| Large earthquakes can not only induce large-scale co-seismic landslide disasters and great changes in landforms,but also lead to lots of mountains being cracked and loosened,forming shattered mountains.Triggered by rainfall,earthquake,and artificial activities,these mountains are easy to be destabilized and turn into chain geological disasters.Due to its long evolution process and complex disaster model,the consensus of the disaster distribution recognition,spatial distribution,progressive failure mechanism,chain disaster model and dynamic process of earthquake shattered mountains is far beyond reached.Therefore,it is urgent to study the disaster pattern and disaster-causing mechanism of earthquake shattered mountain in the strong earthquake area.Longmenshan region,as a typical strong earthquake area around the world,has active geological structure,deep landform,broken rock mass and abundant rainfall,which provides good conditions for the development of geological disasters.This paper takes the shattered mountain of Wenchuan"5.12"earthquake as the research area.Firstly,through the methods of vegetation degradation trend analysis,multi-source and multi-phase remote sensing images,UAV aerial photography and field investigation,we systematically carried out the identification and interpretation of seismic cracked Mountains based on multi-source remote sensing and catalogued them to reveal their spatial distribution,topography and rock mass failure characteristics.Three disaster patterns of the earthquake cracked mountain were condensed.Take the Xiaogangjian landslide in Mianzhu city,Xiangbizui gully debris flow in Pingwu county and Chutougou debris flow in Wenchuan County,based on the methods of engineering geological survey,UAV mapping,mechanical calculation,indoor physical test,mechanical calculation,block discrete element simulation,hydrodynamic simulation method,the dynamic characteristics and disaster causing mechanism of three kinds of shattered mountain-induced disaster chains are deeply studied,the control effect of fracture network in shattered mountain on the initiation mode of earthquake shattered mountain is clarified,the internal mechanism of the transformation of shattered mountain related landslide to other disasters,the erosion effect and discharge amplification effect in the transformation process are clarified,and the comprehensive risk evaluation model of chain debris flow disaster is established.The research results are helpful to further understand the dynamic process of the evolution of earthquake shattered mountain induced disaster chain,and provide a scientific basis for the prevention and mitigation of the shattered mountain related disaster in strong earthquake areas.Based on the above research,the research results can be concluded as follows:(1)Based on the multi-stage remote sensing images of the strong earthquake area,the distribution map of the NDVI(normalized difference vegetation index)was calculated,and the distribution map of the potential earthquake shatterd mountain was established by combining the vegetation degradation trend analysis and the classification of the land cover map.The recognition results are manually corrected by interactive inspection methods such as historical remote sensing image interpretation,UAV aerial photography and field investigation.Finally,124 typical seismic fracture mountain disaster points in the strong earthquake area of Wenchuan earthquake were identified.Through further mining of the data,the spatial distribution,topography and deformation and damage characteristics of the earthquake cracked mountains are revealed.It was found that the distribution area of the disaster points is highly positively correlated with the PGA(peak ground acceleration)isoline,and the continuous deformation of the co-seismic landslide is the main factor.Three disaster modes of shattered mountain related disaster chain are revealed:high-position earthquake shattered mountain landslide to debris flow,high-position seismic fracture landslide to gully-erosion debris flow and multi-source high-position landslide to debris flow.(2)Taking Xiaogangjian landslide as a typical case,based on mechanical analysis,discrete fracture network model and block discrete element method,the initiation mechanism and dynamic process of debris flow in high-position shattered mountain considering random distribution of fractures are revealed.The failure mechanism of Xiaogangjian landslide under high hydrostatic pressure is revealed from the mechanical perspective.Due to the steep terrain of the source area,the rock mass will undergo gravity acceleration after failure,and then collide with the slope,break and disintegrate into smaller blocks,thus forming a large-scale debris flow.During the movement,the furthest horizontal displacement of the sliding mass is1.81 km,and the total duration is about 266 s.At the same time,the simulation reveals the influence of the friction angle of rock fractures and fracture distribution density on the initiation and dynamic characteristics of sliding mass.The larger the friction angle of the joint,the greater the friction energy consumption of the sliding body in the movement process,the smaller the maximum movement distance of the rock block,and the larger the slope angle of the conical accumulation body.With the decrease of the density of the joint surface,the size of the cut rock block increase,resulting in more rock blocks difficult to initiate,thus reducing the damage area of the slope and reducing its accumulation range.(3)Taking Xiangbizui debris flow as a typical case,this paper reveals the formation mechanism,dynamic characteristics and erosion effect of debris flow induced by high-position shattered mountain landslide.The short-term heavy rainfall is the direct cause of debris flow in Xiangbizui gully,and the gully erosion triggered by the landslide from shatterd mountain increased the runout volume of the debris flow.The dynamic process of Xiangbizui debris flow is simulated by fluid numerical simulation method.The Xiangbizui gully debris flow reached the gully mouth at about 160 s after initiation,with the maximum fluid depth of 10.34m and the maximum flow velocity of 18.29 m/s at the narrow and steep gully.After that,it began to accumulate,and the simulation results were in good agreement with the field investigated and calculated results.The difference between the measured value of the maximum mud depth and the simulated value is 6.24～12.45%,and the difference between the calculated peak velocity and the simulated value is-11.37～7.24%,indicating that the model has good reliability.During the movement of debris flow,its erosion effect on gully material sources is continuously strengthened,the maximum erosion depth is 4.86 m,and the total erosion volume is 7.38×10~4 m~3,compared with not considering erosion,the total runout volume of debris flow increases by about 2.5 times,and there is an obvious quadratic function relationship between the runout volume and the released volume.At the same time,the research also reveals the influence of the size of digital elevation model on the simulation results.(4)Taking the Chutou gully debris flow as a typical case,the disaster mechanism and dynamic characteristics of multi-material source debris flow in high-position shattered mountains are revealed.The shattered mountains are distributed at both sides of the wide and gentle gully and initiate when the debris flow occurs,which provides sufficient material source for the debris flow.Based on the numerical simulation method,the convergence process of debris flow material sources to the main gully and branch gully is revealed through the changes of discharge and flow depth.The movement process of debris flow in Chutou gully can be divided into four stages:source convergence in slope-source convergence in branch channel-debris flow in main gully-deposition.In the first stage,all material in the gully rapidly converge to the branch channel,and the overall maximum velocity is 14.96 m/s.At about 2400s,most of sources in the branch channel have converged to the main channel.At about 4800s,the material source of the branch channel converges to the main ditch,and the debris flow in the main gully is dominant at this time.Then,the average velocity of debris flow in the deposition stage is about 5～8 m/s,the total movement time is ablout 2 h and the maximum discharge in the whole process is 414.50 m~3/s.At the same time,about 4/5 of the solid matter stops at the downstream gully,which still has the possibility of large-scale debris flow under the condition of heavy rainfall.(5)The blocking effect in debris flow can be regarded as a discharge superposition effect.By superimposing the discharge curve of debris flow and the block-outburst disacharge curve of landslide dam,combined with the numerical simulation method,this paper revealed the amplification effect of the outburst of landslide dam induced by earthquake shattered mountains on the debris flow discharge.The blocking-outburst effect of landslide dam induced by the earthquake shattered mountain increases the discharge and disaster causing capacity of the debris flow.With the increase of the scale of the blocking points of the landslide dam,the deposition range of the debris flow increases significantly,and the maximum accumulation depth increases from 3.71 m to 8.74 m.At the same time,the cascade blocking of many small blocking points in the gully will also enhance the damage degree of the debris flow and cause the blockage of the Minjiang River. |
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