Combined numerical investigation of the Gangda paleolandslide runout and associated dam breach flood propagation in the upper Jinsha River,SE Tibetan Plateau

A large paleolandslide occurred opposite the Gangda village in the upper Jinsha River, SE Tibetan Plateau. Field geological investigations and remote sensing indicated that the Gangda paleolandslide once blocked the Jinsha River. Evidence of river blocking, including landslide dam relics, upstream lacustrine sediments, and downstream outburst sediments, has been well preserved. To understand the river-blocking event including landslide, dam breach, and associated outburst flooding, optically stimulated luminescence (OSL) dating and numerical simulations were performed in this study. OSL dating results showed that the paleolandslide dam was formed at 5.4?±?0.5 ka BP and breached at 3.4?±?0.3 ka BP, indicating that the dam lasted approximately 2000 years. The discrete element method was used to simulate the dynamics of the Gangda rock landslide based on the restored topography, while a fluid–solid coupling model was performed to simulate the landslide dam breaching and flooding. The fluid–solid coupling model can simultaneously reflect the process of landslide-dam collapse and the propagation of outburst flood. The simulated results indicate that the whole landslide process lasted about 60 s with a peak velocity of 38 m/s. It is significant that the simulated morphology of the residual landslide dam and downstream outburst sediments is consistent with the field observations. The combined numerical investigation in this paper provided new insights into the research of landscape evolution and helped to understand the chain disaster of landslide, dam breach, and flooding.

     

金沙江“10·11”白格特大型滑坡形成机制及发展趋势初步分析

2018年10月11日,西藏昌都市江达县波罗乡白格村发生大规模滑坡,约有3165×104 m3的山体高速冲入金沙江形成堰塞坝,13日9时堰塞坝体被自然泄流冲开,堰塞湖威胁解除。11月3日,在时隔短短23 d后,该滑坡后缘约215×104 m3高位滑体再次发生滑动破坏,高速运动的滑体沿途铲刮坡体并冲入金沙江,再次形成堰塞坝。现场调查研究得出白格滑坡主要是受其后缘逆冲分支断层f₂（不整合接触面）控制,并在长期重力卸荷、降雨和地下水的反复浸润作用影响下,最终整体失稳破坏。通过对滑坡演化过程分析得出,其变形破坏过程可分为5个阶段,即:后缘蠕滑和沉降下错阶段（Ⅰ）、坡体裂缝发展、贯通阶段（Ⅱ）、整体启动"锁固端"剪断阶段（Ⅲ）、高速凌空滑跃阶段（Ⅳ）、碰撞、破碎、堆积成坝阶段（Ⅴ）。一期变形破坏机制模式可归结为蠕滑-下错-剪断-滑跃式,破坏方式表现为推移式,后期临空条件较好,破坏将以牵引式为主。在此基础上,结合残留强变形区块（K1、K2、K3）及其周边影响区形貌特征和变形迹象,对其变形破坏特征和发展趋势进行了预测分析,认为后期强变形区总体将以渐进解体方式破坏为主。     

崩塌诱发其下部滑坡变形过程的分析研究

崩塌冲击或崩积物重力加载作用都可能诱发坡脚滑坡的变形或失稳。在查明坡体结构的基础上,采用3DEC离散元数值模拟方法,对高陡斜坡在地下开采作用下崩塌所产生的机理、失稳模式、破坏规模、运动轨迹进行了全过程模拟,特别是斜坡失稳后和坡脚滑坡的相互作用效应进行了深入分析。结果表明:通过地下开采诱发的崩塌过程模拟及其研究,发现斜坡在地下开采的扰动下会产生大规模的崩塌,其滚石会对滑坡体产生强烈的冲击作用,且所形成的崩积物会对滑坡体产生重力加载作用。再通过监测数据以及现场收集的资料分析得出滑坡的蠕滑变形主要是由于崩积物重力加载作用引起的,且有继续变形的趋势,在暴雨季节时,滑坡的变形速率可能会增大,有潜在大规模滑动的危险,需做好相应的防护工作。     

