The Acheron rock avalanche, Canterbury, New Zealand—morphology and dynamics

The 1100-year-old Acheron rock avalanche deposit lies in an active tectonic setting in Canterbury, New Zealand, and has a volume of ten million cubic metres and a runout distance 3.5 km. The deposit comprises intensely fragmented greywacke rock, and the processes of intense rock fragmentation during runout are postulated to have generated an isotropic dispersive stress. Dynamic simulation shows that the runout can be explained as a flow of dry granular material with a normal coefficient of friction, if the presence of an isotropic dispersive stress within the moving rock debris throughout the runout is assumed. The dispersive stress distribution required to model the rock avalanche runout and match velocities calculated from run-up traces is closely similar to that used to simulate the runout of the much larger Falling Mountain rock avalanche in a similar lithologic and tectonic setting. Both events thus behaved in a fundamentally similar fashion.     

Investigation and dynamic analysis of a catastrophic rock avalanche on September 23, 1991, Zhaotong,China

At 6:10 p.m. on September 23, 1991, a catastrophic rock avalanche occurred in Zhaotong, Yunnan, southwestern China. Over 216 people were killed when the Touzhai village was overwhelmed directly in the path of the landslide. The landslide involved the failure of about 12 Mm³ of jointed basaltic rock mass from the source area. The displaced materials ran out a horizontal distance of 3650 m over a vertical distance of 960 m, equivalent to a Fahrböschung of 14.7°, and covered an area of 1.38 km². To provide information for hazard zonation of similar type of potential landslides in the same area, we used a dynamic model (DAN-W) with three alternative rheological models to simulate the runout behaviour of the displaced landslide materials and found that a combination of the frictional model and Voellmy model could provide the best performance in simulating this landslide. The simulated results indicated that the duration of the movement is estimated at about 175 s for a mean velocity 21 m/s.     

Characteristics and numerical runout modeling of the heavy rainfall-induced catastrophic landslide–debris flow at Sanxicun,Dujiangyan, China,following the Wenchuan Ms 8.0 earthquake

The 2008 Ms 8.0 Wenchuan earthquake triggered a large number of extensive landslides. It also affected geologic properties of the mountains such that large-scale landslides followed the earthquake, resulting in the formation of a disaster chain. On 10 July 2013, a catastrophic landslide–debris flow suddenly occurred in the Dujiangyan area of Sichuan Province in southeast China. This caused the deaths of 166 people and the burying or damage of 11 buildings along the runout path. The landslide involved the failure of ≈1.47 million m³, and the displaced material from the source area was ≈0.3 million m³. This landslide displayed shear failure at a high level under the effects of a rainstorm, which impacted and scraped an accumulated layer underneath and a heavily weathered rock layer during the release of potential and kinetic energies. The landslide body entrained a large volume of surface residual diluvial soil, and then moved downstream along a gully to produce a debris flow disaster. This was determined to be a typical landslide–debris flow disaster type. The runout of displaced material had a horizontal extent of 1200 m and a vertical extent of 400 m. This was equivalent to the angle of reach (fahrböschung angle) of 19° and covered an area of 0.2 km². The background and motion of the landslide are described in this study. On the basis of the above analysis, dynamic simulation software (DAN3D) and rheological models were used to simulate the runout behavior of the displaced landslide materials in order to provide information for the hazard zonation of similar types of potential landslide–debris flows in southeast China following the Wenchuan earthquake. The simulation results of the Sanxicun landslide revealed that the frictional model had the best performance for the source area, while the Voellmy model was most suitable for the scraping and accumulation areas. The simulations estimated that the motion could last for ≈70 s, with a maximum speed of 47.7 m/s.     

Dynamic process of the Wenjiagou rock landslide in Sichuan Province,China

A rock avalanche is a geological event that is always sudden, rapid and with a long run-out, and can result in large loss of lives and property. The Wenjiagou rock avalanche was a high-speed rock landslide caused by a strong earthquake, in Mianzhu, Sichuan Province, southwest China. In this study, we reproduce the movement and deposition processes of the sliding mass by numerical simulation. We analyze the effects of the friction coefficient of each slip surface and the strength of the parallel bonds and contact stiffness between particles on the dynamic process and deposit features using three-dimensional particle flow code (PFC3D). The simulation results agree with the field measurements when the friction coefficient is 0.2, parallel bond strength is 2 MPa, and contact stiffness is 2?×?10⁸ kN/m. The landslide lasted about 115 s from the initial movement to the final deposition at the exit of the valley. The maximum velocity of the sliding mass was 114 m/s.     

