20 March 2014
The Dawlish landslide
The Dawlish landslide is one of the many problems that arose on the mainline railway line that links the southwest of England with London. This is a historic line that takes a very challenging route along the coastline. It has a long history of landslides, as my earlier Landslides in Art posts showed. In the recent winter storms a large section of track washed out, and a series of landslides have also caused significant problems. Clearly Network Rail (the track managers) would like to reinstate the track quickly and to have a long term solution, both of which are difficult. One key problem is a large (reportedly 30,000 tonnes, although some sources have suggested it is even larger) slide above the line. The landslide is shown in a very nice drone / UAV video collected by the BBC and available via the Eyes of Dawlish Facebook page. This is a screenshot of the landslide from that video:
Mitigating the Dawlish landslide
On first inspection the slide looks to be a slump – there is certainly a large displaced block. Network Rail have started an attempt to deal with this landslide that can best described as being impressive and perhaps even audacious. Since the weekend they have been using high pressure fire pumps to push water into the landslide in order to induce failure – if you look carefully you can see this ongoing operation in the image, with several streams of water being pumped onto the slide. Presumably the intention is to fail the landslide onto the track, and then to remove the debris.
This type of approach is occasionally proposed for landslides, but is rarely attempted. There are several reasons for this. First, actually it is rather difficult to get a natural slope to behave as you want it to. So, for example, there is a risk that the slide might partially fail and then stop, leaving an unstable mass that is dangerous to clear. Or the failure might be larger than expected, though that is unlikely here. The greatest danger is that the landslide does not move at all, leaving a now even less stable lump of rock above the asset in question.
I am not sure what is going on at Dawlish. Today the Herald Express reports that the army are involved in the operation, according to the BBC perhaps even using armoured excavators to clear the debris. It will certainly be interesting to see how this pans out over the next few days, and I am in admiration of Network Rail for what they are doing at that site.
18 March 2014
Every year several thousands of people are killed by precipitation-induced landslides, such as the one shown adjacent, which occurred in South Korea a few years ago. Fortunately, this one did not cause any loss of life.
In late November this year, JTC-1 (the joint technical committee on natural slopes and landslide set up by ISSMGE, ISRM and AIEG) will organise a workshop on precipitation-induced landslides. This will be held at the University of Seoul in South Korea from 24 to 26 November. It will be followed on 27 November by a forum on “slope safety preparedness for the effects of climate change” and a half day meeting that will examine recent fatal landslides in Asia. On 28 November there will be a one day field trip.
The full programme is given below:
You will see that this meeting is intended to be a workshop of limited size with lots of discussion rather than a large-scale conference. In my experience this type of event is the most rewarding to attend.
Call for abstracts
The organisers of the meeting have put up an initial website. This includes a call for abstracts, which are due on 31st March. Further details will be provided on the website in due course.
17 March 2014
Mount Haast: A GNS report on the 2013 rock avalanche (which is also known as the Mount Dixon rock avalanche)
Mount Haast rock avalanche
A little over a year ago Neil Wiltshire, a British climber, captured on video a rock avalanche on the flanks of Mount Haast (although at the time this was reported as being from Mount Dixon). I posted a couple of times on this landslide. The full video, surely one of the most remarkable ever collected, is still available on Youtube:
Last year, Graham Hancox and from GNS Science and Roydon Thomson, a local engineering geologist, wrote a report (Hancox and Thomson 2013) on this landslide, which has now been made available online (NB pdf). The report also covers another landslide, which occurred two weeks earlier at Ball Ridge, 9 km from Mount Haast) a fortnight or so earlier. I do not intend to discuss that landslide here.
