Catastrophic Slope Failures Research Articles

COMPARISON AND TIME SERIES ANALYSIS OF LANDSLIDE DISPLACEMENT MAPPED BY AIRBORNE, TERRESTRIAL AND UNMANNED AERIAL VEHICLE BASED PLATFORMS

Abstract. Slow moving deep-seated gravitational slope deformations are threatening infrastructure and economic wellbeing in mountainous areas. Accelerating landslides may end up in a catastrophic slope failure in terms of rapid rock avalanches. Continuous landslide monitoring enables the identification of critical acceleration thresholds, which are required in natural hazard management. Among many existing monitoring methods, laser scanning is a cost effective method providing 3D data for deriving three dimensional and areawide displacement vectors at certain morphological structures travelling on top of the landslide. Comparing displacements between selected observation periods allows the spatial interpretation of landslide acceleration or deceleration. This contribution presents five laser scanning datasets of the active Reissenschuh landslide (Tyrol, Austria) acquired by airborne laser scanning (ALS), terrestrial laser scanning (TLS) and Unmanned aerial vehicle Laser Scanning (ULS) sensors. Three observation periods with acquisition dates between 2008 and 2018 are used to derive area-wide displacement vectors. To ensure a most suitable displacement derivation between ALS, TLS and ULS platforms, an analysis investigating point cloud features within varying search radii is carried out, in order to identify a neighbourhood where common surfaces are represented platform independent or differences between the platforms are minimized. Consequent displacement vector estimation is done by ICP-Matching using morphological structures within the high resolution TLS and ULS point cloud. Displacements from the lower resolution ALS point cloud and TLS point cloud were determined using a modified version of the well-known image correlation (IMCORR) method working with point cloud derived shaded relief images combined with digital terrain models (DTM). The interplatform compatible analyses of the multi-temporal laser scanning data allows to quantify the area-wide displacement patterns of the landslide. Furthermore, changes of these displacement patterns over time are assessed area-wide. Spatially varying areas of landslide acceleration and deceleration in the order of ±15 cm a−1 between 2008 and 2017 and an area wide acceleration of up to 20 cm a−1 between 2016 and 2018 are identified. Continuing the existing time series with future ULS acquisitions may enable a more complete and detailed displacement monitoring using entirely represented objects within the point clouds.

The characteristics and mechanisms of the recent catastrophic landslides in Africa under IPL and WCoE projects

This paper described the recent catastrophic landslides in Sierra Leone, DR Congo, and Nigeria and their probable mechanisms. The landslides occurred at the peak of rainy season on steep mountains predisposed to failure by geologic, climatic, hydrologic, and tectonic factors. The landslides killed over 500 people and displaced more than 2000 people. The DR Congo landslide occurred a few days after the Sierra Leone landslide of August 14, 2017. The Sierra Leone mudslide occurred in the Regent area of Freetown on the steep slopes of Mount Sugar Loaf. Sugar Loaf forms a part of extensive and forested highlands south of the city. The cities of Freetown, Lokoja in Nigeria, Conakry in Guinea, and other cities in Africa have grown in recent times to almost encircle vulnerable highlands that are prone to failure. The internal friction angles of the slope materials were low at all saturation and normal stress levels used in the study. The result correlated with the stability analysis which showed that the factor of safety along the potential sliding surfaces was at critical value at failure. The number of casualties following a slip of the weak slope materials increased because of the housing pattern and the lack of effective drainages in urban cites in Africa. These factors are important considerations for future prevention and mitigating measures aimed at saving Africa from catastrophic slope failures.

Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath

Abstract. Landslide hazard motivates the need for a deeper understanding of the events that occur before, during, and after catastrophic slope failures. Due to the destructive nature of such events, in situ observation is often difficult or impossible. Here, we use data from a network of 58 seismic stations to characterise a large landslide at the Askja caldera, Iceland, on 21 July 2014. High data quality and extensive network coverage allow us to analyse both long- and short-period signals associated with the landslide, and thereby obtain information about its triggering, initiation, timing, and propagation. At long periods, a landslide force history inversion shows that the Askja landslide was a single, large event starting at the SE corner of the caldera lake at 23:24:05 UTC and propagating to the NW in the following 2 min. The bulk sliding mass was 7–16 × 1010 kg, equivalent to a collapsed volume of 35–80 × 106 m3. The sliding mass was displaced downslope by 1260 ± 250 m. At short periods, a seismic tremor was observed for 30 min before the landslide. The tremor is approximately harmonic with a fundamental frequency of 2.3 Hz and shows time-dependent changes of its frequency content. We attribute the seismic tremor to stick-slip motion along the landslide failure plane. Accelerating motion leading up to the catastrophic slope failure culminated in an aseismic quiescent period for 2 min before the landslide. We propose that precursory seismic signals may be useful in landslide early-warning systems. The 8 h after the main landslide failure are characterised by smaller slope failures originating from the destabilised caldera wall decaying in frequency and magnitude. We introduce the term “afterslides” for this subsequent, declining slope activity after a large landslide.

Integration of ground-based radar and satellite InSAR data for the analysis of an unexpected slope failure in an open-pit mine

On 17 November 2016, an unexpected slope failure occurred in an undisclosed copper open-pit mine. The nature of the event urged for a thorough back-analysis of slope monitoring data in order to assess its size and temporal evolution, and to determine whether precursors potentially able to anticipate the failure were present. To this aim, satellite InSAR data spanning over the final 9 months before the event were, in retrospect, acquired and coupled with measurements from a ground-based radar that was in use at the time of the failure. Although progressive deformation was detected by the ground-based radar in correspondence of the two uppermost benches in the pit, the satellite InSAR data revealed that the vast majority of the instability actually involved a large part of natural slope above the mine crest. This sector was not visible by the ground-based radar. Thanks to the short revisit time of the Sentinel-1 mission, clear slope accelerating creep was observed for the first time in satellite InSAR measurements over an open-pit mine. The delimitation of the area featuring accelerating creep behavior matched remarkably the source area of the failure as mapped in the field after the event. Considerations on the volume of the instability and on the development of the failure process (both in space and time) were consequently derived. The results provided a clear example of the value of jointly using ground-based and satellite interferometry to reduce the uncertainties inherent to the identification and characterization of impending catastrophic slope failures.

Coastal cliffs, rock-slope failures and Late Quaternary transgressions of the Black Sea along southern Crimea

Rock-slope failures represent a significant hazard along global coastlines, but their chronology remains poorly documented. Here, we focus on the geomorphology and chronology of giant rockslides affecting the Crimean Mountains along the Black Sea coast. Geomorphic evidence suggests that high (>100 m) limestone cliffs flanking the southern slopes of the Crimean Mountains are scarps of rockslides nested within larger deep-seated gravitational slope deformations (DSGSDs). Such pervasive slope failures originated due to lateral spreading of intensively faulted Late Jurassic carbonate blocks moving atop weak/plastic Late Triassic flysch and tuff layers. By introducing a dating strategy relying on the combination of the uranium-thorium dating (U-Th) of exposed calcareous speleothems covering the landslide scarps with the 36Cl exposure dating of rock walls, we are able to approximate the time interval between the origin of incipient crevices and the final collapse of limestone blocks that exposed the cliff faces. For the three representative large-scale rockslides between the towns of Foros and Yalta, the initiation of the DSGSDs as evidenced by the widening of crevices and the onset of speleothem accumulation was >300 ka BP, but the recent cliff morphology along the coast is the result of Late Pleistocene/Holocene failures spanning ∼20–0.5 ka BP. The exposures of rockslide scarps occurred mostly at ∼20–15, ∼8, ∼5–4 and ∼2–0.5 ka, which substantially coincide with the last major Black Sea transgressions and/or more humid Holocene intervals. Our study suggests that before ultimate fast and/or catastrophic slope failures, the relaxation of rock massifs correlative with karstification, cracks opening, and incipient sliding lasted on the order of 104–105 years. Rapid Late Glacial/Holocene transgressions of the Black Sea likely represented the last impulse for the collapse of limestone blocks and the origin of giant rockslides, simultaneously affecting the majority of the SW coast of the Crimean Peninsula.

