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22 August 2019

Reports of multiple fatal landslides in the Wenchuan area of China

Reports of multiple fatal landslides in the Wenchuan area of China

Various news agencies in China are reporting multiple landslides triggered by extreme rainfall in the Wenchuan area, and adjacent regions, of China.  Xinhua has a fairly basic report, with some imagesMore detail is provided by a report by ECNS:

Nine people were killed and 35 were missing after multiple mudslides struck Aba Tibetan and Qiang autonomous prefecture in Sichuan province on Tuesday, local authorities said.

In Wenchuan itself, ECNS reports:

At least one firefighter was dead and three people were reported missing after multiple mudslides hit Wenchuan County, southwest China’s Sichuan Province, local authorities said.

Whilst Xinhua reports:

At least four people were dead and 11 were reported missing in Wenchuan County after multiple mudslides happened following heavy rainfall. Seven people were missing and 60 were trapped in Wolong special administrative zone in the county.

Gengda Township in Wolong appears to be singled out as having been badly affected, albeit little real detail is available as yet.  This image, via ECNS, is from Gengda township:-

Wenchuan landslide

The aftermath of a debris flow in Gengda township, Wenchuan. Image via ECNS, credit: LIU ZHONGJUN/FOR CHINA DAILY.

 

Meanwhile this image from ECNS shows the aftermath of a debris flow in Banquio village in Sichuan:-

Wenchuan landslide

An image of the aftermath of a debris flow in the Wenchuan area of China. Image via ECNS

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This area was, of course, severely affected by the 2008 Wenchuan earthquake, with disastrous consequences.  The mountains have continued to be very seriously impacted by landslides since then, with significant losses occurring most years.  In a recent paper on this event, Fan et al. (2018) noted that:

Although the slope self-healing process (grain coarsening, consolidation, revegetation) does rebalance the rainfall threshold to that of a pre-earthquake level, the debris flow activity has not yet reached the pre-2008 background levels.

It appears that this situation continues.

Reference

Xuanmei Fan, C. Hsein Juang, Janusz Wasowski, Runqiu Huang, Qiang Xu, Gianvito Scaringi, Cees J. van Westen, Hans-Balder Havenith. 2018. What we have learned from the 2008 Wenchuan Earthquake and its aftermath: A decade of research and challenges, Engineering Geology, 241, 25-32. https://doi.org/10.1016/j.enggeo.2018.05.004. (https://www.sciencedirect.com/science/article/pii/S0013795218307233)

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21 August 2019

The Kavalappara landslide in the western Ghats of India

The Kavalappara landslide in the western Ghats of India

At 7:30 pm on 8th August 2019 the large the Kavalappara landslide in Wayanad, part of the western Ghats in Kerala, India buried a small community.  Whilst rescue operations continue, it has been reported that 46 bodies have been recovered from the debris, whilst it is believed that a further 13 people were killed.  The landslide was triggered by a period of intense monsoon rainfall.  The Indian Express has some details about the precipitation event that initiated the landslide:

Malappuram meteorological subdivision got 189.4 mm rain from August 1 to August 7, 66% more than the normal 114.3 mm. On August 8, the Nilambur rain gauge station, the one nearest to Kavalappara, recorded the highest rainfall of that day in Kerala.

Indian Express also has this image of the landslide, which appears to have been both large and mobile:-

The Kavalappara landslide

The 8th August 2019 The Kavalappara landslide, which killed 59 people. Image via the Indian Express.

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Meanwhile, the Deccan Chronicle has a different perspective on the landslide:-

The Kavalappara landslide

The 8th August 2019 Kavalappara landslide in the Western Ghats of India. Image via the Deccan Chronicle.

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The deep red soils indicate a high degree of weathering.  The upper quarter of the landslide source appears to have exposed bedrock, perhaps suggesting that the initial failure occurred on the interface between weathered and intact rock.  This portion of the slide surface appears to be much steeper than the rest of the landslide source.  The lower three-quarters of the landslide source appear to be situated in weathered soils or regolith.  Note that the flow bifurcated into three streams, presumably controlled by the underlying topography.  This has left an undisturbed island of trees, which is reminiscent of the Pingdi landslide in China that I highlighted yesterday.  The other flow path has cut a separate track.