The characteristics of the Mount Haast landslide
Although the video and subsequent analyses are the main sources of information about the landslide, the report also includes this image, collected by Anna Seybold, od the landslide in motion:
This image is really interesting as it appears to capture the transition from turbulent rock avalanche to rapid sliding. If you view this image alongside the video it is clear that the front of the landslide appears to have just started to slide. The report suggests that at its peak the landslide was moving at about 150 km/h. The landslide characteristics are as follows:
- Source area width: 200-250 m
- Source area height: 350-400 m
- Source thickness: 5-25 m
- Initial volume: 575,000-980,000 cubic metres
- Average velocity (from seismic data): 160 km/h
- Runout distance: 2.9 km
- Deposit volume: 2 million cubic metres (NB this is larger than the source volume because of entrainment en route)
The report has very nice before and after images of the landslide:
But perhaps the most useful part of the report is the combination of these two images, the one on the lefty showing the landslide track and the one on the right providing a long section of the slide:
What is beautifully shown in this image are the four phases of movement:
- Initial freefall
- Rock avalanche, characterised by a steep track and dust deposits on both sides. When the gradient reduced this transitioned to…
- Rapid sliding, during which the main body developed very clear flow lines
- Slow sliding, in which the deposit developed the lobate frontal structure and entrained fresh snow and ice.
There is a great deal more in the report, including a very interesting discussion about causation and triggering. I recommend that you download a copy.
Hancox, G.T. and Thomson, R. 2013. The January 2013 Mt Haast Rock Avalanche and Ball Ridge Rock Fall in Mt Aoraki/Mt Cook National Park, New Zealand. GNS Science Report 2013/33. 26 pp.
13 March 2014
The Mount La Perouse landslide: a Pleaides image
Last week Marten Geertsema flew up to the Mount La Perouse landslide in Alaska. He has very kindly made a set of images that he took from the air and on the ground available. Marten and Colin Stark also arranged the acquisition of a Pleaides high resolution satellite image. Colin has kindly generated this image of the landslide, which gives a fantastic overview of the slide:
The source area of the Mount La Perouse landslide
This image shows the source area of the Mount La Perouse landslide. Note the structure in the rocks behind the failure scar. The general geological structure here is an anticline, the core of which has been eroded out by the glacier to form the valley. Thus, the basal shear surface of the landslide is the natural structure in the rocks, with the form of the anticline allowing the plane of weakness to daylight. The rear scar appears to have been defined by a sub-vertical joint. Thus, the landslide itself is a giant wedge; it is perhaps inevitable that this slope was showing some signs of distress prior to failure:
The material that formed the Mount La Perouse landslide
The rock material that formed the initial failure of the landslide was gabbro according to the geological maps (hat-tip to Colin for digging that information out). However, upon impact at the foot of the slope this is likely to have shattered and then to have entrained snow and ice. The resulting deposit at the foot of the slide is thus a complex mixture, which appears to consist of a matrix of dirty ice (possibly slush during the movement?) with clasts of intact ice and gabbro (note the handle of an ice axe for scale):
The Mount La Perouse landslide track
An interesting aspect of the Mount La Perouse landslide is its behaviour upon impacting the valley floor. The satellite images show considerable super-elevation – i.e. it ran up the far side of the valley before turning to flow downhill. This image shows the run up on the opposite valley side:
The slide then travelled down the valley, but with a rather complex flow pattern en route:
There is a great deal of work to do to understand this landslide properly, and it does deserve a detailed investigation. Unfortunately, snowfall is likely to mean that it will soon be difficult to work on it.
11 March 2014
In the next week or so I’ll be giving or chairing a number of lectures and events:
1. Yorkshire Geotechnical Group, Leeds
I’m giving a talk tonight to the Yorkshire Geotechnical Group entitled “Managing very large landslides”. This will be held at the University of Leeds in Lecture Theatre A, School of Civil Engineering, LS2 9JT starting at 6:30 today (11th March). This is the abstract for the talk:
Landslides in high mountain areas, such as the Himalayas and the Southern Alps of New Zealand, have the potential to cause very high levels of damage. In most cases they are so large that it is impossible to mitigate them, such that other approaches need to be developed to manage the hazard. This talk will focus on three case studies. The first will explore the Attabad landslide, which in 2010 blocked the Hunza valley in Northern Pakistan, threatening 25,000 people with a dam burst flood. The presenter was involved in a six month long project to manage the risk as the water level approached the overtopping point, which included the setting up of warning systems and the relocation of large numbers of people. The second will explore the Gayari rock avalanche in Siachen, Pakistan, which killed 142 soldiers based at an army camp. The author was involved in a programme of work to find and recover the remains of the victims, all of whom were buried 25 m below rock and ice debris from the landslide. The third will examine the threats posed by the Utiku landslide in New Zealand, which is a very large but slow moving slide that threatens a railway and a strategic highway. In each case the talk will examine the threats that the landslides posed and the lessons that can be learnt from their management.
