This blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings. The blog is written on a personal basis by Dave Petley, who is the Wilson Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom.

This blog is a personal project that does not seek to represent Durham University.

Wednesday, 30 September 2009

Debris flow damage from Typhoon Morakot in Taiwan

In addition to visiting the Shiaolin landslide in Taiwan at the weekend, we also managed to visit a couple of other places that had suffered damage during typhoon Morakot. Damage from debris flows and river floods occurred extensively throughout the upland areas of southern Taiwan, as these images show:

The government has set aside about US$5 billion for reconstruction. Unfortunately this area is threatened by another typhoon, called Parma:

Sunday, 27 September 2009

Photos and text from a visit to the Shiaolin landslide in Taiwan

The aim of this post is to present an initial interpretation of the Shiaolin landslide, based upon a couple of days of fieldwork by Chris Massey and myself over the past two days. The interpretation here, which I have written (so don't blame Chris!) should be considered to be provisional at best. At the end of the post there are some thoughts about future work. We have no intention at this stage of publishing our analysis - that is for others to do - and so are happy for others to develop the ideas presented here. If so, we would just ask for acknowledgement in any resulting publications and that you let us know what you have found.

Disclaimer - please note that this is a personal assessment based on a very provisional walk over survey. Far more work is needed to produce a definitive evaluation - this is not that! Please do not use this in any formal sense - that is not its purpose.

Geological setting
The basic geology of the landslide is an interbedded sandstone and mudstone sequence. The structure is quite complex - on the west side of the river the beds are steeply dipping, as the image below shows. The steeply dipping bedding is clearly evident on the left (west) side of the image:

Steep dips are also seen at the toe of the slope on the east side of the river near to the location of the old bridge. Further up the slope the bedding is difficult to discern, and we did not have time to get that far. There are clear planar surface visible - we interpreted these as being bedding, perhaps dipping approximately parallel to the slope, although these could have been joints. On the south edge of the landslide bedding is visible, but as the slope is steep we could not get a proper idea of the dip and dip direction. The upshot of this is that it is clear that the simple geological sections that appear to indicate a simple syncline and with failure occurring as a dipslope may not be supported by field evidence. An initial interpretation would be that there is a fault running through the valley close to the toe of the slope (probably on the right side of the image above), but this needs a great deal more detailed work.

Landslide morphology
This is an attempt to calculate some crude initial details and statistics of the landslide:

Landslide type: Rock-slide/debris-flow
Volume: 30 to 45 million cubic metres
Source area: 1.4 square kilometres
Depth (average): 20 to 30 m
Distance from crown to assumed toe (western side of river): 3,400 m
Vertical fall: 1,000 m
Angle of reach: 16°

The morphology of the slide is indicated below. To the left is the original image, to the right is the same image with annotation. The labels are meant to be indicative only - a much more robust mapping exercise would be needed to do this properly.

(Base image taken from a helicopter by Mr Chi)

The key points are as follows:
  1. There are effectively two slides at this site - the main one to the south (in the centre of the image above), and the other to the north, which is a much smaller (though still very substantial) slide. The slides are divided by a bedrock ridge for essentially their entire length;
  2. The main slide is a structurally controlled failure in bedrock. A large promontary high on the slope has failed and collapsed. At this point the failure is quite deep. The detachment surfaces are likely to be joints and/or bedding.
  3. The debris from the main failure has travelled directly downslope, unfortunately through the location of the village. In this central part the landslide appears to have been highly disrupted and to have bulldozed everything in its path;
  4. Most of the debris was deposited in the river channel. The river was blocked near to the old bridge, but there would also have been substantial amounts downstream. Much of this debris has been removed in the subsequent flood. It is likely that this is also the case for the remains of the village;
  5. The main rockslope failure was followed by a complex series of debris flows that have deposited finer grained, matrix supported materials on top of, and on the flanks of, the main slide deposit. The flows form distinct lobes that could be mapped. In some cases there is intact vegetation associated with them;
  6. The final phase of deposition was of fluvially reworked sediments.
The main rockslide
The main event appears to have been a catastrophic failure of a large area of the upper part of the landslide. Failure was without doubt structurally controlled, as evidenced by the surfaces visible in the scar:

This failure transitioned into a high speed failure that ran straight down the hill, across the first terrace and into the river, taking the lower terrace with it. The remains of this deposit are clear, including large boulders This one is on the river bed, hence the fine sediment and the rounding):

The deposit is clast (i.e. rock) supported, implying a rockslide mechanism, although there is some finer material, primarily derived from crushing and fragmentation. There are some highly shattered tree remains visible as well:

This part of the landslide appears to have run across the upper terrace, leaving some rockslide material behind, without eroding the terrace substantially. In places terraces gravels are still visible, overlain with rockslide material and underlain by bedrock:

The lower terrace has been mostly removed, and there is little evidence of debris from the village.This might indicate that it was incorporated into the landslide mass that was deposited in the river channel. Most of this debris has been removed by the dam break flood.

