25 May 2013
So where is the lateral margin of the Laxiwa Hydropower Station landslide?
A few days ago I posted about the worrying landslide that has developed on the banks of the reservoir created by the Laxiwa Hydropower Station in China, as highlighted in a recent paper by Zhang et al. (2013). As a reminder, this is the Google Earth image of the slope, with the rear scarp of the landslide very clearly visible:
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There are three aspects of this landslide that are worth examining in more detail. All are reasons to be concerned:
1. Evidence for the presence of an ancient landslide before construction of the dam
On Google Earth there is also a set of images of the site before the dam was constructed, dated May 2004. This image is below, taken from the same perspective as the image above:
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Careful examination of the image shows that the landslide scarp was already present – in other words this is an ancient landslide that predates the construction of the dam. A closer view of the scarp suggests that this might be a quite ancient feature – it appears to be quite weathered and degraded. Nonetheless, there can be no doubt that this is an old landslide scarp:
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It is surprising to me that a dam would be constructed in such close proximity to a large landslide, or that extensive works were not undertaken to stabilise the mass. This is a very large landslide – as a rough guide the vertical extent is about 700 m, it is about 1000 m from crown to toe, and it is over 1500 m wide. A rapid collapse of such a large block would be disastrous. So what happened? Was this mass not identified as being a landslide before the dam was constructed – I don’t see how that can be the case as the scarp is so obvious. Or was it thought that the lake would not affect the stability of the block? This is also somewhat surprising, and if so has proven to be incorrect.
2. So where is the lateral margin of the landslide?
A second interesting issue that emerges is the location of the lateral margin of this block. This is another view of the 2004 image; the rear scarp is clearly visible running across the plateau:
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The obvious location of the lateral margin is down the gully that has the huge accumulation of scree at the toe. A logical hypothesis would be that the shear surface is highly fractured, which has allowed active erosion, creating the gully. The concern that arises from this is the location of the dam, as shown in the more recent image:
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The good news is that the dam is probably (though not definitively) off the landslide. The worry is that the lateral scarp of the landslide is with 300 m of the dam at the crest of the structure. This is disturbingly close should a large landslide occur, and it would be interesting to model the change in the stress state of the abutment in the event of a large movement of the landslide.
3. Other possible landslides in the valley
The mass identified is not the only possible landslide in the valley. On the other bank, directly opposite the landslide above, is this mass as shown in the 2004 image:
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There are two candidate landslides here – I have delineated them below:
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Landslide 1 is think quite obvious; landslide 2 is more subtle. In reality these are probably two portions of the same landslide complex that has originated from the large arcuate scarp above. These are rather different landslides from the one on the other bank, with lower gradient sliding surfaces for example. If we look at the recent satellite image, it appears that extensive works have been undertaken on landslide 1; perhaps unsurprising given the proximity of the dam. This appears to include regrading of the slope and possibly some drainage works.
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Much less work appears to have been done on landslide 2 though. This is a much smaller landslide than is the one on the other bank, and at the moment there are no obvious signs of movement, but this slope needs to be monitored very carefully too. A rapid movement event on this slope would be very interesting given the proximity of the dam.
The landslide problems at this dam site are quite extraordinary. At the Japan Geoscience Union meeting this week we had a discussion about how reservoir bank landslides are managed in Japan. The verdict was that when these problems are identified the lake should be drawn right down (carefully) in order to reduce groundwater levels in the slopes and to ensure that any wave generated by a collapse would not overtop the dam. The water level in this satellite image looks to be high, although of course a different approach may have been adopted since then.
Reference
Zhang, D., Wang, G., Yang, T., Zhang, M., Chen, S., & Zhang, F. (2012). Satellite remote sensing-based detection of the deformation of a reservoir bank slope in Laxiwa Hydropower Station, China Landslides, 10 (2), 231-238 DOI: 10.1007/s10346-012-0378-9
24 May 2013
A round-up of recent landslide incidents, including two American children tragically killed by a mudslide whilst collecting fossils
The following landslide incidents caught my eye in the last few days:
1. A tragic mudslide in St Paul, Minnesota
Two young children were killed on Wednesday, and another was seriously injured, when a mudslide struck a school trip collecting fossils in Lilydale Regional Park. It is a little unclear as to exactly what happened, but it appears that the collapse was triggered by heavy rainfall. To lose young children in such circumstances is desperately sad; one can only imagine the shear sense of panic amongst the teachers and children after the event. Collecting fossils is a really important activity that must continue. However, exposure of fossils that can be collected requires both soft rocks and active erosion, which means that care must be taken with regard to landslides, especially during heavy and/or prolonged rainfall.