Runout and entrainment analysis of an extremely large rock avalanche—a case study of Yigong,Tibet, China

An extremely large rock avalanche occurred on April 9, 2000 at Yigong, Tibet, China. It started with an initial volume of material of 90?×?10⁶ m³ comprising mainly of loose material lying on the channel bed. The rock avalanche travelled around 10 km in horizontal distance and formed a 2.5-km-long by 2.5-km-wide depositional fan with a final volume of approximately 300?×?10⁶ m³. An energy-based debris flow runout model is used to simulate the movement process with a new entrainment model. The entrainment model considers both rolling and sliding motions in calculating the volume of eroded material. Entrainment calculation is governed by a second order partial differential equation which is solved using the finite difference method. During entrainment, it is considered that the total mass is changed due to basal erosion. Also the profile of the channel bed is adjusted accordingly due to erosion at the end of each calculation time step. For Yigong, the profile used in the simulation was extracted from a digital elevation model (DEM) with a resolution of 30 m?×?30 m. Measurements obtained from site investigation, including deposition depth and flow height at specific location, are used to verify the model. Ground elevation-based DEM before and after the event is also used to verify the simulation results where access was difficult. It is found that the calculated runout distance and the modified deposition height agree with the field observations. Moreover, the back-calculated flow characteristics based on field observations, such as flow velocity, are also used for model verifications. The results indicate that the new entrainment model is able to capture the entrainment volume and depth, runout distance, and deposition height for this case.     

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.     

ICL/IPL activities in West Africa: landslide risk assessment and hazard mapping approach

The ICL/IPL Project achieved results in capacity building, investigation of landslides in West Africa and also evaluated some other slope movements in the region. These include the catastrophic rock–debris avalanche at the Cameroon–Nigeria border and the Iva Valley landslides in Enugu. During the avalanche, an estimated 100 M m³ of rock and debris was moved more than 2 km from the source of the slide at 600 m above sea level to the toe in the valley in a few minutes. The materials range from mud and soil debris to blocks of rocks up to 20 m in diameter. The grain size of moved material tended to increase upslope and closer to the head scarp though it tended to decrease again close to and at the source area. Nature and composition of the basement bedrock with foliation planes dipping in the direction of slope, dominant joint sets oriented perpendicularly to the foliation, the nature of weathered material and high relief were strong factors in the avalanche. Field studies identified 43 landslides at the Iva Valley area of Enugu state, which were shallow, short run-out movements with slip-surface depth less than 2 m. The shallow slides and the avalanche are triggered by water infiltration in slopes with high topographic gradient. The soil saturation leads to a reduction of the shear strength of the soil because of a rise in pore water pressure. These landslides are known to occur during or after intense rainfalls at the beginning or at the tail end of rainy season.     

Experimental and numerical investigations of a catastrophic long-runout landslide in Zhenxiong,Yunnan, southwestern China

On 11 January 2013, a catastrophic landslide of ～0.2 million m³ due to a prolonged low-intensity rainfall occurred in Zhenxiong, Yunnan, southwestern China. This landslide destroyed the village of Zhaojiagou and killed 46 people in the distal part of its path. The displaced landslide material traveled a horizontal distance of ～800 m with a vertical drop of ～280 m and stopped at 1520 m a.s.l. To examine the possible mechanism and behavior of the landslide from initiation to runout, the shear behavior of soil samples collected from the sliding surface and runout path was examined by means of ring shear tests. The test results show that the shear strength of sample from the sliding surface is less affected by shear rate while the shear rate has a negative effect on the shear strength of runout path material. It is suggested that the source and runout path materials follow the frictional and Voellmy rheology, respectively. Post-failure behavior of the landslide was modeled by using a DAN-W model, and the numerical results show that the selected rheological relationships and parameters based on the results of ring shear tests may provide good performance in modeling the Zhenxiong landslide.     