Sedimentology and geomorphology of a large tsunamigenic landslide, Taan Fiord, Alaska

On 17 October 2015, a landslide of roughly 60×106m3 occurred at the terminus of Tyndall Glacier in Taan Fiord, southeastern Alaska. It caused a tsunami that inundated an area over 20km2, whereas the landslide debris itself deposited within a much smaller area of approximately 2km2. It is a unique event in that the landslide debris was deposited into three very different environments: on the glacier surface, on land, and in the marine waters of the fjord. Part of the debris traversed the width of the fjord and re-emerged onto land, depositing coherent hummocks with preserved source stratigraphy on an alluvial fan and adjacent moraines on the far side of the fjord. Imagery from before the landslide shows that the catastrophic slope failure was preceded by deformation and sliding for at least the two decades since the glacier retreated to its current terminus location, exposing steep and extensively faulted slopes. A small volume of the total slide mass remains within the source area and is topped by striated blocks (>10m across) and standing trees that were transported down the slope in intact positions during the landslide. Field work was carried out in the summer of 2016, and by the time this paper was written, almost all of the supraglacial debris was advected into the fjord and half the subaerial hummocks were buried by glacial advance; this rapid change illustrates how highly active sedimentary processes in high-altitude glacial settings can skew any landslide-frequency analyses, and emphasizes the need for timely field investigations of these natural hazards.

The Benner pass rock avalanche cluster suggests a close relation between long-term slope deformation (DSGSDs and translational rock slides) and catastrophic failure

In mountain ranges deep-seated gravitational slope deformations (DSGSDs) and extremely rapid mass wastings of rock >105m3 in volume (catastrophic rock-slope failures, CRF) are present, yet their mutual relation is poorly documented. Near the Brenner Pass (1370m asl) in the eastern Alps, five catastrophic rock-slope failures of medium- to high-grade metamorphites are clustered (‘Brenner Pass Cluster’; BPC), and three of them are related to DSGSDs. The catastrophic rock-slope failures involved volumes from 12 to 110Mm3 and show fahrboeschung angles of 10–27°. Numerical dating (14C, 234U/230Th) suggests that all catastrophic slope failures of the BPC occurred between ≤13.5 and 6.2ka. Three of the CRF events may have occurred during the Younger Dryas (12.7–11.7ka), whereas two events occurred during the Holocene. Backwater basins dammed up by the CRFs range from 2.5km2 (Ridnaun rock avalanche) to 15.5km2 (Stilfes rock avalanche).Three of the catastrophic rock-slope failures are associated with and developed as a partial failure of a DSGSD. This suggests that progressively slow deformation of slopes ultimately exceeded a stability threshold, resulting in catastrophic rock-slope failures. The initial kinematic mechanisms of failure vary between large-scale toppling, wedge sliding, and planar sliding and are strongly controlled by the structural setting of the slopes.A direct connection of catastrophic mass wasting with specific palaeoclimatic conditions (e.g., phases of enhanced precipitation) is not indicated; however, this does not exclude specific meteorological situations (e.g., occurrence of short-term heavy rainfall) that may have expedited slope instability and perhaps even triggered catastrophic events.Attempts to correlate catastrophic rock-slope failures with specific palaeoclimatic regimes are still encumbered by substantial methodical uncertainties and imprecisions as well as the scarcity of dated CRF events. The mapped distribution of CRFs unequivocally indicates that structural predisposition is the most significant long-term control in forming CRF clusters.

Giant landslides and highstands of the Caspian Sea

Abstract The history of Quaternary sea-level changes in the Caspian Sea, the world’s largest lake, is partly enigmatic, and so is the geomorphic response of its coasts. Late Pleistocene transgressions during the Early Khvalynian (ca. 40–25 ka) inundated extensive portions of the flat, low-lying semi-desert of western Kazakhstan. Cliffs cut during these highstands form a prominent escarpment tens of kilometers to several hundred kilometers from the present coast of the Caspian Sea. Satellite images, digital terrain analysis, and field mapping reveal that >300 giant landslides intersect with this escarpment. More than 100 of these slope failures mobilized volumes >108 m3 along basal failure planes with gradients as low as ∼5°. All landslides share characteristics of lateral rock spreads involving competent limestones overlying weak and plastic claystones. From relative stratigraphy and new 14C data, we infer that catastrophic slope failure of over 41 km3 occurred mostly during Pleistocene Caspian sea-level highstands, while several landslides may have been reactivated or entirely originated during the Holocene. This largest cluster of terrestrial mass wasting in a tectonically quiescent setting offers an opportunity to understand how landslides erode low-relief landscapes subject to oscillating sea levels.

