19 December 2014
The Ventnor landslide
One of the most interesting landslides in the UK lies under the town of Ventnor, which is located on the southern side of the Isle of Wight in southern England. This is a large, slow moving landslide that nonetheless causes considerable damage, particularly in periods of accelerated movement. Over quite a long period, the Isle of Wight Council worked with Halcrow (now CH2M HILL) to understand the landslide by monitoring its movement patterns. In recent years I have worked with a part time PhD student, Jon Carey (who is now at GNS Science in New Zealand), and Roger Moore at Halcrow, to understand the relationships between the movement of the landslide and pore water pressures. That work is a part of Jon’s PhD thesis (which is available online), and has just been published in the journal Landslides (Carey et al. 2014).
Perhaps the most interesting aspect of this landslide is a large graben structure that is opening up at the crest of the landslide. This is one of the figures from the paper, showing a geomorphological map of the landslide together with, bottom left, the graben structure:
In the paper we examined some very detailed monitoring data that was collected in a study led by our co-author, Roger Moore, between 1998 and 2002. During this time, Halcrow monitored the pore water pressure at depth using piezometers located in deep boreholes and the opening of the graben using crackmeters (to measure horizontal movements) and settlement cells (to measure vertical movements). We found that the movement of landslide can be divided into two key components. In the background is long-term creep of the landslide, at rates of about 5 to 10 mm per year, regardless of the groundwater conditions. This indicates that the landslide is in a condition of marginal stability / instability. However, when the groundwater level rises due to periods of heavy rainfall the landslide moves more rapidly – up to about 35 mm per year – but in a complex manner. This is captured in the diagram below, which is part of Fig. 6 from the paper:
The upper panel of the diagram shows the movement of the landslide as indicated by extension of the graben sides and subsidence of the graben floor. The lower panel shows the same data for the crackmeter, but expressed as a displacement rate, and the measured groundwater level. It is clear that as the groundwater level increases the landslide movement rate goes up, and vice versa. However, across the entire dataset we found that the relationship between the pore water pressure and movement rate was not simple, and in particular that sometimes the movement rate remained elevated even as pore water pressures reduced.
At Ventnor there has long been discussion of the likelihood that the landslide might transition into a more rapid movement event. Our data suggests that because the landslide is occurring on a very well-developed basal shear plane, and because the toe of the landslide is buttressed by large landslide blocks, this is unlikely without some fundamental shift in material behaviour.
Carey, J.M., Moore, R. and Petley, D.N. 2014. Patterns of movement in the Ventnor landslide complex, Isle of Wight, southern England. Landslides. doi: http://dx.doi.org/10.1007/s10346-014-0538-1
18 December 2014
Amazing but watch with caution: camera phone footage of the deadly Banjarnegara district landslide in Indonesia
Camera phone footage of the deadly Banjarnegara district landslide in Indonesia
Hidden in the depths of Youtube is a quite amazing video, uploaded on 13th December, of the Banjarnegara district landslide in Central Java, Indonesia. The current known loss of life from this event is 83 people, 25 people remain missing. I cannot guarantee the authenticity of this video, although I’ve certainly not seen it before. I must caution you to watch with care – whilst there are no directly horrific scenes, the sense of utter chaos and panic is disturbing – really disturbing. This is far from easy viewing. The video can be found here, and as usual I have embedded it below:
A key question of course is whether this is genuine – well first it is clearly a landslide disaster. This is the best screenshot that I have found from the video:
This is from 38 seconds into the video, when a wave of material (very clearly a flow) can be seen. A comparison with this image (via Voice of America), from a different angle (and note this is very foreshortened), suggests to me that it probably is the same landslide, but this needs confirmation:
The video stops half way through – the text thereafter says “thank you for watching; thanks for his visit”, or something similar.
The video does capture the utter chaos of the situation during a catastrophic landslide, and should be a spur to all of us involved in landslide work. The losses from this landslide include 20 school children, 11 more children aged less than 10 years old and two elementary school teachers.
17 December 2014
Sierra Cucapah earthquake
On 4th April 2010 a Mw=7.1 earthquake struck the Sierra Cucapah range of northern Mexico. I blogged about that earthquake at the time (though the blog post didn’t survive the migration to the AGU website so well – it works a bit better on my original blog site) because the amazing dust clouds generated by the landslides triggered by the earthquake were captured on a mobile phone camera, and were subsequently uploaded to youtube:
At the time I had a post-doctoral researcher, Dr John Barlow, working with me at Durham. He and I, together with other colleagues, became intrigued by the landslides triggered by this earthquake for a number of reasons. First, we wanted to know just how many slides would generate this sort of effect. Second, there is very little data on landslide triggering in an earthquake in an arid regime. And third, it has been hypothsised that in an arid, seismically-active area most of the erosion might be generated by earthquake-induced landslides. So we applied to the Natural Environment Research Council (NERC) in the UK, and obtained a grant to study these landslides.
