FORMATION CONDITIONS OF LOW FREQUENCY DEBRIS FLOW IN WESTERN SICHUAN MOUNTAINOUS AREA: A Case Study of Xiangzhagou Debris Flow in Jiuzhaigou County

In recent years, low-frequency debris flow disasters often occurred in western Sichuan mountainous area, causing massive loss of life and property with great harm. Through the in-depth investigation and visit to Xiangzhagou debris in Jiuzhaigou County, the authors analyze the topographic and geomorphic features of gully area, as well as material compositions and outbreak characteristics of debris flow. It is concluded that the materials of debris flow are mostly initiated from gully bed and closely related to forest cutting and wood decay in early years. Besides, concentrated heavy rainfall and steep terrain also contribute to the formation of debris flow. Comparing with the medium- and high-frequency debris flows, the low-frequency debris flow has the characteristics of being hidden, sudden and dangerous. The gully areas with a logging history and corresponding topographic conditions in western Sichuan mountainous area should be the focus of investigation and prevention of low-frequency debris flow disasters.