2. The report on the Johnson’s Landing landslide in BC
On 12th July 2012 a landslide struck the small community of Johnsons Landing in BC, Canada. I covered the landslide extensively at the time (including this memorable video) – it was an unusually large and rapid slide that engulfed the houses below, killing four people:
BC Government image
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The full report is available here – I shall blog again on this in due course – and there is a nice news report about it here. The salient points are:
- The landslide was triggered by record rainfall and snowmelt;
- It was the largest landslide in the region for 12,000 years;
- It could not have been foreseen;
- A high level of threat remains, such that 18 properties are considered to lie in the path of a potential landslide, with five of those being at a very high level of risk.
3. The Bingham Canyon landslide impacts continue
The enormous cost of the Bingham Canyon landslide continue to be felt. Kennecott Utah Copper, the mine operators, have today released information about the “initial workforce implications” of the landslide, which make grim reading. About 100 employees have lost their jobs, with the company warning that more redundancies will follow over the next month.
4. Landslides and the Day of Judgement
According to Arab News, one sign that the Day of Judgement is nigh is that three landslides will strike. In the words of the article:
One will occur in the East, one in the West and one in the Arabian Peninsula. Not much further information has been given concerning these events—and therefore not much can be added. However, the well-known Hadith exegete Ibn Hajar does note that landslides are a well-known occurrence and have occurred often. Therefore, he says, it is likely that the nature of these three landslides which will occur shortly before the Day of Judgment will be of a much greater magnitude and severity, setting them apart from what occurs customarily in this world.
Please be assured that I will keep readers posted should such events occur.
5. Landslides in Hong Kong during heavy rainfall
During heavy rainfall early on Wednesday morning, there were 19 reported minor landslides in Hong Kong:
David Wong vis SCMP
David Wong via SCMP
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Those who follow my Twitter feed ( @davepetley ) will know that I was in Hong Kong at the time, en route to Japan. I can confirm that the rainfall was somewhat intense!
23 May 2013
A wonderful new progressive failure rockslide video
With a hat-tip to Lélé, who pointed this out to me, below is probably the best video of a progressive failure event that has appeared to date. It has been posted on the rts.ch website. This is a landslide that occurred at Riddes in Southwest Switzerland on Friday. It had volume of about 60,000 cubic metres. I have tried to embed it, but WordPress does funny things (life was so much easier on Blogger sometimes!), so if it doesn’t work take a look here:
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The block was presumably creeping, generating the collapses at the margins. The event (rockfall) rate noticeably increases before final failure.
It appears to me that there is still a great deal of material to leave this slope.
17 May 2013
Was the Bingham Canyon landslide the largest historic non-volcanic landslip in North America?
Kennecott Utah, used with permission
Various media agencies are reporting a story about the Bingham Canyon landslide of a few weeks ago, suggesting that this might be the largest landslide in North America in historic times. This has been prompted by some work by Jeff Moore of the University of Utah, who has compared the landslide with other events in North America. Jeff kindly emailed me about the analysis. First, to set the record straight, his analysis is that Bingham Canyon is the largest historic, non-volcanic landslide in North American history. This is an important qualifier as the 1980 Mount St Helens eruption started with a flank collapse that had a volume of about 2.9 to 3.7 billion cubic metres, and so was much larger
Jeff’s calculation for Bingham Canyon looks like this: the estimated mass is 165 million tons, which corresponds to about 55 million cubic metres of source volume and 65-70 million cubic metres of deposit (allowing for bulking of the mass during movement). In comparison, these are the largest recorded non-volcanic landslides in North America according to Jeff’s review:
- Mt. Stellar AK – Sept 14, 2005 – 50 million cubic metres of rock and ice total, but initial detachment = 10-20 million cubic metres of rock.
- Mt. Steele Yukon – July 24, 2007 – between 28 and 80 million cubic metres including a “significant volume of ice”, modeled as 50 million cubic metres deposit.
- Mt. Meager BC Canada – 48 million cubic metres – 6 August 2010 – turned into 12 km debris flow, no deaths, ~largest in Canada.
- Hope slide BC Canada – 47 million cubic metres, January 9, 1965, 4 people killed – two seismic events noted, previous largest in Canada.
