地基雷达干涉测量技术在滑坡灾后稳定性评估中的应用

针对2014-09-01大树场镇大面积山体滑坡灾后稳定性,从IBIS-L地基雷达形变测量原理和关键技术(步进频率连续波和合成孔径雷达)入手,给出地基InSAR数据处理流程,获得了滑坡灾后高精度、高时空分辨率的形变演化特征,测量精度达到亚mm级。地基InSAR结果表明,滑坡体滑动幅度较大的区域位于滑坡体左侧中上部(120mm)和右侧中上部(75mm)。滑动主要由堆积松散土在裂隙水、雨水等作用下造成,产生较大次生灾害的可能性较小。     

Effects of river flow velocity on the formation of landslide dams

Natural dams are formed when landslides are triggered by heavy rainfall during extreme weather events in the mountainous areas of Taiwan.During landslide debris movement, two processes occur simultaneously: the movement of landslide debris from a slope onto the riverbed and the erosion of the debris under the action of high-velocity river flow. When the rate of landslide deposition in a river channel is higher than the rate of landslide debris erosion by the river flow, the landslide forms a natural dam by blocking the river channel. In this study, the effects of the rates of river flow erosion and landslide deposition(termed the erosive capacity and depositional capacity, respectively) on the formation of natural dams are quantified using a physics-based approach and are tested using a scaled physical model.We define a dimensionless velocity index vde as the ratio between the depositional capacity of landslide debris(vd) and the erosive capacity of water flow(ve).The experimental test results show that a landslidedam forms when landslide debris moves at high velocity into a river channel where the river-flow velocity is low, that is, the dimensionless velocity index vde 54. Landslide debris will not have sufficient depositional capacity to block stream flow when the dimensionless velocity index vde 47. The depositional capacity of a landslide can be determined from the slope angle and the friction of the sliding surface, while the erosive capacity of a dam can be determined using river flow velocity and rainfall conditions. The methodology described in this paper was applied to seven landslide dams that formed in Taiwan on 8 August 2009 during Typhoon Morakot,the Tangjiashan landslide dam case, and the YingxiuWolong highway K24 landslide case. The dimensionless velocity index presented in this paper can be used before a rainstorm event occurs to determine if the formation of a landslide dam is possible.     

Influences of the Wenchuan Earthquake on sediment supply of debris flows

The 5.12 Wenchuan Earthquake and the subsequent rainstorms induced a large number of landslides, which later were transformed into debris flows. To evaluate the effect of the earthquake on the sediment supply of debris flows, eight debris flow basins near Beichuan City, Sichuan Province, China were chosen as the study area. The area variations of the debris flow source after the Wenchuan Earthquake and the subsequent rainstorm are analyzed and discussed in this paper. Interpretations of aerial photographs (after the 5.12 Wenchuan Earthquake) and SPOT5 images (after the rainstorm event of September 24, 2008) as well as field investigations were compared to identify the transformation of landslide surface in the study area, indicating that the landslide area in the eight debris flow basins significantly increased. The loose sediment area on the channel bed increased after the rainstorm event. In order to estimate the relationship of the landslide area with the rainfall intensity in different return periods, a model proposed by Uchihugi was adopted. Results show that new landslide area induced by heavy rainfall with 50-year and 100-year return period will be 0.87 km2 and 1.67 km2, respectively. The study results show the Wenchuan earthquake had particular influences on subsequent rainfall-induced debris flow occurrence.     

A catastrophic debris flow in the Wenchuan Earthquake area,July 2013: characteristics,formation, and risk reduction