中国西藏金沙江白格滑坡的地质成因分析

2018年10月11日发生的金沙江白格滑坡是中国西藏继2000年易贡滑坡以来社会影响最大的滑坡事件。许多学者对该滑坡的形成条件、稳定性和监测预警等进行了研究报道,但对滑坡的地质成因研究比较薄弱。本文在区域地质分析、现场调查测绘和综合研究的基础上,重点从断裂作用控制斜坡岩体结构、水与蚀变软岩夹层作用促进结构面弱化、卸荷作用控制滑坡规模和失稳方式等方面,提出了金沙江构造缝合带混杂岩体岸坡在持续重力作用下的失稳机理,对金沙江沿岸滑坡隐患早期识别和风险防控具有一定指导意义。     

The mechanism of a railway landslide caused by rainfall

This paper presents the study of a landslide induced by rainfall above a railway line in terms of site investigations, laboratory tests, finite element, and limit equilibrium analyses. The purpose of the study is to identify the cause and failure mechanism so as to prevent the recurrence of such failure. The slope concerned was composed of 3∼4 m thick fissured clay overlying highly weathered mica marble interbedded with quartz schist. The slope was cut into two steps and was protected by a masonry revetment and a thin masonry retaining wall at the toe. The region concerned suffered a whole month’s light drizzle in September, after which a rainfall of an intensity of 80 mm (in 48 h) occurred 5 days prior to the failure that happened on October 5, 2004. The rainfall infiltration and a failed drainage system behind the retaining wall resulted in high saturation of the clay and finally led to an undrained failure. The short-term slope stability using total stress parameters was examined by the finite element and limit equilibrium analyses. The factors of safety of the slope obtained by the finite element and limit equilibrium analysis were 0.935 and 0.854, which are in agreement with the occurrence of failure. Since the slide took place through the interface of the soil and rock, the exposure of rock has significant effect on the stability.     

Causes of catastrophic failure of Tam Pokhari moraine dam in the Mt. Everest region

The moraine dam of the Tam Pokhari glacial lake breached on 3 September 1998 and caused a catastrophic flood in the downstream areas. To learn from the event, a field survey was conducted. The survey team found that a landslide, which is considered to be responsible for the outburst flood, occurred in the northeast-facing slope of the moraine dam. The dam internal structure played a crucial role in forming a landslide that triggered the excess overflow and finally the breach of the dam. The internal structure of the dam was made of alternating layers of finer and coarser sediments inclining at 30° downstream and layers are truncated in the upslope direction by a huge pile of unconsolidated and structureless moraine materials. Since the upstream slope angle of the dam i.e., 40° is larger than the angle of repose i.e. 35° of sediments, the increased pore water pressure in the dam triggered a landslide. The rainfall and seismological activities of that particular day, which hit the record high, were crucial in triggering the failure. It is estimated that the dam’s north and northeast-facing slopes completely slid involving about 30,000 m³ of sediment mass of unconsolidated moraine materials above the shear plane. A slope stability analysis was also performed. The calculated safety factor was 0.85, and the calculated slip circle agreed with the shear plane marked in the dam. About 18 million cubic metres of water was swiftly released due to the sudden breach of the moraine dam.     

