8 November 2013
As I write typhoon Haiyan is tracking across the Philippines. Jeff Masters reports that this is likely to be the most intense landfalling tropical cyclone on record; as such the damage on the areas affected is likely to be extreme. As I have noted previously, whilst it is the wind that tends to grab the attention of the media (and wind speeds of 310 km/h are clearly devastating), most of the damage is done by water, in the form of the storm surge, waves, floods and landslides. In terms of the latter, unfortunately the island of Leyte, across which the storm has just tracked, is very landslide prone. Indeed, Leyte was the location of one of the most devastating landslides in recent times, when the Guinsaugon landslide of 2006 caused 1126 deaths:
Interestingly, the Guinsaugon landslide occurred as a result of ten days of rainfall that deposited over 2,000 mm of precipitation in total. Typhoon Haiyan is also likely to bring very high rainfall totals, although the high rate of movement of the storm means that durations should hopefully be comparatively short. This is a simulation of rainfall totals along the track of the storm from NOAA:
Note that the forecast from Typhoon Haiyan is for a peak total of 52.5 inches (1300 mm). Taking Leyte alone (and that is not the only part of the Philippines to be affected by this storm), landslide hazard levels are very high. This is a rainfall induced landslide hazard map, from the National Mapping and Resource Information Agency, for Leyte:
Note in particular the very high levels of hazard on the eastern-facing slopes of the upland areas. Due to the orographic effects, these areas are likely to have received extremely high rainfall totals. It would be a surprise to me if there are not very serious landslide problems in this area. A general idea of what might be expected from Typhoon Haiyan can be gained from revisiting the typhoon Bopha / Pablo disaster, also in the Philippines, of December 2012. We may well not get a proper idea of the magnitude of the losses in the upland areas of Leyte until tomorrow; I suspect that the impacts will be very serious.
7 November 2013
I missed this story at the time, but it is worth highlighting here. Back in early August, golfers at the Matterhorn Golf Club in Taesch, Switzerland faced a slightly unusual hazard when a boulder fell from an adjacent mountain to land on one of the greens. The oobgolf.com website has the best set of images, from which these are taken:
In the comments below the article, several people are confused about how it ended up in the middle of the green. I would suspect that the boulder was both bouncing and rotating – in this case the tabular shape of the rock would create a very stable motion (a bit like a wheel bouncing down a slope). This would also be why the crater has that elongated shape, and why the boulder has bounced out of it.
6 November 2013
When I was studying Physics at school, we were shown the famous regelation experiment in which a wire is placed over a block of ice and a weight is attached to each end. Though time the pressure on the wire causes melting of the ice, and the wire slowly cuts its way through the block, and eventually the weights and wire fall to the ground. As the wire passes through, the water refreezes, such that the wire appears to pass magically through the ice.
There are various versions of this experiment on Youtube, of which this is about the best:
The interesting part of this experiment starts at about 1:22 and ends at 1:40 (in the speeded up sequence). Of course when the wire finally cuts through the ice the weights collapse to the floor with a great crash – the very last moments before this are shown below:
This experiment, though fun, illustrates an important aspect of first time failures in rock slopes. If you weren’t watching the experiment closely then the collapse looks spontaneous – I remember when we were shown the experiment one of my class rates immediately asked “What caused that?”. However, the collapse had no trigger – it was a spontaneous event caused by a progressive process. Now, if towards the end of the experiment that same classmate had leant on one of the weights then he would have triggered the collapse. However, he didn’t cause the failure, which was generated by the wire cutting its way through the block.
The Tumbi Quarry landslide brings this into sharp focus. Very often in the aftermath of large slope failures we spend a great deal of time looking for a landslide trigger. We analyse rainfall records, look at seismicity, examine temperature data and investigate the actions of people. Sometimes we even look at a combination of all these things. And, lo and behold, eventually we find the event, or combination of events, that caused the collapse to occur at that specific moment in time. Occasionally, we can’t even find a landslide trigger, as was the case in the Mount Cook landslide in 1991.
