Landslide Risk Assessment In Wanzhou County, Three Gorges Reservoir

Landslide is a common geo-hazard phenomenon while the disastrous consequence caused by it is a social and economy problem that threatens safety of lives and also damages environment, property and resources. Therefore landslide hazard is composed by landside and human social and economy activities. The disaster and harm caused by landslides rank only second to earthquakes. The impact area of single landside is limited, but the frequency of occurrence is much higher than earthquake. With the population increasing and land utilization expansion in the past three decades, landslide and it’s hazard chain (such as landslide generated waves, landslide dams and landslide barrier lakes) has caused a mass of casualties and economy losses. China is a country with fragile geological environment and multiple geo-hazards. In recent years, data from China Geological Survey manifest that 20000 geo-hazards happened, results in 1200 injuries and deaths and direct economic loss reaches high as 4.143 billion RMB every year in average. In six types of geo-hazards (landslides, collapse, debris flow, surface collapse, ground fracture and surface subsidence), landslides take the highest proportion (50.6% ～86.1%) and caused averaged economic loss more than 2 billions.After 21 century, the landslide disaster prevention work faces severer situation in our country, includes obviously increasing seismic activities, extreme climate and more human activities for the urbanization development. In the severe situation, disaster prevention goal can’t be reached without early warning mechanism and risk control, for the traditional engineering treatment can hardly satisfy geo-hazard prevention requirement. Landslide risk prediction and risk control is an extension research from geology, engineering management and monitoring to risk assessment, prediction and management, which is another important method for disaster prevention and reduction. Our country has a vast area with complexes hazards and multiple disaster mechanics, while researches for landslide risk are still in a starting stage and further systematic researches need to be enhanced soon.The thesis firstly discusses landslide risk assessment and management system domestic and abroad, puts forward a series of key problems in risk evaluation like landslide classification, temporal scale and spatial scale etc. Based on that, it describes main points, techniques and methods for domestic landslide risk research. Next, it takes Wanzhou County in Three Gorges Reservoir for a case study and completes the risk tracking research on spatial scale from the whole County to urban area and to Sunjia landslide. At the same time, it considers the temporal scale for landslide risk evaluation, carries on landslide dynamic risk evaluation based on triggering factors’ return period. On account of above research, the thesis gets some achievements and conclusions as below:(1) Summarize and discuss landslide risk assessment and management system and technique processes, put forward landslide risk assessment technique processes considering temporal and spatial scale.Summarize current international mainstream, widely accepted landslide risk assessment and management system and manifest the basic concepts and frameworks of it, which is firstly proposed by landslide risk research institutes such as AGS, JTC-land so on, and been modified and improved with time. Put forward three key problems in the risk assessment (landslide classification, temporal scale and spatial scale), and deeply analyses the basic elements in it (basic data, models, key processes and temporal and spatial problems). Based on above research, the thesis puts forward landslide risk assessment technique processes considering temporal and spatial scale.(2) Comprehensively introduce the main contents of landslide risk assessment, intensively study quantitative landslide risk assessment methods in different spatial scales.Comprehensively introduce the main contents of landslide risk assessment includes basic data and landslide inventory, susceptibility evaluation, hazard evaluation, elements identification, value assessment and vulnerability evaluation etc. Based on three different spatial scales in region, specific area and individual landslide, it describes magnitude and frequency method (M-F) for landslide frequency and probability, from which it proposes the thinking of regional landslide susceptibility to hazard. For a specific area landslide hazard, it discusses rainfall influence to the slope stability considering steady flow and transient flow. For individual landslide, it mainly discusses reliability theory application in landslide failure probability and analyzes landslide run-out and Tsunami Squares numerical simulation theory.Summarize the vulnerability evaluation in different spatial scale and put forward structure vulnerability assessment considering the impulsive force between landslide and elements. For frame structure, it defines structure vulnerability as the ratio of landslide impulsive energy to structure and the maximum impulsive energy the structure can bear. The former is obtained from TS method and the latter is simulated by ANSYS modeling. For masonry structure, structure vulnerability is defined as landslide impulsive force to structure and the maximum impulsive force the structure can bear. The former is obtained from empirical equations and the latter is getting form’masonry structure design standard’.