Berm design to reduce risks of catastrophic slope failures at solid waste disposal sites.

Existing waste disposal sites are being strained by exceeding their volumetric capacities because of exponentially increasing rates of municipal solid waste generation worldwide, especially in densely populated metropolises. Over the past 40 years, six well-documented and analyzed disposal sites experienced catastrophic failure. This research presents a novel analysis and design method for implementation of a series of in-situ earth berms to slow down the movement of waste material flow following a catastrophic failure. This is the first study of its kind that employs a dynamic landslide analysis model, DAN-W, and the Voellmy rheological model to approximate solid waste avalanche flow. A variety of single and multiple berm configuration scenarios were developed and tested to find an optimum configuration of the various earth berm geometries and number of berms to achieve desired energy dissipation and reduction in total waste material runout length. The case study application of the novel mitigation measure shows that by constructing a series of six relatively inexpensive 3 m high earth berms at an optimum distance of 250 m from the slope toe, the total runout length of 1000 m and associated fatalities of the Leuwigajah dumpsite catastrophic failure in Bandung, Indonesia, could have been reduced by half.

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.

Nondestructive natural gas hydrate recovery driven by air and carbon dioxide.

Current technologies for production of natural gas hydrates (NGH), which include thermal stimulation, depressurization and inhibitor injection, have raised concerns over unintended consequences. The possibility of catastrophic slope failure and marine ecosystem damage remain serious challenges to safe NGH production. As a potential approach, this paper presents air-driven NGH recovery from permeable marine sediments induced by simultaneous mechanisms for methane liberation (NGH decomposition) and CH4-air or CH4-CO2/air replacement. Air is diffused into and penetrates NGH and, on its surface, forms a boundary between the gas and solid phases. Then spontaneous melting proceeds until the chemical potentials become equal in both phases as NGH depletion continues and self-regulated CH4-air replacement occurs over an arbitrary point. We observed the existence of critical methane concentration forming the boundary between decomposition and replacement mechanisms in the NGH reservoirs. Furthermore, when CO2 was added, we observed a very strong, stable, self-regulating process of exchange (CH4 replaced by CO2/air; hereafter CH4-CO2/air) occurring in the NGH. The proposed process will work well for most global gas hydrate reservoirs, regardless of the injection conditions or geothermal gradient.

Forecasting landslide hazard by the 3D discrete element method: A case study of the unstable slope in the Lushan hot spring district, central Taiwan

Catastrophic landslides and related phenomena are commonly facilitated by the subtropical climate with frequent typhoons and recurrent earthquakes in Taiwan's mountainous areas. One area susceptible to potentially catastrophic landslides is located at Lushan in central Taiwan, which is famous for its hot springs and tourism. The northern slope above the hot spring district slips gradually and frequently due to heavy rainfall. For safety reasons, the slip-affected area has been under constant borehole monitoring by government agencies, and a controversial public debate has arisen over permanent evacuation of the hot spring district. In this study, we attempt to simulate possible scenarios of catastrophic slope failure using slip geometries derived from the monitoring data, and assess potential landslide impact areas by a discrete element method using the PFC3D code. In the worst case scenario, the Lushan hot spring district is predicted to be destroyed by debris in 20s. Besides, the planned regional emergency refuge is rapidly jeopardized by flooding resulted from landslide-dammed lakes. This study addresses catastrophic slope failure under heavy rainfall conditions given a range of friction coefficients and varied continuity of the failure surfaces. It is noted that the PFC3D code has limitations in modeling all complex mechanisms of landslide, particularly in modeling loss of material shear strength due to increases in pore pressure. Nevertheless, the numerical simulation results by the 3D discrete element method provide scenario-based runout paths, particle velocities and landslide-affected areas, which are useful information for decision support and future landslide hazard assessment.