- Frank slide NWT Canada – Apr 29, 1903 – 30 million cubic metres – 70-90 deaths – Turtle mountain area today.
- Madison River Canyon (Earthquake lake) Montana – Aug 17, 1959 – 28-33 million cubic metres – 28 deaths.
- Lituya Mountain AK – June 11, 2012 – 5-60 million cubic metres – no deaths, poor volume estimate from deposit on glacier.
- Lituya Bay AK – July 9, 1958 – 30 million cubic metres – 5 killed from tsunami.
I cannot really disagree with this list, but would point out a couple of things. The first is that of course there are larger ones in pre-historic times. Indeed the largest of them all in terms of ancient landslides is the deeply bizarre Heart Mountain landslide in Wyoming, which has a volume of about 3,400 billion cubic metres. But as this is over 40 million years old it does not count as being historic. I think the landslide at Seaward in Alaska triggered by the 1964 earthquake had a volume of about 210 million cubic metres, although the vast majority of this was underwater.
So I think he is probably right that this is the largest historic, non-volcanic, terrestrial landslide in North America in recorded history. That is quite remarkable given that it is man-made. Indeed I have been trying to work out whether this is the largest manmade landslide in history. The obvious candidate is the 1984 Ok Tedi landslide in Papua New Guinea (Griffiths et al. 2004), which was also triggered by mining. However, that appears to have had a volume of about 35 million cubic metres, so it smaller. Can anyone come up with anything larger?
It is a surprise to me that large, historic landslides in North America are so small! The Daguangboa landslide, triggered by the 2008 Wenchuan earthquake, is over 1 billion cubic metres in volume, and this list includes many that are larger than Bingham Canyon. I wonder why this is the case?
Finally, I think it would be fun to start to compile a list of the largest landslides of the 21st Century. Any suggestions?
14 May 2013
How to escape a landslide
Various news reports suggest that a couple in Alaska had a very lucky escape from a landslide on Sunday. The best news report that I have come across is on Alaska Public Media, which also has this image of the landslide (credited to Kevin Knox):
Alaska Public Radio / Kevin Knox
Kevin Knox and Maggie Gallin were staying at Redoubt Lake Cabin in Alaska when, in Kevin’s words:
“We had just tied the boat up and Maggie was in the cabin, and it just let loose — a huge piece off of the side of the mountain. I yelled for Maggie to run, to get out of the cabin. We started running down the beach.”
“We were running along the lakeshore and got thrown into the water, trees kind of toppling on top of us. We both popped up three or four feet from each other. Then we got our wits about us and just tried to hunker down.”
Interestingly, the report indicates that they saw precursory rockfalls the evening before. The couple were extremely luck to escape the landslide; sadly it appears that their dog was killed. The couple were rescued by floatplane a few hours after the landslide.
The website of the former cabin suggests that it would have offered little protection from a landslide. This is a Google Earth perspective view of the slope that failed:
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There are few if any obvious signs of instability on the image. From the photo the left side of the landslide appears to be a wedge failure in bedrock; this in turn has destabilised regolith / colluvium on the right side.
It is quite rare to outrun a landslide in the way that these people have; they are exceptionally fortunate.
13 May 2013
A very dangerous reservoir bank landslide in China
In a technical note that was recently published in the journal Landslides, Zhang et al. (2013) have highlighted a very interesting landslide that is developing on the flanks of the resevoir associated with the Laxiwa Hydropower Station in China. This HEP station consists of the Laxiwa Dam, a very large concrete arch dam in Qinghai Province on the Yellow River, which was commissioned in 2009, and associated reservoir and infrastructure. Construction of the dam has not been completed, but once it is finished it will be 250 m high with a storage capacity of about one cubic kilometre of water.
Large landslides on reservoir banks are a highly sensitive issue. In 1963 a very large landslide into the Vajont reservoir in northern Italy killed about 2500 people; subsequently, dam builders have taken great measures to avoid a repeat of such a problem. Fortunately, despite very large-scale dam construction over the last 50 years, there has been no large-scale recurrence of the Vajont tragedy. This may well be a combination of good management and a certain amount of luck; many people feel that with the current boom in the construction of large dams we may be pushing this luck to the limit.