The heavy rainfall that triggered this landslide caused widespread damage.  At Puthumala, also within Wayanad district, 17 people  were killed when 53 houses, a mosque, a temple and a shop were buried by another large landslide.

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20 August 2019

A high resolution Planet Labs image of the Pingdi landslide in China

A high resolution Planet Labs image of the Pingdi landslide in China

The 23rd July 2019 Pingdi landslide in Shuicheng County in the city of Liupanshui in Guizhou Province, China is thought to have killed 51 peopleThis was a large, complex landslide associated with heavy rainfall.  As is usual in China, after the initial reports of the losses and the heroic acts of the rescuers, little information has emerged as to the causes of the landslide.  We can expect that it will be analysed in a research paper in due course, but this is unlikely to emerge for a year or so.

Planet Labs have however captured a high quality satellite image of the site, which they have kindly made available to me.  Once again I am indebted to Rob Simmon for his help with this.  The image was captured on 2nd August 2019:-

Pingdi landslide

A Planet Labs SkySat image of the Pingdi landslide in Guizhou Province China. Image collected on 2nd August 2019, copyright Planet Labs, used with permission.

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There are aspects of this landslide that are interesting:

  • The road crossing the upper part of the landslide is clear.  I have previously suggested that this may have played a role in the initial failure.  Note the apparent fresh cuts on the upslope side of the road, suggesting recent widening.  It is clear that a number of these cut slopes have also undergone small scale failures.  I continue to wonder if this road may have been a key part of the failure process;
  • There appears to be a number of other smaller-scale failures located in the vicinity of the landslide.  This may indicated very localised, extreme rainfall;
  • The runout behaviour of the toe of the landslide is quite complex, as this part of the image shows:
Pingdi landslide

A Planet Labs SkySat image of the toe of the Pingdi landslide in Guizhou Province China. Image collected on 2nd August 2019, copyright Planet Labs, used with permission.

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Further up the slope the landslide track bifurcated and then recombined before running out.  On the right side of the images parts of the landslide debris have entered and followed existing channels, in one case crossing and blocking another road,  To me this suggests quite high mobility.

Reference and acknowledgement

Thanks to Robert Simmon for his help in acquiring and processing the image.

Planet Team (2019). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/

 

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8 August 2019

The South Tahoma Glacier/Tahoma Creek Outburst Flood & Debris Flow on 5th August 2019

The South Tahoma Glacier/Tahoma Creek Outburst Flood & Debris Flow on 5th August 2019

With thanks to Scott Beason, Park Geologist, Mount Rainier National Park.

On Monday evening, 5th August 2019, an outburst flood and debris flow occurred at Mount Rainier National Park. Between approximately 6:48 – 7:58 PM PDT (8/6/2019 01:48 – 2:58 UTC), the park’s seismographs picked up four separate debris flow sequences in the Tahoma Creek area. Scott notes that:

“On arrival in the park on Tuesday morning, I observed extreme sediment transport/hyperconcentrated flows at the road bridge, approximately 13 km downstream of the glacier. Once I got in the office, I checked the seismic records and saw a clear debris flow signature (see below). After checking with the USGS/Cascades Volcano Observatory, who concurred on the debris flow signature, we closed the park’s West Side Road (which parallels Tahoma Creek and is downstream of the South Tahoma Glacier) until we could undertake a ground and aerial reconnaissance to determine the source and additional hazards”

Mount Rainier debris flow

The seismogram record that captures the Mount Rainier Debris Flow. Provided by Scott Beason. Data from Pacific Northwest Seismic Network (PNSN) and USGS/Cascade Volcano Observatory.

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Seismic data shows that the debris flows had four discrete pulses (see the earthquake spectra image and seismic records below) and Scott has found numerous debris flow levees and boulder fields in the field.

Mount Rainier debris flow

The earthquake spectra for the Mount Rainier debris flow event. Image provided by Scott Beason. Data from Pacific Northwest Seismic Network (PNSN) and USGS/Cascade Volcano Observatory.

