Objective The Qilian Mountains serve as a crucial ecological security barrier in western China. In recent years, frequent extreme rainfall events have made the large amounts of loosely structured, low-shear-strength slag heaps left from historical mining activities in the eastern section highly prone to instability, thereby further intensifying gully erosion and associated disaster risks, and posing a serious threat to regional ecological security. Taking the ''9·03'' 2024 slag debris flow event in Jinyue ravine, Xining, as an example, this study aims to reveal the disaster amplification effect and cascading mechanism of slag heaps on gully erosion under extreme rainfall conditions.

Methods Field investigations and laboratory experiments were conducted. The Iverson debris flow initiation model, a hydrodynamic analysis model, and the FLO-2D model were applied to perform a comparative analysis of two scenarios (with and without slag) in the Jinyue ravine of eastern section of the Qilian Mountains, and to explore their respective disaster processes. Combined with multiple typical mining area cases in the eastern section of the Qilian Mountains, the cascading disaster amplification mechanism induced by slag heaps in this region was revealed.

Results The waste silica-mine slag heap in Jinyue ravine was characterized by a loose structure, low shear strength, and a low safety factor. Under the extreme rainfall event, the slag heap became unstable and failed, resulting in a landslide. The landslide mass subsequently entered the channel, constricting the flood conveyance pathway and supplying a large amount of loose sediment source for the debris flow. Consequently, the bulk density of the debris flow increased by approximately 20%, transforming from a diluted debris flow under natural conditions into a sub-viscous debris flow with greater destructive power. The combined changes in topography and sediment source jointly increased debris flow intensity. Flow velocity increased by 43% and peak discharge by 59% compared to the natural state. Overall, the debris flow inundation area expanded, and high-hazard zones became dominant.

Conclusions Under extreme rainfall conditions, slag heaps amplify the disaster impact of the ''slag heap instability–erosion amplification–disaster chain development'' process in ravines. Targeted optimization of slag heap management and strengthened debris flow risk prevention and control are recommended, thereby providing a theoretical basis for disaster prevention and ecological security barrier construction in the eastern section of the Qilian Mountains and other regions with similar geological and climatic backgrounds.