The landslide that Zhang et al. (2013) lies on the flanks of the Laxiwa reservoir, a little way upstream of the dam. The dam itself is readily visible on Google Earth – put 36.071667, 101.18722 into the search location and it will take you there. The landslide is on the south bank about 500 m upstream of the dam – go to 36.0954, 101.185. The slope started to move soon after impoundment was initiative – the Google earth image of late 2010 shows very substantial levels of movement across the newly formed rear scarp:
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The sits at the top of the mass that is moving – the unstable block is shown below:
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An intial visual inspection of this site rings alarm bells – the landslide is very large, the terrain is steep, and the proximity to the dam means that the wave would not attenuate substantially before reaching the dam. This becomes even more alarming when the detail from the paper is examined. In March 2010 the scarp was about 20 metres high, seven months later it had extended by a further 7 m. Detailed investigations are ongoing, but at the time of writing the slip surface had not been identified. Nonetheless, the estimated volume is about 120 -150 million cubic metres. The study used InSAR data to look at the slope – before impoundment the data suggest that the slope was not moving.
This slope is at best deeply worrying. It is hard to predict future behaviour without much more detailed analyses, but a catastrophic collapse cannot be ruled out. As the paper states:
“Although the precision in the estimation of the landslide volume and the wave height needs to be specified, the possible damage to the dam as well as to the downstream area could be devastating.”
Reference
Zhang, D., Wang, G., Yang, T., Zhang, M., Chen, S., & Zhang, F. (2012). Satellite remote sensing-based detection of the deformation of a reservoir bank slope in Laxiwa Hydropower Station, China Landslides, 10 (2), 231-238 DOI: 10.1007/s10346-012-0378-9
12 May 2013
The Wenchuan earthquake, five years on
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Five years ago today, on 12th May 2008, Sichuan Province was struck by the devastating M=7.9 Wenchuan earthquake. This earthquake was the landslide event of a generation, with over 60,000 slides being triggered, causing over 20,000 of the 80,000 fatalities. In the aftermath of the earthquake the army fought a desperate battle to drain the barrier lake at Tangjiashan, which threatened a million people.
Today I am in Chengdu attending a conference to mark the anniversary of the earthquake. The meeting, organised by the Chengdu University of Technology, aims to share information about earthquake-induced landslides. Meanwhile, the Chair of the conference, Professor Runqiu Huang, has written a really nice article in the current edition of Nature Geoscience (Huang and Fan 2013) about the landslides triggered by the earthquake. Unfortunately it is behind a paywall. Part of the paper reflects upon the unexpected or unanticipated dimensions of the landslide problem. Three key themes emerge:
- “Not enough attention was paid to the cascade of geohazards following the earthquake”. The paper argues that, in particular, the barrier lakes caused a huge problem, which drained resources from the emergency effort.
- “The longer-term effects of a similar cascade of potential hazards were not fully taken into account in the post-seismic hazard assessment and in the selection of sites for the reconstruction of destroyed buildings”. In the aftermath of the earthquake newly-constructed towns were flooded when post-seismic debris flows blocked rivers. Elevated levels of landslide activity are expected to persist for anothertwo decades.
- “The long-term impact of the 2008 Wenchuan earthquake on sediment flux in the affected watersheds was also underestimated initially”. The ways that rivers respond to the additional sediment load produced by the earthquake are complex, but very often the bed aggrades, leading to floods. This means that the effects of the earthquake can extend beyond the area affected by shaking.
In coming years we will see further earthquakes in mountain areas, with similar effects. We have gained a huge amount of knowledge from Wenchuan, albeit at a terribly high cost in human lives. I wonder though whether our improved knowledge is really leading to better preparedness for the next big event.
Reference
Huang, R., & Fan, X. (2013). The landslide story Nature Geoscience, 6 (5), 325-326 DOI: 10.1038/ngeo1806
11 May 2013
First news of a costly landslide in Mizoram, India
Various news reports are emerging of a substantial landslide in Laipuitland, Mizoram, in the far east of India. The Hindu reports that eight people have been confirmed as having been killed, a further 11 are missing and that nine people were injured. The trigger appears to have been a thunderstorm.
More information should emerge in the next 24 hours.
6 May 2013
Analysing the Bingham Canyon mine landslide part 2: the landslide track
This is part two of a three-part series looking at the Gingham Canyon mine landslide. Part 1 is here.