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Mount Rainier debris flow

Detailed seismic record for the Mount Rainier debris flow event. Image provided by Scott Beason. Data from Pacific Northwest Seismic Network (PNSN) and USGS/Cascade Volcano Observatory.

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Scott and his colleagues were able to collect images of the aftermath of  the event via a helicopter flight and fieldwork. These are available via a Flickr page – they are fabulous:-

Mount Rainier debris flow

The bulking zone of the Mount Rainer debris flow, Image by Scott Beason via Flickr.

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Initial satellite image analysis by Scott suggests that the outlet stream from the South Tahoma changed locations:-

Mount Rainier debris flow

An initial analysis, by Scott Beason, of the source of the Mount Rainier debris flow.

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Scott’s initial hypothesis that this was a catastrophic change that then rapidly incised into debris-covered stagnant ice and ground-cored moraine just downstream and between bedrock “steps” below the glacier. Once it had enough momentum, it then rapidly bulked up into little ice age ground moraine several km’s below the terminus. The run out ended about 6.5 – 7 km’s downstream where the boulder fields were deposited.

Interestingly, this event is remarkably similar to the 2015 debris flow sequence.

There is much more to write about this event, and I will return to it in the next few days.  In the meantime, many thanks to Scott for highlighting it to me, and for the amazing data.

Wow!

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7 August 2019

Deviot, Tasmania: the impact of a slow moving landslide on home owners

Deviot, Tasmania: the impact of a slow moving landslide on home owners

In Australia today, news agencies are reporting a dispute between the government and home-owners over the impact of a reactivated, slow-moving landslide.  The location is Deviot in Tasmania, where a landslide is slowly destroying five properties.  It is a common, and extremely sad story of the impact that a landslide has had on everyday lives.

The location is -41.226, 146.924 – there is good Google Earth imagery of the site, taken in early 2016:-

Deviot landslide

The site of the landslide at Deviot in Tasmania. Imagre via google Earth, collected in January 2016.

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In 2013, during a period of heavy winter rainfall, the landslide at this site in Deviot reactivated, causing structural damage to a house.  Further significant damage occurred in 2016, with the movement affecting five houses.  At least one has had to be demolished.  Such an event is usually a disaster for the property owners, as in most jurisdictions domestic insurance does not cover landslide damage.  The level of impact of the landslide is dramatic:-

Deviot landslide

Structural damage caused by the Deviot landslide in 2016. Image by Nick Daking (the property owner) via ABC News.

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This story, which is far from unique, has come to prominence because the property owners allege that the local authority were fully aware of the landslides at this site, but failed to inform the property owners of the hazard.  The situation is being considered in the Upper House of the Tasmania Government.

Interestingly, there is a detailed geotechnical report on the site online (NB PDF), written this year by a geotechnical consultant, Dr Derek Pennington.  This includes a map, from 2011, which shows the extent of the relict (historic) landslide and the portion that has reactivated, causing the property damage:-

Deviot landslide

The 2011 MRT geomorphological map showing the Deviot landslide. Extracted from the report by Dr Derek Pennington.

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The report also demonstrates that the landslide reactivated during unusually wet periods:

Deviot landslide

The rainfall record showing periods in which the Deviot landslide reactivated. Figure extracted from the report by Dr Derek Pennington.

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This situation is a tragedy for the householders, who will have suffered huge financial loss and enormous stress.  It is simply the case that an ordinary property owner cannot possibly anticipate this sort of hazard.  Whilst I cannot really comment on the rights or wrongs of the case going before the parliament, we have to recognise that the system is not working correctly when potential property owners have no way to know that their site is at risk, and that there is no mechanism to insure against the risk.  This needs to be addressed as it is occurs again and again around the world.

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6 August 2019

July 2019: a record-breaking landslide month for fatal landslides

July 2019: a record-breaking month for fatal landslides

Yesterday, the European Union’s Copernicus Climate Change Service announced that July 2019 was the warmest month on record, continuing the long period of unsustainable global heating to which we are subjecting our planet.  The statistics are stark – last month was 1.01 C warmer than the average July temperature between 1981 and 2010. The previous record was set in July 2016.  The odds are strongly in favour of the new record being broken within the next few years.

