14 December 2018
The Nyixoi Chongco rock avalanche in Tibet
In a new paper in the journal Landslides, Wang et al. (2018) describe the Nyixoi Chongco rock avalanche in Tibet. I have not seen this spectacular rock avalanche in the literature before this paper; it is an excellent addition to the inventory of such events. The dry, low vegetation environment of the Tibetan Plateau has meant that the rock avalanche is beautifully preserved in the landscape. Whilst the paper itself is a little vague on the exact location, I have managed to track it down to 29.534, 90.263, a spot that has excellent Google Earth imagery:-
According to Wang et al. (2018), the Nyixoi Chongco rock avalanche involved a mass of 28 million cubic metres descending a vertical distance of 885 metres, and traveling a total of 4.6 km horizontally, representing exceptional mobility. The landslide deposit is about 50 metres thick. The authors do not give an estimated age for the rock avalanche, nor a definite trigger mechanism, but the proximity to active faults makes them suspect that an earthquake might have been involved. Wang et al. (2018) note that a nearby fault is thought to have generated a magnitude 7.0 earthquake about 2,200 years BP; this would have induced sufficiently intense shaking to have triggered a rock avalanche of this type.
The Nyixoi Chongco rock avalanche can be better appreciated in this Google Earth oblique view:-
I have placed the marker at the crown of the landslide scar, which has a scoop morphology. Below this is a very clear transportation zone, stripped by the passage of the landslide. The deposit starts from the foot of the slope and extends down the valley. Note the clear hummocks in the landslide mass, which are very common for large rock avalanches. Wang et al. (2018) note that the landslide has many of the common characteristics of large rock avalanche masses, including a very coarse (i.e. boulder-rich) carapace (the surface layer), and a highly fragmented main landslide mass. The base of the landslide mass shows almost no mixing with the valley deposits, and has sedimentary structures that suggest that these deposits were unsaturated when the landslide was deposited. The authors suggest that the movement of the landslide was not associated with any of the complex mechanisms sometimes postulated for these huge rock avalanches, in this case involving simple shear between the landslide mass and the valley floor.
It is interesting to note that this means that the landslide could have high mobility without needing liquefaction or anything more exotic, a finding that is important in the context of understanding other large events.
Wang, YF., Cheng, QG., Shi, AW. et al. 2018. Sedimentary deformation structures in the Nyixoi Chongco rock avalanche: implications on rock avalanche transport mechanisms. Landslides https://doi.org/10.1007/s10346-018-1117-7
13 December 2018
InSARNORWAY: a national deformation map for Norway
Late in November the Geological Survey of Norway launched InSARNORWAY, a new national level deformation map for the whole of the country. This is, as far as I know, the first such national level project, although there have been some proof of concept studies previously. The new map uses multiple epochs of InSAR data (satellite radar data) to generate a map of deformation, accessible via a free web portal. The resource allows the user to map the deformation and to generate graphs of change through time. It is fascinating and exciting.
One of the questions that we have been pondering for a while is the degree to which all rock slopes deform through time, and how fast these rates might be. Sitting below that are nested questions, such as the geological controls, the effects of weather and the evolution of failure. A resource of this type allows us to start to answer these key questions. As an example, I was browsing the maps, and came across this area of deformation near to Aarset (location 62.198, 7.217):-
Here the warm colours are areas of more active deformation. The data suggest that on the west side of a mountain in this area there is some active movement. This is the Google Earth imagery of the same area:-
And this is the plot of the data from one of the points in this area:-
So the InSARNORWAY data suggest that the over the course of 38 months the slope has moved about 50 mm downwards and towards the west. This is entirely logical in terms of the configuration of the slope. So, on the face of it this appears to be a creeping rockslope. That is not to say that it is dangerous in any way, this is type of creeping behaviour probably quite normal for large, steep rockslopes and so is no cause for alarm. The rate appears to be quite linear with time – again this is not unexpected. But for the first time it is possible to characterise this behaviour over a large area.
This of course creates a capability to identify large numbers of creeping rockslopes. It is likely that, in most cases, prior to a large event they will go through a phase of acceleration, so systematic monitoring (perhaps with artificial intelligence) could allow this to be detected. And of course using InSARNORWAY we can now start to understand what is “normal” creep across large areas, allowing us to understand and determine when behaviour is changing.
InSARNORWAY is an extraordinary and very exciting development. I would love to see similar resources for other countries.