The point is not that triggers are unimportant – of course they are, especially in slopes in weak materials, and it is sensible to try to understand the relationship between the number of landslides and the magnitude of large triggering events in any landslide-prone environment. However, for individual first time failures in rock slopes the search for a landslide trigger is often a red herring. The reality is that the slope has weakened through time until it is on a hair-trigger; when a large enough event comes along the collapse occurs. If no event occurs then the slope collapses spontaneously; this is a true progressive failure.
So, spending time looking for a trigger for the Tumbi Quarry landslide is interesting and worthwhile, and I have no problem with this sort of work. However, we must first recognise that the basic question should be whether there is a landslide trigger at all, and if so what? It may well be the case that no landslide trigger can be identified. Second, we should not allow the search for a landslide trigger to distract us from the real issue – i.e. what were the processes that led to the rock mass becoming unstable, such that the trigger became effective?
To me the key issues at Tumbi Quarry remain unanswered, and the multiple victims of the landslide mean that they deserve more attention.
5 November 2013
This is Part 19 of my occasional Landslides in Art series, this time featuring the Dorset artist Geoff Townson. Part 18 is here.
Geoff Townson is an artist based in that classic landslide area, the West Dorset. Given that he has both a degree and a PhD in geology, and he worked in the oil industry for almost 30 years, it is not surprising that much of his work (though not all by any means) features the geological aspects of landscapes. And given that he lives in such close proximity to many active landslides, it is equally unsurprising that landslides feature heavily in his work.
Geoff has a good website that shows his many works – his paintings are well-priced and some are available as cards as well. If you browse his Dorset Landscapes and Dorset Seascapes sections you’ll see a variety of landslide-related pictures. I thought I’d feature three here, all of which are within the classic landslide group on the West Dorset coast. First, this one is entitled “Evans Cliff Landslip Charmouth – Overview”:
Second is “Golden Cap from the Beach”:
And third is “Winter Walk – Black Ven”:
In my view Geoff’s great skill is his ability to capture the drama of the landscape – at times these landslides can be truly forbidding in their appearance – and also a sense of the dynamic nature of the landslide systems.
4 November 2013
Last year, and subsequently, I blogged on a number of occasions about the January 2012 Tumbi Quarry landslide in Papua New Guinea. In a nutshell, this very large landslide occurred in an aggregate quarry that had been used by contractors working for Esso Highlands (Exxon) in the construction of a pipeline. Everyone involved in the pipeline project denied that the quarry was in any way a factor in the landslide, even though this is by far the most likely cause. This image below shows the landslide, which killed at least 25 people and ruined the livelihoods of many more.
It seems to me that it is inconceivable to think that Esso / Exxon did not investigate this landslide themselves. It is interesting to note that they have not released the results of such an investigation.
Anyway, a paper (Robbins et al. 2013 – sadly not open access, and not in either the Reading University or the Met Office online repositories) has appeared in the journal Landslides which explores the Tumbi Quarry landslide. The paper has been co-authored by meteorologists and geologists, although as far as I can see none have a long publication track record in landslide research. The paper essentially examines the question of the trigger of the landslide, comparing the likelihood of a rainfall trigger versus the effects of a distant seismic event. However, in the introduction the paper also says that:
This paper gives a brief description of the landslide, with the main aims being: (1) to review the rainfall accumulations in the Tagali Valley prior to the landslide event, (2) review the likelihood that seismicity could have played a role in the failure and (3) provide an overview of the landslide in relation to the land use and activities at the quarry site.
In terms of the description of the landslide, the paper does a very reasonable job, although it does not really explore the possibility that the rear scarp of the landslide had an element of wedge failure (this would be an important consideration in stability modelling). Of course the meat of the paper is an investigation as to the role of rainfall triggering. The analysis here is comprehensive and interesting. The Tumbi Quarry landslide was a deep-seated failure, so one would expect that the landslide would respond to long term rainfall trends rather than short duration intense rainfall. The paper suggests that the months and days immediately prior to the failure did include the sort of rainfall events that have been associated with landslides in Papua New Guinea on previous occasions. However, the long term rainfall trend was below average, and the paper notes that:
The recurrence of below average monthly rainfall, particularly in November and December, and the timing of the slide at the start of the wet season, rather than the end, suggests that groundwater would have been close to the dry season low point or even lower, given the exceptionally low rainfall in August 2011. It seems unlikely therefore that groundwater played a significant role in the failure
The paper also looks at the role of seismicity in triggering the landslide – this is an interesting issue. They have examined the earthquake catalogue for events in Papua New Guinea in the 90 days prior to the landslide. The conclusion is, rightly, that seismicity played no role. On 14th December 2011 PNG was affected by an M=7.1 earthquake, but this occurred a month before the landslide at an epicentral distance of 478 km. The time delay and the distance both render it highly unlikely that this earthquake played a role. On the day of the landslide PNG was affected by a M=4.7 earthquake, but this occurred 446 km from the landslide site. Again, it is highly unlikely that this played any role. Thus, the seismic trigger hypothesis should be rejected.
