Experimental Investigation on Debris Flow Gully Bank Instability Triggered by Coupled Rainfall and Flood

Gully bank instability provides critical sediment supplies for debris flow hazards in alpine canyon regions. The physical processes of unconsolidated bank failure driven by coupled rainfall and flood scouring require further experimental characterization. This study performs flume model experiments under two test scenarios, that is, flood loading with and without antecedent rainfall. Macroscopic failure features and subsurface hydrological responses are monitored via a dual perspective visual system and pore‐water pressure sensors. Results indicate that flood‐driven gully bank failure manifests as typical retrogressive landslides caused by cantilever undercutting. External toe erosion destroys basal support and reshapes bank geometry, whereas flood infiltration increases pore water pressure and reduces matric suction to weaken soil shear strength. Although antecedent rainfall may degrade soil mechanical properties by raising moisture content, earlier landslide initiation and larger failure magnitude are observed under the no‐rainfall condition, with the main failure processes completing within 90 s. Rainfall drives fine particles to migrate downward and fill subsurface voids, increasing soil compactness. This improved antierosion capacity partially offsets the moisture‐induced strength loss, resulting in a smaller overall failure volume during the rainfall present test. Derived from simplified laboratory conditions, these findings offer mechanistic insights into rainfall and flood triggered gully bank instability and support hazard related research for alpine debris flow catchments.