo seepage due to poor particle gradation and loose structure, yet most existing studies focus solely on overtopping-induced breaching mechanisms, neglecting the potential influence of pre-breaching seepage. Seepage may alter the dam's erodibility, structural stability, and material composition, thereby affecting the overtopping breaching process. Through flume experiments, this study investigates the breaching mechanisms of cohesionless landslide dams with different gradations within the same particle size range under coupled seepage-overtopping conditions. The results demonstrate that pre-breaching seepage significantly impacts breaching dynamics. Within a specific particle size range, compared to pure overtopping, seepage reduces downstream slope stability, increases material erodibility, shortens breaching duration, amplifies peak discharge, and advances the timing of peak flow. As the median particle size (D₅₀) increases, the amplification effect of seepage on peak discharge initially increases then decreases, the advancement of peak flow timing diminishes, and the breach erosion rate declines. When D₅₀ is sufficiently large, seepage has negligible effects on breach development. For smaller D₅₀, seepage markedly accelerates breach widening and deepening. Furthermore, coupled seepage-overtopping extends the downstream deposition area and exacerbates channel erosion due to differences in sediment sorting. These findings highlight the critical role of seepage in landslide dam breaching, providing a scientific basis for hazard prevention and mitigation.