Numerical modeling of debris avalanche propagation from collapse of volcanic edifices

Debris avalanches produced from the collapse of volcanic edifices are destructive events that involve volumes up to two orders of magnitude larger (cubic kilometer) than most non-volcanic rock and debris avalanches. We replicate the motion and spreading of several volcanic collapses by means of a depth-averaged quasi-3D numerical code. The model assumes a frictional internal rheology and a variable basal rheology (i.e frictional, Voellmy and plastic). We back analyzed seven case-studies against observations reported in the literature to provide a set of calibrated cases. The ASTER and SRTM satellite-derived digital elevation models were used as topographic data. The numerical model captures the main features of the propagation process, including travel distance, lateral spreading and run up. At varying triggering factors and material characteristics the best fitting parameters span in a narrow interval and differ from those typical of non-volcanic rock and debris avalanches. The bulk basal friction angles (the sole parameter required in the frictional rheology) range within 3° and 7.5° whereas typical values for non-volcanic debris avalanches vary from 11° to 31°. The consistency of the back analyzed parameters is encouraging for a possible use of the model in the perspective of hazard mapping. The reconstruction of the pre-event topography is critical, and it is associated to large uncertainty. The quality of the terrain data, more than the resolution of the DEMs used, is relevant for the modeling. Resampling the original square grid to larger cell sizes determines a low increase in the back analyzed rheological parameters, as a result of the lower roughness of the terrain.     

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.     

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 Cascade rock avalanche: implications of a very large Alpine Fault-triggered failure,New Zealand

Catastrophic deep-seated rock slope failures (RSFs; e.g., rock avalanches) can be particularly useful proxies for fault rupture and strong ground motion, and currently represent an underappreciated hazard of earthquakes in New Zealand. This study presents observations of the previously undescribed Cascade rock avalanche (CRA), a c. 0.75 km³ single-event, long-runout, catastrophic failure interpreted to have been coseismically triggered by a large to great earthquake c. 660 AD on the Alpine Fault. Despite its size and remarkable preservation, the CRA deposit has been previously identified as a terminal moraine and fault-damaged outcrop, highlighting the common misinterpretation of similar rock avalanche deposits. Comparisons are drawn between the CRA and other Alpine Fault-attributed rock avalanches, such as the better-studied c. 860 AD Round Top rock avalanche, to re-assess coseismic rock avalanche hazard. Structural relationships indicate the rock mass comprising the CRA may have formerly been a portion of a larger (c. 3 km³) RSF, before its catastrophic collapse on a deep-seated gravitational collapse structure (sackung). Sackungen and RSFs are common throughout the Southern Alps and other mountainous regions worldwide; in many cases, they should be considered potential precursors to catastrophic failure events. Two masses of rock in the Cascade River Valley show precursory signs of potential catastrophic failures of up to c. 2 km³; a similar mass may threaten the town of Franz Josef.     

Results of Back-Analysis of the Propagation of Rock Avalanches as a Function of the Assumed Rheology

Summary. Numerical simulation can provide a useful tool for investigating the dynamics of phenomena like rock avalanches, within realistic geological contexts and in the framework of a better risk assessment and decision making. Difficulties in numerical modelling of a heterogeneous moving mass are mainly linked to the simulation of the complex behaviour assumed by the mass during propagation. The numerical code RASH3D, based on a continuum mechanics approach and on the long wave approximation, is used to back-analyse two cases of rock avalanches: Frank (1903, Canada) and Val Pola (1987, Italy). The two events are characterised by approximately the same volume (about 30 × 10⁶ m³) while the run out area morphologies are widely different. Three alternative “rheologies” (Frictional, Voellmy and Pouliquen) are used. Comparison among obtained results underlines that the validation of a “rheology” requires not only a good agreement between the numerical simulation results and the run out area boundaries but also in term of depth distribution of the mass in the deposit. In case of a Frictional rheology, the obtained calibrated dynamic friction angle values are in a range of 15 ± 1° for both the cases; while assuming a Pouliquen or a Voellmy rheology it emerges a different behaviour of rheological parameters for each of the considered events. Besides the calibration of rheological parameters to better back-analyse each of the considered events, it is investigated how the behaviour due to the assumed rheology is influenced by the geometry of the run out area (e.g. narrow or broad valley). Authors’ addresses: Marina Pirulli, Department of Structural and Geotechnical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy; Anne Mangeney, Equipe de Sismologie, Institut de Physique du Globe de Paris, Université Denis Diderot, 4 Place Jussieu, 75005 Paris, France     

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.     

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.     