This is a hazardous area, and we were strongly advised that it would be risky to spend much time in the mountains (these are human rather than natural hazards of course), so we undertook the research using remote sensing. The results (Barlow et al. 2014) have just been published in a paper in the journal Geomorphology. In the paper we calculated the volume of material that was moved into the mountain chain by the earthquake. For a thrust-type of event, this can be substantial as the mountains can be uplifted by the movement of the fault. However, in the case of the Mexico earthquake this volume would be expected to be quite small because most of the predominantly strike-slip movement of the fault. In fact, in this case we found that there was more subsidence than uplift – i.e. the net effect of the earthquake was to reduce the mountain volume.
We also mapped the landslides triggered by the earthquake. Despite the appearance from the video, there was not very widespread landsliding in the mountains – we only identified 452 landslides in total. However, we can use this to estimate the total volume of soil and rock released by landslides, which comes out at about 2.6 million cubic metres, again not a big total.
We also looked at the controls on the landslides triggered in the earthquake. We found a very strong control from slope gradient – i.e. steep slopes were much more likely to fail – and from peak ground acceleration (i.e. those slopes that had been shaken most intensely were most likely to collapse). So, if you were living on a steep slope close to the fault (where the shaking is most intense) you were at a high level of risk. But interestingly, we also found that there was a strong influence exerted by slope orientation. In particular, those slopes orientated perpendicular to the fault were four times more likely to fail than those orientated parallel to the fault. The reasons for this are not clear at present.
From the work that we’ve undertaken we have been able to generate graph of the occurrence of landslides in relation to distance from the fault in the mountains:
The bars show the number of landslides per square kilometre, which is at a peak alongside the fault and then declines with distance. The black line shows average slope angle, which also declines away from the fault. The relationship between the landslides and the distance from the fault is generally quite simple, but at about 1.5 km from the fault the occurrence of landslides notably declines. The cause is valley orientation – this is a section of the mountains in which the valleys are orientated perpendicular to the fault, such that the slopes are parallel, as shown by the grey line on the graph. Thus landslide occurrence was much lower in this zone.
So, in conclusion, the youtube video was the inspiration to undertake a really interesting study of the landslides triggered by this significant earthquake in Mexico. As a result we now understand landslides triggered by earthquakes a little bit better.
Barlow, J., Barisin, I., Rosser, N., Petley, D., Densmore, A. and Wright, T. 2014. Seismically-induced mass movements and volumetric fluxes resulting from the 2010 Mw = 7.2 earthquake in the Sierra Cucapah, Mexico, Geomorphology, Available online 24 November 2014, http://dx.doi.org/10.1016/j.geomorph.2014.11.012.
16 December 2014
Oso: The SR 530 Landslide Commission report
Yesterday, the Oso / SR 530 Landslide Commission released its final report – it is available online as a PDF. It its a very interesting document, carefully crafted and sensible in its recommendations. I would recommend that anyone interested in landslide, or indeed other geophysical, disasters reads the report as its findings are almost universally applicable. It is notable that the commission notes that there were many successes in the response, but that inevitably not everything went as smoothly as it could have done. Some aspects are almost bizarre – for example, the commission notes that:
On March 23, 2014, the second day following the landslide, Chief Willy Harper, District 25 (Oso), made a request to Chief Eric Andrews, Northwest Regional Coordinator for the Washington State Fire Defense Board, for a mobilization of state resources. Chief Andrews assessed the situation per state mobilization guidelines and made a formal request to the Washington State Patrol (WSP) for state fire service mobilization (all-hazards or state mobilization) under RCW 43.43.960 – -.964. This request was denied by WSP due to their legal counsel’s interpretation that state fire service mobilization resources and funding is available only for fire disasters.
Not surprisingly the commission recommends that this strange anomaly is put right.
However, in the context of this blog the most interesting aspects pertain to the landslide hazard and risk elements. Here the Commission makes a three key recommendations (amongst many others):
1. Support a Statewide Landslide Hazard and Risk Mapping Program
The Commission recommends the Legislature significantly expand data collection and landslide mapping efforts, which will provide the foundation for sound public and private land-use planning and decision-making.
The report provides more detail about how this should be undertaken, in particular with the use of lidar data. It is also suggests prioritization of transportation corridors, residential areas, urban growth areas, emergency evacuation routes and some forest lands. The work should be undertaken by the State Geological Survey, be subject to peer review and be overseen by a technical advisory group. The Commission illustrates the structure of this programme with the following diagram:
It is hard to disagree with this recommendation, and indeed it is reported that the Governor has requested a budget of $36 million for “improved mapping and other landslide mitigation measures”. Whether this budget is granted is a different matter of course. My only criticism of this recommendation is in terms of the use of the information. Hazard maps are only useful if there is clarity in how and when they are to be used, and by whom. In particular it is perhaps slightly surprising that there is no mention of risk assessment or mapping, but perhaps this is considered to be a step too far at this point.
