Abstract: The massive debris flows of the Minjiang River small watershed triggered by persistent rainstorm blocked Minjiang River, destroyed road and affected residents, causing enormous economic loss and casualties on July 10, 2013 in Miansi town, Wenchuan county. In this paper, the risks of debris flows in eight main debris flow gullies were assessed via fuzzy comprehensive evaluation based on field investigation, remote sensed imagery interpretation and terrain data processing. Wayao gully,Xindian gully and Sucun gully are of low risk;Anjia gully, Caopomozi gully and Huaxi gully are of medium risk, Daxi gully and Cutou gully are of high risk. Furthermore, Cutou gully which is of high risk was simulated under the actual rainfall frequency using FLO-2D. The outbreak scale of debris flow and the risk characteristics of deposition fan were obtained. Finally, the simulation results were verified, and hazard zoning map was made through the risk assessment. According to statistical calculation, the area of high risk zone accounted for 61% of the deposition fan area of Cutou gully, medium risk zone 19%, and low risk zone 20%. The risk assessment in the present paper offers a reliable reference for debris flow hazard assessment, disaster forecasting, engineering treatment and hazard zone partition in the study area.

Key words: debris flow, Miansi town, fuzzy comprehensive evaluation method, numerical simulation, hazard assessment