Some observations on the prediction of the dynamic parameters of debris flows in pyroclastic deposits in the Campania region of Italy

Over the last 20 years, many tools have been developed for the prediction of the post-failure behaviour of rapid landslides. However, as pointed out by several researchers, knowledge may be improved by the performance of back-analyses using different models and the evaluation of their reliability. This paper reports the back-analysis, conducted using numerical models, of 57 rapid landslides that have occurred in the Campania region. The back-analysis has been performed using the 2-D DAN_W code (version 2003) with two different rheological models: the Voellmy and the frictional models. The latter has been immediately discarded because it did not match the observed data. Instead, using the Voellmy model, the best-fit values of the parameters (friction μ and turbulence ξ) for different types of flow (channelled, un-channelled and mixed flows) have been researched. With these values a parametric study has been carried out on four representative slope profiles of the Campania region, enabling the prediction of runout, velocity and depth of flow (dynamic parameters) of potential debris flows.

Characteristics and numerical runout modelling of a catastrophic rock avalanche triggered by the Wenchuan earthquake in the Wenjia valley,Mianzhu, Sichuan,China

The 2008 Wenchuan earthquake triggered more than 100 rock avalanches with volumes greater than 10 million cubic metres. The rock avalanche with the longest runout amongst these destructive landslides occurred in the Wenjia valley, Mianzhu, Sichuan, China. The landslide involved the failure of about 27.5 million cubic metres of sandstone from the source area. The displaced material travelled about 4,170 m with an elevation descent of about 1,360 m, equivalent to a fahrböschung of 16.9° and covered an area of 1.5 million square metres, with the final deposited volume of approximately 49 million cubic metres. The catastrophic event destroyed the village of Yanjing, killed 48 people and buried some houses at the mouth of the Wenjia valley. On the basis of a detailed field investigation, we introduce basic characteristics of the rock avalanche and find that the rock avalanche resulted in two run-ups and a superelevation along the runout path, and downslope enlargement due to the entrainment of path materials. A numerical model (DAN3D) is used to simulate the post-failure behaviour of the rock avalanche. By means of trial and error, a combination of the frictional model and Voellmy model is found to provide the best performance in simulating this rock avalanche. The simulation results reveal that the rock avalanche had a duration of about 240 s and an average velocity of 17.4 m/s.     

The prehistoric Mt Wilberg rock avalanche, Westland, New Zealand

The Mt Wilberg rock avalanche in Westland, New Zealand occurred before 1300 AD and may have occurred as a consequence of an Alpine fault earthquake in ca. 1220 AD or earlier. Its ∼40 × 10⁶ m³ deposit may have briefly obstructed the Wanganui River, but only about 25% of its surface morphology still survives, on terraces isolated from river erosion. The landslide appears to have moved initially as a block, in a direction controlled by a strong rock mass at the base of the source area, before disintegrating and spreading across terraces, fans, and floodplains. Rock avalanche deposits in Westland have relatively short expected lifetimes in the rugged terrain and high rainfall of the area; hence, the hazard from such events is under-represented by their current remnants.     

A 36Cl age determination for Mystery Creek rock avalanche and its implications in the context of hazard assessment, British Columbia, Canada

The Sea to Sky Corridor has experienced hundreds of historic and prehistoric landslides. The most common types of historical landslides are rock falls and debris flows, which are relatively small in volume but can be damaging. These types of failures are more common in the southern part of the corridor, between Horseshoe Bay and Porteau, where infrastructure has been built in close proximity to steep slopes. Farther north, fewer landslides have been reported historically, but those that have been recorded are usually large and date to prehistoric time (e.g., Cheekye fan and Mystery Creek rock avalanche). As part of a Geological Survey of Canada surficial geology and landslide inventory mapping study, Mystery Creek rock avalanche, near Whistler, British Columbia, was sampled for ³⁶Cl dating. Samples were collected from three large flat boulders of quartz diorite in the rock avalanche deposit to test a correlation with the previously reported radiocarbon age of 800 ± 100 years BP on charcoal. One sample revealed a mean age of 2,400 years and the other two, 4,300 and 4,800 years, respectively. These new results point to four possible interpretations: (1) Mystery Creek landslide is about 800 years old; (2) Based on the overlapping 2σ uncertainties, the rock avalanche took place between 2,200 and 3,600 years ago; (3) The rock avalanche deposit is 2,400 years old and the other two blocks are too old; and (4) The rock avalanche is between 4,300 and 4,800 years old. Although there is strength in numbers and it is likely that the age varies between 4,300 and 4,800 years, we favor the second interpretation where the age range is broader and statistically significant for all three samples. Moreover, at this time, we favor discounting the radiocarbon age based on a greater number of samples analyzed for ³⁶Cl analysis and lack of detailed information on the charcoal sampling. The causes and triggers of the Mystery Creek rock avalanche remain unknown, but direct glacial debuttressing can be ruled out. Some of the causes are likely a combination of the regional tectonic setting which produced preferential planes of weakness reflected in the trend of major faults, headscarp, and reverse scarps. Yearly cycles of freezing and thawing are considered a plausible cause based on present-day climate records. Finally, a large earthquake still remains a possible trigger because of the active tectonic setting and the presence of potentially contemporaneous landslides in the same area. Mystery Creek rock avalanche and other historic and pre-historic landslides contributed to validation of a heuristic rock fall/rock slide/rock avalanche susceptibility mapping study, in which their headscarps correlated well with medium-high to high susceptibility zones. In terms of hazard assessment, Mystery Creek rock avalanche, although pre-historic in age, occurred in present-day climatic and geological conditions. This poses a threat to infrastructure such as the Sea to Sky Highway, railway, and power line.     