(3) Evaluate landslide susceptibility of Wanzhou County based on statistics method.Analyzing the Wanzhou County geography conditions, geo-environment, landslide types and influence factors, it choses susceptibility index factors, grades states and analyze the factors correlations. It builds up a susceptibility evaluation index system including slope angle, aspects, slope structure, lithology, geological structure, water effects and land usage, etc. The landslide susceptibility in Wanzhou County is evaluated by information value method and logistic regression method. Comparison of the two methods shows that the results are similar and the accuracy is high as to 87%. Landslide susceptibility of Wanzhou County shows:1:most high susceptibility areas are construction lands; 2:high susceptibility stratum concentrates on the second (J2s) and third ) section of mid Jurassic Shaximiao group; 3:high susceptibility area basically belongs to reservoir influence area, and the closer to river the higher susceptibility it is; 4:high susceptibility area mainly locates in the Wanzhou urban area.(4) Evaluates landslide hazard and risk in Wanzhou area considering rainfall, realizes regional landslide susceptibility to hazard evaluation research.Statistical analysis 50 years’rainfall data in Wanzhou County and the correlation between rainfall and landslide, the landslide events shows an obviously positive correlation to rainfall events for two decades from 1978-2003. Using Gumbel Distribution function to fit probability distribution curve of maximum rainfall per year, it gets the annual rainfall of return period in 5 years,10 years,20 years and 50 years. Calculates landslide spatial probability, temporal probability and hazard with different return period and it conclude that:1:the longer the return period is, the higher the hazard spatial probability is; 2:For the regional landslide hazard equals to spatial probability times temporal probability, so the longer return period is, the higher spatial probability is but at the same time the lower the temporal probability is. This lead to the annual hazard has no rules to follow; 3:in a same return period but different susceptibility level, the hazardous is apparently different and hazard in high susceptibility area is obviously higher than it in low susceptibility area. Based on above research, it realizes the whole regional landslide susceptibility to hazard evaluation, and gets the landslide hazard evaluation on 10 years temporal scale, and then completes the landslide hazard research in 10 years’ return period based on elements investigation, value estimate and vulnerability analysis.(5) Based on hydrology transient flow TRIGRS model, it completes the Wanzhou urban area landslide hazard and risk evaluation and realizes dynamic risk assessment.Statistical analysis the characteristic of landslides in urban area and take a full consideration of engineering treatment, it proposes that the accumulative landslide influence by reservoir level and rainfall is the main hazard in urban area. According to rainfall extreme value distribution theory, it gets the daily maximum rainfall,5 days maximum accumulate rainfall,10 days maximum accumulate rainfall. Calculate the dynamic hazard of accumulative landslide by TRIGRS model under above rainfall conditions with consideration of reservoir water influence and engineering treatment.(6) Analyzing the influence factors and disintegration model of Sunjia Landslide, it simulates the potential debris flow run-out by Tsunami Squares method under extreme rainfall condition. Considering the hazard intensity and elements’ resist disaster ability, it conducts quantitative or quasi-quantitative vulnerability assessment and then complete Sunjia landslide risk assessment.Field investigation provides a detailed data for analyzing Sunjia Landslide failure influence factors and disintegrate mode. The influence factors include steep topography, interbedding of sandstone and soft water proof shale, landslide toe excavation, and rainfall effects. Sunjia landslide trail fails first under heavy rainfall and push the middle and front part slides farther. Seeing from the landslide post failure distribution, it would form a debris flow under a 100 year return period rainfall. It models the Run-out of potential debris flow by Tsunami Squares method and completes the Sunjia debris flow risk evaluation.The Run-out results provide debris flow average velocity, sliding thickness and influence area, combine the elements susceptibility, it carries out quantitative and quasi-quantitative vulnerability assessment. Building vulnerability is calculated by the ratio of sliding impulsive energy to impact resistance. The quantitative risk evaluation results show that the population casualties risk under the debris flow will be 33 and the economy losses will be 26.58 million RMB.(7) Taking Wanzhou County in Three Gorges Reservoir for example, it carries out three different spatial scales risk assessment research from the whole region, to urban area and to Sunjia landslide, realizing the spatial risk location research from large region to specific area and the assessment from coarse results to fine results.Based on the proposed landslide risk assessment process, it conducts three different spatial scales risk assessment research from the whole Wanzhou County to urban area and to individual landslide. Statistical analysis is used to evaluate landslide risk in the whole county and the results show that the high risk area mainly locates in the urban area and the strip terrain on one side of Tiefeng mountain anticline. Having the whole region risk, we locate the high risk at urban area, and assess the accumulative landslide risk in this area by deterministic model considering rainfall and reservoir water fluctuates. The results shows that the urban area risk is comparatively lower than it in the whole region risk prediction. The reason lies on the neglect of the positive effects brought by engineering treatment during whole region risk assessment. The high risk area on the strip terrain on one side of Tiefeng mountain anticline locates Sunjia landslide, which would be trigger into debris flow by 100 years return period rainfall. The predicted population casualties risk will be 33 and the economy losses will be 26.58 million RMB. The thesis realizes the spatial risk location research from large region to specific area and the assessment from coarse results to fine results.