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28 March 2020

The 28 June 2010 Guanling landslide in China

The 28 June 2010 Guanling landslide in China

On 28 June 2010 the Guanling landslide in Guizhou, China killed 99 people.  Triggered by heavy rainfall, the landslide had a volume of almost a million cubic metres and a travel distance of about 1.4 km.  The landslide is described in an article in the Canadian Geotechnical Journal (Xing et al. 2015).  Another paper, Yin et al. (2011) includes this aerial image of the landslide source and track:-

The Guanling landslide

Aerial image of the source and track of the Guanling landslide. Image from Yin et al. (2011).

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The landslide appears to have occurred early in the 2010 summer monsoon.  Xing et al. (2015) include in their paper the following rainfall information for the days leading up to the landslide.  On 28 May this area received 260 mm of rainfall:-

Rainfall from the 28 June 2010 Guanling landslide

Rainfall data for the 28 June 2010 Guanling landslide, from Xing et al. (2015).

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Note that in an average year, the area has received 309 mm of rainfall by 28 June, so this was an exceptional event. This was the highest rainfall total on record for this area at the time of the Guanling landslide.

The landslide itself occurred at about 2 pm when a mass of Triassic sandstone collapsed.  According to  Xing et al. (2015), the landslide travelled across the valley floor and partially super-elevated on the far side, before descending the slope to impact first Yongwo village, where 21 houses were destroyed. The landslide transitioned into a rock avalanche, travelling rapidly down the valley.  Close to the valley floor, the topography had a sharp bend; the rock avalanche super-elevated once more, striking the village of Dazhai, where it destroyed 17 houses.  Yin et al. (2011) estimate that it had a peak velocity of about 45 metres per second (about 160 km per hour).

The runout distance and velocity of the landslide were unusually high.  Yin et al. (2011) suggest that deposits in the floor of the valley through which the Guanling landslide passed were saturated, providing a low friction surface that accounts for the exceptional mobility.

Every year there are a small number of these large, highly destructive landslides in high mountain areas.  The Guanling landslide illustrates the difficulties of identifying potential locations and forecasting their behaviour.

References

Xing, A., Wang, G., Li, B., Jiang, Y., Feng, Z. and Kamai, T. 2015.  Long-runout mechanism and landsliding behaviour of large catastrophic landslide triggered by heavy rainfall in Guanling, Guizhou, China. Canadian Geotechnical Journal, 52 (7), 971-981. https://doi.org/10.1139/cgj-2014-0122.

Yin, Y., Sun, P., Zhu, J. et al. 2011. Research on catastrophic rock avalanche at Guanling, Guizhou, China. Landslides 8, 517–525. https://doi-org/10.100/s10346-011-0266-8.

 

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27 March 2020

EGU2020: sharing landslide research online

EGU2020 won't look like this

The main exhibition hall from EGU2019. Image via Selfrag.

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EGU2020: sharing landslide research online

The EGU2020 General Assembly was scheduled to run from 3 to 8 May 2020 in Vienna.  It will come as a surprise to no-one that the meeting has been cancelled in light of the Covid-19 crisis.  Delegates are having their registration fees refunded.

Cancellation of EGU2020 was essential and inevitable, but as the largest annual landslide conference it’s a blow to the community.  To try to mitigate the impact of the loss of the meeting, the organisers have put together a plan to share the research online.  This takes the form of two different elements:

1. Those scheduled to present in any form, whether an oral contribution, a poster or one of their PICO talks, will have the opportunity to upload materials on to a dedicated web site.  The system opens for upload on 1 April.  It will allow comments and feedback should the authors wish.

2. On the day of the scheduled sessions, EGU “will open a chat channel per scheduled session, where authors and attendees can actively discuss the presentation materials. Please note that this does not involve live presentations or streaming.”