The best overview image of the track of the landslide is this one (used with permission courtesy of the Kennecott Utah flickr site
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Unfortunately, the images of the slope before the landslide were taken from a slightly different angle, but they are very useful as a reference point:
http://www.flickr.com/photos/riotinto-kennecottutahcopper/8644405220/in/photostream
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The first thing to note is that the block that failed does not extend to the floor of the mine – in fact it toes out about half way up the slope. The landslide has run across the lower slope without inducing any major failure in this area. So, on the lower slope it is still possible to see the remains of the mining benches, although they have been eroded from the passage of the landslide:
http://www.flickr.com/photos/riotinto-kennecottutahcopper/8644405220/in/photostream
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The unusual orientation of the shear surface is clear from the image below, also from Kennecott Utah. The mine manager (see end of this post) explained that the failure occurred in a thin sedimentary band within the quartzites.
http://www.flickr.com/photos/riotinto-kennecottutahcopper/8639731985/in/photostream
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Note how the sliding surface, which is formed from an existing plane of weakness, is orientated towards the camera rather than into the pit. This would have made release of the landslide much more kinematically difficult. This might imply that the first landslide was a wedge failure in the lower part of the slide area, which then released the block moving on this comparatively unfavourable slide plane. Analysis of the radar data would be very interesting in order to understand this. Most of the debris followed a track that was defined by this basal sliding plane before in effect free-falling to the floor of the mine and spreading out:
http://www.flickr.com/photos/riotinto-kennecottutahcopper/8643310015/sizes/o/in/set-72157633216160914/
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Another interesting aspect of this slide is the way that some material has flowed over the lateral boundary of the landslide:
http://www.flickr.com/photos/riotinto-kennecottutahcopper/8643310015/sizes/o/in/set-72157633216160914/
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The obvious interpretation of this is that it is late movement of debris, but the images that show the slide surface well do not suggest that this is the case:
http://www.flickr.com/photos/riotinto-kennecottutahcopper/8644406138/sizes/z/in/photostream/
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So it is likely that this is material that broke of the side of the main slide blocks during their movement and then followed the fall line.
Finally, Lee Allison from the Arizona Geological Survey has a video interview with Ted Himebaugh, general manager of the mine about the landslide:
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The intreview starts 18 minutes into the film. There are two really interesting points here:
1. He noted that the movement started about two and a half months ago.
2. They did not expect that the movement would extend to the pit bottom. The latter is perhaps slightly surprising given that he says that they identified correctly the volume that was moving.
He correctly notes that the most important aspect of this was that the safety system performed perfectly. This is a good demonstration that it is possible to manage even these very large pits very safely.
1 May 2013
We are recruiting a post-doctoral researcher to work on earthquake-induced landslides
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My colleague Professor Alex Densmore and I are currently recruiting a Post-Doctoral researcher to work with us on a project on earthquake-induced landslides. This post, which will start on 1st October 2013, is a two month position that is part of the consortium team on the ‘Earthquakes Without Frontiers’ project, funded by the NERC-ESRC Increasing Resilience to Natural Hazards Programme. The aim of the project is to increase resilience to continental earthquakes across the Alpine-Himalayan mountain chain, through a linked trans-disciplinary partnership of physical scientists, social scientists, policy specialists, and regional and national partner organisations. The overall project involves:
- Characterisation of the physical earthquake hazard across the region, including better understanding of the locations of active faults and strain accumulation, as well as better assessment of the likely locations, extent, and long-term impacts of secondary hazards such as landsliding;
- Assessment of pathways to resilience in the partner countries, including a full mapping of the societal, cultural, economic, and governance factors that enhance or erode resilience along with an understanding of existing efforts to build resilience and the ways in which that information has been used;
- Development of effective policy and strategies for intervention to increase resilience to future damaging earthquakes.
This new post will contribute primarily to aspect (i), and will focus in particular on delivering an enhanced understanding of coseismic landsliding, and of new web-based forecasting tools for end-users. This work will comprise two separate strands:
1. Development of a process-based approach to the forecasting of coseismic landslides that improves upon current empirical approaches; and
2. Construction of a novel tool for tracking the temporal evolution of landslide material.
The work will be focused on three primary field areas: Nepal and northern India; southern Kazakhstan; and central China. The post-holder will be based in the Department of Geography, Durham University, but will be expected to work closely with team members at the other institutions within the consortium (Cambridge, Oxford, Leeds, Hull, Northumbria, the British Geological Survey, and the Overseas Development Institute), as well as with wider members of the partnership.
Full details of the post are available on the Durham University jobs website: http://www.dur.ac.uk/jobs/ or please feel free to email me at: d.n.petley@durham.ac.uk
Dave Petley is the Wilson Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom. His blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
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