Geomechanics of a snowmelt-induced slope failure in glacial till

An integrated approach involving field, laboratory and numerical investigations was undertaken to study the progressive deformation mechanism of a slope in glacial till above the Town of Alta, Utah that experienced catastrophic failure due to rapid snowmelt in June 2010. Detailed geometry of the slope surface and of the exposed sliding surface obtained from global positioning system surveying together with strength and stress–strain parameters derived from laboratory triaxial tests on undisturbed samples of glacial till collected from the landslide site were employed with finite-element modeling to examine the effects of an increase in snowmelt-induced perched water table on the yielding behavior of the slope prior to catastrophic failure. The numerical results indicate gradual development of a plastic yield zone along the sliding surface with progressive rise in the perched water table above the toe of the slide mass, which was spreading out throughout the slide mass at the stage corresponding to the onset of catastrophic slope failure. The evolution of the slope safety factor obtained from limit-equilibrium stability analyses in relation to the development of the finite-element computed yield zone along the sliding surface is also discussed.

The 1963 Vajont Landslide: 50th Anniversary

This editorial is intended to bring to our minds the 50th Anniversary of the Vajont Landslide occurred on 9 October 1963 at 10.39 pm in Italy, in the Dolomites of Friuli, on the borders of the Veneto Region and about 100 km north of Venice. A mass of approximately 270–300 million m of rock and debris collapsed into the reservoir generating a wave that over-topped the 261.6 m high double curved arch dam built across a V-shaped gorge (Fig. 1). The flood wave dropped into the Piave Valley destroying the town of Longarone and other villages nearby. More than 2,000 people (the real number is not known) were killed. The Vajont landslide is considered to be one of the most catastrophic slope failures and is an outstanding and valuable reference case history for the study and back analysis of the complex instability mechanisms which generally characterize deep-seated landslides and rock slopes. This dramatic event is also of particular importance for the understanding of the influence of mountainside reservoirs on the stability of the adjacent slopes, when deep-seated landslides are present which were not disclosed or fully investigated at the design stage. At the same time, interest stems from the need to find the most appropriate investigation techniques to be adopted at the design stage, including suitable numerical modelling methods to be used for the analysis of instability modes and simulation of the interaction between the rock mass and the new infrastructures to be built. Also the re-analysis of the available database on the triggering and subsequent propagation of the landslide may shed light on monitoring methods—with conventional and advanced technologies— used as key components of hazard assessment. This is of particular relevance for those countries where the need for energy and development leads to the construction of new infrastructures, including dams, of unprecedented sizes. It is noted that this editorial is only intended to remind us what is considered to be the starting point for the development of modern Rock Mechanics and Rock Engineering, when in rock as well as in soil, according to Terzaghi ‘we were over-stepping the limits of our ability to predict the consequences of our actions’ (Hoek 2007). With the intent to account for the most recent and updated studies on the Vajont landslide, the Editor has asked the help of Professor Paolo Paronuzzi from the University of Udine who, together with his research group, has been and is carrying out a comprehensive research project on different aspects and debated questions still posed on the Vajont catastrophic landslide.

Cosmogenic nuclide dating of rocksildes/rock avalanches compared with other dating methods