I’ve listed the landslide hazard sessions in EGU2020 below.  The times are local to Vienna:

NH3.1 Large slope instabilities: characterisation, dating, triggering, monitoring and modelling
Convener: Giovanni Crosta | Co-conveners: Federico Agliardi, Masahiro Chigira, Fabio Vittorio De Blasio

Displays

| Attendance Tue, 05 May, 08:30–10:15 | Display Tue, 05 May, 08:30–19:30

Orals

Convener: Michael Krautblatter | Co-conveners: Axel Volkwein, Anne Voigtländer, Matthew Westoby, Andreas Ewald

Displays

| Attendance Thu, 07 May, 16:15–18:00 | Display Thu, 07 May, 08:30–19:30

Orals

| Thu, 07 May, 14:00–15:45

NH3.4 Hydrological and geomorphological processes in natural and human-modified slopes and landslides

Convener: Thom Bogaard | Co-conveners: Giacomo Pepe, Massimiliano Bordoni, Stella Moreiras, Roberto Greco

Displays

| Attendance Thu, 07 May, 08:30–10:15 | Display Thu, 07 May, 08:30–19:30Orals

Convener: Marcel Hürlimann | Co-conveners: Velio Coviello, Xiaojun Guo, Roland Kaitna

Displays

| Attendance Mon, 04 May, 14:00–18:00 | Display Mon, 04 May, 08:30–19:30

Orals

Convener: Filippo Catani | Co-conveners: Xuanmei Fan, Fausto Guzzetti, Binod Tiwari

Displays

| Attendance Wed, 06 May, 08:30–12:30 | Display Wed, 06 May, 08:30–19:30Hall X2

Orals

Convener: Antonio Abellan | Co-conveners: Janusz Wasowski, Masahiro Chigira, Oriol Monserrat, Jan Burjanek

Displays

| Attendance Wed, 06 May, 14:00–18:00 | Display Wed, 06 May, 08:30–19:30

Orals

Convener: Lorenzo Solari  | Co-conveners: Corey Froese, Peter Bobrowsky, Davide Bertolo, Mateja Jemec Auflič, Federico Raspini, Veronica Tofani

Displays

| Attendance Thu, 07 May, 08:30–10:15 | Display Thu, 07 May, 08:30–19:30Hall X2

Orals

Convener: Paola Reichenbach | Co-conveners: Andreas Günther, Mihai Micu

Displays

| Attendance Thu, 07 May, 14:00–18:00 | Display Thu, 07 May, 08:30–19:30

Orals

Convener: Luca Piciullo | Co-conveners: Stefano Luigi Gariano, Helen Reeves, Samuele Segoni

Displays

| Attendance Tue, 05 May, 10:45–12:30 | Display Tue, 05 May, 08:30–19:30
| Attendance Tue, 05 May, 14:00–15:45 | Display Tue, 05 May, 08:30–19:30

Orals

| Tue, 05 May, 08:30–10:15

NH3.12 Landslides and Soil Erosion in a Changing Climate: Analysis, Trends, Uncertainties and Adaptation Solutions

Convener: Guido Rianna | Co-conveners: Stefano Luigi Gariano, Fausto Guzzetti, Alfredo Reder, Monia Santini

PICOs

| Thu, 07 May, 10:45–12:30

Ironically, for those of us with day jobs that make attending conferences difficult at that time of year difficult, this is really helpful.  I shall look forward to engaging with the EGU2020 meeting from my state of self isolation.

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26 March 2020

The Imom rockslide in Central Asia

The Imom rockslide in Central Asia

Central Asia is probably the finest place on Earth to view and understand giant rockslides and rock avalanches.  The combination of young, rugged mountains, high uplift rates, previous glaciation and very active seismicity means that it is the ideal terrain for these giant landslides.  Furthermore, the arid climate means that the rates of removal of the landslide deposits, and the rate of weathering of the landslide scars, results in exceptional preservation of them in the landscape.  My friend Alexander Strom has worked tirelessly on the documentation of these landslides, and on their analysis, as well as introducing them to the landslide community via his field trips.  It is to my eternal regret that I have been unable to attend as yet.

