Study on initiation patterns and critical conditions of debris flows in typical small watersheds on the North piedmont of the Qinling Mountains

Debris flow is a typical geological hazard in the mountainous areas of the northern slope of the Qinling Mountains, and its initiation mechanism and critical conditions have distinct regional characteristics. This study comprehensively analyzed the controlling effects of factors such as rainfall intensity, slope angle, and dry density on the debris flow initiation process through field investigations and systematic indoor flume model tests. The results show that the initiation of debris flows in the study area primarily manifests in three typical patterns: steady-state liquefaction initiation, landslide-transformation initiation, and retrogressive erosion initiation. Among these, landslide-transformation initiation is most common under conditions of rainfall intensity greater than 60 mm/h and lower dry density (1.20–1.30 g/cm 3 ). The initiation time increases with increasing dry density and decreases significantly with increasing slope angle and rainfall intensity. When the slope angle exceeds 40° and the rainfall intensity reaches 90 mm/h, it is more likely to trigger large-scale debris flow disasters. The study determined the critical conditions for debris flow initiation in this region: the threshold for mass water content is 22.73%–31.21%, the threshold for cumulative rainfall is 54.75–118.05 mm, and the threshold for starting time is 48.9–131.7 min. Comparative analysis with other regions in China reveals that this critical water content is higher than that for typical debris flows in southwestern China, while the critical cumulative rainfall is lower than that on the Loess Plateau. The initiation pattern identification methods and quantitative critical indicators established in this study can provide a scientific basis for risk assessment and early warning of debris flow disasters on the northern slope of the Qinling Mountains.