Initiating Mechanism Analysis And Risk Assessment On De-bris Flow In Earthquake Stricken Areas

Initiating Mechanism Analysis and Risk Assessment on Debris Flow in Earth-quake Stricken Areas China is one of the few countries there are the worst earthquakes in the world. On 12 May 2008, at Wenchuan County in Sichuan Province, the Richter 8.0 earthquake occurs. Not only did it cause heavy casualties and huge property losses, but also serious damage on the ecological environment there. By this earthquake nu-merous landslides, avalanches, and potentially unstable slopes and other adverse geo-logical bodies develop gradually, at the meantime provided favorable conditions for the occurrence of debris flow in earthquake stricken area. In sight of the large number of people threatened by debris flow, it will result in significant casualties and economic losses once the debris flow breaks out, which impact the reconstruction and economic development of mountainous areas, so the Initiating Mechanism analysis on debris flow is becoming increasingly important.In the night of August 13, 2010, the major region of the Wenchuan earthquake: Beichuan, Guangyuan, Wenchuan, Yingxiu and other places, broke a regional debris flow, which resulted in heavy casualties, thousands of acres of farmland, housing and engineering facilities being washed away and buried. The debris flow in the earthquake region is very large, and the destructive power is so strong that can not be compared by other kinds of debris flow. Through this regional debris flow we can see that the "5.12" Wenchuan earthquake debris flow has entered a new active period. In the next period of time, debris will be the major geological disasters in the earthquake region. The big problem we're facing in the post-disaster reconstruction is that in what condition will the debris flow In the earthquake region (that have occurred or potential) start. So to carry out the research in the trigger mechanism of the debris flow in the earthquake region has the important theoretical significance and practical significance. Given the current domestic research in the trigger mechanism of the debris flow in the earthquake is relatively few, this paper, by means of researching on the 14 debris flow on the highway parts in the region of the "5.12" Wenchuan earthquake, combined the investi-gation with the high-resolution remote sensing image data, based on research of the debris flow of the regional geological setting, the geological environmental conditions, the terrain conditions of the debris flow, the loose solid material conditions, water con-ditions and the relationship of human activities, carried out a systematic study on the trigger mechanism of debris flow in the earthquake region. And the main results are as follows:(1) Debris flows in the study area are mostly presented in type of storm-valley debris flow ,largely in medium and large scale . Affected by the earthquake, the to-pography and hydrometeorological conditions, the impact is characterized as variety of formation simultaneousness, poor grain sorting and so on.(2)The formation of debris flow in the study area have the following three conditions: the rich solid material conditions; the favorable terrain conditions and fa-vorable hydrodynamic conditions.①The rich solid material conditions: The material sources of the debris flow in the study area were mainly generated by erosion, and which can be divided into two categories. One is the centralized material source, mainly by the gravitational deposits, the gully deposits, and the old debris flow depos-its formed, which is characterized by the distribution of concentration, the larger num-ber, the high speed of supply in debris flow. Based on the research on the debris flow which has already occurred, the supply usually occurs on the beginning of the debris flow. The other one is the distributed material source. As the study area in the earth-quake zone, the mountain was mostly squeezed a bit more broken, a lot of cracks oc-curred in the mountain, and the basin of rock and soil which for many years suffered from water erosion formed the source material, and which is characterized by the gen-eral distribution area, the source material for long-term slow in the process, added gradually into the formation.②Favorable terrain conditions: One is that the study area is in the mountain valley. Through the terrain conditions, the favorable topography can provide the potential energy for the debris flow, which gives the energy which is needed for the erosion, transportation and accumulation; the second is that in the evo-lution stage of the slope or channel in the researching region. It provides a sufficient number of substances (water and soil).③The third is the favorable hydrodynamic conditions: if in the same area or the same debris flow, in the rich source material and favorable terrain conditions, if there is a lot of raining participation, it is most likely to have a landslides. The main annual rainfall in the study area is above 1000 mm, which has already surpassed some of our semi-arid and sub-humid areas where the activity of the debris flow is strong. Coupled with the characteristics of the concentration of the rainfall and the heavy rainstorms, which indicates that most of the rainy season pre-cipitation in the study area has meet the demand for water of the strong activity of the debris flow.(3) Through artificial rainfall experiment in different conditions, the real situa-tions of the study area are simulated. Analysis shows that:①In different particle size distribution, the content of more than 50% of whole weight for the 5-2mm diameter of the fine group of soil slippery, and start the body most likely runoff; Large size or small size is too small and too much content are not conducive to large-scale soil slippery and start production flow.②In different slope angles, disasters are more easily oc-curred in greater slope.③In different rainfall, if the total rainfall remains unchanged, discontinuous rainfall is more prone to incur mudslides. This precipitation experiment shows that area contains sediments mainly in the 5-2mm diameter fine-grained soil groups, with steep slope during the discontinuous raining, is mostly like to break out debris flow disaster, while most of the region of this study are meet this standard, which can be drawn from the study, this area is in the high risk of debris flow.(4)Of the whole region ,by using the gray relational method to conduct an over-all evaluation of the study area showed that the debris flow , at the integrated river ba-sin area, sediment supply length ratio, the main channel length, cut density, rainfall conditions and other factors, only to achieve an very combination of these factors will incur the debris flow. This comprehensive study contains14 debris flow gullies, based on gray correlation method, evaluated risk level of each gully, provided some useful references for analyzing and evaluation of the debris flow, especially after the earth-quake.