In mountain ranges deep-seated gravitational slope deformations (DSGSDs) and extremely rapid mass wastings of rock > 105 m3 in volume (catastrophic rock-slope failures, CRF) are present, yet their mutual relation is poorly documented. Near the Brenner Pass (1370 m asl) in the eastern Alps, five catastrophic rock-slope failures of medium- to high-grade metamorphites are clustered (‘Brenner Pass Cluster’; BPC), and three of them are related to DSGSDs. The catastrophic rock-slope failures involved volumes from 12 to 110 Mm3 and show fahrboeschung angles of 10–27°. Numerical dating (14C, 234U/230Th) suggests that all catastrophic slope failures of the BPC occurred between ≤ 13.5 and 6.2 ka. Three of the CRF events may have occurred during the Younger Dryas (12.7–11.7 ka), whereas two events occurred during the Holocene. Backwater basins dammed up by the CRFs range from 2.5 km2 (Ridnaun rock avalanche) to 15.5 km2 (Stilfes rock avalanche).Three of the catastrophic rock-slope failures are associated with and developed as a partial failure of a DSGSD. This suggests that progressively slow deformation of slopes ultimately exceeded a stability threshold, resulting in catastrophic rock-slope failures. The initial kinematic mechanisms of failure vary between large-scale toppling, wedge sliding, and planar sliding and are strongly controlled by the structural setting of the slopes.A direct connection of catastrophic mass wasting with specific palaeoclimatic conditions (e.g., phases of enhanced precipitation) is not indicated; however, this does not exclude specific meteorological situations (e.g., occurrence of short-term heavy rainfall) that may have expedited slope instability and perhaps even triggered catastrophic events.Attempts to correlate catastrophic rock-slope failures with specific palaeoclimatic regimes are still encumbered by substantial methodical uncertainties and imprecisions as well as the scarcity of dated CRF events. The mapped distribution of CRFs unequivocally indicates that structural predisposition is the most significant long-term control in forming CRF clusters.

Early Holocene (8.6 ka) rock avalanche deposits, Obernberg valley (Eastern Alps): Landform interpretation and kinematics of rapid mass movement

In the Obernberg valley, the Eastern Alps, landforms recently interpreted as moraines are re-interpreted as rock avalanche deposits. The catastrophic slope failure involved an initial rock volume of about 45million m³, with a runout of 7.2km over a total vertical distance of 1330m (fahrböschung 10°). 36Cl surface-exposure dating of boulders of the avalanche mass indicates an event age of 8.6±0.6ka. A 14C age of 7785±190calyr BP of a palaeosoil within an alluvial fan downlapping the rock avalanche is consistent with the event age. The distal 2km of the rock-avalanche deposit is characterized by a highly regular array of transverse ridges that were previously interpreted as terminal moraines of Late-Glacial. ‘Jigsaw-puzzle structure’ of gravel to boulder-size clasts in the ridges and a matrix of cataclastic gouge indicate a rock avalanche origin. For a wide altitude range the avalanche deposit is preserved, and the event age of mass-wasting precludes both runout over glacial ice and subsequent glacial overprint. The regularly arrayed transverse ridges thus were formed during freezing of the rock avalanche deposits.

Is climate change responsible for changing landslide activity in high mountains?

ABSTRACTClimate change, manifested by an increase in mean, minimum, and maximum temperatures and by more intense rainstorms, is becoming more evident in many regions. An important consequence of these changes may be an increase in landslides in high mountains. More research, however, is necessary to detect changes in landslide magnitude and frequency related to contemporary climate, particularly in alpine regions hosting glaciers, permafrost, and snow. These regions not only are sensitive to changes in both temperature and precipitation, but are also areas in which landslides are ubiquitous even under a stable climate. We analyze a series of catastrophic slope failures that occurred in the mountains of Europe, the Americas, and the Caucasus since the end of the 1990s. We distinguish between rock and ice avalanches, debris flows from de‐glaciated areas, and landslides that involve dynamic interactions with glacial and river processes. Analysis of these events indicates several important controls on slope stability in high mountains, including: the non‐linear response of firn and ice to warming; three‐dimensional warming of subsurface bedrock and its relation to site geology; de‐glaciation accompanied by exposure of new sediment; and combined short‐term effects of precipitation and temperature. Based on several case studies, we propose that the following mechanisms can significantly alter landslide magnitude and frequency, and thus hazard, under warming conditions: (1) positive feedbacks acting on mass movement processes that after an initial climatic stimulus may evolve independently of climate change; (2) threshold behavior and tipping points in geomorphic systems; (3) storage of sediment and ice involving important lag‐time effects. Copyright © 2011 John Wiley & Sons, Ltd.