12 December 2018
Fenshui, China: a fatal landslide, caused by humans?
On Sunday a rock slope failure affected an area of Fenshui township in Xuyong County, Sichuan Province in China, killing five people and injuring a further seven. The landslide, which happened in a period of heavy rainfall, destroyed a number of residential properties.
Xinhua has quite a number of images of the aftermath, most of them focusing on the heroic actions of the government rescue teams, which is quite normal in the Chinese state media. Asia One has a much more interesting image of the landslide site:-
The landslide appears to have occurred in quite intensely weathered rock. The main scar originated high on the slope, which appears to the seat of the failure. The landslide has entrained some slope materials en route, though it seems to me that the volume of debris at the slope toe is quite modest. And note also that the properties appear to have collapsed, perhaps as a result of impacts, rather than being buried. This is probably the reason why the proportion of victims of the landslide that survived was quite high for a landslide event.
Most interesting to me is that the slope appears to be traversed by a road about a third of the way from the top. The construction on the right side of the image suggests that this is quite new. The toe of the failure of the slope appears to me to coincide with the level of the road. I wonder if this slope was cut prior to the landslide and, if so, whether this was a factor in the events? Poor road construction techniques often lead to landslides.
10 December 2018
Spantik basecamp – that was close!
In August Shanyan Anwer posted the following video onto Youtube, showing an extraordinary near-miss for a camp, and its occupants, from a large boulder:-
Near-misses do not come much more near than that. The video was apparently shot at Spantik basecamp, located between Baltistan and Hunza in northern Pakistan. In fact the video, which is very short, is worth closer inspection. Early in the video the boulder can be seen heading down the slope towards the camp:-
Just how close this rock came to the camp, and the person holding the camera, can be seen here:-
Next of course it comes terrifyingly close to another person:-
This video does emphasise a few points about rockfalls. The boulder is tabular in shape, which in some circumstances can generate extreme mobility. This happens when the boulder starts to rotate around its shortest axis, behaving like a wheel. In such conditions boulders are comparatively stable, skipping down the slope in large bounces. As the boulder itself is uneven, and the surface is very rough, the path is somewhat unpredictable. In this case the boulder appears to bounced away from the people located downslope of the camera, in part because it struck softer debris, which absorbed some of the energy of the rock.
There is little doubt that this was a very lucky escape. That boulder could have struck any of the people or any of the tents.
Thanks to Fabien for pointing this one out to me. I had somehow missed it during the summer! It is a remarkable video.
7 December 2018
The Guazi landslide: an interesting ancient landslide reactivated by a hydroelectric power scheme in China
The Guazi landslide: an interesting ancient landslide reactivated by a hydroelectric power scheme in China
An interesting article in the journal Landslides (Zhao et al. 2018) highlights the Guazi landslide in Heishui County, Sichuan, China. This is a large, ancient landslide located sandstone and phyllite rocks in mountainous terrain on the slopes above the Heishui River. In 2008 a major hydroelectric power scheme was developed downstream of the landslide site, consisting of the Maoergai Dam, a rock-filled embankment dam providing a hydraulic head of about 170 metres, and associated infrastructure. The dam was completed on 20th March 2011, and impoundment of the reservoir was started. On 2nd September 2011, six months after impoundment was initiated, movement of the Guazi landslide was noted.
The current state of the Guazi landslide can be seen in the Google Earth oblique image below:
The landslide is about 950 metres from the crown of the ancient slide (marked with the landslide symbol) to the water level, and it is about 550 metres wide. According to Zhao et al. (2018), the ancient landslide has a volume of about 13.5 million m3, with a depth of up to 65 metres. The landslide is marked by tension cracks at the crown, which appeared to widen during the 2008 Wenchuan earthquake. The authors interpret it as a creeping landslide prior to the construction of the dam.
The reactivation in 2011 occurred on the lower slopes, creating a new, very large tension crack. Zhao et al. (2018) provide the following annotated diagram to illustrate both the ancient landslide and the reactivated lower portion. The displacement across the new tension crack is marked as the dislocation zone:-
Interestingly, I would place the crown of the ancient landslide further upslope than the annotated photograph suggests, but the higher feature may be a secondary slide on the rear scarp. Zhao et al. (2018) note that the landslide reactivated during the first phase of impoundment through to December 2011, primarily in the lower third of the slope. In some places the tension cracks are now 20 metres wide. The slide has been monitored since late 2011; the paper reports that in the period from then to September 2012 some parts of the landslide moved a further 30 cm or so. Measurement using inclinometers suggest that the movement is quite shallow. The upper section of the landslide shows little additional movement.