Characteristics of a rapid landsliding area along Jinsha River revealed by multi-temporal remote sensing and its risks to Sichuan-Tibet railway

The Sichuan-Tibet railway goes across the Upper Jinsha River, along which a large number of large historical landslides have occurred and dammed the river. Therefore, it is of great significance to investigate large potential landslides along the Jinsha River. In this paper, we inspect the deformation characteristics of a rapid landsliding area along the Jinsha River by using multi-temporal remote sensing, and analyzed its future development and risk to the Sichuan-Tibet railway. Surface deformations and damage features between January 2016 and October 2020 were obtained using multi-temporal InSAR and multi-temporal correlations of optical images, respectively. Deformation and failure signs obtained from the field investigation were highly consistent. Results showed that cumulative deformation of the landsliding area is more than 50 cm, and the landsliding area is undergoing an accelerated deformation stage. The external rainfall condition, water level, and water flow rate are important factors controlling the deformation. The increase of rainfall, the rise of water level, and faster flow rate will accelerate the deformation of slope. The geological conditions of the slope itself affect the deformation of landslide. Due to the enrichment of gently dipping gneiss and groundwater, the slope is more likely to slide along the slope. The Jinsha River continuously scours the concave bank of the slope, causing local collapses and forming local free surfaces. Numerical simulation results show that once the landsliding area fails, the landslide body may form a 4-km-long dammed lake, and the water level could rise about 200 m; the historic data shows that landslide dam may burst in 2–8 days after sliding. Therefore, strategies of landslide hazard mitigation in the study area should be particularly made for the coming rainy seasons to mitigate risks from the landsliding area.

     

大型反倾库岸岩体变形过程及破坏机制数值模拟

随着边坡地质结构、地层岩性和诱发因素的不同,反倾岩体的主控变形破坏模式都会发生改变。金沙江龙蟠边坡岩体主要为反倾向千板岩和砂岩互层状结构,坡脚河床深厚覆盖层构成了边坡的软弱支座。本文运用离散单元法模拟了大型反倾库岸岩体在漫长的地质历史时期中的变形演化过程,并基于反倾岩体变形的时效性观点,引入强度折减法分析边坡在不同阶段的剪切屈服区扩展情况及相应的稳定性状态。结论表明龙蟠边坡变形岩体是重力弯曲蠕变为主导的成因机制,并归纳提出了软基效应和互层效应共同作用下的大规模反倾岩体的累进性剪切破坏模式,俗称龙蟠模式。     

基于颗粒流方法的堆积层滑坡运动过程模拟

以颗粒流离散元为研究方法对勉县杨家湾十组堆积层滑坡破坏方式与运动过程进行数值模拟研究。通过PFC2D双轴模拟试验所标定的岩土体宏观模拟参数与室内试验所获取的宏观实测参数进行对比,确定堆积层滑坡所需的颗粒细观参数,然后将标定的细观参数代入堆积层滑坡模型,对滑坡破坏方式及运动过程进行模拟研究。结果表明:滑坡破坏在初始阶段蠕滑变形累积,滑坡体挤压坡脚,直至坡脚产生剪切破坏,并向上牵引发展,使得滑坡整体顺接触面破坏下滑并堆积于坡脚,表现为典型的牵引式渐进破坏,结果与实际情况基本吻合。研究认为,采用颗粒流方法对堆积层滑坡破坏与运动过程的模拟研究具有较高的适用性,对该类滑坡防治具有一定的参考意义。      

Evaluation of potential landslide damming: Case study of Urni landslide,Kinnaur, Satluj valley,India

This work aims to understand the process of potential landslide damming using slope failure mechanism,dam dimension and dam stability evaluation. The Urni landslide, situated on the right bank of the Satluj River, Himachal Pradesh(India) is taken as the case study. The Urni landslide has evolved into a complex landslide in the last two decade(2000-2016) and has dammed the Satluj River partially since year 2013,damaging ～200 m stretch of the National Highway(NH-05). The crown of the landslide exists at an altitude of ～2180-2190 m above msl, close to the Urni village that has a human population of about 500.The high resolution imagery shows ～50 m long landslide scarp and ～100 m long transverse cracks in the detached mass that implies potential for further slope failure movement. Further analysis shows that the landslide has attained an areal increase of 103,900 ± 1142 m^2 during year 2004-2016. About 86% of this areal increase occurred since year 2013. Abrupt increase in the annual mean rainfall is also observed since the year 2013. The extreme rainfall in the June, 2013; 11 June(～100 mm) and 16 June(～115 mm),are considered to be responsible for the slope failure in the Urni landslide that has partially dammed the river. The finite element modelling(FEM) based slope stability analysis revealed the shear strain in the order of 0.0-0.16 with 0.0-0.6 m total displacement in the detachment zone. Further, kinematic analysis indicated planar and wedge failure condition in the jointed rockmass. The debris flow runout simulation of the detached mass in the landslide showed a velocity of ～25 m/s with a flow height of ～15 m while it(debris flow) reaches the valley floor. Finally, it is also estimated that further slope failure may detach as much as 0.80 ±0.32 million m^3 mass that will completely dam the river to a height of 76±30 m above the river bed.     