Challenges in Understanding the Hydrologic Controls on the Mobility of Slow‐Moving Landslides

Slow‐moving landslides are a wide‐spread type of active mass movement, can cause severe damages to infrastructure, and may be a precursor of sudden catastrophic slope failures. Pore‐water pressure is commonly regarded as the most important among a number of possible factors controlling landslide velocity. We used high‐resolution monitoring data to explore the relations of landslide mobility and hydrologic processes at the Heumöser landslide in Austria, which is characterized by continuous slow movement along a shear zone. Movement rates showed a seasonality that was associated with elevated pore‐water pressures. Pore pressure monitoring revealed a system of confined and separated aquifers with differing dynamics. Analysis of a simple infinite slope mobility model showed that small variations in parameters, along with measured pore pressure dynamics, provided a perfect match to our observations. Modeling showed a stabilizing effect of snow cover due to the additional load. This finding was supported by a multiple regression model, which further suggested that effective pore pressures at the slip surface were partially differing from the borehole observations and were related to preferential infiltration and subsurface flow in adjacent areas. It appears that in a setting like the Heumöser landslide, hydrologic processes delicately influence slope mobility through their control on pore pressure dynamics and the weight of the landslide body, which challenges observation and modeling. Moreover, it appears that their simplicity, and especially their high sensitivity to parameter variations, limits the conclusions that can be drawn from infinite slope models.

Catastrophic slope failure and its origins: Case of the May 2010 Girová Mountain long-runout rockslide (Czech Republic)

One of the largest long-runout landslides in the modern history of the Czech Republic originated on the southern slopes of the Girová Mountain (Outer Western Carpathians) at the end of extreme rainfalls in May 2010. The structurally predisposed Girová Mountain rockslide occurred very quickly within fault-weakened, deeply-weathered flysch bedrock. Its main phase, during which a wedge-like block collapsed in the upper part of slope, lasted for a few hours. The overloaded slope collapsed and its motion lasted for the following 3days. The May 2010 Girová Mountain rockslide is an excellent case of a recurrent long-runout landslide nested within a deep-seated gravitational slope deformation with a long history of Late Quaternary mass movements. The chronology of slope failures within the studied slope was reconstructed by means of radiocarbon dating and dendrogeomorphological analysis. At least one Holocene long-runout landslide (7.4calka BP) and several smaller failures (~1.5 and ~0.6calka BP) preceded this recent catastrophic failure. Dendrogeomorphological analysis showed that the failure had been preceded by at least 80-year-long creep movement that accelerated over the years due to extreme rainfall events. Despite the fact that extreme precipitation of May 2010 represented the last incremental change before catastrophic collapse of the slope, its main reason was gradual weakening of rock massif and concentrated creeping (with some accelerated phases) in the upper part of the slope deformation.

Megafans and outsize fans from catastrophic slope failures in Alpine glacial troughs: the Malser Haide and the Val Venosta cluster, Italy

Abstract A cluster of exceptionally large sediment fans occurs in Val Venosta, a glacial trough in the east-central Alps, Italy. Its 59 tributary valleys generate 49 fans with volume:catchment area ratios varying across four orders of magnitude. Geomorphological and statistical analysis distinguish ‘allometric’ and ‘anomalous’ fans. Catastrophic massive slope failure origins are suggested for the anomalous cases. They comprise ‘outsize fans’ and ‘megafans’, the latter attaining 400 m cone height and 2700 m radius, and dominating the trough. Above most fans, evidence is found for source cavities of comparable volume. Reconstruction of the missing sides and heads of two tributary valleys reveals lost mountains 700 m deep. They are credible sources for the Malser Haide, a globally significant 11 km-long megafan with an estimated volume of 1650 Mm 3 , and the St Valentin outsize fans. Generally, anomalous fans occur where landslides are funnelled, comminuted and controlled through ‘debouchures’ high enough above the trough floor for conoidal deposition. Although sedimentological data are sparse, these fans may represent a new category of catastrophic slope failure outcome, mimicking conventional sediment fans of incremental origin. The Val Venosta cluster is the largest in the Alps, with concentrated glacial erosion in conducive geology among the possible factors explaining anomalous fan incidence.