强震区高位滑坡远程灾害特征研究——以四川茂县新磨滑坡为例

近年来,在汶川地震等强震区常发生一种特大的高位滑坡地质灾害,它从高陡斜坡上部位置剪出并形成凌空加速坠落,具有撞击粉碎效应和动力侵蚀效应,导致滑体解体碎化,从而转化为高速远程碎屑流滑动或泥石流流动,并铲刮下部岩土体,使体积明显增加。新磨滑坡就是这种典型,它发生于2017年6月24日,滑坡后缘高程约3450m,前缘高程约2250 m,高差1200 m,水平距离2800 m,堆积体体积达1637×10~4m~3,摧毁了新磨村村庄,导致83人死亡。新磨滑坡地处叠溪较场弧形构造带前弧西翼,母岩为中三叠统中厚层变砂岩夹板岩,是1933年叠溪Ms7.5级震中区(烈度X度)和汶川Ms8.0级强震区(烈度IX度),形成震裂山体。滑源区分布多组不连续结构面,将厚层块状岩体分割成碎裂块体,在高程3150~3450 m区间形成明显的压裂鼓胀区,特别是存在2组反倾节理带,具有典型的"锁固段"失稳机理。滑坡体高位剪出滑动,连续加载并堆积于斜坡体上部,体积达390×10~4m~3,导致残坡积岩土层失稳并转化为管道型碎屑流;碎屑流高速流滑至斜坡下部老滑坡堆积体后,因前方地形开阔、坡度变缓,转化为扩散型碎屑流散落堆积,具有"高速远程"成灾模式。据此,可建立强震山区高位滑坡的早期识别方法,当陡倾山脊存在大型岩质高位滑坡时,应当考虑冲击作用带来的动力侵蚀效应和堆积加载效应,特别是沿沟谷赋存丰富的地下水时,发生高速远程滑坡的可能性将明显增加。因此,在地质灾害调查排查中,在高位岩质滑坡剪出口下方的斜坡堆积体上的聚居区等应划定为地质灾害危险区。在强震山区地质灾害研究中,不仅应采用静力学理论分析滑坡的失稳机理,而且应采用动力学方法加强运动过程的成灾模式研究。     

Dynamics of the Niumiangou Creek rock avalanche triggered by 2008 Ms 8.0 Wenchuan earthquake, Sichuan, China

The Niumiangou Creek rock avalanche was triggered by an M_s 8.0 earthquake that happened on 12 May 2008 in the Sichuan Province, China. The rock avalanche traveled a horizontal distance of 3.0 km over a vertical elevation difference of 0.89 km, equivalent to a coefficient of friction of only 0.29. The travel path of the rock avalanche can be divided into three segments: (1) failing and disintegrating, (2) flying, (3) flowing. In the failing and disintegrating segment, the rock slope failed because of the coupled action of horizontal and vertical force of the earthquake, then smashed into the opposite mountain and disintegrated. In the flying segment, the disintegrating rock mass changed direction and flew into the Lianhuaxin Creek, which was different from the previous research results that concluded rock debris flowed in Lianhuaxin Creek. A great amount of air trapped and compressed under the rock debris acted as air cushion and supported the rock debris to fly a further distance. In the flowing segment, the rock debris flowed on the ground surface in Niumiangou Creek. The flowing velocity has been estimated from the maximum elevation and runup according to the damaged trimlines of the debris. The saturated fine material in Niumiangou Creek entrained by the failed debris mass is thought to have contributed to the long runout of the debris. The Niumiangou Creek rock avalanche is one of the three longest rock avalanches triggered by Wenchuan earthquake. The conclusions of the paper have implications for hazard assessment of potential rock avalanches in the earthquake area and the other similar mountainous area in west China.     

Research on catastrophic rock avalanche at Guanling, Guizhou, China

On June 28, 2010, a catastrophic rock avalanche occurred after an extreme rainstorm at Guanling with N 25°59′10′′ in latitude and E 105°16′50′′ in longitude, Guizhou, China. This rock avalanche has a long run-out distance of 1.5 km, with 1.75 million cubic meters of debris instantly burying two villages and resulting in 99 deaths. It originated in the coal measure strata, with the upper part of limestone and dolomite, the middle part of the sandstone with gentle inclination, and the lower part of shale and mudstone, together locally with coals. This kind of unique structure, with hard resistant caprock overlying softer ductile rocks, coupled with the central outflow region at the contact zone, has catastrophic potential for rock avalanches and creates challenges for engineering geological/hydrogeological analysis. The topography showed that the hillside slopes were steeper at the upper portion but gentler in the lower portion, looked like the shape of a “boot.” The upper steep landform easily led to slope instability due to its high static shear stresses, and the wide middle and lower parts provided kinematic conditions for long run-out. Transformation of the larger potential energy into kinetic energy contributed to the formation of a rapid long run-out rock avalanche. The rainfall from June 27 to 28 was the apparent trigger of this catastrophic avalanche. The measured rainfall of more than 310 mm within 24 h exceeded the local historical records that were recorded over the last 60 years. The pore pressure on discontinuities of sandstone had an effect on the slope stability. The valley runoff supplied a saturated base for the long run-out debris, inducing an additional increase of the terminus distance and the increased velocity of the avalanche movement.     