Zhao et al. (2018) note that at the moment the landslide does not show signs of a larger scale reactivation. But, they note that:
“At present, the Guazi landslide is stable overall; however, confirming its stability in the future is difficult, and long-term monitoring is necessary”
I agree wholeheartedly. In a translational, flexural toppling landslide such as this movement of the toe is likely to be progressively destabilising the system through time. This does not mean that a large-scale reactivation is inevitable, but it cannot be ruled out. And of course this is yet another example of an unanticipated major landslide at a hydroelectric dam project.
Zhao, B., Wang, Y., Wang, Y. et al. (2018). Triggering mechanism and deformation characteristics of a reactivated ancient landslide, Sichuan Province, China. Landslides https://doi.org/10.1007/s10346-018-1111-0
5 December 2018
Malibu: Planet Labs images of the first round of mudflows
One of the potentially disastrous impacts of the recent wildfires in California is the vulnerability of the now bare soil to highly destructive mudflows. Similar processes have been seen elsewhere in California, and indeed worldwide. The fires have occurred at the start of the period in which extremely heavy rainfall, associated with atmospheric river events, can occur. The potential impacts are serious.
Planet Labs have captured a high resolution SkySat image of a part of Malibu following the Woolsey fire and then the heavy rainfall last week. They have kindly provided three images of the area. The first image, collected on 31st August 2017, predates the fire:-
The second image was collected on 28th November 2018, after the fire:-
The extensive fire damage to the vegetation in the hilly area is very clearly seen. This is this damage that has made the area vulnerable to heavy rainfall. Finally, this image shows the same area after the first round of heavy rainfall:-
The image shows extensive stripping of top soil by the heavy rainfall, and the formation of gullies, especially on the right side of the image. This is a more detailed image of this area:-
This is a remarkable image, showing extensive shallow stripping and gullying, with the gullies providing direct connection to the river channels. The ways in which huge numbers of small flow structures all connect to the same channel provides the potential for the massive downslope impacts.
The climate emergency (sometimes less well described as climate change) is very much in the news, and the landslide community is worrying about he impacts on mass movements. This example is a beautiful, tragic illustration of the problem – increased drought is increasing the risk of forest fires; increased rainfall intensities then allow more dynamic mobilisation of the debris to form mudflows.
1 December 2018
First images of slope failures triggered by the M=7.0 Alaska earthquake
The M=7.0 Alaska earthquake on 30th November 2018 was sufficiently large to trigger some slope failures, even though it was comparatively deep (41 km). Such an earthquake would be expected to generate shaking over a large area, but probably with reasonably modest peak ground accelerations. The USGS ground failure model suggests that only a small area might have been subjected to slope failures, but much of the area affected had limited potential for large-scale landslides due to the low relief topography:-
There may be some landslides to the east of the epicentre in the rugged terrain there. Some images of slope failures have started to emerge though. @AOC-Security has tweeted this image of a lateral spread affecting a road embankment on Vine Road in Wasilla:-
Meanwhile David Ruffini has tweeted an image of the same feature in daylight:-
This lateral spread has been caught in a spectacular airborne image by Marc Lester:-
Josh Bierma tweeted an image of another apparent lateral spread affecting a road:-
That event could have been far worse for the occupants of the car. And Ryan Hobbs tweeted an image of yet another such lateral spread affecting a highway:-
Ryan Hobbs has also tweeted an image of rockfalls affecting a road in the earthquake affected area:-
25 November 2018
Planet Labs SkySat image of the Yarlung Tsangpo valley-blocking landslide
In the last few days Planet Labs have succeeded in capturing a gorgeous SkySat (very high resolution) image of the Yarlung Tsangpo landslide dam. As a reminder, this is a large landslide in Tibet, which originated from a rock and ice avalanche on a glacier. The events has occurred on two occasions in recent weeks, both of which have blocked the valley.
The image is below:-
The location of the breach of the dam is very clear, meaning the the blockage is not impounding a significant amount of water now. Note that there is still a large volume of sediment in the channel though; presumably this will be eroded away in time. This erosion is ongoing – compare the colour of the river water on the upstream and downstream sides of the blockage.