Interestingly, last month I also recorded the highest monthly number of fatal landslides.  Regular readers will know that I have been collating this data since September 2002, a dataset that is now approaching 17 years in duration.  This has formed the basis of a number of papers over the years (e.g. Petley 2012), most recently with my post-doctoral researcher Dr Melanie Froude (Froude and Petley 2018).  The data for the period 2004-2017 can be mapped using our online tool.

In 2019 I recorded a total of exactly 100 fatal landslides, breaking the previous record by a considerable margin.  These landslides killed 358 people.  By comparison, in 2018 I recorded just 51 fatal landslides, causing 148 fatalities.  The graph below shows the cumulative total of fatal landslides, and the resulting deaths, for the year to date (correct as to 5th August 2019), but note that this data has not been subjected to the validation process that will allow it to be included in the main dataset as yet):-

Fatal landslides in July 2019

The cumulative graph for fatal landslides, and the resulting fatalities, in 2019 to date.

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The black line is the number of landslides that I have recorded worldwide that have led to the loss of at least one life, whilst the grey line is the cumulative total of the number of resulting fatalities.  The dashed lines indicate the start and end of July.  The rapid acceleration in the number of fatal landslides at the start of the month is clear.  This reflects the onset of the South Asian monsoon and is the normal pattern.  However, the number of landslides recorded in unprecedented.  In this period, there have been many landslides in South Asia (most notably Nepal, India and Bangladesh), but I have also recorded events in, for example, UK, Norway, USA, Myanmar, Tajikistan, Thailand, Guinea, Switzerland, Brazil, China and elsewhere.

At this stage it is not clear as to the cause of this surprising number of fatal landslides.  It has long been suggested that increased global temperatures will lead to higher rainfall intensities (put simply, a warmer atmosphere can hold more moisture), and there is strong evidence that this is occurring.  Most of the landslides in July 2019 were triggered by short duration, high intensity rainstorms.  Whilst a single month does not prove the link by any means, July 2019 serves to illustrate once again the challenges that we will face in the future.

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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5 August 2019

Grandview Surf Beach: a fatal rock topple in California

Grandview Surf Beach: a fatal rock topple in California

On Friday a small rock topple occurred on sandstone bluffs on Grandview Surf Beach in Encinitas to the north of San Diego, in California, USA.  The topple appears to have occurred without identified precursors.  Three people were tragically killed in this landslide.  This image, from Channel News Asia, provides an overview of the event:-

Grandview Surf Beach rockfall

The rock topple at Grandview Surf Beach in California on 2nd August 2019. Image via Channel News Asia.

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Reports indicate that the three victims were all women from the same family, who were local to the area.

The images above suggest that the failure mechanisms on these cliffs are quite complex, with rock toppling on the steep lower parts of the bluffs and shallow slips on the slightly lower angled upper sections.  In this case the landslide appears to have been primarily the result of a large (some reports suggest about 10 m long) slab that underwent a toppling failure.  Some of the debris from the upper part of the slope might also have been incorporated.  This image, from Flipboard, gives a better sense of this failure event:-

Grandview Surf Beach rockfall

Overview of the aftermath of the rock topple at Grandview Surf Beach on 2nd August 2019. Image via Flipboard.

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The images above both suggest that the sea erodes a notch at the foot of the cliff at this site (such a notch can be seen on the left side of the image above), destabilising the face of the cliff.  This sets up a classic toppling scenario.  Note that the sandstones that form this cliff have little jointing, a situation that allows larger slabs to form and that sets up failures that can be rapid and catastrophic.

Coastal cliffs can be dangerous.  There often seems to be a presumption that rockfalls occur primarily in periods of poor weather.  I am not sure that there is much evidence to support that assumption.

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1 August 2019

Sogn og Fjordane: a series of large landslides in Norway

Sogn og Fjordane: a series of large landslides in Norway

Exceptional rainfall in both the United Kingdom and Norway have triggered substantial flood and landslide problems this week. In the UK, a large-scale evacuation remains in place as a result of the failure of the spillway of the Toddbrook Reservoir in Derbyshire.  Efforts are underway to lower the reservoir level, but concerns remain that the dam could fail.