A very detailed analysis of these landslides is available in a detailed and lavishly illustrated book (Strom and Abdrakhmatov 2018), which provides large numbers of case studies.  Unfortunately, at US$180 per copy this is beyond the reach of most of us, but substantial sections are available online via Google Books.  This provides staggering insight into the diversity of giant landslides in Central Asia.

One nice example is the Imom rockslide.  This is located at 37.692, 72.327 and is beautifully clear in Google Earth:-

The Imom rock avalanche

Google Earth image of the Imom rockslide in Central Asia.

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This is a large landslide – Strom and Abdrakhmatov (2018) suggest it is about 15 million cubic metres – with a very clear scar and a large deposit, a part of which is on the valley floor. The landslide is about 2.4 km from the scar to the front edge of the deposit, and the height difference is about 700 metres.  Interestingly, the authors hypothesise that the material at the front of the landslide may be moraine that was perched on the hillside and that has been bulldozed by the landslide to form a part of the deposit.

Reference

Strom, A. and Abdrakhmatov, K. 2018. Rockslides and Rock Avalanches of Central Asia – Distribution, Morphology, and Internal Structure. Elsevier, 1st Edition. ISBN: 9780128032046.

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25 March 2020

Fatal landslides in 2019

Fatal landslides in 2019

The slowdown in daily activity caused by the Covid-19 crisis, and the cancellation of my planned trip to GNS Science in New Zealand last week, has provided the opportunity to complete the analysis of the fatal landslide database for 2019.  Regular readers will know that this is a dataset that I’ve been collecting since September 2002, and which has formed the basis of a number of research publications (Petley 2012 and Froude and Petley 2018) for example, as well as many posts to this blog.  The ways in which I collect this data are described in detail in the papers (Froude and Petley 2018 is open access).

The final statistics look like this:

  • Number of non-seismic fatal landslides: 463
  • Number of non-seismic landslide fatalities: 3270

In addition, I recorded 14 landslides triggered by earthquakes, with 27 fatalities, but this is undoubtedly a significant underestimate in both respects.

The graph below shows the cumulative total number of fatal landslides through 2019:-

Fatal landslides in 2019

The cumulative total number of fatal landslides (black line, right hand axis), and the resultant fatalities (grey line, left hand axis), for 2019.

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I have delineated the most intense period of the summer monsoon in Asia with the vertical black lines – during this time the number of landslides increases dramatically.  As usual the fatality data is much more noisy, dominated by a comparatively small number of large events, such as the Brumadinho tailings dam failure in Brazil.  The graph shows that the Asian monsoon remains the dominant factor in determining the total number of landslides in a given year; outside of the summer monsoon landslides occur at a steady (but increasing) rate.

In terms of the number of fatal landslides, 2019 was the third worst in my dataset.  The record is still 2010 with 496 fatal landslides, whilst second place is held by 2009 with 488.  The average is 382 fatal landslides per year.

On first inspection the higher number in 2019 was possibly a consequence of:

  1. A higher than normal incidence of monsoon induced landslides;
  2. The continuing rise in landslides associated with mining;
  3. A much higher than normal incidence of fatal landslides in Africa.

I will need to undertake more work to verify these hypotheses.

References

Froude, M. J. and Petley, D. N. 2018. Global fatal landslide occurrence from 2004 to 2016. Natural Hazards and Earth System Sciences, 18, 2161-2181, https://doi.org/10.5194/nhess-18-2161-2018.

Petley, D.N. 2012. Global patterns of loss of life from landslides. Geology 40 (10), 927-930.

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24 March 2020

Tendepo: a major landslide in Tambul-Nebilyer district in Papua New Guinea

Tendepo: a major landslide in Tambul-Nebilyer district in Papua New Guinea

On Saturday 21 March 2020 a major landslide occurred at Tendepo in Tambul-Nebilyer district in Papua New Guinea, resulting in 12 fatalities.  There is an accessible and useful account of this event on The Watchers website, which notes that the landslide was triggered by ongoing heavy rainfall, with further events considered to be possible in the coming days.

