On September 5, 2019, the Veslemannen unstable rock slope (54,000 m
3
) in Romsdalen, Western Norway, failed catastrophically after 5 years of continuous monitoring. During this period, the rock ...slope weakened while the precursor movements increased progressively, in particular from 2017. Measured displacement prior to the failure was around 19 m in the upper parts of the instability and 4–5 m in the toe area. The pre-failure movements were usually associated with precipitation events, where peak velocities occurred 2–12 h after maximum precipitation. This indicates that the pore-water pressure in the sliding zones had a large influence on the slope stability. The sensitivity to rainfall increased greatly from spring to autumn suggesting a thermal control on the pore-water pressure. Transient modelling of temperatures suggests near permafrost conditions, and deep seasonal frost was certainly present. We propose that a frozen surface layer prevented water percolation to the sliding zone during spring snowmelt and early summer rainfalls. A transition from possible permafrost to a seasonal frost setting of the landslide body after 2000 was modelled, which may have affected the slope stability. Repeated rapid accelerations during late summers and autumns caused a total of 16 events of the red (high) hazard level and evacuation of the hazard zone. Threshold values for velocity were used in the risk management when increasing or decreasing hazard levels. The inverse velocity method was initially of little value. However, in the final phase before the failure, the inverse velocity method was useful for forecasting the time of failure. Risk communication was important for maintaining public trust in early-warning systems, and especially critical is the communication of the difference between issuing the red hazard level and predicting a landslide.
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.
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