2017年8月28日贵州纳雍县张家湾镇普洒村崩塌特征与成因机理研究

2017年8月28日10时30分许,贵州省纳雍县张家湾镇普洒村后山约49.1×104 m3的山体发生高位崩塌,高速运动的碎屑物质沿途铲刮坡面原有松散崩滑堆积物,最终形成体积约为82.3×104 m3的堆积体,摧毁坡脚的普洒村大树脚组和桥边组居民区,造成26人遇难,9人失踪。通过对灾害发生现场进行详细的地质调查,本文综合运用无人机航拍、地面合成孔径雷达监测等技术手段,对普洒村崩塌体特征进行了详细描述,初步阐述了崩塌发生的动力学过程和成因机理,并对周边受崩塌体失稳影响而产生的欠稳定区岩体特征的危险性进行了分析评价。初步研究结果认为,崩塌源区岩体在下部巷道采煤的影响下产生拉张裂缝,之后在长期重力作用下,山体开始变形、破碎,最终整体失稳破坏。崩塌体从小规模掉块开始到整体失稳破坏、远程运动,直至最终停积,整个过程用时约7分21秒,其中主崩塌体失稳用时约26 s,远程运动距离约788 m,是一处典型的高位崩塌-碎屑流。深入研究普洒村崩塌的形成过程和成灾机理,对我国西南山区存在的与普洒村崩塌体类似条件的灾害隐患点的减灾防灾工作,具有重要的指导意义。     

The AD 1717 rock avalanche deposits in the upper Ferret Valley (Italy): a dating approach with cosmogenic 10Be

On 12 September AD 1717, a rock volume larger than 10 million m³ collapsed onto the Triolet Glacier, mobilized a mass composed of ice and sediment and travelled more than 7 km downvalley in the upper Ferret Valley, Mont Blanc Massif (Italy). This rock avalanche destroyed two small settlements, causing seven casualties and loss of livestock. No detailed maps were made at the time. Later investigators attributed accumulations of granitic boulders and irregular ridges on the upper valley floor to either glacial deposition, or the AD 1717 rock avalanche, or a complex mixture of glacial deposition, earlier rock avalanche and AD 1717 rock avalanche origin. In this study, we present cosmogenic ¹⁰Be exposure ages from nine boulders in the extensive chaotic boulder deposit with irregular ridges, two from Holocene glacier‐free areas, and one from a Little Ice Age moraine. Exposure ages between 330 ± 23 and 483 ± 123 a from eight of nine boulders from the chaotic deposit indicate that at least seven were deposited by the AD 1717 rock avalanche. The other three boulders yielded ¹⁰Be exposure ages of 10 900 ± 400, 9700 ± 400 and 244 ± 97 a, respectively. Our results are in good agreement with the existing chronology from dendrochronology and lichenometry, and radiocarbon analysis of wood samples, but not with older ¹⁴C ages from a peat bog in the upper part of the valley. Based on the new age control, the rock avalanche deposits cover the whole bottom of the upper Ferret valley. Copyright © 2012 John Wiley & Sons, Ltd.     

The August 27, 2014, rock avalanche and related impulse water waves in Fuquan,Guizhou, China

At about 8:30 p.m. on 27 August 2014, a catastrophic rock avalanche suddenly occurred in Fuquan, Yunnan, southwestern China. This landslide and related impulse water waves destroyed two villages and killed 23 persons. The impulse waves occurred after initiation of the landslide, caused by the main part of the slide mass rapidly plunging into a water-filled quarry below the source area. The wave, comprising muddy water and rock debris, impacted the opposite slope of the quarry on the western side of the runout path and washed away three homes in Xinwan village. Part of the displaced material traveled a horizontal distance of about 40 m from its source and destroyed the village of Xiaoba. To provide information for potential landslide hazard zonation in this area, a combined landslide–wave simulation was undertaken. A dynamic landslide analysis (DAN-W) model is used to simulate the landslide propagation before entering the quarry, while Fluent (Ansys Inc., USA) is used to simulate the impulse wave generation and propagation. Output data from the DAN-W simulation are used as input parameters for wave modeling, and there is good agreement between the observed and simulated results of the landslide propagation. Notably, the locations affected by recordable waves according to the simulation correspond to those recorded by field investigation.     