公伯峡面板堆石坝流变变形的反演分析

采用沈珠江等提出的流变模型,根据公伯峡面板堆石坝坝体填筑竣工到蓄水前的沉降监测资料,进行反演分析得到相应的流变参数,并用该参数对大坝应力变形进行了计算分析。沉降点监测值和计算值变动规律比较一致,说明了流变模型的有效性;由于坝体堆石料的流变,面板的受力变形在运行期内呈现一定规律性的变化,并逐渐趋于稳定     

炭质泥质灰岩饱水流变试验研究

采用岩石直剪流变仪对重庆武隆鸡尾山滑坡滑带(炭质泥质灰岩)进行了岩石饱水直接剪切流变试验.通过分析流变试验结果,从而得出滑带炭质泥质灰岩的剪应力-剪切位移曲线,进而得到软岩的长期强度.结果表明:炭质泥质灰岩饱水条件下与自然状况下流变强度和瞬时剪切强度参数相比,饱水状况下的长期强度有明显降低,饱水状况下炭质泥质灰岩摩擦系数比自然流变降低13.87%,比瞬时剪切的摩擦系数低40.91%; 饱水状况下炭质泥质灰岩黏聚力比自然流变降低13.81%,比瞬时剪切获取的黏聚力降低36.67%.因裂隙损伤扩展,饱水条件下长期抗剪强度比自然状况下长期抗剪强度有所降低,但是没有直剪流变试验长期抗剪强度与瞬时抗剪强度相比降低显著,为深入认识和分析鸡尾山滑坡滑带软岩的流变力学特性提供重要的试验和依据.     

Transport process and mechanism of the Hongshiyan rock avalanche triggered by the 2014 Ludian earthquake,China

The Hongshiyan rock avalanche is a remarkable landslide disaster with approximately volume of 12?×?10⁶ m³, triggered by the 2014 Ms. 6.5 Ludian earthquake in Yunnan Province, China. This study conducted a comprehensive analysis by the model based on discrete element (DEM-based) numerical simulation to understand the transport process and mechanism for this rock avalanche. The simulation results showed that the transport process of the rock avalanche depends on the input seismic duration and motion. The average velocity of the rock avalanche sharply increases to peak value of 27 m/s and then gradually decreases to zero, and 64% and 36% of the total energy are dissipated by collision and friction, respectively. In this process, the progressions from simple disintegration along pre-existing discontinuities to fragmentation that creates new fracture surface are documented, and gradual increase of the fragmentation degree over time results in the decrease of fragment size and the formation of well-graded and narrower-interval gradation. This fragmentation evolution creates a conductive condition to the development of internal shear, and is closely associated with the dense flow regime that dominates the main body of the rock avalanche but presents discontinuous distribution along the flow thickness direction. In addition, further analyzing the simulated results indicates that more likely effects of fragmentation on mobility of rock avalanches depend on fragmentation-induced special flow structure, which makes a rock avalanche in a flow state with lower friction and lower energy consumption.

     

Magnitude and frequency relations: are there geological constraints to the rockfall size?