2. A visiting lecture at Durham from Dr Sergio Sepulveda
Sergio Sepulveda from the University of Chile is currently working with us in Durham on a range of projects. He will be giving a seminar on Thursday 13th March entitled “Landslides induced by mega thrust vs. shallow crustal earthquakes: Examples from the Chilean Andes”. This will be held in room W007 in the Department of Geography at Durham University, DH1 3LE. All welcome, but please email me at firstname.lastname@example.org to let me know you are coming (so that we have enough room!). This is the abstract for the talk:
Earthquake-induced landslides tend to show different patterns of both size and geographic distribution depending on the seismic source mechanism. Examples from investigations of recent earthquakes in Chile at different latitudes from the Atacama desert down to Patagonia show how landslides triggered by moderately large (M 6.0-7.0), inland shallow crustal earthquakes tend to have higher density and larger volumes in comparison with those induced by large magnitude (M 7.5-9.0), megathrust earthquakes along the subduction plate boundary. These observations raise the question on the origin of prehistoric, giant size landslides widely distributed in the Andes uplands, which seem to be related with regional crustal faults. As many of these faults have recently found to be active, they may pose an unexpected landslide hazard for local communities.
3. Cafe Scientifique, Cockermouth, Cumbria
I will be leading a Cafe Scientifique event in Cockermouth in Cumbria on Landslides on Tuesday 18th March at 7:30 pm. Details here.
10 March 2014
UBC distinguished lecture powerpoint file: Earthquake-induced landslides – lessons from Taiwan, Pakistan, China and New Zealand
UBC Geological Engineering distinguished lecture
Last week I was fortunate to have the opportunity to deliver the annual UBC Geological Engineering distinguished lecture, entitled “Earthquake-induced landslides – lessons from Taiwan, Pakistan, China and New Zealand”. At the time I promised to make the Powerpoint file available for download, so, I have uploaded it onto Authorstream. The file can be found here and should be visible as a preview below:
7 March 2014
Birling Gap, Sussex, 4th March 2014
Birling Gap in Sussex, on the south coast of England, has suffered high rates of erosion in the storms of this winter. On 4th March Eddie Mitchell caught on camera a substantial collapse event, as shown in the screenshot above. This is well-described in a Sussex Express article, and is available on Youtube:
The collapse event is quite interesting. If you look carefully you will see that it was initiated at the foot of the cliff, after which the collapse rapidly propagates upslope. The area that collapsed had been cordoned off because a large crack had appeared across the cliff top. Rockfalls are the usual erosional process on this section of cliff; this is undoubtedly just one of many this winter, which has seen seven years of erosion in two months. Sadly this is now threatening a set of historic cottages, whose loss now seems inevitable within a few years at most.
Erosion on the cliffs of Birling Gap is a serious problem. The National Trust have a nice video about the issues:
San Leo, Italy, 27th February
Thanks to Doug Stead and Margherita Cecilia Spreafico for highlighting the video to me.
26 February 2014
The Mount La Perouse landslide in the news
The Mount La Perouse landslide is now attracting interest from the media, which is rather cool. This has been driven at least in part by the publication yesterday of images of the landslide collected by Landsat 8 and published by NASA yesterday. The best thing about this is that we now have a before and after view of the landslide.
Mount La Perouse before the landslide
This is an image of the location of the landslide, collected on 27 May 2013:
Mount La Perouse after the landslide
This is the Landsat 8 image of the landslide, collected on 23 February 2014:
The mobility of the Mount La Perouse landslide
It has been suggested by some that flowing across ice and snow might be a factor in determining the mobility of very large landslides. Back in 2002 Francois Legros wrote a very nice paper in which he collated a dataset on very large landslides, and then compared their mobility (for example the maximum distance that they traveled in relation to the maximum height difference from the landslide crown to the landslide toe). I have plotted the approximate position of the Mount La Perouse landslide on his graph below (apologies for the poor quality scan!):
The data suggests that this landslide was at the higher end of the mobility scale on this criteria. Interestingly, the morphology of the toe of the landslide is very similar to that of the Mount Dixon rock avalanche in New Zealand. The final movement phase for that event consisted of sliding across the snow and ice base at a low rate, as the video below shows:
Thus, it may be that this type of movement contributes significantly to the mobility of large landslides moving across ice
25 February 2014
The Mount La Perouse rock avalanche
The Mount La Perouse rock avalanche nine days ago is a very interesting landslide. My original post, based on the seismic data of Colin Stark, Clément Hibert and Goran Ekstrom is here; the first images of it, as collected by Drake Olson, are here.
