Study On Source Erosion, Movement Characteristics And Risk Assessment Of Debris Flows

Inrecentyears,affectedbytheglobalclimatedramaticchangeandintenseearthquakeac tivity,thefrequencyandextentofdamageofsecondarygeologicaldisasters,suchascollapses,landslidesanddebrisflow,havebeenbecomingmoreandmoreserious.Amongthem,becaus eattheendofchainofdisasters,debrisflowhasobviouslag,andcausesaseriousnegativeimpa ctoninfrastructure,suchasvillages,roads,waterconservancyandhydropower. Plenty of landslides and collapse with high potential energy and shattered mountains have been formed and deposited in the channel and slope after Wenchuan earthquake. Under combined effect of gravity and rainfall, shattered surfaces start to extend and penetrate. Gradually they develop into a penetrating fracture face which breaks the internal structure of rock and soil. As a result, this process turns sources of the potential shattering cracks into that of new landslides so that loose source further increase. So these loose sources resulted in the phenomenon that debris flows became one of the major secondary geological disasters after earthquake. Due to the different physical properties between meizoseismal and non-seismic sources, debeis flows have these characteristics that reducing critical condition of initiating, the scale become larger and the cycle time become longer. So it is very urgent and necessary that research on the debris flows.Based on the fieldwork and existing research, this paper uses a variety of methods such as GIS, dynamics, morphology to study characteristics of debris flow in strong earthquake zone in Yingxiu, including source distribution, erosion migration and accumulation characteristics, and the danger. The study includes the following sections:(1)We collect multiphase remotesensingimage, and the use ENVI and Arc GIS software to do Geometric correction, image mosaic and enhancement process. Besides, combining field investigation with literature work, we extract remote sensing information from remotesensingimage, including spatial location of the landslide collapse, projected area, and boundary feature. What’s more, based on the actual sample volume, this paper use MATLAB programming to analysis the relationship of landslide volume and projected area by linear regression. Finally, we get a regression equation: V=0.222A1.038.(2)Basing on spatial analysis tools of Arc GIS and Globalmapper software to statistical analysis the distribution characteristics, time distribution characteristics, the formation conditions of the sources in study area. Through the statistical analysis we know that: ①According to analysis and identification of minerals and crack of granite, it indicates that the difference rebound between minerals in granite. So, it is favorite to the production and development of crack among minerals and in the inner mineral with the unloading of the confining pressure. So landslides and collapses mainly occur in granite rocks, the development degree of landslides and collapses in granite rocks is much large than sanstone, slate, phyllite and other soft rocks. ②the smaller the vegetation cover, the greater occurrence possibility of the landslide. ③the main geological environment between high incidence of geological disasters factors as: slope between 35° and 50°, aspect between 100° and 180°, elevation between 1400 m and 2800 m. ④the distribution of earthquake-triggered source had marked locked segment effects, back slope effects, distance effects, hangingwall effects and fault dislocation effects. ⑤the sources show the feature of linear distribution along the rivers.(3)Basing on Newton’s law, energy conservation law, infiltration property curve method, momentum conservation law and so on to analysis erosion of debris flows, through the analysis we know that:① this paper obtains the relationship between the depth of surface runoff and the rainfall intensity and the critical conditions of different initiation mechanisms. ②The depth of surface runoff is negatively correlated with slope gradient, the velocity of overland flows achieve maximum value when slope is 60° and the critical slope gradient of erosion estimated theoretically with its range between 42.4° and 48.1°. ③source erosion can be classified to rain splash erosion, sheet foot erosion, rill erosion and gully erosion, and obtain erosion models of each type.④channel has this process of evolution: undercutting the toe of slope→ slope collapse→ channel blockage→ dam break, meanwhile, this paper also establishes erosion models of lateral erosion.(4)Basing on analysis methods of soil-rock ratio, φ value and fratal to analysis movement and deposite characteristics. Through the analysis we know that debris flow in the overall performance show characteristics of erosion in upstream and sedimentation in downstream. Meanwhile,this paperselectssoil-rock ratio and granularity dimension value as parameters to juge erosion or sedimentation. Through the analysis of fratal, we know that: the particle size of debris flow deposits gradually attenuated at different positions and the transport-fractal model was set up.Furthermore, the grain-size fractal dimension calculation method of particles by different transportation modes was put forward.Fratal dimension can be used to react gradation characteristic, contents of coarse grain and porosity. It shows that if the value of fractal dimension increases, contents of coarse grain, permeability and the porosity will decrease and nonuniformity coefficient will increase.(5)According to the analysis of the engineering effects of debris flow by using statics, this paper establishes comptutional models of different accumulation state of debris flow. Then, according to the cross-sectional shape of channel, the channel is classified to U type channel, V type channel( on both sides of the channel is linear), V type channel( on both sides of the channel is parabola). Furthermore, accoding to the analysis of blocking ability of different accumulation state of debris flow, obtains corresponding calculation methods of accumulation of debris flow.(6)Combining the characteristics of the study area with the maturity models of risk assessment of debris flow, analysis of all the factors what has a potential impact on debris flow. Debris flow hazard assessment for debris flow scale, the frequency of debris flow as main factors, basin area, main channel length, maximum elevation difference, gully density, vegetation coverage, 8 evaluation factors of the study area as the indicators of the hazard assessment of debris flows. Furthermore, determining the weigh of evaluation index and establishing the model of the hazard assessment of debris flow by using AHP. Thus, this paper obtains debris flow hazard values under the conditions of different rainfall frequency. Then, the nonlinear regression model is established by using MATLAB and SPSS, finally get the relationship hazard value and the rainfall frequency.