That article includes this image of the landslide:-

The Tendepo landslide

Tendepo: the aftermath of the major landslide in Tambul-Nebilyer district in Papua New Guinea on 21 March 2020. Image by Jux Ablis via The Watchers.

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Based upon this image this appears to be reasonably deep-seated, possibly rotational slip in deeply weathered residual soil on a slope that has been deforested. The displaced block appears to have disintegrated to generate a major, probably rapid, earthflow.  The mobility of the landslide appears to have been quite high.

This image, from the same article, provides an alternative view of the scar of the landslide:-

Tendepo: the scar of the major landslide

Tendepo: the scar of the major landslide in Tambul-Nebilyer district in Papua New Guinea on 21 March 2020. Image by Caligen Giluwe Ice via The Watchers.

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The National (a Papua New Guinea newspaper) has an account of the landslide:-

The landslide, which began around 1 am, also washed away 13 homes, domestic animals and food gardens … “People who were awake at that time heard a big noise and found out it was a landslide,” Yanga said. He said it was the first time for him to witness a landslide destroying lives and properties. “We are shocked at the moment because this is the first time to witness people die like that in our Tendepo tribe,” Yanga said.

Papua New Guinea is no stranger to major landslides – for example, a few years ago I covered in detail the outrageous 2012 Tumbi Quarry landslide, which killed at least 25 people, and more recently the 25 February 2018 Mw=7.5 earthquake triggered many landslides, causing an unknown number of fatalities.

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23 March 2020

Alborz mountains in Iran: an extraordinary rockfall video

Alborz mountains in Iran: an extraordinary rockfall video

On 20 March 2020 a major rockfall / boulder roll occurred in the Alborz mountains of Iran.  It was caught on a video that has been posted to Youtube:-

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As the video shows, this boulder rolled through a village, which is reported to called “Pit Sara” in Savadkuh County in Mazandaran Province.  The rockfall (or boulder roll) was triggered by heavy rainfall.  It reportedly destroyed four houses, two cattle sheds a bridge and four vehicles.  Reports suggest that the volume of the rock was about 30 cubic metres.

This still from the video shows the moment that the boulder ploughed through one of the houses.  Fortunately, the village had been evacuated ahead of the boulder roll event:-

Alborz mountains boulder roll

The extraordinary boulder roll in the Alborz mountains of Iran on 20 March 2020. Still from a video posted to Youtube.

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The video also captures the aftermath of the boulder roll event, showing total destruction of anything in its path:-

The aftermath of the rockfall in the Alborz Mountains of Iran

The aftermath of the rockfall in the Alborz Mountains of Iran on 20 March 2020. Still from a video posted to Youtube.

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Wikipedia describes the Alborz Mountains thus:-

The Alborz range, also spelled as Alburz, Elburz or Elborz, is a mountain range in northern Iran that stretches from the border of Azerbaijan along the western and entire southern coast of the Caspian Sea and finally runs northeast and merges into the Aladagh Mountains in the northern parts of Khorasan … Mount Damavand, the highest mountain in Iran measuring 5,610.0 m (18,405.5 ft), is located in the Central Alborz Mountains.

There is a grey literature paper on landslides in the Alborz Mountains (NB pdf), which indicates that this is an area with a very significant landslide issue, triggered by both rainfall (as in this case) and earthquakes.

This event is quite reminiscent of the well-known and very spectacular 21 January 2014 Tramin (Termino) rockfall / boulder roll in the South Tyrol in Italy.

