RISK ANALYSIS AND CHARACTERISTICS OF TYPICAL SMALL WATERSHED DEBRIS FLOW IN WENCHUAN EARTHQUAKE STRICKEN AREA

Yindongzi gully is a typical small watershed debris flow in earthquake stricken area where 14 debris flow events happened from 2009 rainy season beginning to the end of rainy season in 2013,which is the result of Wenchuan earthquake and subsequent heavy rainfall for several years. Therefore, it is very significantly to study the mechanism and procession of debris flow for understanding its long-term activities and riskness in earthquake disturbed area. The characteristics of material supplies and rainfall condition have been analyzed by filed investigation, experiment and collecting antecedent rainfall events reference, and calculating some parameters of debris flow, such as density, velocity, etc. Finally, the riskness of Yindongzi gully is analyzed according to the volume of material supplies presently and,providing some advices for debris flow prevention. The conclusion are that(1):After Wenchuan earthquake, Yindongzi debris flow is a frequently and diluted debris flow, and frequency is 3.5 times a year while density is 1.5~1.6tm-3.(2)During 7 years of water erosion the total volume of material supplies decreased from 35.93104m3 to 23.02104m3, and the percentage of fine particles that smaller than 10mm reduced from 27.04%to 18.22%,which indicate that Yindongzi debris flow events will be diluted debris flow type.(3)When rainfall duration more than 8 hours, the mean rainfall intensity will be 11.53mmh-1 that less than the average value of the whole Wenchuan earthquake striken area. Therefore, Yindongzi gully in high dangerous condition presently.