Meanwhile, in Norway record-breaking rainfall triggered a series of large landslides, primarily in Sogn og Fjordane county.  One of these appears to have caused a fatality when a driver was caught in the flow at the foot of the slope at Årnes, with the car being swept into the Jølstravatnet lake.  As yet the victim has not been recovered. Aftenposten has an article about this event, based upon the eye-witness report of a woman on the other side of the lake.  It includes this image of the aftermath of the landslide:-

Sogn og Fjordane landslide

One of the landslides at Sogn og Fjordane in Norway on 30th July 2019. Image from Aftenposten.

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Once again this appears to be a comparatively small failure on the upper slope, possibly consisting of a failure at the intact rock / regolith interface, which has then entrained slope material to become a debris avalanche.

Elsewhere, there was a series of similar failures. The most dramatic may well have occurred at Vassenden in Jølster, in which the landslide appears to have split the community in half.  The image below is from NK, which has an excellent gallery of pictures of the landslides:-

Sogn og Fjordane landslide

The landslide at Vassenden in Norway on 30th July 2019. Image by Hallstein Dvergsdal, via NK.

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NRK has a report of another series of the landslides, this time at Halvgjerda.  The image below, also from NRK, shows the aftermath of the series of failures:-

Sogn og Fjordane landslides

The landslides at Halvgjerda in Sogn og Fjordane on 3oth July 2019. Image via NRK

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A local lad, Jakob Slåtten, captured one of these in a dramatic video, which can be seen on the NRK page. It’s worth a look.

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31 July 2019

The best landslide videos of 2019

2019 landslide videos

A still from a video on Youtube showing a tractor being hit by a boulder. No casualties were reported.

The best landslide videos of 2019

The (Northern Hemisphere) summer of 2019 is proving to be exceptionally active from a landslide perspective, with multiple events worldwide.  Indeed, to date I have recorded 96 fatal landslide events in July alone, smashing the previous record of 88.  This seems to reflect a large number of intense rainfall events, including an active summer monsoon in both South and East Asia.  The upshot is that a number of good landslide videos are emerging.  Rather than posting them iteratively, I thought it would be good to set up a post to collate them. I will endeavour to keep this updated as new videos emerge.

Whilst the focus will be on landslides during 2019, I will also post interesting videos from previous events too, where I have not featured them previously.  The first on the list is one of these.  Please use the comments to highlight any video that should be included.

1. An intriguing video of a valley-blocking landslide from Hunza, Pakistan (in 2018)

Thanks to Matt Moore for highlighting this one.  I have never seen a slide quite like this – it is a beautiful illustration of the ways in which slope controls movement rate, and multiple pulses of activity can combine to create a complex deposit:

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2. The debris flows at the Tour de France

It’s not often the case that landslides stop a major sporting event.  The extreme rainfall in the French mountains last week triggered debris flows, which were well-captured by the huge numbers of spectators along the route:-

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3. Extreme erosion of fill in Canada

Thanks to Peter Weisinger for highlighting this one.  The journalist Tareq Hadhad in Canada tweeted a beautiful video of rapid erosion, and bank collapse, of fill in Antigonish, Canada.

 

Frustratingly, but oh so predictably, the tweet drew the ire of the ridiculous climate change conspiracy community.  Sigh!

4. Genuinely terrifying “shooting stones” in northern India

If you ever want a reminder of why you should never risk getting in the way of a rockfall, this is it:

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5. Boulder vs tractor in Sikkim

It wasn’t a fair fight. Fortunately no-one was hurt, but this was probably economically ruinous for the owner of the vehicle:-

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6. A landslide near Champawat in India

This landslide reportedly occurred on 31st July in Champawat in Uttarakhand, India.  The slide is almost majestic in progress:-

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7. Six amazing landslide videos from Bhutan

This is a remarkable collection of landslide videos from Bhutan.  The fifth one is particularly interesting:-

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8. Cut slope failures in residual soil in Solan, India

Unfortunately, it is hard to argue that this is anything other than poor design:-

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9. A very near miss for a group of Kayakers on Lake Superior

This bluff collapse occurred at Pictured Rocks National Lakeshore on Lake Superior on 13th August 2019.  It was a near miss for the kayakers.  Remarkably, this collapse was captured from a drone and from the boats:

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There is also a decent video that captures the drone footage in more detail:-

Note the flyrock!

