Suchergebnisse

Abstract. Landslides triggered by rainfall are very common phenomena in complex tropical environments such as the Colombian Andes, one of the regions of South America most affected by landslides every year. Currently in Colombia, physically based methods for landslide hazard mapping are mandatory for land use planning in urban areas. In this work, we perform probabilistic analyses with r.slope.stability, a spatially distributed, physically based model for landslide susceptibility analysis, available as an open-source tool coupled to GRASS GIS. This model considers alternatively the infinite slope stability model or the 2.5-D geometry of shallow planar and deep-seated landslides with ellipsoidal or truncated failure surfaces. We test the model in the La Arenosa catchment, northern Colombian Andes. The results are compared to those yielded with the corresponding deterministic analyses and with other physically based models applied in the same catchment. Finally, the model results are evaluated against a landslide inventory using a confusion matrix and receiver operating characteristic (ROC) analysis. The model performs reasonably well, the infinite slope stability model showing a better performance. The outcomes are, however, rather conservative, pointing to possible challenges with regard to the geotechnical and geo-hydraulic parameterization. The results also highlight the importance to perform probabilistic instead of – or in addition to – deterministic slope stability analyses.

Abstract. The Köfels rockslide in the Ötztal Valley (Tyrol, Austria)
represents the largest known extremely rapid landslide in metamorphic rock
masses in the Alps. Although many hypotheses for the trigger were discussed
in the past, until now no scientifically proven trigger factor has been
identified. This study provides new data about the (i) pre-failure and
failure topography, (ii) failure volume and porosity of the sliding mass, and
(iii) numerical models on initial deformation and failure mechanism, as well
as shear strength properties of the basal shear zone obtained by
back-calculations. Geographic information system (GIS) methods were used to
reconstruct the slope topographies before, during and after the event.
Comparing the resulting digital terrain models leads to volume estimates of
the failure and deposition masses of 3100 and 4000 million m3,
respectively, and a sliding mass porosity of 26 %. For the 2D numerical
investigation the distinct element method was applied to study the
geomechanical characteristics of the initial failure process (i.e. model
runs without a basal shear zone) and to determine the shear strength
properties of the reconstructed basal shear zone. Based on numerous model
runs by varying the block and joint input parameters, the failure process of
the rock slope could be plausibly reconstructed; however, the exact geometry
of the rockslide, especially in view of thickness, could not be fully
reproduced. Our results suggest that both failure of rock blocks and
shearing along dipping joints moderately to the east were responsible for the
formation or the rockslide. The progressive failure process may have taken
place by fracturing and loosening of the rock mass, advancing from shallow
to deep-seated zones, especially by the development of internal shear zones,
as well as localized domains of increased block failure. The simulations
further highlighted the importance of considering the dominant structural
features of the rock mass. Considering back-calculations of the strength
properties, i.e. the friction angle of the basal shear zone, the results
indicated that under no groundwater flow conditions, an exceptionally low
friction angle of 21 to 24∘ or below is required to
promote failure, depending on how much internal shearing of the sliding mass
is allowed. Model runs considering groundwater flow resulted in approximately
6∘ higher back-calculated critical friction angles ranging from
27 to 30∘. Such low friction angles of the basal
failure zone are unexpected from a rock mechanical perspective for this
strong rock, and groundwater flow, even if high water pressures are assumed,
may not be able to trigger this rockslide. In addition, the rock mass
properties needed to induce failure in the model runs if no basal shear zone
was implemented are significantly lower than those which would be obtained
by classical rock mechanical considerations. Additional conditioning and
triggering factors such as the impact of earthquakes acting as precursors
for progressive rock mass weakening may have been involved in causing this
gigantic rockslide.