In the Wenchuan Earthquake area,many co-seismic landslides formed blocking-dams in debris flow channels. This blocking and bursting of landslide dams amplifies the debris flow scale and results in severe catastrophes. The catastrophic debris flow that occurred in Qipan gully(Wenchuan,Southwest China) on July 11,2013 was caused by intense rainfall and upstream cascading bursting of landslide dams. To gain an understanding of the processes of dam bursting and subsequent debris flow scale amplification effect,we attempted to estimate the bursting debris flow peak discharges along the main gully and analyzed the scale amplification process. The results showed that the antecedent and triggering rainfalls for 11 July debris flow event were 88.0 mm and 21.6 mm,respectively. The event highlights the fact that lower rainfall intensity can trigger debris flows after the earthquake. Calculations of the debris flow peak discharge showed that the peak discharges after the dams-bursting were 1.17–1.69 times greater than the upstream peak discharge. The peak discharge at the gully outlet reached 2553 m~3/s which was amplified by 4.76 times in comparison with the initial peak discharge in the upstream. To mitigate debris flow disasters,a new drainage channel with a trapezoidal V-shaped cross section was proposed. The characteristic lengths(h1 and h2) under optimal hydraulic conditions were calculated as 4.50 m and 0.90 m,respectively.     

The formation of the Wulipo landslide and the resulting debris flow in Dujiangyan City,China

The Wulipo landslide, triggered by heavy rainfall on July 10, 2013, transformed into debris flow,resulted in the destruction of 12 houses, 44 deaths, and 117 missing. Our systematic investigation has led to the following results and to a new understanding about the formation and evolution process of this hazard. The fundamental factors of the formation of the landslide are a high-steep free surface at the front of the slide mass and the sandstone-mudstone mixed stratum structure of the slope. The inducing factor of the landslide is hydrostatic and hydrodynamic pressure change caused by heavy continuous rainfall. The geological mechanical model of the landslide can be summarized as "instability-translational slide-tension fracture-collapse" and the formation mechanism as "translational landslide induced by heavy rainfall". The total volume of the landslide is 124.6×104 m3, and 16.3% of the sliding mass was dropped down from the cliff and transformed into debris flow during the sliding process, which enlarged 46.7% of the original sliding deposit area. The final accumulation area is found to be 9.2×104 m2. The hazard is a typical example of a disaster chain involving landslide and its induced debris flow. The concealment and disaster chain effect is the main reason for the heavy damage. In future risk assessment, it is suggested to enhance the research onpotential landslide identification for weakly intercalated slopes. By considering the influence of the behaviors of landslide-induced debris flow, the disaster area could be determined more reasonably.     

Impacts of future climate change (2030-2059) on debris flow hazard: A case study in the Upper Minjiang River basin,China

An increase in extreme precipitation events due to future climate change will have a decisive influence on the formation of debris flows in earthquake-stricken areas. This paper aimed to describe the possible impacts of future climate change on debris flow hazards in the Upper Minjiang River basin in Northwest Sichuan of China, which was severely affected by the 2008 Wenchuan earthquake. The study area was divided into 1285 catchments, which were used as the basic assessment units for debris flow hazards. Based on the current understanding of the causes of debris flows, a binary logistic regression model was used to screen key factors based on local geologic, geomorphologic, soil, vegetation, and meteorological and climatic conditions. We used the weighted summation method to obtain a composite index for debris flow hazards, based on two weight allocation methods: Relative Degree Analysis and rough set theory. Our results showed that the assessment model using the rough set theory resulted in better accuracy. According to the bias corrected and downscaled daily climate model data, future annual precipitation (2030-2059) in the study area are expected to decrease, with an increasing number of heavy rainfall events. Under future climate change, areas with a high-level of debris flow hazard will be even more dangerous, and 5.9% more of the study area was categorized as having a high-level hazard. Future climate change will cause an increase in debris flow hazard levels for 128 catchments, accounting for 10.5% of the total area. In the coming few decades, attention should be paid not only to traditional areas with high-level of debris flow hazards, but also to those areas with an increased hazard level to improve their resilience to debris flow disasters.     

