Research On Formation And Transformation Mechanism Of Landslide-induced Debris Flow Disaster Chain

The landslides is one of the most common geological hazards in the alpine and gorge areas,where the geological structure is active,the terrain changes greatly,the rock mass is severely damaged,and the water and heat conditions are complex.Recently,an increasing number of cases have shown that landslides may transform into debris flows under certain conditions(such as Wulipo landslide-induced debris flow in Dujiangyan,Sichuan;Chamoli glacial avalancheinduced debris flow in India).The transformation of disaster has significantly changed the disaster dynamics,significantly increasing the affected area and destructive capacity,which seriously threatens the major infrastructure projects and the safety of people’s lives.Carrying out research on the evolution process and formation mechanism of the landslide-debris flow disaster chain is of great significance to guarantee the development of social and economic of mountainous areas,and it is also a scientific premise for developing the theory research in complex valley disaster chain generation.This study takes the landslide-debris flow disaster chain as the research object and establishes a database of landslide-debris flow disaster chains based on 57 typical cases.We analyze the generating conditions of landslide-debris flow and summarize the evolution models of seven landslide-debris flow disaster chains.The two key physical processes of the formation of the landslide-debris flow disaster chain have been developed,and their internal physical mechanisms were further explored by both physical and numerical experiments.Based on the above research results,a numerical model of the landslide-debris flow disaster chain evolution process was developed,which considers the phase change of the flow regime.The main results obtained in this research are as follows:(1)A database of 57 landslide-debris flow disaster chains was established,and their spatial distribution,triggering factors,topography,solid materials,erosion,liquid materials,volume,altitude,runout distance,and apparent friction angle was discussed.The results indicate the transformation conditions of landslide-induced debris flow including both the solid materials,liquid material,and topographical conditions.Attention should be paid to both the initial state and triggering factors when considering disaster conversion.(2)According to the evolution process,the landslide-debris flow disaster chain can be divided into two categories: direct conversion and indirect conversion.The direct conversion type can be further divided into four categories based on their physical mechanism: Ⅰ.movement conversion mode,Ⅱ.water source replenishment mode,Ⅲ.impacting liquefaction mode,andⅣ.shearing liquefaction model.The indirect transformation type can be divided into three categories: Ⅰ.landslide surface erosion model,Ⅱ.landslide dam failure model,and Ⅲ.landslide lateral erosion model.All these seven models include two key physical processes of landslide fragmentation and water-soil coupling.(3)To explore the influence of the fragmentation process on the formation of the disaster chain,a flume experiment was carried out,and the results indicate that both the decrease in particle size and the increase in the slope would enhance the mobility of the landslide.The particle size variety caused by internal shear changes the spatial distribution of resistance,making the landslide transition from sliding to flowing.The physical mechanism was further revealed by ring shear tests based on the Discrete Element Method(DEM).The results showed that the particle size was vertically stratified under the high-speed shearing,and the fine particles were enriched in the sliding zone,which enhanced the fluidity of the landslide.The steady-state friction coefficient decreases with the increase of particle fragmentation and shear rate.The steady-state friction coefficient expression,which is logarithmically related to the inertia number,is further proposed to describe the resistance weakening effect caused by the disintegration of the landslide in the process of landslide-debris flow transformation.(4)To explore the effect of water on the transformation process,The two-phase collapse experiment was carried out and the results show that the state of the landslide can be divided into five states: granular state,cemented state,plastic state,fluid-plastic state,and fluid state with the increase of water content.Adding moisture before saturates will significantly improve the soil strength;continuing to add moisture will significantly improve soil mobility.To explore its physical mechanism,numerical experiments based on CFDEM coupling were carried out.The results show that viscosity and solid concentration will significantly affect the movement regime of the disaster mass.The Stokes number St and density ratio r can capture the landslidedebris flow transformation process.In the starting stage,the Stokes number is low,indicating that it is in the landslide stage dominated by viscous force;in the moving stage,the Stokes number is large,indicating that it is in the debris flow movement stage dominated by inertial force.(5)Based on the Voellmy model,a Voellmy-D model considering fluid regime transition is proposed and applied to the shallow water equation based on depth integration method.Taking the Lagang landslide and Wangcang landslide-debris flow disaster chain as examples,the key parameters of the model are calibrated,and the accuracy and applicability of the model are discussed.The results show that the Voellmy-D model is more realistic for the simulation of the start-up stage of the landslide.The μ(K)model can reflect the resistance weakening and the regime transition process of the two physical processes of landslide disintegration and water-soil coupling,and the key parameter λ can better describe the impact of landslide disintegration.In this study,the landslide-debris flow hazard chain has been preliminarily explored from the aspects of disaster-pregnancy background,evolution process,and physical mechanism,which may improve the existing theory of formation and transformation of the landslide-debris flow hazard chain.The Voellmy-D resistance model proposed in this paper may further provide a theoretical basis for the risk assessment of the valley disaster chain.