Landsat 8 passed over the landslide on Saturday; nature was kind to us by providing a cloud-free day, so the landslide has been images well. Colin Stark has run a quick initial process of the data, which reveal the following parameters for the Mount La Perouse rock avalanche:
- Maximum length (from the crown to the toe: about 7,400 metres
- Maximum elevation (i.e. height of the crown of the source area): about 2,800 metres
- Minimum elevation (i.e. the height of the toe of the landslide): about 1060 metres
Colin has draped the Landsat 8 data on to a digital elevation model to provide a view of the dynamics of the landslide:
Some additional images of the Mount La Perouse rock avalanche
Drake Olson, via Marten Geertsema at UNBC, has provided some additional photographs of the landslide. First, this is a view of the landslide from the air. Note the remarkable super-elevation of the landslide (i.e. the way in which it has run up the sides of the valley):
This is a really vast landslide. To give an impression of the scale, this is the landslide toe. If you look carefully you will just be able to see Drake’s aircraft:
We are all indebted to Drake for collecting these wonderful images. Here he is sitting on the landslide debris:
This post uses images and information collected by Colin Stark. He would like to acknowledge NSF, which funded the research under the following schemes:
- Geomorphology and Land-use Dynamics & Geophysics programs under award #1227083
- Division of Civil, Mechanical, and Manufacturing Innovation and the Hazards SEES program under award #1331499
22 February 2014
The Mount La Perouse Rock avalanche: background
Earlier this week I posted the initial results of the tool developed by Goran Ekstrom and Colin Stark, which indicated that a rock avalanche had occurred in Alaska on Sunday. We were waiting for Landsat 8 to collect imagery today in the hope of locating it. However, yesterday pilot Drake Olson found it on the flank of Mount La Perouse. Marten Geertsema has generously acted as the conduit of information about the landslide, and Drake has very kindly provided some photographs of the landslide too. Two satellite passes are due this weekend, so hopefully we’ll get some images from that source too.
The approximate location from the seismic data
The seismic data indicated that the landslide was located somewhere is the region of coordinates 58.68, -137.37,which is this area of Alaska. Interestingly, this is within 10 km of the Mount Lituya rock avalanche of two years ago:
Drake Olson and the location of the Mount La Perouse landslide
Today helicopter pilot Drake Olson spotted the landslide. It is on the flank of Mount La Perouse at about 58.542 -137.01:
First information and images of the Mount La Perouse rock avalanche
This is a big landslide. It appears that a near vertical wall of the mountain has collapsed and then turned into a rock avalanche that has entrained snow and ice. The run out distance is about 4.8 km. The collapse occurred at about 3000 m elevation; unfortunately this is above the snow line, so the landslide has already started to be covered by snow. With thanks to Drake Olson, this is the source:
So it is clear that a near-vertical wall on the flank of the mountain collapsed. Note the clear runup on the other side of the corrie, after which the landslide has flowed downslope. The initial part of the track has a quite low gradient, after which it has flowed down a steeper slope. This is the track:
At the toe the landslide has spread out. This image shows the toe of the landslide with some quite beautiful flow lines:
These wonderful images perhaps don’t give a feel for the thickness of the deposit, which is up to about 13 m. Drake even landed at the toe of the landslide to get a feel for the constituent materials. This image shows that they consist of ice and rock, suggesting that the landslide has entrained a large amount of snow and ice as it travelled downslope:
Unfortuantely the landslide will disappear beneath snow very quickly. Already there is a few centimetres of fresh snow on it – this is the dust layer generated by the landslide, with fresh snow on the surface:
I must thank Drake Olson for both locating the landslide and for providing the images; Marten Geertsema for forwarding the information to me; and Colin Stark, Goran Ekstrom and Clément Hibert for spotting this in the first place and for allowing me to break the news about it.