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20 March 2020

Sinking ships to stop erosion

Sinking ships to stop erosion

A wide range of approaches have been developed to try to reduce or stop erosion and landslides, with mixed success and (in some cases) no lack of controversyGloucestershire Live has published a nice article about an approach that is new to me, but provided to be surprisingly successful. Along the banks of the River Severn, in southwest England, ships were sunk to protect the riverbanks and to stop erosion.  This was undertaken in a number of locations, but one such spot lies at Purton (located at 51.736, -2.457), and is visible on Google Earth:-

Sinking ships to stop erosion

Sinking ships to stop erosion: Google Earth imagery of the Purton ships graveyard. The ships are buried in the banks between the River Severn to the north and the canal to the south.

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There is a similar situation at Wainlodes (location 51.930, -2.224).

This is what the ships look like on the ground:-

The Purton ships graveyard

Sinking ships to stop erosion: the Purton ships graveyard. Image via Gloucestershire Live.

The Gloucestershire Live article explains the rationale:-

In the case of Wainlodes an ancient crossing point on the river, the barges were sunk just after the First World War to slow the stream that was eating away at the hill causing regular landslips. 

At Purton the need to arrest erosion was much more pressing. In about 1910 the Severn changed its course. Serious and rapid erosion of the bank took place that threatened to break through to the Sharpness to Gloucester Ship Canal, which was a busy commercial waterway bringing timber, grain, oil and other commodities to Gloucester Docks from all over the world. Action had to be taken quickly.

The canal authority bought the hulks of vessels that were at the end of their working life from the owners to use as breakwaters. Tugs took the hulks at full tilt to the place where they were to be beached, then cast off and let them run aground, leaving the man at the tiller marooned. He had to hole the hulk, then be taken off.

It was a remedy that worked. The hulks, also known as wingdykes, slowed the velocity of the river, which then deposited sediment around the wrecks. This reduced the stream further, which meant that more silt was deposited and so the cycle continued until today many of the craft that were beached decades ago now stand on dry, reclaimed land.

Once the plan was proven to work, more vessels were beached. In 1945 there were nine of them, by about 1952 there were 21 and then more were grounded in the 1960s.

This is an unusual but seemingly highly effective way to stop erosion and thus to prevent landslides.  Erosion of riverbanks is proving to be a substantial problem in many parts of the world at the moment; novel approaches may be needed elsewhere too.

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19 March 2020

The May 1971 Saint Jean Vianney landslide disaster

The May 1971 Saint Jean Vianney landslide disaster

One of my daily landslide feeds threw up an interesting reflection today on the May 1971 Saint Jean Vianney landslide disaster in Quebec, Canada.  This landslide, which occurred with little warning at 10:15 pm on 4 May 1971, affected the eastern side of the town, destroying 40 homes and killing 30 people.  This extraordinary image, taken in the aftermath of the landslide, shows the scale of the failure.  Note the houses lying within the landslide mass:-

St. Jean Vianney landslide

The aftermath of the 1971 St. Jean Vianney landslide in Canada. Image published by Gendisasters.

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There are many resources about this landslide available online, and the landslide was described in an article published in the Canadian Geotechnical Journal (Tavenas et al. 1971). The landslide occurred in the Champlain Clay, now known to be a classic sensitive or quick clay. Such materials can generate spectacular retrogressive landslides – I have featured a number of more recent examples on this blog.

This was an enormous landslide – Tavenas et al. (1971) give a volume of 6.9 million cubic metres – and the image below from the paper provides an overview of the full extent of the slide:-

Vertical aerial image of the Saint Jean Vianney Landslide

Vertical aerial image of the Saint Jean Vianney landslide, from Tavernas et al. (1971)

 

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The likely cause of the landslide was a spell of warmer weather, which drove a thaw, followed by two spells of heavy rainfall, one in late April and the second on 3-4 May 1971.  On 24 April a smaller landslide occurred on the periphery of what became the main landslide a few days later.

Tavernas et al. (1971) used eye-witness reports to try to reconstruct the sequence of events for the Saint Jean Vianney landslide. This provides a fascinating set of accounts, starting with this one:

Mr. R. Girard and Mr. J. Tremblay, living in the new development of Saint-Jean-Vianney, reported that their dogs started to behave abnormally at 7.00 p.m. on May 4th, being extremely nervous. Mr. Girard described the behavior of his dog as the same as during a thunderstorm.