2. Establish a Geologic Hazards Resilience Institute
The Commission recommends the Governor explore the creation of a geologic hazards resilience institute to address education, outreach, and research needs, professional practice guidelines, and other geologic issues impacting Washington communities.
In my view this is the most interesting recommendation, and the one that could have the greatest long term impact. The vision is for an institute that provides support by:
Assisting tribal, state, and local governments to establish programs and staffing to address local geologic hazards.
Providing accurate information on geologic hazards and risks relevant to land use planners as well as to the general public.
Identifying needs and providing training for geohazard specialists; for example, ICS training, and other training that assures successful emergency response.
Establishing public information response protocol for emergencies.Enhancing public education and awareness programs and partners.
Identifying long-term research and education/outreach funding partners.Conducting educational, outreach, and research activities.
If such an institute were to be established then Washington State could become a world leader in hazard management, a very exciting prospect.
3. Conduct Landslide Investigations
The Commission recommends the Department of Natural Resources (DNR) Division of Geology and Earth Sciences, Washington State Department of Transportation (WSDOT), Snohomish County, and the US Geological Survey (USGS) conduct landslide investigations to characterize the mechanisms that activated the landslide and to understand the stability of the landslide mass.
I cannot over-emphasize just how important this is. Undertaking a proper, detailed assessment of this landslide, and others as they occur, using a combination of detailed mapping, drilling, monitoring, lab testing and modelling is vital if we are to understand where such hazards will occur in the future. Opinion remains divided as to whether the incredibly destructive, fast and long runout of the landslide could have been foreseen (I remain convinced that it could have been), but we still need to understand properly the mechanisms that allowed this behaviour to occur. Proper forensic investigations are the key. Interestingly, this will be discussed in detail at the AGU Fall Meeting on Friday – I wish I could be there for both the poster session and presentations).
So overall, I think that this is an excellent report that deserves to be both read widely and to be implemented properly. It will be interesting to see what happens to the budget requests over the next few months.
15 December 2014
The Banjarnegara landslide
Late on Friday night a large and apparently very rapid landslide was triggered by heavy rainfall at Banjarnegara in Central Java, Indonesia. Latest reports suggest that 32 bodies have been recovered to date, with a further 76 people thought to be missing. The likelihood of their having survived the landslide is low; one can only hope that, as is often the case, it turns out that at least some of those reported missing were elsewhere at the time of the landslide. A further 15 people were injured, 11 of those seriously.
This image, from the Straits Times, shows the landslide site from close to the toe:
As well as being a tragedy, this is an interesting landslide for two reasons. First, the slide appears to have been a very mobile earthflow – in fact there appears to be two lobes with an untouched area between. Second, the landslide appears to be surprisingly deep-seated in the source area judging by the size of the lateral scarp on the left side of the lower part of the source area. It seems that this deep-seated nature created a very large mobile mass that overran the village.
The source area of the landslide is quite well illustrated in this AP image via CTV:
As far as I can see the slide has occurred in deeply weathered residual soil, with no evidence of bedrock, or a bedrock – regolith contact, in any of the images that I’ve seen. I wonder if this was a static liquefaction type process to generate the exceptional mobility. An alternative might be an undrained loading process generated by an initial failure near to the crown of the landslide, which would also explain the mobility, although the depth of the failure surface might be harder to explain.
Whatever the cause, this is a landslide that deserves detailed analysis. Java is a global hotspot for landslide deaths because of the toxic combination of steep slopes, volcanic soils, heavy rainfall and lots of people. There is an urgent need to understand better the mechanics of these large and immensely damaging landslides there.
4 December 2014
Yesterday I highlighted the very large Domkar Monastery landslide in Tibet, which failed in a somewhat spectacular manner last month. I have managed to track down a good image of the slope that failed, from the Tibetan Trekking website:
It is not clear to me whether this mass is loess or an old landslide body – I am still tending towards the former, but it could be the latter. The failed body has deeply incised drainage lines on each side, but little sign of erosion or excavation at the toe.
It is quite interesting to look at the Google Earth archive imagery of the site, taken from a perspective view. This is November 2007:
This is May 2009 (in black and white):
This is November 2010, which post-dates the April 2010 Yushu earthquake that affected this area and damaged the monastery:
The most obvious change is the development of works beside the road in the bottom right hand corner of the image – I wonder what this is? But the remarkable thing is that there is really no sign in these three images of the development of the landslide except perhaps that the rear scarp formed along the line of the track at the rear of the settlement. Even after the 2010 earthquake the landslide does not seem to have progressed, although it would be interesting to see an image from earlier this year. But from these images there is little sign that an incipient collapse was developing. I suspect that this supports the notion that this is a loess failure – loess can be a collapsible material, which means that it can be subject to rather spectacular landslides.