基于滑带完整性指标的三峡库区堆积层滑坡失稳破坏判据

滑坡稳定性评价是滑坡防治中关键问题之一, 滑坡失稳破坏判据研究可为滑坡稳定性评价提供支持。为了提高滑坡稳定性评价的精度, 以滑坡深部位移监测数据为基础的滑坡失稳破坏判据不失为一种有效方法。基于滑坡深部位移监测数据, 引入滑带完整性指标, 推导得到了滑带完整性指标与滑坡抗剪强度参数之间呈正比例关系; 运用滑坡稳定性计算方法和三峡库区堆积层滑坡简化模型, 获得了滑带完整性指标与滑坡的稳定性系数之间呈正比例关系。建立了考虑滑带完整性指标的三峡库区堆积层滑坡失稳破坏判据, 即: 当滑坡滑带土完整性指标大于滑坡滑带土完整性指标的临界值时, 滑坡处于稳定状态; 当滑坡滑带土完整性指标小于滑坡滑带土完整性指标的临界值时, 滑坡发生失稳破坏。以三峡库区典型堆积层滑坡——五尺坝滑坡为例, 通过实例分析发现该判据具备可靠性, 对堆积层滑坡适用性好。研究成果表明, 滑带完整性指标失稳破坏判据可以用于评价滑坡的稳定状态, 为滑坡失稳破坏判据研究提供了一种新思路。      

The critical rainfall characteristics for torrents and debris flows in the Wenchuan earthquake stricken area

Critical rainfall assessment is a very important tool for hazard management of torrents and debris flows in mountainous areas. The Wenchuan Earthquake 2008 caused huge casualties and property damages in the earthquake-stricken area, which also generated large quantities of loose solid materials and increased occurrence probabilities of debris flows. There is an urgent need to quantify the critical rainfall distribution in the area so that better hazard management could be planned and if real time rainfall forecast is available, torrent and debris flow early-warning could be issued in advance. This study is based on 49-year observations （1954-2003） of up to 678 torrent and debris flow events. Detailed contour maps of 1 hour and 24 hour critical rainfalls have been generated （Due to the data limitation, there was insufficient 10 minute critical rainfall to make its contour map）. Generally, the contour maps from 1 hour and 24 hours have similar patterns. Three zones with low, medium and high critical rainfalls have been identified. The characteristics of the critical rainfall zones are linked with the local vegetation cover and land forms. Further studies and observations are needed to validate the finding and improve the contour maps.     

Debris flow in metropolitan area — 2011 Seoul debris flow

A large number of debris flows occurred simultaneously at around 8:30 to 8:50 a.m. on July 27, 2011, at the center of Seoul, Korea. This area is located in the southern part of Seoul and is a densely populated district. As a result of the debris flow event, 16 people were killed, 30 houses were buried, and 116 houses were damaged around Umyeon Mountain, a relatively small mountain with a height of 312.6 m. Since the debris flow event, field investigations on the initiation and transportation zones of debris flows have been carried out. Rainfall data were collected from the automatic weather stations (AWSs) which are operated by the Korea Meteorological Administration (KMA). Video files recorded by residents were also acquired and used to analyze the flow characteristics of the debris flow. Field investigation shows that about 40 debris flows occurred around Umyeon Mountain and most of the debris flows were initiated by small slope failures. The effects of the precipitation that triggered the debris flows were analyzed as well. A landslide hazard map which considers slope gradient and aspect, strength of soil, hazard record, rainfall conditions, and vegetation, was constructed and compared with the initiation zones of debris flows.     

Numerical simulation on a tremendous debris flow caused by Typhoon Morakot in the Jiaopu Stream,Taiwan

In August 2009,Typhoon Morakot brought a large amount of rainfall with both high intensity and long duration to a vast area of Taiwan.Unfortunately,this resulted in a catastrophic landslide in Hsiaolin Village,Taiwan.Meanwhile,large amounts of landslides were formed in the Jiaopu Stream watershed near the southeast part of the Hsiaolin Village.The Hsiaolin Village access road（Provincial Highway No.21 and Bridge No.8） was completely destroyed by the landslide and consequent debris flow.The major scope of this study is to apply a debris flow model to simulate the disaster caused by the debris flow that occurred in the Jiaopu Stream during Typhoon Morakot.According to the interviews with local residents,this study applied the destruction time of Bridge No.8 and Chen＇s house to verify the numerical debris flow model.By the spatial rainfall distributions information,the numerical simulations of the debris flow are conducted in two stages.In the first stage（before the landslide-dam failure）,the elevation of the debris flow and the corresponding potential damages toward residential properties were investigated.In the second stage（after the landslidedam failure）,comparisons of simulation results between the longitudinal and cross profiles of the Jiaopu Stream were performed using topographic maps and satellite imagery.In summary,applications of the adopted numerical debris flow model have shown positive impact on supporting better understanding of the occurrence and movement of debris flow processes.     