Abstract. Landslide forecasting and early warning has a long tradition in landslide
research and is primarily carried out based on empirical and statistical
approaches, e.g., landslide-triggering rainfall thresholds. In the last
decade, flood forecasting started the operational mode of so-called ensemble
prediction systems following the success of the use of ensembles for weather
forecasting. These probabilistic approaches acknowledge the presence of
unavoidable variability and uncertainty when larger areas are considered and
explicitly introduce them into the model results. Now that highly detailed
numerical weather predictions and high-performance computing are becoming more
common, physically based landslide forecasting for larger areas is becoming
feasible, and the landslide research community could benefit from the
experiences that have been reported from flood forecasting using ensemble
predictions. This paper reviews and summarizes concepts of ensemble
prediction in hydrology and discusses how these could facilitate improved
landslide forecasting. In addition, a prototype landslide forecasting system
utilizing the physically based TRIGRS (Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability) model is presented to highlight how
such forecasting systems could be implemented. The paper concludes with a
discussion of challenges related to parameter variability and uncertainty,
calibration and validation, and computational concerns.

Abstract. In the morning of 23 August 2017, around 3×106 m3 of
granitoid rock broke off from the eastern face of Piz Cengalo, southeastern Switzerland.
The initial rockslide–rockfall entrained 6×105m3
of a glacier and continued as a rock (or rock–ice) avalanche before evolving into a
channelized debris flow that reached the village of Bondo at a distance of
6.5 km after a couple of minutes. Subsequent debris flow surges followed in
the next hours and days. The event resulted in eight fatalities along its
path and severely damaged Bondo. The most likely candidates for the water
causing the transformation of the rock avalanche into a long-runout debris
flow are the entrained glacier ice and water originating from the debris
beneath the rock avalanche. In the present work we try to reconstruct
conceptually and numerically the cascade from the initial rockslide–rockfall to the first debris flow surge and thereby consider two scenarios in
terms of qualitative conceptual process models: (i) entrainment of most of
the glacier ice by the frontal part of the initial rockslide–rockfall
and/or injection of water from the basal sediments due to sudden rise in
pore pressure, leading to a frontal debris flow, with the rear part largely
remaining dry and depositing mid-valley, and (ii) most of the entrained
glacier ice remaining beneath or behind the frontal rock avalanche and
developing into an avalanching flow of ice and water, part of which overtops
and partially entrains the rock avalanche deposit, resulting in a debris
flow. Both scenarios can – with some limitations – be numerically
reproduced with an enhanced version of the two-phase mass flow model
(Pudasaini, 2012) implemented with the simulation software r.avaflow, based
on plausible assumptions of the model parameters. However, these simulation
results do not allow us to conclude on which of the two scenarios is the more
likely one. Future work will be directed towards the application of a
three-phase flow model (rock, ice, and fluid) including phase transitions in
order to better represent the melting of glacier ice and a more appropriate
consideration of deposition of debris flow material along the channel.

Abstract. Glacial lake outburst floods (GLOFs) are among the most
concerning consequences of retreating glaciers in mountain ranges worldwide.
GLOFs have attracted significant attention amongst scientists and
practitioners in the past 2 decades, with particular interest in the
physical drivers and mechanisms of GLOF hazard and in socioeconomic and other
human-related developments that affect vulnerabilities to GLOF events. This
increased research focus on GLOFs is reflected in the gradually increasing
number of papers published annually. This study offers an overview of recent
GLOF research by analysing 594 peer-reviewed GLOF studies published between
2017 and 2021 (Web of Science and Scopus databases), reviewing the content and
geographical focus as well as other characteristics of GLOF studies. This
review is complemented with perspectives from the first GLOF conference (7–9
July 2021, online) where a global GLOF research community of major
mountain regions gathered to discuss the current state of the art of
integrated GLOF research. Therefore, representatives from 17 countries
identified and elaborated trends and challenges and proposed possible ways
forward to navigate future GLOF research, in four thematic areas: (i) understanding GLOFs – timing and processes; (ii) modelling GLOFs and GLOF
process chains; (iii) GLOF risk management, prevention and warning; and (iv) human dimensions of GLOFs and GLOF attribution to climate change.