Analysis of the rainfall-triggering mechanism and movement characteristics of debris flows in the southwest mountainous area

To elucidate the initiation mechanisms and movement characteristics of debris flows in the mountainous areas of southwestern China, a laboratory physical model test for rainfall-induced debris flow initiation was designed and established based on field survey data from typical debris flow gullies along the Jiuzhaigou–Mianyang Expressway. Numerical simulations were used to replicate the entire process of rainfall infiltration, initiation, and transport, clarifying regional debris flow triggering mechanisms. The results indicate that under varying rainfall intensities, the source materials exhibit progressive failure characterized by “rainwater infiltration → toe erosion → retrogressive erosion → main body collapse.” Under 10-year (P = 10%) and 20-year (P = 5%) rainfall return periods, toe erosion and retrogressive erosion dominate, while under 50-year (P = 2%) and 100-year (P = 1%) return periods, retrogressive erosion and main body collapse become more pronounced. The soil infiltration rate decreases with prolonged rainfall duration. Before reaching saturated water content (approximately 35%), toe erosion predominates, whereas after saturation, retrogressive erosion becomes dominant. Soil loss leads to a sharp decline in pore water pressure and earth pressure curves, typically occurring after 60 min of rainfall. The movement velocity of destabilized soil particles increases with slope gradient. After slope toe particles become unstable, they gradually migrate forward, and the interparticle bonding model degenerates to a linear model. Changes in porosity reflect the mesoscopic evolution process of “structural adjustment → compaction → failure and expansion.” Within 400 s after the initiation of the Tangshang Gully debris flow, most source materials reach the deposition area, forming a triangular accumulation body. During downward movement, the materials may impact bridge piers near the Muzuo Tunnel, cause riverbank sedimentation, and force channel migration toward the opposite bank, potentially triggering landslides on the opposite side. Consequently, protective measures combining check dams and deflection walls are proposed, demonstrating significant economic benefits.