Rainfall, landslide and debris flow intergrowth relationship in Jiangjia Ravine

Jiangjia Ravine is a world-famous debris flow valley in Dongchuan,Yunnan Province,China.Every year large numbers of landslides and collapses happened and caused enormous damages to people’s properties and lives.With longtime observation and testing in Jiangjia Ravine we had found out one kind of special landslide which had the characteristics of landslide and collapse.Landslide and collapse supplied sufficient materials for debris flow.When a debris flow broke out,some kind of intergrowth existed among rainfall,landslide and debris flow.In order to study the intergrowth and some key parameters,we carried out artificial rainfall landslide tests and model experiments to observe the phenomena such as collapse,surface slide and surface flow.By observing the experimental phenomena and monitoring water contents,the transformation process among landslide deposits and debris flow under the condition of rainfall had been analyzed.Research results revealed the relationship of this kind of intergrowth among rainfall,landslide and debris flow in Jiangjia Ravine.Meanwhile,it was found that this kind of intergrowth relationship existed only when the moisture content was in a certain range.That is,the critical state seemed to be existed in the transformation process.     

Characteristics of clustering debris flows in Wenchuan earthquake zone

Clustering debris-flow events, namely many debris flows simultaneously triggered by a regional rainstorm in a large-scale mountainous area, occurred in four regions of Wenchuan earthquake stricken areas in 2008 and 2010. The characteristics of the clustering debris flows are examined with regard to triggering rainfall, formation process, and relationship with the earthquake by field survey and remote sensing interpretation. It is found that the clustering events occurred nearly at the same time with the local peak rainstorms, and the rainfall intensity-duration bottom limit line for clustering debris flows is higher than the worldwide line. It means that more rainfall is needed for the occurrence of the clustering debris flows. Four kinds of major formation processes for these debris flows are summarized： tributary-dominated, mainstream- dominated, transformation from slope failures, and mobilization or liquefaction of landslide. The four regions has a spatial correlation with the strong- quake-influenced zone with the peak ground acceleration = 0.2 g and the seismic intensity 〉 X.     

Landslide and basin self-organized criticality in the Lushan Hot Spring area

Defining a basin under a critical state(or a self-organized criticality) that has the potential to initiate landslides,debris flows,and subsequent sediment disasters,is a key issue for disaster prevention.The Lushan Hot Spring area in Nantou County,Taiwan,suffered serious sediment disasters after typhoons Sinlaku and Jangmi in 2008,and following Typhoon Morakot in 2009.The basin’s internal slope instability after the typhoons brought rain was examined using the landslide frequency-area distribution.The critical state indices attributed to landslide frequency-area distribution are discussed and the marginally unstable characteristics of the study area indicated.The landslides were interpreted from Spot 5 images before and after disastrous events.The results of the analysis show that the power-law landslide frequency-area curves in the basin for different rainfall-induced events tend to coincide with a single line.The temporal trend of the rainfallinduced landslide frequency-area distribution shows 1/f noise and scale invariance.A trend exists for landslide frequency-area distribution in log-log space for larger landslides controlled by the historical maximum accumulated rainfall brought by typhoons.The unstable state of the basin,including landslides,breached dams,and debris flows,are parts of the basin’s self-organizing processes.The critical state of landslide frequency-area distribution could be estimated by a critical exponent of 1.0.The distribution could be used for future estimation of the potential landslide magnitude for disaster mitigation and to identify the current state of a basin for management.     

A Regional-Scale Method of Forecasting Debris Flow Events Based on Water-Soil Coupling Mechanism