A strange aspect of this paper is that the discussion of the role of quarry and land use changes in triggering the landslide is accorded just one paragraph, even though this is flagged early in the paper as being one of the core aims. The paragraph adds little to that already known; it highlights that the landslide occurred in the area that was being quarried, such that “[t]he additional extraction of material from the base of the steepest part of the slope may have weakened the integrity of the unit above…One factor which could be considered the most likely to have acted as a causal influence is the over-steepening of the existing steep slope by the extraction of additional material.” The report notes that an IESC report from November 2011 “suggests that the site was ‘benched and slopes have been stabilised such that the quarry is safe and could be reoccupied should this be required in the future’ (D’Appolonia S.p.A. 2011b). Such measures would have been put in place to increase the stability of the slope and quarry area following material extraction.” This may well be the case, but it is really important to note that benching would not increase the stability of a slope that was prone to the type of failure that actually occurred – indeed in some cases it could reduce the stability (if for example the toe of the slope was trimmed back to create the benches).
To me the most interesting aspect of this paper is the cross-section of the site, which is Fig. 3 in the paper:
The cross-section picks out that the landslide occurred on a steeply dipping slide surface with a near vertical release surface at the rear. It is highly likely that removal of material from the toe of such a setting would decrease the stability of the slope, and it is equally unlikely that benching the slope would increase stability. This is a quarry setting that would alarm any geologist with a knowledge of landslides.
I don’t want to be critical of this paper – the core aim is to analyse the rainfall conditions leading up to the landslide, and the paper does this really well. As such it is a useful contribution, as is the analysis of the likelihood of seismic triggering. However, in deep-seated landslides the actual trigger is often a sideshow – it is literally no more than the straw breaks that camel’s back. The key issue is how the slope became destabilised. The team have not undertaken the sort of fieldwork at the site that would allow a definitive analysis of the causes of the landslide, so understandably this is covered only lightly and with understandable caution. However, it is really important to recognise that this analysis does not let the operators of the quarry off the hook – indeed, the cross-section suggests that it is far more likely that the quarry was the cause of the problem. If the operators thought that stability of this site could be assured through benching the slope then they were sadly mistaken. Of course, it is also not possible to say that the quarry was responsible for the landslide. This is simply unknowable based upon the information to hand, and it could only be resolved with a proper investigation. Throughout the last two years I have avoided pinning the blame on any particular cause, and I would reiterate again that we simply do not know why this landslide happened. What I find unbelievable is that a slide that killed so many people has not been properly investigated. That is surely a scandal.
Joanne C. Robbins, Michael G. Petterson, Ken Mylne, & Joseph O. Espi (2013). Tumbi Landslide, Papua New Guinea: rainfall induced? Landslides, 10, 673-684 DOI: 10.1007/s10346-013-0422-4
29 October 2013
Last week I posted one of the images kindly provided by Vaibhav Kaul of the aftermath of the Kedarnath disaster. These were taken on a recent visit by Vaibhav to the area, on foot of course – a truly epic journey. He has kindly allowed me to post more of them here. For reference, it is worth looking at the earlier post in which I reconstructed the events of that day in June, and the one in which I blended eye-witness reports with an overall narrative.