Be exposure ages of a rock avalanche and a late glacial moraine in Alta Valtellina, Italian Alps

A rock avalanche deposit was investigated in order to understand the chronological evolution of geological hazards and to evaluate the interaction of the triggering geodynamic processes in the valley Val Viola, Italian Alps. The deposit is situated west of the Alpe Dosdé, in a permafrost area with deep-seated gravitational deformations (DSGD) along a tectonic line. Based on its geomorphologic context, the rock avalanche was first interpreted as a result of slope stress release without exact timing. This hypothesis was tested by measuring the ¹⁰Be exposure date of quartz from one boulder from the rock avalanche. The age of 7430±460 years places the event in the early Holocene. The timing of the last deglaciation was constrained using the inner late glacial moraine of a moraine doublet in the valley Alpe Dosdé situated at an altitude between 2140 and 2120 m a.s.l. west of the rock avalanche. The ¹⁰Be concentrations of quartz yield minimum exposure ages of 11,480±670 and 10,850±820 years. Different proposals for potential triggering factors of the rock avalanche include (a) melting of the local valley glacier and slope stress release in the Val Viola, likely to play a minor role as trigger, because of the time delay between the deglaciation and the rock avalanche event. More likely are (b) enhanced crustal seismicity induced by post-glacial regional isostatic glacial rebound coupled with tectonic stress or/and (c) climate conditions with higher temperatures around 7430±460 years, resulting in an upwards movement of the permafrost limit and destabilization of the rock walls.     

Formation mechanism and evolution process of the Chada rock avalanche in Southeast Tibet,China

The Chada rock avalanche is a prehistoric high-elevation giant rock landslide located in the Boshula Mountains, Lhorong County, Southeast Tibet. It is composed of conglomerates with a volume of 6.62?×?10⁶ m³ and has a height difference of 1450 m and a transport distance of 3155 m. The accumulational landform shows characteristics indicating rock avalanches. With a unique red conglomerate as the marker of landslide movement, we combined the results of geological surveys, aerial surveys, and engineering geological drilling to determine the entrainment and geomorphic features of the rock avalanche. The rock avalanche was divided into the main scarp, entrainment zone (residual deposit, mixed deposit, and impact fragmentation areas), transport zone (compressed, local landslide, and longitudinal ridge areas), and deposit zone. The sequence of deposits in the valley indicates that the rock avalanche formed before the first-stage terrace and after the second-stage terrace. Combined with 3D numerical simulation, four movement stages were obtained: (1) the rock mass was broken and disintegrated due to progressive failure, initiating high-speed sliding; (2) the sliding mass scraped the thick previous slope material and formed oblique ridges by forward extrusion and lateral friction; (3) the 4.95?×?10⁶ m³ sliding mass was compressed and decelerated to form bending ridges, and the 1.67?×?10⁶ m³ sliding mass continued to move through the channel; and (4) the sliding mass extended to form longitudinal ridges in the channel and hummocks in the valley. The rock avalanche accelerated three times and decelerated three times during its motion.

     

Failure mechanism and kinematics of the deadly June 24th 2017 Xinmo landslide,Maoxian, Sichuan,China

At 5:38 am on the 24th June, 2017, a catastrophic rock avalanche destroyed the whole village of Xinmo, in Maoxian County, Sichuan Province, China. About 4.3 million m³ of rock detached from the crest of the mountain, gained momentum along a steep hillslope, entrained a large amount of pre-existing deposits, and hit the village at a velocity of 250 km/h. The impact produced a seismic shaking of ML = 2.3 magnitude. The sliding mass dammed the Songping gully with an accumulation body of 13 million m³. The avalanche buried 64 houses; 10 people were killed and 73 were reported missing. The event raised great concerns both in China and worldwide. Extensive field investigation, satellite remote sensing, UAV aerial photography, and seismic analysis allowed to identify the main kinematic features, the dynamic process, and the triggering mechanism of the event. With the aid of ground-based synthetic aperture radar monitoring, the hazard deriving from potential further instabilities in the source area has been assessed. The preliminary results suggest that the landslide was triggered by the failure of a rock mass, which had been already weakened by the M_s 7.5 Diexi earthquake in 1933. Several major earthquakes since then, and the long-term effect of gravity and rainfall, contributed to the mass failure. The high elevation, slope angle, and vegetation cover in the source area hinder geological field investigation and make hazard assessment difficult. Nonetheless, monitoring and prevention of similar collapses in mountainous areas must be carried out to protect human lives and infrastructures. To this aim, the integrated use of modern high-precision observation technologies is strongly encouraged.     