A debris flow forecast model based on a water-soil coupling mechanism that takes the debrisflow watershed as a basic forecast unit was established here for the prediction of disasters at the watershed scale.This was achieved through advances in our understanding of the formation mechanism of debris flow.To expand the applicable spatial scale of this forecasting model,a method of identifying potential debris flow watersheds was used to locate areas vulnerable to debris flow within a forecast region.Using these watersheds as forecasting units and a prediction method based on the water-soil coupling mechanism,a new forecasting method of debris flow at the regional scale was established.In order to test the prediction ability of this new forecasting method,the Sichuan province,China was selected as a study zone and the large-scale debris flow disasters attributable to heavy rainfall in this region on July 9,2013 were taken as the study case.According to debris flow disaster data on July 9,2013 which were provided by the geo-environmental monitoring station of Sichuan province,there were 252 watersheds in which debris flow events actually occurred.The current model predicted that 265 watersheds were likely to experience a debris flow event.Among these,43 towns including 204 debrisflow watersheds were successfully forecasted and 24 towns including 48 watersheds failed.The false prediction rate and failure prediction rate of thisforecast model were 23% and 19%,respectively.The results show that this method is more accurate and more applicable than traditional methods.     

Forecast method of multimode system for debris flow risk assessment in Qingping Town, Sichuan Province, China

The Wenchuan Earthquake of May 12,2008 triggered large numbers of geo-hazards.The heavy rain on 13 August 2010 triggered debris flows with total volume of more than 6 million cubic meters and the debris flows destroyed 500 houses and infrastructure built after the Wenchuan Earthquake.The study area Qingping Town was located in the northwestern part of the Sichuan Basin of China,which needs the second reconstructions and the critical evaluation of debris flow.This study takes basin as the study unit and defines collapse,landslide and debris flow hazard as a geo-hazard system.A multimode system composed of principal series system and secondary parallel system were established to evaluate the hazard grade of debris flow in 138 drainage basins of Qingping Town.The evaluation result shows that 30.43% of study basins(42 basins) and 24.58% of study area,are in extremely high or high hazard grades,and both percentage of basin quantity and percentage of area in different hazard grades decrease with the increase of hazard grade.According to the geo-hazard data from the interpretation of unmanned plane image with a 0.5-m resolution and field investigation after the Wenchuan Earthquake and 8.13 Big Debris Flow,the ratio of landslides and collapses increased after the Wenchuan Earthquake and the ratios of extremely high or high hazard grades were more than moderate or low hazard grades obviously.23 geo-hazards after8.13 Big Debris Flow in Qingping town region all occurred in basins with extremely high or high hazard grades,and 9 debris flows were in basins with extremely high hazard grade.The model of multimode system for critical evaluation could forecast not only the collapse and landslide but also the debris flow precisely when the basin was taken as the study unit.     

Hybrid simulation of the initiation and runout characteristics of a catastrophic debris flow

On 13 August 2010, a catastrophic debris flow with a volume of 1.17 million m³ occurred in Xiaojiagou Ravine near Yingxiu town of Wenchuan county in Sichuan Province, China. The main source material was the landslide deposits retained in the ravine during the 2008 Wenchuan earthquake. This paper describes a two-dimensional hybrid numerical method that simulates the entire process of the debris flow from initiation to transportation and finally to deposition. The study area is discretized into a grid of square zones. A two dimensional finite difference method is then applied to simulate the rainfall-runoff and debris flow runout processes. The analysis is divided into three steps; namely, rainfall-runoff simulation, mixing water and solid materials, and debris flow runout simulation. The rainfall-runoff simulation is firstly conducted to obtain the cumulative runoff near the location of main source material and at the outlet of the first branch. The water and solid materials are then mixed to create an inflow hydrograph for the debris flow runout simulation. The occurrence time and volume of the debris flow can be estimated in this step. Finally the runout process of the debris flow is simulated. When the yield stress is high, it controls the deposition zone. When the yield stress is medium or low, both yield stress and viscosity influence the deposition zone. The flow velocity is largely influenced by the viscosity. The estimated yield stress by the equation, τ _y = ρghsin θ, and the estimated viscosity by the equation established by Bisantino et al. (2010) provide good estimates of the area of the debris flow fan and the distribution of deposition depth.     

Comparison of the entrainment rate of debris flows in distinctive triggering conditions

Debris flows can be extremely destructive because they can increase in magnitude via progressive entrainment. In this paper, a total of 18 landslide-type debris flows and 268 channelized debris flows in Wenchuan earthquake and Taiwan region, as well as other regions were collected to analyze the entrainment rate of debris flows in each triggering condition. Results show that there is a power relationship between volume of initial triggered mass and final deposited debris for landslide type debris flow. The debris flows during 2008 and 2013 in Wenchuan earthquake-region have smaller entrainment rate than that from 2001 t0 2009 in Taiwan. The entrainment rate of debris flow events from 2001 to 2009 in Taiwan shows a decaying tendency as elapsed time. Comparison of the entrainment rate in the two earthquake-hit regions with other regions proves that entrainment rate has a close relation with major sediment availability and secondary rainstorm conditions.     

