Formation conditions and the disaster modes of debris flows along middle and upper reaches of the Bailongjiang River Basin

Debris flows are extremely serious along middle and upper reaches of Bailongjiang River Basin. It is of great scientific significance and practical value to study the disaster mode and hazard mode of debris flows in this area for disaster prevention, land space planning and ecological civilization construction. Based on the analysis of formation conditions of 241 debris flows in the middle and upper reaches of Bailongjiang River Basin, the disaster-forming, hazard modes and prevention of debris flows are analyzed. The results show that: (1) Debris flows are greatly affected by topography and landform, with height differences greater than 1000 m accounting for 74.3% of the total, and the longitudinal ratio of the main ditch bed being >200‰ accounting for 93.4% of the total. (2) Most of landslide sources are mainly distributed along the Pingding-Huama Fault Zone in a strip shape. Debris flows material source mainly comes from the collapse and landslide of the soft strata such as phyllite, metamorphic sandstone, residual slope deposit and Loess. The distribution of sediment source has little relationship with the slope aspect direction of underlying bedrock. (3) According to the landform characteristics, the debris flows in this area mainly include gully debris flows (69.7%) and slope debris flows (30.3%). According to the mode of solid material supply, the main debris flows disaster mode is gravity erosion supply type (71%), slope erosion supply type (10%) and gully erosion supply type (19%). (4) The main hazard modes of debris flows are burst-erosion, alluvial-silting, climbing-stacking, erosion-collapse, silting-burying, and barrier-secondary hazard. (5) The gravity erosion-recharge type debris flow is mainly to prevent and control the major disaster bodies in the gully. Slope erosion-recharge type debris flows are mainly based on blocking and drainage works. Gully erosion-recharge type debris flows are mainly based on desilting and drainage works.