Characteristics and formation mechanism of a catastrophic rainfall–induced rock avalanche–mud flow in Sichuan, China, 2010

A catastrophic rock avalanche–mud flow was triggered by the heavy rainfall in Sichuan, China, on July 27, 2010. A mass of strongly weathered basalts with a volume of ∼480,000 m³ was initiated from a valley side slope and then moved downstream along the valley, entraining a large amount of unconsolidated substrate and bilateral materials and colluviums. The entrainment increased the volume of slide to ∼1.0 million m³ and may also enhance the mobility of the landslide. Approximately 30 min after the first failure, the deposits of the rock avalanche in the steepest part of the valley started to creep slowly down as a mud flow. It reached a small town at the foot of the slope after several hours, causing the damage of 92 houses and the urgent evacuation of 1,500 people. The field investigation, mapping, grain size test, and aerial photo interpretation were applied to analyze the dynamic process and the formation mechanism of the landslide. The strongly weathered and fragmented basalts as well as the most vulnerable combination of joint sets were revealed to be the most contributive factors. The antecedent torrential rainfall is the direct trigger, which affected the slope stability in three aspects: induced debris flow that scoured the toe of the sliding surface of rock avalanche; caused the increase of the slope unit weight, and penetrated into the steep joints reducing the strength of the materials.     

三轴压缩蠕变试验下石英云母片岩各向异性蠕变特性研究

对石英云母片岩进行三轴压缩蠕变试验,研究丹巴水电站石英云母片岩的三轴蠕变特性及其各向异性特性。按轴向荷载与层理面关系加工成平行组和垂直组2组试件,开展分级加载方式进行不同围压条件下的蠕变试验。试验研究表明,石英云母片岩具有蠕变特性,包括瞬时变形、衰减蠕变、稳定蠕变和加速蠕变阶段;采用带Kachanov蠕变损伤律的蠕变模型来描述石英云母片岩的蠕变特性,并进行蠕变参数辨识,拟合结果显示此模型能很好地描述石英云母片岩的蠕变特性;根据试验结果获得2组试件的长期屈服强度、破坏形态、瞬时变形参数和稳定蠕变速率,分析表明石英云母片岩的蠕变力学特性具有明显的各向异性特性。层理面与轴向荷载垂直的试件较层理面与轴向荷载平行的试件的强度、弹性剪切模量、体积模量和黏性系数相对较大,表现出较高的抗变形和抗破坏能力;平行组以剪切破坏为主,垂直组破坏时出现侧向鼓胀现象,显示出延性破坏的特点;2组石英云母片岩试件的瞬时应变量和稳定蠕变速率都随着应力水平的提高而增大。     