From this, and other accounts, Tavernas et al. (1971) proposed that instability started at about 7 pm, with the first major failure occurring at about 10:15 pm. Over the next 45 minutes the landslide appears to have undergone a series of retrogressive events; by 11pm most of the houses that were lost had been destroyed.

There are many harrowing stories from survivors of this landslide,  The Montreal Gazette article includes two such accounts:

Resident François Richard told our reporter he was in his living room watching the hockey game when he heard shouting outside. “He walked 600 metres down his street and saw houses falling one by one.”

One lucky woman survived by crawling onto the roof of her car after it had fallen into the crater. But there were few such stories. Many families were trapped in the liquid mud, which then solidified.

This event greatly increased awareness of the hazards posed by sensitive clays, although sadly such failures continue to occur occasionally, especially in Norway, Sweden and Canada.

Reference

Tavenas, F., Chagnon, J-Y., and La Rochelle, P. 1971. The Saint-Jean-Vianney Landslide: Observations and Eyewitnesses Accounts.  Canadian Geotechnical Journal, 8, 463-478, https://doi.org/10.1139

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17 March 2020

The catastrophic lahars from Mount Kelud in 1919

The catastrophic lahars from Mount Kelud in 1919

On 19 and 20 May 1919 a catastrophic eruption occurred on Mount Kelud in East Java, Indonesia. This major eruption, one of the most deadly of the 20th Century, is estimated to have killed 5,160 people. The vast majority of those died in lahars (volcanic debris flows) triggered by the eruption. This was also one of the worst landslide disasters of the 20th Century.

There is a really interesting paper about the social impact of this event (Mawiyato and Sasmita 2019) available online, open access.  They point out that this is a particularly interesting lahar event in part because of the detailed accounts of the impacts.  The underlying problem at Mount Kelud is the presence of a crater lake, which at the time had an estimated volume of 40 million cubic metres.  The eruption displaced this lake, which entrained large volumes of ash, generating the enormous lahars.  On Researchgate there is a map of the deposits left by these landslides:

Mount Kelud lahars

The lahars generated by the Mount Kelud eruption in 1919. Image posted to Researchgate by Kelvin Rodolfo, modified from an unpublished monograph of the Japan International Cooperation Agency

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Mawiyato and Sasmita (20189) provide an account of the lahars, with a focus on those that travelled southwards to Blitar:

Bladak Dam of Kali Lahar, which was built to reduce the lahar floods of Mount Kelud, was destroyed. With a speed of 60 km/h, the flows of lahar reached Blitar in less than an hour and destroyed everything standing in its way. The scale of damage reached dozens of kilometers of the volcano. Many villages were reported to have been flattened to the ground and even some of them were wipe out from the map. Among the destroyed vilages were Sumbersari, Salam, Ngoran villages in Udanawu district. Villages along the road stretching from Panataran Temple to Blitar were badly devastated and also Omboh, Sidareja and Sumberejo villages of Wlingi.

To illustrate the magnitude of the impact of these lahars, Mawiyato and Sasmita (2019) describe the events in a prison located in the path of the lahars.  About 900 prisoners were trapped in their cells. Of these only about 100 were able to escape the lahar, but they were quickly caught by the flows and were killed.  There is perhaps no worse prospect than that of being trapped in a locked cell as it fills with boiling mud.

The devastation of the lahars are also well described.  For example, this is the account of the damage in the town of Blitar:

Instantly Blitar lost its form as a town, turned into a sea of lahar. It was dark as being covered by cloud resulting from the swift ash rain. The height of the lahar was approximately 1.6 m, houses around the town square were all damaged, many walled houses were collapsed. The Chinese, Dutch, Javanese settlements whose house-buildings between were somewhat distant could be said to be non-existent. Only the houses of brick walls that coincided and hand-in-hand were not so damaged, but the walls were broken down, such as the house of the resident master, the district house, the post office, the bank office, the clinic, the hotel, the detention center and the other.

