Material sources supplying debris flows in Jiangjia Gully

Debris flow susceptibility is usually evaluated through material supplies and topographic features and the formation of debris flow is ascribed to the factors over the whole valley, almost ignoring detailed distribution of material sources and variety of behaviors in different tributaries. So far, there is not a comprehensive picture of debris flow developing from source tributaries to mainstream channel. Debris flow in Jiangjia Gully (JJG) exhibits diversity of surges and vivid scenarios concerning the forming and developing mechanisms. This study takes JJG as an example to explore how the materials are distributed in the valley and how the spatial heterogeneity of material distribution and tributary evolution influence the forming of variety of debris flow surges. It is found that most materials are distributed in tributaries of active stages with evolution index between 0.55 and 0.65; the occurrence of debris flow relies more on the spatial distribution of source tributaries than on the quantity of the material. Local conditions of tributaries, such as the concentration of materials, the granular structure of soils, and the locations receiving frequent rainfalls, are the very factors governing a debris flow event. It is the spatial heterogeneity of sources and material supplies that result in the variety of debris flow surges in JJG. Similar mode is believed to occur in debris flows in other regions.