The Chorabari Glacier source of the second debris flow
The source of the second, more damaging debris flow, was a breach in the glacial moraine that formed a dam to create a temporary lake, known as Chorabari Tal (Gandhi
Sarovar), on the flank of the glacier. This image shows one of the two tongues of the glacier. The valley on the left side of the image on the flank of the glacier is that of Chorabari Tal (Gandhi Sarovar). The breach is clearly visible, as is the path that the water followed:
The track of the second debris flow
Viewed from below the town, the track of the main debris flow down the slope above the town is very clear. Most of the water and debris flowed down the left side of the image, a smaller component over-spilled the ridge to flow down towards the right side. The flows recombined above the town:
The impact on the town
The debris flow then struck the town with devastating consequences. At the upslope end of the town was located the temple, which escaped with comparatively minor damage. This is probably due to a combination of an extremely robust structure and, possibly, the protective effects of a boulder immediately upstream of the main building:
Downstream the buildings were far less fortunate:
Effects below Kedarnath
The debris flow then passed down through the valley below the town, where the effects of the debris flow were devastating. This is the channel upstream of the small village of Rambara – note the slope failures on the flanks of the channel triggered by undercutting by the main flow:
The most shocking impact of this disaster is the effect on Rambara. This is Rambara before the debris flow (source):
This is the site of the village now:
28 October 2013
One of the ways that landslides destroy buildings is by forcing a collapse due to progressive loading of the structure. This is a very interesting process that is perhaps surprisingly poorly studied. So, a new video that has appeared on Youtube is very welcome. There is very little information on the website about this event other than that it occurred in Thailand. The landslide appears to be a large earthflow. The relentless way in which the landslide loads the building is quite interesting.
At the start, the building is clearly being loaded at the rear, and the top portion is starting to shear:
By the time the house starts to topple, the upper portion of the building has been pushed forward a significant distance, leading to failure of the columns at the front of the building, which then topples forward:
Failure of the near end of the building causes the entire structure to progressively collapse.
26 October 2013
The Kedarnath disaster in Uttarakhand, India in June remains the worst landslide accident of 2013 to date. As a taster for a fuller post next week, Vaibhav Kaul has made the following photograph available to me. He recently trekked up to the site and collected a set of images, of which this is one. It is posted here with his agreement:
The image shows Kedarnath town in the bottom left corner. To understand the processes,please refer to my earlier post reconstructing the accident. On the slopes above are the smaller debris flows triggered by the rainfall event in June – these were not responsible for the losses. Upstream of the town to the right are the two debris flow tracks – the left hand one,which originated from the glacial lake overflow event, split into two components (clearly visible on the photograph). On the right side of the image lie the two debris covered strands of the Charobari glacier; the other (first) debris flow came from a slope failure that is out of site of this image and then flowed down the right hand lateral margin of the glacier before striking the town. This track is just visible towards the right of the image and then above the town.
25 October 2013
Review of a paper: A risk society? Environmental hazards, risk and resilience in the later Middle Ages in Europe
The management of natural hazards is a major activity in modern society, although often it occurs in a way that is not particularly obvious. Thus, for example, major road projects in upland areas generally include substantial investment in slope stability measures to prevent landslides and rockfalls from endangering road users – in many cases these can form a large part of the project budget. Of course there are more obvious approaches too – in the news at the moment is both the apparent failure of local government officials in Japan to make people aware of the hazard posed by landslides as typhoon Wipha bore down on Japan last week, with tragic consequences, and the resulting high-profile evacuations today as Typhoon Francisco threatens to bring rain to the same area. Recently, an colleague in the Archaeology department here at Durham, Chris Gerrard, and I were mulling over whether people in the past took the same general approaches to managing hazards as we do today. If they did, how sophisticated were those approaches, and how successful were they?
That conversation led to a research paper that was recently published the in journal Natural Hazards. The paper has been published in full Open Access form, so you can download the paper as a PDF or view it on the screen. In the paper we focused on the Middle Ages – which we defined as the period 1000-1500 AD – in Europe. This was a period of great change in Europe as modern societies started to emerge. It was also a period of great upheaval, not least because of the effects of repeated epidemics that devastated large swathes of Europe, of which the Black Death in 1348-50 was the worst – it is estimated that over 30% of the population of Europe lost their lives. In this context, it might be thought that managing environmental hazards was almost insignificant. That was most definitely not the case.
A risk society?