There exists a transition between rockfalls, large rock mass failures, and rock avalanches. The magnitude and frequency relations (M/F) of the slope failure are increasingly used to assess the hazard level. The management of the rockfall risk requires the knowledge of the frequency of the events but also defining the worst case scenario, which is the one associated to the maximum expected (credible) rockfall event. The analysis of the volume distribution of the historical rockfall events in the slopes of the Solà d’Andorra during the last 50 years shows that they can be fitted to a power law. We argue that the extrapolation of the F-M relations far beyond the historical data is not appropriate in this case. Neither geomorphological evidences of past events nor the size of the potentially unstable rock masses identified in the slope support the occurrence of the large rockfall/rock avalanche volumes predicted by the power law. We have observed that the stability of the slope at the Solà is controlled by the presence of two sets of unfavorably dipping joints (F3, F5) that act as basal sliding planes of the detachable rock masses. The area of the basal sliding planes outcropping at the rockfall scars was measured with a terrestrial laser scanner. The distribution of the areas of the basal planes may be also fitted to a power law that shows a truncation for values bigger than 50 m² and a maximum exposed surface of 200 m². The analysis of the geological structure of the rock mass at the Solà d’Andorra makes us conclude that the size of the failures is controlled by the fracture pattern and that the maximum size of the failure is constrained. Two sets of steeply dipping faults (F1 and F7) interrupt the other joint sets and prevent the formation of continuous failure surfaces (F3 and F5). We conclude that due to the structural control, large slope failures in Andorra are not randomly distributed thus confirming the findings in other mountain ranges.     

角闪斜长片麻岩流变力学特性研究

为了解小湾水电站枢纽区角闪斜长片麻岩的流变力学特性,采用岩石全自动三轴流变伺服仪对角闪斜长片麻岩进行三轴流变力学试验。试验结果表明,强度较高的角闪斜长片麻岩会发生流变现象,尤其在高应力水平条件下,其流变特性明显;外荷载超过岩样的长期强度时,随着时间的逐渐增加试样变形经历典型流变3个阶段,并最终加速流变破裂。依据岩样稳态流变阶段的流变速率与应力水平的关系,提出了一种确定长期强度的方法。在Burgers流变模型中加入统计损伤,并用Mohr-Coulomb准则确定的岩石长期强度作为开始进入非线性加速流变阶段的阀值,得到相应的非线性损伤流变模型。利用岩石全程三轴流变试验结果,采用优化后的算法,对非线性损伤流变模型相应的参数进行了辨识,结果表明,建立的岩石统计损伤非线性流变变模型与流变试验结果吻合较为理想,可以较为准确地反映角闪斜长片麻岩的流变力学特征     

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.     

A non-hydrostatic model for the numerical study of landslide-generated waves

This paper proposes and demonstrates a two-layer depth-averaged model with non-hydrostatic pressure correction to simulate landslide-generated waves. Landslide (lower layer) and water (upper layer) motions are governed by the general shallow water equations derived from mass and momentum conservation laws. The landslide motion and wave generation/propagation are separately formulated, but they form a coupled system. Our model combines some features of the landslide analysis model DAN3D and the tsunami analysis model COMCOT and adds a non-hydrostatic pressure correction. We use the new model to simulate a 2007 rock avalanche-generated wave event at Chehalis Lake, British Columbia, Canada. The model results match both the observed distribution of the rock avalanche deposit in the lake and the wave run-up trimline along the shoreline. Sensitivity analyses demonstrate the importance of accounting for the non-hydrostatic dynamic pressure at the landslide-water interface, as well as the influence of the internal strength of the landslide on the size of the generated waves. Finally, we compare the numerical results of landslide-generated waves simulated with frictional and Voellmy rheologies. Similar maximum wave run-ups can be obtained using the two different rheologies, but the frictional model better reproduces the known limit of the rock avalanche deposit and is thus considered to yield the best overall results in this particular case.     

Changing debris flow activity after sudden sediment input: a case study from the Swiss Alps

On 27 December 2011, a rock avalanche in the upper Val Bondasca in the southern Swiss Alps deposited 1.5–1.7 million m³ of rock debris. The following summer, debris flow activity in Val Bondasca was unusually high with four events after a 90‐year period of debris flow inactivity. This was an exceptional situation for the valley. Analysing the 2012 events, the long‐term record of meteorological conditions such as rainfall intensity and duration, in comparison with debris flow activity, suggests that the meteorological conditions in summer 2012 would not have triggered the high intensity debris flow events without additional sediment input. Consequently, the suddenly increased debris availability can be considered a major factor in these events. Interestingly, rainfall events of similar magnitude in the subsequent years 2013–2015 did not trigger additional debris flow events, indicating that debris flow initiation thresholds are increasing again, back towards pre‐rock avalanche levels. This study aims to help in understanding the so far poorly understood temporal evolution of debris flow triggering thresholds and the effect of sudden changes in sediment availability.