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30 July 2019

The Mount Zandila (Valpola) rock avalanche

The Mount Zandila (Valpola) rock avalanche

This post was kindly provided by Fabio Luino of the Consiglio Nazionale delle Ricerche, Istituto di Ricerca per la Protezione Idrogeologica, Turin, Italy

Email: fabio.luino@irpi.cnr.it.

Introduction

The Mount Zandila landslide (also known as the Valpola rock avalanche) is possibly the most important landslide in Europe in the last 40 years.  It occurred on 28th July 1987 in the valley of the Adda River (Valtellina) in the Central Alps, 130 km NE of Milan. The landslide, which involved about 34 million cubic metres of rock, killed 29 people in the valley below. This post is intended to provide a brief overview of this important event.

Geology and geomorphology

The slope of Mount Zandila affected by the rock avalanche is mainly composed of igneous and metamorphic rocks with limited superficial deposits. The outcropping rocks were predominantly diorite, gabbro and orthoquartzite, with lesser amounts of gneiss and amphibolite; moraine, debris and alluvial deposits are generally situated in the less steep areas. The rocks were heavily fractured and deeply etched by erosion and rockfall processes.

Like most Alpine valleys, Valtellina shows the typical features of a glacial valley with a superimposed fluvial morphology. In this zone, the average ridge elevation reaches 2500 to 3000 m, while the valley bottom lies roughly at 1000 to 1100 m.

Pre-existing instability

A prehistoric landslide, identified from archive aerial photographs dating from 1954 to 1981, had previously affected the upper part of the slope between 2100 and 2360 m elevation. This feeature, consisting of a steep unvegetated slope about 100 m long is easily recognisable on the imagery below.  This appears to represent the sliding surface of a previous landslide on this slope:-

Mount Zandila landslide

Archive image of the site of the Mount Zandila landslide. An unvegetated area (marked by arrow) was clearly present prior to the 1987 failure event.  This is interpreted as being the scar from an earlier landslide event.

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The period prior to the failure

Between 15th and 19th July 1987, rainfall in the Valtellina totaled 229 mm, of which 124 mm occurred on 18th July (Govi & Turitto, 1992). Several landslides, mostly soil slips, were triggered on the slopes in the tributary valleys. During the rainstorm of 18th July, the streams on the flanks of the landslide were affected by numerous debris flows, which continued to occur through to midday the following day. The displaced masses dammed the Adda River, creating a lake of almost 100,000 m² about 100 m upstream from the northern border of the incipient landslide.

Three days before the collapse, a discontinuous tension crack was sighted by a guide. The crack ran for about 600 m along the scarp at an altitude of 2200 m, coinciding with the slide surface of the old landslide on the eastern slope of Mount Zandila.

Between 25th and 27th July, several rockfalls on the eastern slope occurred, including 98 falls in only 24 hours the day before the collapse (Govi & Turitto, 1992). Inhabitants from the villages of Morignone, San Antonio, Poz and Tirindrè were evacuated in response.

The collapse event

At 7:23 a.m. on 28th July 1987, a large mass of rock, with a volume of about 34 million m³, detached from the eastern slope of Mount Zandila (Figure 2). The image below shows the source zone of the landslide:

Mount Zandila landslide

The source area of the 28th July 1987 Mount Zandila landslide.

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The displaced mass, including the prehistorical slide and the bedrock, moved in two short phases: the first came down as a northward slide of the upper part of the slope (marked 6 in the diagram below), whilst the second occurred in a single rapid displacement towards the east into the Adda valley floor (marked 7 below).  This failure inundated the village of Morignone:-

Mount Zandila landslide

Figure 3 – Interpretative maps of the Mount Zandila rock avalanche. 1) Main scarp; 2) debris mostly coarse blocks; 3) debris mostly fine material; 4) strips of old clayey lacustrine deposits mixed with debris landslide; 5) places of maximum accumulation thickness; 6) falling direction of rock slide; 7) falling direction of rock avalanche; 8) climbing direction on the opposite slope and diversions of the displaced mass; 9) falling back directions; 10) muddy wave drifts; 11) cloudy waters tracks and throwing alluvial debris at AEM waste water; 12) muddy wave tracks at the foot of the slopes; 13) cutting down direction of the trees on the borders of the devastated area; 14) basin upstream the displaced mass (from Govi & Turitto, 1992, modified).