In the paper we have tried to demonstrate first that Europe was, perhaps unsurprisingly, affected by a wide range of disasters in this period, including major earthquakes, wind storms, floods, volcanic eruptions, landslides, tsunamis and famines. The illustration below, from the Konstanzer Weltchronik, a fourteenth-century ‘world chronicle’ produced at Konstanzer in Germany, shows the effects of the Basel earthquake in 1356:
In the paper we tried to estimate the costs of these disasters – this is a topic upon which we are now working, but a reasonable estimate seems to be in the range of 250,000 to half a million people over 500 years. This might seem low, but remember that the population of Europe at this time was just 39 to 70 million people.
It might be tempting to think that the standard response to such disasters in the Middle Ages was through religion – after all, even now we sometimes term these disasters “Acts of God”. It is true that that religion played a major role in creating a framework for understanding these events. Populations prayed that disasters would not occur, and that they would recover quickly when they happened, and religious leaders used events to encourage the population to higher levels of devotion. However, alongside this was a rapidly developing attempt to provide rational explanations for hazardous events – for example Hegenburg wrote in the 14th Century that:
Earthquakes arise from the fact that in subterranean caverns and especially those within hollow mountains, earthly vapours collect and sometimes these gather in such enormous volumes that the caverns can no longer contain them. They batter the walls of the caven in which they are and force their way into another and still another cavern until they fill every space in the mountain…If they cannot reach the surface they give rise to great earthquakes.
As an aside this is strangely similar to the utterly bizarre, and bogus, explosive gas theory for the generation of the Wenchuan earthquake (see this pdf for example).
Alongside this increasingly rational explanation for hazardous events was a surprisingly sophisticated system for managing risk. In the European Middle Ages, hazard mitigation was commonplace, with Italy very much leading way. For example after Florence flooded in 1333 the city authorities formed a committee to manage the repairs, they provided tax relief to victims (especially on food) and they organised the distribution of food. In modern soceties we share the costs of disasters through risk sharing – this is essentially the role of the insurance industry for example – and societies in the Middle Ages did likewise, especially in cities, through for example the organisation of fraternities and religious guilds that offered help in-kind, loans and/or stipends. Charitable giving was also common at all levels of society, and many societies also tried to spread the costs of disasters over time by storing a proportion of the harvest on a large-scale.
Sitting alongside these societal responses were structural measures to prevent hazard impacts. The image below, from the paper, shows the interior of the onastic church at Clara-a-Velha in Coimbra (Portugal). To escape the floods from the nearby River Mondego, the nuns initially raised the floor of the church. When this was unsuccessful they extended the church upwards – the people in the photo are on the partially reconstructed upper storey. Note the staining from the floods on the supporting columns:
In the town of Kootwijk in the Netherlands) the villagers fought against drifting sand, which smothered their fields, by erecting screens over 100 m) long. Eventually they appear to have given up the fight and relocated inland.
In modern society we also use hazard adaptation as a key mechanism for managing risk – thus for example we enforce building codes in seismically-active areas to try to reduce the likelihood that buildings will collapse. Hazard adaptation was surprisingly prevalent in the Middle Ages too, with the authorities imposing requirements on for example building quality to try to reduce losses. After parts of the city of Pisa were by fire in 1158, the civic authorities tried to reduce the risk of a repeat by demolishing wooden porches and balconies, whilst elsewhere thatched roofs were replaced with tiles. Inevitably, some disasters drove the relocation of people, but the archaeological evidence suggests that this was perhaps less common than might be expected – generally people “built back” in the same location.
So, in the paper we concluded that modern rsik management for natural hazards has its origin in historic practices, and indeed that essentially almost all of the techniques we use today were widespread at that time. There is no doubt that modern approaches are more complex and are based on a better understanding of both the hazard processes and the ways in which their effects can be reduced, but societies in the Middle Ages were also remarkably sophisticated and organised. As we say in the paper, “faith was no barrier to mitigation, and although medieval society may not have been the best protected against environmental hazards or the best resourced or claim a complete understanding of the risks it faced, it was also perhaps not the most frightened”. It is not a defining characteristic of modern societies that we manage risk in a sophisticated way, this has been the case for centuries.