Monitoring and early warning of the 2012 Preonzo catastrophic rockslope failure

In this paper, we describe the investigations and actions taken to reduce risk and prevent casualties from a catastrophic 210,000 m³ rockslope failure, which occurred near the village of Preonzo in the Swiss Alps on May 15, 2012. We describe the geological predisposition and displacement history before and during the accelerated creep stage as well as the development and operation of an efficient early warning system. The failure of May 15, 2012, occurred from a large and retrogressive instability in gneisses and amphibolites with a total volume of about 350,000 m³, which formed an alpine meadow 1250 m above the valley floor. About 140,000 m³ of unstable rock mass remained in place and might collapse partially or completely in the future. The instability showed clearly visible signs of movements along a tension crack since 1989 and accelerated creep with significant hydromechanical forcing since about 2006. Because the active rockslide at Preonzo threatened a large industrial facility and important transport routes located directly at the toe of the slope, an early warning system was installed in 2010. The thresholds for prealarm, general public alarm, and evacuation were derived from crack meter and total station monitoring data covering a period of about 10 years, supplemented with information from past failure events with similar predisposition. These thresholds were successfully applied to evacuate the industrial facility and to close important roads a few days before the catastrophic slope failure of May 15, 2012. The rock slope failure occurred in two events, exposing a compound rupture plane dipping 42° and generating deposits in the midslope portion with a travel angle of 39°. Three hours after the second rockslide, the fresh deposits became reactivated in a devastating debris avalanche that reached the foot of the slope but did not destroy any infrastructure. The final run-out distance of this combined rock collapse–debris avalanche corresponded to the predictions made in the year 2004.     

Source characteristics of long runout rock avalanches triggered by the 2008 Wenchuan earthquake,China

The May 12, 2008 Wenchuan, China Earthquake which measured M_w = 8.3 according to Chinese Earthquake Administration – CEA (M_w = 7.9 according to the USGS) directly triggered many landslides, which caused about 20,000 deaths, a quarter of the total. Rock avalanches were among the most destructive landslides triggered by this seismic event, and have killed more people than any other type of landslide in this earthquake. The Donghekou rock avalanche, one example of a catastrophic avalanche triggered by the Wenchuan earthquake, occurred in Qingchuan and buried one primary school and 184 houses, resulting in more than 780 deaths, and in addition, caused the formation of two landslide dams, which formed barrier lakes.Combining aerial images (resolution of 0.5 m) with field investigations, this paper lists some parameters of 66 cases in one table, and details source characteristics of six typical cases. It has been found that most of the long runout rock avalanches have source areas with high relief and steep inclination, causing the debris in the travel courses to accelerate. There was also a large amount of saturated Holocene-age loose deposits formed by a river or gully that existed in the travel courses. Comparison studies indicate that saturated Holocene loose deposits in the travel courses could be the most important factor for the causes of the long runout characteristic of the rock avalanches especially when they traveled over gentle or even flat ground surfaces.Furthermore, the relationships among the relief slope gradient, runout and covered area are investigated, and a threshold line for predicting the maximum horizontal runout distance under certain change in elevation is presented.     

Numerical runout simulation of debris avalanches in the Faroe Islands, North Atlantic Ocean

The Faroe Islands in the North Atlantic Ocean are susceptible to flow-type landslides in coarse-grained highly organic colluvium. Following several hazardous debris avalanche events, research work has been initiated to quantify landslide risk. A central task in this work is to predict landslide runout behavior. From numerical simulation of four debris avalanches, this study provides a first screening of which rheology and appertaining input parameters best predict runout behavior of debris avalanches in the Faroe Islands. Three rheologies (frictional, Voellmy, and Bingham) are selected and used for individual back analysis of the events in the numerical models BING and DAN3D. A best fit rheology is selected from comparing predicted and observed landslide runout behavior. General back analysis to identify the optimal input parameters for the chosen rheology is performed by cross validation, where each debris avalanche is modeled with input parameters from the three other events. Optimal input parameters are found from the model run producing the most accurate runout length and velocity. The Bingham is selected as the best fit rheology, a result differing from similar studies of coarse-grained landslides. A reason for why particularly the frictional rheology proves unsuitable is its tendency to produce too long runout lengths of the low-weight runout material, a result showing important limitations for using the frictional rheology in DAN3D. Optimal Bingham input parameters are τ _y ?=?980 Pa and μ _b ?=?117 Pa/s. However, future studies performed in 2D models are needed for precise parameterization before results can be used for landslide risk assessment.