The image provides detail of the chronology of the emplacement of the landslide and of subsequent modification of the landslide mass. It is notable that the landslide spread out both upstream and downstream of the junction with the main channel. Some erosion of the landslide track can now be seen as well. The original landslide mass would have comprised of both ice and sediment; the former will now be thawing, leaving just the mineral matter in the valley.
The reasons for this large glacial landslide remains unclear, but large rockslides and glacial landslides are becoming more common as the climate change crisis rapidly increases rates of melting on glaciers and in permafrost slopes around the world.
Reference and acknowledgement
Thanks to Robert Simmon, Senior Data Visualization Engineer, and his colleagues at Planet Labs for capturing this image and for processing it.
Planet Team (2018). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/
Video: a spectacular double flow-type failure from Kerala, India
I have come across a spectacular video of flow-type landslide, apparently collected in Kerala, India. The video, which was posted to Youtube on 21st November 2018, has no detailed explanation associated with it. That is unfortunate as it is genuinely spectacular. The video should be visible below:-
The landslide appears to occur in two phases, initially a dynamic failure that splinters and destroys the trees bordering the river:-
Note that this landslide has clearly stated much higher up the slope as after the slide a track is visible disappearing into the distance. This initial failure is then followed by a very large, very rapid flow type failure along the track of the first landslide:-
This second phase is much more rapid, with a higher volume, than the first. It is sufficiently mobile that the flow crosses the river, even though it is in flood, and super-elevates to the level of the observers. This could have been very serious.
The origin of this double failure is not clear, but I am reminded of the very dramatic Lantau island landslides in Hong Kong in 2008, one of which was also caught on video. These started as comparatively small, shallow failures high on the hillside that transitioned into large-volume, channelised debris flows that entrained significant amounts of material. Note that in the case of the Kerala landslide in this video that material involved appears to be deeply weathered (red in colour) soil and rock, which might also hint at a similar mechanism.
It is rare to capture a channelised flow such as this coming straight at the camera. That they cause such high losses is unsurprising in light of this video.
22 November 2018
Pre-failure analysis of the Fagraskogarfjall landslide
Back in July I wrote about the Fagraskogarfjall landslide, a very large failure that occurred in western Iceland, inundating a large area of farmland. The volume of the failure is at the upper end of the scale – up to 20 million cubic metres – and it extended over 2.5 km. Fortunately this remote area of Iceland is sparsely inhabited, so there was no loss of life.
Large landslides such as this are generally predated by a long period of creep deformation – slow movement in which the failure develops progressively through time, often associated with the development of the final failure surface. This creates an opportunity to anticipate the location of major failures is creep deformation can be assessed. Whilst simple in concept, this is hard in practice. Measuring creep is difficult over large areas; we don’t really understand how different types of creep operate (do all slopes creep for example?); and can we differentiate between creep that might lead to catastrophic failure and creep that will not?
The advent of InSAR – the measurement of surface deformation using radar satellite data – potentially provides the answer to some of these questions. If it is possible to measure deformation over large areas on a regular basis then it may be possible to both understand background rates of creep and to identify those areas that might suffer a major failure. This is without doubt one of the most exciting areas of landslide science at the moment. I have noted previously that InSAR now provides the basis for measuring creep deformation with high levels of precision, and that wide area InSAR techniques are becoming available. Later this month Norway will launch a nationwide ground deformation map, a prospect that is immensely exciting.
NPA Satellite Mapping have looked at InSAR data for the period leading up to the failure of the Fagraskogarfjall landslide. Their results are available in a PDF article. In this case they have used Differential InSAR between July 2017 and July 2018. Their results, shown below, indicate that the parts of the slope were moving at up to 2 metres per year in the period up to the failure event:-
There are several interesting aspects to this. First, of course, is that pre-failure creep of the Fagraskogarfjall landslide was detected. Second, the landslide showed differential movement, with the greatest accumulation of strain in the central portion of the landslide, in effect generating bulging that increased susceptibility to failure. And this, in this case the whole of the slope was moving (sometimes failure occurs in a portion of the slope, which then triggers failure through retrogression upslope or loading of lower slope portions). Thus, the InSAR data provides significant insight into deformation processes and the mechanisms of the slope failure.
Thanks to NPA Satellite Mapping for allowing me to highlight this example, and to Adam Thomas for bringing it to my attention. The analysis was undertaken using Copernicus Sentinel data.
Planet Team (2018). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://api.planet.com