Hazard Assessment On Rainstorm Induced Debris Flows In Beichuan County Of Wenchuan Earthquake Affected Area

China is one of the few countries in the world which suffer most serious geological disasters, and landslides, Debris flows, landslips and other geological disasters wreak havoc in the western region. Debris flow, as a major disaster in the mountain, has a great impact on the normal life of mountain people. Due to the large number of the crowd, as soon as an outbreak of Debris flow in a mountain town , it will result in significant casualties, economic losses, destruction of urban functions, and people's safety and national property will be in danger, restricting the mountain-building and economic development. Therefore,Debris flow research and disaster prevention and reduction are becoming more and more important. We must strengthen the research, assessment, forecasting of Debris flow and early warning for disaster reduction; organize and implement economic, effective and reasonable prevention, in order to prevent the occurrence of disasters and to reduce disaster losses to a minimum as far as possible.In order to organize the implementation of Debris flow disaster prevention and disaster mitigation projects more scientific and reasonable, the most important task is having a basic knowledge and evaluation of the possibility, risk, endanger scope and extent of harm. Rapid reflection of Debris flow activity in the status and its development trends, provides an important scientific basis to guide the decision-making of urban disaster relief and mitigation of Debris flow.5.12 Wenchuan earthquake in Beichuan caused so serious injuries that the old county town almost completely destroyed. Accumulation of a large number of loose material caused by the earthquake has provided good condition for the occurrence of secondary disasters. A heavy rain on September 23, 2008 made the county which just suffered a major earthquake in trouble again. 214 geological disasters, including 72 landslides will undoubtedly make people who are in the process of rebuilding their homes in Beichuan in another dangerous situation. Therefore, this article makes a hazard assessment of the Debris flow in Beichuan County, the evaluation model uses the particular combination mode. On one hand, use aviation disaster image maps to analysis the developing characteristics of 72 hardest hit Debris flow ditch on the 5.12 earthquake in Beichuan County, and access to of the evaluation indicators of the Debris flow risk factors, make the sensitivity analysis of each indicators. Then make use of spatial analysis tools to pile up superimposed layers of each sensitivity factor, get the zoning map of Debris flow risk in Beichuan County. The risk area of the study will be divided into three levels, namely, high-mud-rock flow-prone areas, Debris flow-prone areas, low-mud-rock flow-prone areas. On the other hand, use a currently widely used LIU Xi-lin and Professor Tang Chuan's single-channel ofDebris flow channel, respectively evaluate the return risk period of Debris flow once in every 20 years and 50 years of each channel. At the same time, carry out a detailed research on a typical Debris flow channel of Wei Ravine area, on its formation area, circulation area and accumulation area. Through source conditions, terrain conditions and water conditions, analyze the reasons for its formation, and reached the main predisposing factors of the Debris flow.Combining the results of the regional evaluation, put forward feasible control measures for Beichuan County, and provide an effective basis for the site-selection and reconstruction of new Beichuan County. For high-risk areas, because they are near Beichuan - Yingxiu rupture zone, taking into account also the possibility of aftershocks, the area is recommended to avoid the relocation of people, and it should not be occupied. For the medium and low-risk areas, through engineering measures, biological measures and risk management measures as well as the combined effect of measures to to achieve the best effect by prevention. To strengthen safety education to the residents so that we could recognize the danger of Debris flow, may seek to minimize the risk.