金沙江中游巴塘县地质灾害发育特征及成灾规律分析

金沙江流域地处青藏高原东南缘,地跨我国地势一、二阶梯过渡带,地质环境条件脆弱,属地质灾害高易发区。巴塘县位于金沙江中游,目前调查发现各类地质灾害隐患点486处,以泥石流和不稳定斜坡为主。其中泥石流151处,不稳定斜坡133处,崩塌109处,滑坡93处。通过对巴塘县地质灾害详细调查与测绘,对地质灾害的发育特征、分布规律及其影响因素进行了深入研究,结果表明：（1）巴塘县地质灾害发育类型多,点多面广、密度大,分布不均衡,成条带、群片状分布;（2）地质灾害的分布与地形地貌有密切的关系。主要沿金沙江高山峡谷区及其支沟流域的深切河谷区、丘状高原与峡谷区的地形转折带集中分布。大多数的不稳定斜坡、崩塌、滑坡发育在高程2500～3500m。（3）地质灾害受地质构造控制,时空分布差异明显。地质灾害隐患点集中沿巴塘－莫西活动构造带、金沙江构造带分布。（4）不同的岩性决定了地质灾害的类型。滑坡、不稳定斜坡主要发生在第四系松散土体中,崩塌主要发育于岩浆岩、玄武岩、火山岩、细砂岩等硬岩、较硬岩岩体;软的石英片岩、绢云片岩、绿片岩为主的岩组,岩石破碎,为泥石流提供丰富的物源。     

Applying discontinuous deformation analysis to assess the constrained area of the unstable Chiu‐fen‐erh‐shan landslide slope

Chiu‐fen‐erh‐shan landslide is a remarkable slope failure occurred during the Chi‐Chi earthquake in 1999. In November of 2002, abnormal geomorphologic features, including buckling and ground subsidence, were observed on the lower slope of the Chiu‐fen‐erh‐shan landslide. This study attempts to assess the constrained area of a future collapsing on the slope using a dynamic discrete numerical analysis method, discontinuous deformation analysis (DDA). The simulation results show that the depression in front of the toe of the slope provides a space for arresting the whole sliding rocks when only the unstable lower slope fails. However, as the whole slope slides, the rock fragments move farther into the memorial park and can impact other facilities resulting in the enlarging of constrained area. The authority should prohibit people from entrancing the constrained area in the rainy season. Copyright © 2006 John Wiley & Sons, Ltd.     

An anthropogenic landslide dammed the Songmai River,a tributary of the Jinsha River in Southwestern China

On March 20, 2019, a landslide (named Yagu landslide) occurred in eastern Tibetan Plateau. It produced a 10-m-high dam, resulting in a lake on the Songmai River, a tributary of the Jinsha River. This paper describes this slope failure and analyzes the process and cause of the landsliding based on the combination of Google Earth images, PlanetScope satellite optical images, field photography and geologic data. It is speculated that this event was likely induced by local human activity, such as quarrying rather than natural factors. This example raises a challenging issue whether the ongoing projects along the Jinsha River can induce landslides. In addition, the emergency responses of the government and the effort for risk removal of the dammed lake are presented.

     

The characteristics and failure mechanism of the largest landslide triggered by the Wenchuan earthquake, May 12, 2008, China

Strong earthquakes are among the prime triggering factors of landslides. The 2008 Wenchuan earthquake (M _w = 7.9) triggered tens of thousands of landslides. Among them, the Daguangbao landslide is the largest one, which covered an area of 7.8 km² with a maximum width of 2.2 km and an estimated volume of 7.5 × 10⁸ m³. The landslide is located on the hanging wall of the seismogenic fault, the Yingxiu–Beichuan fault in Anxian town, Sichuan Province. The sliding mass travelled about 4.5 km and blocked the Huangdongzi valley, forming a landslide dam nearly 600 m high. Compared to other coseismic landslides in the study area, the Daguangbao landslide attained phenomenal kinetic energy, intense cracking, and deformation, exposing a 1-km long head scarp in the rear of the landslide. Based on the field investigation, we conclude that the occurrence of the landslide is controlled mainly by the seismic, terrain, and geological factors. The special location of the landslide and the possible topographic amplification of ground motions due to the terrain features governed the landslide failure. The effects of earthquakes on the stability of slopes were considered in two aspects: First, the ground shaking may reduce the frictional strength of the substrate by shattering of rock mass. Second, the seismic acceleration may result in short-lived and episodic changes of the normal (tensile) and shear stresses in the hillshopes during earthquakes. According to the failure mechanism, the dynamic process of the landslide might contain four stages: (a) the cracking of rock mass in the rear of the slope mainly due to the tensile stress generated by the ground shaking; (b) the shattering of the substrate due to the ground shaking, which reduced the frictional strength of the substrate; (c) the shearing failure of the toe of the landslide due to the large shear stress caused by the landslide gravity; and (d) the deposition stage.     