3 December 2014
The Domkar Monastery landslide
In mid November the ever impressive Tibet Earthquake twitter feed (@aam868) posted a number of reports about a landslide at the Domkar Monastery in Qinghai, eastern Tibet. This landslide, appears to have occurred on 2nd November 2014, is both spectacular and catastrophic. The Domkar Monastery is located at 33.01N, 97.14E:
The site appears to have undergone a massive rotational slip with an enormous displacement across the back scar: From the Tibet Express, these are the before and after shots:
Note the damage to the access road. Interestingly, this site was severely damaged by the Yushu earthquake in 2010; rebuilding had only been completed this year. In fact, if you look carefully at the before image it is clear that the back scar of the landslide had already formed, with substantial displacement have already occurred. The lateral scarp of the landslide has cut through the main temple, with major part of the building having been completely destroyed. China News has this image of the landslide, showing some detail of the damage. A translation of the text confirms that there were no injuries:
The material forming the landslide appears to be loess, which is prone to large-scale landslides. It would be interesting to know if there had been any construction works at the toe of the slope – often rotational landslides of this type are triggered by unloading of the foot of the slope.
1 December 2014
The Railway Children – Landslides in the Movies Part 2
This is Part 2 in my new series on Landslides in the Movies. Part 1 is here. The 1970 movie The Railway Children, which was based on a novel of the same name by Edith Nesbit, stars a very young Jenny Agutter. Probably the most famous scene in this film occurs when a landslide blocks the railway line at a time when a train is approaching. The three children run up the line to flag down the approaching train, thus preventing a disaster. This scenario is far from impossible – indeed I have featured landslide-induced derailings of trains on many occasions on this blog, and some of them do indeed have catastrophic consequences.
The landslide scene in the film is shown in this sequence on Youtube:
If nothing else it makes you realise how far special effects have come in the last 45 years. The movement starts as a translational slide – note how the trees remain perfectly upright as they displace – although the movement of the trees whilst the grass around them remains static is quite amusing in some ways. A really nice element is that the landslide occurs on a cut slope that clearly has an inadequate retaining structure at the toe – it looks like poor engineering by the construction crew to me. The should never have expected to retain a slope of this size with old railway sleepers. In addition, the way that the wooden posts topple onto the line suggests that they had very shallow foundations – surely a recipe for disaster:
It is this poor engineering that probably accounts for the collapse on a beautiful sunny day and in a dry condition (note the dust that is kicked up by the collapse at various points). The boulders on the line at this point is a nice touch, suggesting some precursory deformation.. Of course the sequencing of the failure is a little odd, in that the upslope section moves before there is much deformation in the retaining structure – there is clearly a complex process at work here.
I’m not sure that the film crew managed to depict the post-failure landslide scar particularly well – the deformation at the start suggests that the scar should extend right up the slope.
Finally, I think that the children need to study natural hazards at school, given that they diagnose the event as an earthquake.
30 November 2014
It has been a long time since I posted in my Landslides in Art series, so this one is well overdue. The last posting in this series was made almost a year ago. I really don’t know very much about this artist, except that she is based in Taipei in Taiwan, and uses the pseudonym Harlequin or Harlequipan. The painting in question appeared on her blog, which new seems to be defunct, four years ago. It is simply titled Landslide:
To me this appears to depict the type of channelised flow failures that are common on the steep slopes in the Central Mountains of Taiwan, triggered both by heavy rainfall and by earthquakes, such as this one from the 1999 earthquake:
I would welcome suggestions for future posts in this series.
18 November 2014
Rabenstein, South Tirol
A large landslide occurred at Rabenstein in the South Tyrol northern Italy (this is the German-speaking area of northern Italy). Fortunately it was caught on video and is now available on both Liveleak and Youtube – I have embedded the Youtube version below because it is easier:
The reported volume of this latest landslide is 20,000 cubic metres. The very large amount of precursory deformation is notable, as is the very rapid fragmentation of the rock mass as the collapse developed:
This is not the first landslide event at this site – Liveleak also has a smaller, earlier landslide event, which I think occurred on 30th September:
This second video is worth watching for the movement of the enormous boulder at the foot of the slope, which was safely captured by an engineered runout zone.
An update on the Mannen Landslide in Norway
Meanwhile the Mannen landslide in Norway has now clearly entered a steady secondary creep phase of movement. The Rauma Kommune website posted an update in Norwegian two days ago, which includes this movement graph:
The commentary suggests that continued low temperatures and dry conditions suggest that little change in the rate of movement is expected in the days ahead.