四川凉山平川镇“7.14”泥石流灾害的气象成因

以盐源县的泥石流发生为例,研究了凉山地区出现局地强降雨引发的中型泥石流灾害的气象成因。运用天气学方法,使用NCEP再分析资料以及卫星云图和雷达回波等遥感资料,从环流形势、强对流不稳定、卫星云图和雷达回波图像演变特征,研究了台风登陆时凉山地区强降雨引起泥石流灾害形成的气象成因,并讨论了应用多普勒雷达回波对局地强降水及泥石流灾害短时临近监测和预报预警的思路。     

Geometrical feature analysis and disaster assessment of the Xinmo landslide based on remote sensing data

At 5:39 am on June 24, 2017, a landslide occurred in the village of Xinmo in Maoxian County, Aba Tibet and Qiang Autonomous Prefecture(Sichuan Province, Southwest China). On June 25, aerial images were acquired from an unmanned aerial vehicle(UAV), and a digital elevation model(DEM) was processed. Landslide geometrical features were then analyzed. These are the front and rear edge elevation, accumulation area and horizontal sliding distance. Then, the volume and the spatial distribution of the thickness of the deposit were calculated from the difference between the DEM available before the landslide, and the UAV-derived DEM collected after the landslide. Also, the disaster was assessed using high-resolution satellite images acquired before the landslide. These include Quick Bird, Pleiades-1 and GF-2 images with spatial resolutions of 0.65 m, 0.70 m, and 0.80 m, respectively, and the aerial images acquired from the UAV after the landslide with a spatial resolution of 0.1 m. According to the analysis, the area of the landslide was 1.62 km2, and the volume of the landslide was 7.70 ± 1.46 million m3. The average thickness of the landslide accumulation was approximately 8 m. The landslide destroyed a total of 103 buildings. The area of destroyed farmlands was 2.53 ha, and the orchard area was reduced by 28.67 ha. A 2-km section of Songpinggou River was blocked and a 2.1-km section of township road No. 104 was buried. Constrained by the terrain conditions, densely populated and more economically developed areas in the upper reaches of the Minjiang River basin are mainly located in the bottom of the valleys. This is a dangerous area regarding landslide, debris flow and flash flood events Therefore, in mountainous, high-risk disaster areas, it is important to carefully select residential sites to avoid a large number of casualties.     

Landslide inventory and susceptibility modelling using geospatial tools,in Hunza-Nagar valley,northern Pakistan

A comprehensive landslide inventory and susceptibility maps are prerequisite for developing and implementing landslide mitigation strategies. Landslide susceptibility maps for the landslides prone regions in northern Pakistan are rarely available. The Hunza-Nagar valley in northern Pakistan is known for its frequent and devastating landslides. In this paper, we have developed a landslide inventory map for Hunza-Nagar valley by using the visual interpretation of the SPOT-5 satellite imagery and mapped a total of 172 landslides. The landslide inventory was subsequently divided into modelling and validation data sets. For the development of landslide susceptibility map seven discrete landslide causative factors were correlated with the landslide inventory map using weight of evidence and frequency ratio statistical models. Four different models of conditional independence were used for the selection of landslide causative factors. The produced landslides susceptibility maps were validated by the success rate and area under curves criteria. The prediction power of the models was also validated with the prediction rate curve. The validation results shows that the success rate curves of the weight of evidence and the frequency models are 82% and 79%, respectively. The prediction accuracy results obtained from this study are 84% for weight of evidence model and 80% for the frequency ratio model. Finally, the landslide susceptibility index maps were classified into five different varying susceptibility zones. The validation and prediction result indicates that the weight of evidence and frequency ratio model are reliable to produce an accurate landslide susceptibility map, which may be helpful for landslides management strategies.