Apart from the loss of life, the lahars and ashfall caused massive damage to the extensive coffee farms on the flanks of the volcano, and to other agricultural areas.

Mount Kelud erupted again in 1966, killing more than 200 people.  To manage the hazard, a set of tunnels, known as the Ampera Tunnels, were constructed to manage the water levels in the crater.

Reference

Nawiyanto and Sasmita, N. 2019. The Eruption of Mount Kelud in 1919: Its Impact and Mitigation Efforts. In: 1st International Conference on Social Sciences and Interdisciplinary Studies (ICSSIS 2018). Atlantis Press. https://doi.org/10.2991/icssis-18.2019.25.

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16 March 2020

An analysis of the Piz Cengalo landslide

An analysis of the Piz Cengalo landslide

In August 2017 a large landslide initiated on the Piz Cengalo mountain in Switzerland.  It travelled 6.5 km before striking the village of Bondo.  In total it was responsible for eight fatalities.

In an article just published in the journal NHESS, which is open access, Mergili et al. (2020) provide a detailed description and simulation of the landslide.  The first part of the paper provides a really good description of the event.  The landslide was initiated at 09:31 local time on 23 August 2017 when about 3.6 million cubic metres broke off from the eastern face of Piz Cengalo. The sequence of events is shown in the images below, from Mergili et al. (2020):-

Piz Cengalo

The sequence of events for the Piz Cengalo landslide, from Mergili et al. (2020). Original caption:- The 2017 Piz Cengalo–Bondo landslide cascade. (a) Scarp area on 20 September 2014. (b) Scarp area on 23 September 2017 at 09:30 LT, 20 s after release, in frame of a video taken from the Capanna di Sciora. Note the fountain of water and/or crushed ice at the front of the avalanche, most likely representing meltwater from the impacted glacier. (c) Upper part of Val Bondasca, where the channelized debris flow developed. Note the zone of dust- and pressure-induced damage to trees on the right side of the valley. (d) Traces of the debris flows in Val Bondasca. (e) The debris cone of Bondo after the event. Image sources: © Daniele Porro (a), © Diego Salasc/Reto and Barbara Salis (b), and © swisstopo (c–e).

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After detaching, the landslide impacted the glacier below and entrained about 600,000 cubic metres of ice.  In a manner that was similar to the Mount Haast landslide, the mass behaved initially as a rock avalanche.  However, downstream it evolved into a debris flow that swept down the valley to strike the village of Bondo.  Over the next day or so a series of nine debris flow surges struck the village.

Interestingly, two different scenarios have been proposed to explain the behaviour of the landslide, and in particular the formation of the first debris flow.  These are described in the schematic diagram below, from Mergili et al. (2020):-

Piz Cengalo rock avalanche

The two scenarios for the behaviour of the Piz Cengalo rock avalanche, from Mergili et al. (2020).

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In the first scenario (S1), the front of the rock avalanche became highly saturated, and transitioned into the first debris flow.  In the second (S2), the rock avalanche stalled mid-valley, but was overtopped by an avalanching flow of entrained ice and rock fragments, to form the debris flow.

Mergili et al. (2020) have attempted to simulate these two scenarios.  Interestingly, the models indicate that both are plausible, and the simulations do not allow us to determine which is likely to be correct.

The behaviour of these complex landslides remains challenging to understand and model, but is really important if future hazards are to be understood.  The impacts of global heating, which are increasing the frequency of these large events, means that this remains a really important research question.

Reference

Mergili, M., Jaboyedoff, M., Pullarello, J. and Pudasaini, S. P. 2020.  Back calculation of the 2017 Piz Cengalo–Bondo landslide cascade with r.avaflow: what we can do and what we can learn. Natural Hazards and Earth System Sciences, 20 (2), 505-520.  DOI: 10.5194/nhess-20-505-2020.

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