Formation analysis and hazard assessment of the landslide-debris flow disaster chain in small watersheds of the reservoir type

Frequent disaster chains from landslides and debris flows in the tectonically active southwest of the Loess Plateau significantly impact local settlement safety and economic development. This paper proposes a method that integrates a hydrological stability model for landslides with FLO-2D numerical simulation to predict the reservoir type landslide-debris flow disaster chain under various rainfall conditions, based on the amplifying effects of landslides on debris flow disasters during extreme rainfall events. The results indicate that the construction of reservoir is a key factor triggering landslides. The calculated rainfall threshold for landslide reactivation ranges from 0.0–122.1 mm/d, meaning that under a 20-year return period, 98.5% (1750.0 × 10⁴ m³) of landslides will reactivate and become material sources of debris flow. Therefore, under the influence of heavy rainfall, landslides slide into the reservoir, forming debris flows, which serve as a model for the landslide-debris flow disaster chain evolution. Simulation results for the Yuling Gully debris flow under different return periods indicate that the volume of debris flow under a 100-year return period is equivalent to the sum of volumes under both 20-year and 50-year return periods, while the area of high-hazard areas is 2.7 times greater than that under a 20-year return period. Therefore, it is crucial to emphasize the investigation of debris flow disaster chains in small watersheds that contain reservoirs, as well as to enhance disaster prevention and early warning systems to ensure public safety.