Christopher M. Gerrard, & David N. Petley (2013). A risk society? Environmental hazards, risk and resilience in the later Middle Ages in Europe Natural Hazards, 69 (1), 1051-1079 DOI: 10.1007/s11069-013-0750-7
23 October 2013
Regular readers will know that the lack of attention that is paid to potential and actual landslide impacts during earthquakes in upland areas is a real hobby-horse of mine. Time and again we see the situation in which there is a lack of preparedness for landslides, causing huge disruption to the response and recovery operations, even though the threat was entirely forseeable. It is pleasing to see increased interest in the science of this issue in recent years, with a succession of good papers exploring both the mechanics of the landslide process (which is a very complex problem) and the likely occurrence of landslides. This week, a paper has been electronically released on the BSSA website, to appear in a forthcoming edition of the journal, which examines the likely impact of landslides in the event of a Mw=7.0 earthquake of a “Seattle earthquake” – i.e. a quake on the Seattle Fault in Seattle, Washington. The paper, Allstadt et al. (2013) uses synthetic broadband seismograms to model shallow landslides in the area likely toi be affected by such an earthquake. Such an analysis is complex and computationally extremely intensive. I should also note that the technique uses the so-called Newmark method to model the slope behaviour. Newmark is basically the best technique that we have at our disposal at present, and so the team were right to do this, but in my view it is somewhat deficient in terms of the ways in which it models slope behaviour. We need a better technique; the trouble is that at the moment we do not have one.
Putting those concerns to one side, the novel element of this is the use of the synthetic seismic data. However, given that this is a landslide blog I am going to leave that for the seismologists to discuss and analyse, and instead look at what the work shows us in terms of likely landslide impacts in such a Seattle earthquake. Figure 1 from the paper, shown below, is a map of the Seattle area showing in red the areas that have been identified by the city as being prone to landslides and in black triangles the Seattle Fault. The last significant earthquake on that fault was about AD900; the recurrence period of earthquakes is in the range 200 to 12,000 years:
The researchers ran two different scenarios for the landslides associated with a Seattle earthquake. In the first they assumed that the earthquake occurred when conditions were dry. In this case the earthquake generated just 4,977 landslides covering a source area of about 0.2 square kilometres. On the other hand, when the scenario was run for saturated conditions – in this case they generated 30,000 landslides covering 1.9 square kilometres in the city. It has long been hypothesised that antecedent weather conditions play a strong role in determining coseismic landslide distributions – this study supports that view. Fortunately, the extreme ends of the spectrum represented by these scenarios are unlikely – the probability is that the earthquake would occur when groundwater levels are somewhere between the two.
The consequences of the landslides in either scenario would be very serious indeed. In the best case scenario, shown in the figure below from the paper, landslides would impact heavily on the coastal bluffs of Seattle and in scattered locations around the south side of the city. In the worst case scenario, also shown below, the southern half of Seattle would suffer extremely high rates of landsliding, and even the northern half of the city would be affected.
The paper then considers the consequences of these landslides in terms of infrastructure impacts. The authors calculated the numbers of buildings at risk from these landslides (although not all such structures would be affected in an actual event). For even the best case scenario over 1,000 buildings lie in zones with elevated hazard ratings, of which 400 were in the two highest hazard classes. In the worst case scenario 8,000 buildings were in an elevated hazard zone, of which 5,000 were in the highest hazard classes. A further 8,500 buildings are in zones that might be affected by landslide debris runout. Of course this means that many roads, railways and pipelines are also at risk.
The authors acknowledge in the paper that because our understanding of seismically-triggered landslides is so poor there is considerable uncertainty in these analyses, and much more work is needed. I agree with this, but the research is clearly flagging a major and important hazard that deserves attention. I should also add that in one respect the research plays down the risk though. Experience from elsewhere suggests that the first really heavy rainfall event after the earthquake induces many more landslides, and that landslide activity would remain at a highly elevated level for years or even decades after a Seattle earthquake. Thus, we would expect to see many additional landslides after the main shock, and these landslides are likely to be very damaging.
Allstadt, K., Vidalem J.E., & Frankel, A.D. (2013). A Scenario Study of Seismically Induced Landsliding in Seattle Using Broadband Synthetic Seismograms Bulletin of the Seismological Society of America : 10.1785/0120130051