The Oeschinensee rock avalanche,Bernese Alps,Switzerland: a co-seismic failure 2300 years ago?

Landslide deposits dam Lake Oeschinen (Oeschinensee), located above Kandersteg, Switzerland. However, past confusion differentiating deposits of multiple landslide events has confounded efforts to quantify the volume, age, and failure dynamics of the Oeschinensee rock avalanche. Here we combine field and remote mapping, topographic reconstruction, cosmogenic surface exposure dating, and numerical runout modeling to quantify salient parameters of the event. Differences in boulder lithology and deposit morphology reveal that the landslide body damming Oeschinensee consists of debris from both an older rock avalanche, possibly Kandertal, as well as the Oeschinensee rock avalanche. We distinguish a source volume for the Oeschinensee event of 37 Mm³, resulting in an estimated deposit volume of 46 Mm³, smaller than previous estimates that included portions of the Kandertal mass. Runout modeling revealed peak and average rock avalanche velocities of 65 and 45 m/s, respectively, and support a single-event failure scenario. ³⁶Cl surface exposure dating of deposited boulders indicates a mean age for the rock avalanche of 2.3 ± 0.2 kyr. This age coincides with the timing of a paleo-seismic event identified from lacustrine sediments in Swiss lakes, suggesting an earthquake trigger. Our results help clarify the hazard and geomorphic effects of rare, large rock avalanches in alpine settings.     

The 1979 Abbotsford Landslide, Dunedin, New Zealand: a retrospective look at its nature and causes

The Abbotsford Landslide of 8 August 1979 occurred in an urban area of Dunedin, New Zealand, causing much damage to houses and urban infrastructure. Rapid failure occurred after weeks of preliminary movements, resulting in the formation of a approximately 5 million m³ block slide. It caused the loss of 69 houses, with an overall cost of about NZ $10–13 million. After several months of investigations, a commission of inquiry found that unfavorable geology (weak clay layers in a 7°-dip slope) was the underlying cause of the landslide. An old sand quarry at the toe of the slope and a leaking water main above the slide area were found to be man-made factors that contributed to the failure. Slope stability analysis showed that after sand excavation (approximately 300,000 m³), the water table had to rise 0.3 m less for failure to occur. Because the quarry closed 10 years before the landslide occurred, it is concluded that a long-term rise in groundwater levels because of the increased rainfall over the previous decade and leakage from the water main controlled the timing of the failure and, in this sense, are considered to have triggered the landslide.     

金沙江上游快速隆升河段复杂结构岩体灾变特征与机理

横断山脉北麓金沙江上游河段沟壑纵横,水能资源丰富。中、晚更新世以来,快速隆升的新构造活动导致该河段复杂结构岩体在重力场的持续作用下灾变频繁。笔者阐述了该河段高地应力的基础地质背景与金沙江板块构造结合带蛇绿岩套的复杂结构岩体基本特征,提出了快速隆升河段的基本认知,建议将≥5 mm/a作为快速隆升河段的界限值;列举了21.4 km河段内不同时期、不同类型4处大规模堵江事件的证据和基本特征,阐述了其与快速隆升之间的关系;运用地质过程机制法分析了4个堵江体的致灾机理,指出早期堵江残体为未来人类工程活动的潜灾体。