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The mass roared up the opposite slope of the valley to about 300 m above the valley floor, then split into two parts, diverted upstream and downstream:

Mount Zandila landslide

General view of the landslide area showing the 2100-m stretch where the valley  was dammed. The flow of water in the channel generated a barrier lake. For this reason inhabitants living downstream had to be evacuated, due to the very high risk of an overtopping and breaching of the dam.

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The mass that moved downstream travelled almost 1400 m from the impact point. The mass plunged into the small lake, displacing alluvial debris and muddy water to a height of 140 metres. The impact unleashed a large wave that moved quickly upstream. Eyewitnesses reported that the wave travelled 1,000 m in about 30 seconds (Govi & Turitto, 1992). Mud marks were surveyed at a maximum height of 95 m near the source, decreasing to 15 m northward at a distance of about 1,300 m. The villages of Poz, San Antonio and Tirindrè were destroyed. More than 2 km upstream, 27 people were killed in the village of Aquilone, which had not been completely evacuated:-

Mount Zandila landslide

Extent of Mount Zandila rock avalanche. The mass rapidly fell towards the Adda valley bottom destroying Morignone. The material moving upstream fell into the small lake splashing the muddy water and the alluvial materials: a large wave moved very quickly sweeping away the other four villages.

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Just before the wave impact, survivors saw the bell tower of San Antonio church shatter from the violent blast, which also blew down also the trees of the opposite slope over 300 m.

A dark dust cloud, extending up to 2 km elevation on the opposite side of the valley and upstream to Massaniga creek, was observed.  It reportedly disappeared about 20 sec later (Azzoni et al., 1992).

No seismic activity was recorded before the collapse. The landslide generated a seismic signal recorded by instruments over 150 km away for a period of about 30 seconds, comparable to a M=3.9 earthquake (Govi, 1989).  The seismograms indicate the detachment of the mass occurred in 18 seconds and the fall in 23 seconds.

The impacts and the remedial works

The valley was dammed by a estimated volume of about 40 million cubic metres more than 2 km long. The average thickness of the accumulation was 30 to 60 m, with a maximum of 90 m:-

Mount Zandila landslide

The Adda valley floor, looking downstream, after the landslide. The valley was dammed by a debris accumulation 2,100 m long and about 200-700 m wide: the great part of this mass was 30 to 60 m deep, with a maximum local thickness of 90 m. The displaced mass moved up to the opposite slope (on the right), attaining a height of 300 m above the valley floor.

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It was made up of material ranging in size from fine debris to coarse blocks up to 10 m across (Azzoni et al., 1992), but mostly consisting of deposits from silt-sand to boulders 30 cm in diameter.

The flow of water in the channel upstream from the landslide dam formed a lake. After thirty days, following an intense rainstorm, the lake filled to a volume of about 20 million cubic meters. Downstream inhabitants had to be evacuated, due to the very high risk of an overtopping and breaching of the dam. As an emergency pumping system proved inadequate for reducing the likelihood of overtopping, a spillway was built at the top of the blockage to control the discharge and to evaluate the resistance of landslide dam to down cutting. The induced flow did not cause significant erosion of the dam, probably due to the gentle slope of the spillway. However, the downstream face of the blockage was reinforced and two diversion tunnels were fully opened the following year.

References

Azzoni A., Chiesa S., Frassoni A. & Govi M. (1992) – The Valpola landslide. Engineering Geology, Elsevier Science Publishers B.V., Amsterdam, 33, 59-70

Govi M. (1989) – The 1987 Landslide on Mount Zandila in the Valtellina, northern Italy. Landslide News, Japan landslide Soc., 3, 1-3.

Govi M. & Turitto O. (1992) – La frana di Val Pola del 1987 in alta Valtellina. In Vallario A. “Frane e territorio”, Napoli, Liguori Ed., 392-414.

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