Just upstream from the old bridge (and from the village) the river channel is quite narrow. Landslide debris is found on top of the terrace on the far side of the river. Above this debris the slope has been stripped of trees and vegetation. This could be the outcome of a possible displacement wave. It appears that the main channel was blocked at this location by the landslide debris:

Subsequent debris flows
There is lots of evidence of subsequent debris flows overlying the main rockslide debris. These vary in character but all have a fine matrix with clasts. Some of these debris flows appear to contain little vegetation other than very splintered wood:

However, in other locations large amounts of vegetation are present, some of it still quite green with foliage:

On the margins of the slide these flows have bifurcated, leaving areas with in situ vegetation in place. In some cases there is evidence that rocks have travelled over these vegetated surfaces, leaving blocks caught in tree remains:

The dam break and flood
We did not really spend much time examining the remains of the subsequent flood. However, there are two houses left standing from the southern edge of the village. These show the after effects of the flood very clearly, even though the river was very wide at this point:

Downstream the effects are severe, with damage to bridges, embankments and even the river bed. For example, the central spans of this bridge have been washed away:

Future studies
The text here is no substitute for a detailed examination of this site. We are sure that this is being undertaken by our colleagues in Taiwan. For example, to understand this landslide properly there is a need for a full geological and geomorphological mapping exercise, including a structural geology assessment. It would be great to see an attempt to reconstruct the multiple events by mapping out the different lobes. The landslide should also have been recorded on the seismometer network, so an analysis of this might help to constrain what happened, and indeed to allow the event to be tied to the rainfall events that have been recorded from this storm. An interpretation of stallite and aerial imagery from before the slide would also be good. There is a need to reconstruct the terrain before and after the slide - using aerial images from before and ideally LIDAR data from the present. This would allow a calculation of the mass balances of the slide and of the geometry. Later on stability assessment based upon geotechnical testing could be undertaken, and ultimately even full modelling (although it is not clear what purpose this would serve). We are sure that all of the above is planned or under way (and a lot more as well) - the results will be very interesting.

A final disclaimer
All of the above is based on no more that a brief walk over survey. None of what is written here should be considered to be authorative or a substitute for a detailed survey. What is written here is the personal opinion of the author only and should not be used formally in any way.

Comments and thoughts welcome of course!

Friday, 25 September 2009

First images from a visit to the Shiaolin landslide

Today, with two colleagues, I have visited the site of the Shiaolin landslide in Taiwan, which killed about 500 people on 8th August. I will write a longer analysis soon (we are due to go back tomorrow), but for now I will just provide a before and after image.


After (the village was at the foot of the slope):

And this is the aftermath of the flood caused by the collapse of the landslide dam:

Wednesday, 23 September 2009

An interesting landslide website: Observatoire Multidisciplinaire des Instabilités de Versants' (OMIV)

In France there is an organisation called OMIV (Observatoire Multidisciplinaire des Instabilités de Versants), which is charged with the responsibility to "conduct geomorphologic, geologic, hydrologic, geophysical and seismic studies related to long-term monitoring, understanding of processes, and assessment of hazards at several active landslides". Perhaps most interestingly, they make the data from their various measurements available online, which provides a great research and teaching tool. The website is here:

The key aspect of the site is detailed four case studies of landslides in France. In each case detailed information is provided on each landslide, together with a quite detailed explanation of the slide. One example is the La Valette mudslide, which is this one (Google Earth image):

For each of the landslides it is possible to download an array of information, including maps, seismic datasets, movement data, meteorological data, groundwater data, etc. It is quite a resource, and its all available free of charge.

Tuesday, 22 September 2009

After Morakot

A new blog, called After Morakot, has been started to highlight the impact of Typhoon Morakot in Taiwan. The most recent post describes a journey by the author into the village of Namsia, which was one of the most seriously affected villages in the typhoon. The images and the commentary are remarkable.

I reproduce here two of the images, of the village of Mintsu - there are many more on the blog:

Rainfall disaster in the Cape Verde Islands

Cape Verde is an island state consisting of an archipelago of islands located in the North Atlantic 600 km from the coast of West Africa. It is a pretty poor country of about 500,000 people, which rarely makes the news. Over the weekend the islands were hit by very heavy rainfall, triggering extensive flooding and landslides. According to this article, at least three people appear to have been killed, and there has been extensive damage. The town of Covaoda on San Nicolau appears to have been particularly badly hit.

Thanks to Rebecca Sharples, who sent these images of the disaster to me. The situation, which as far as I know, has not been reported internationally, looks very serious:

Saturday, 19 September 2009

Special session on Typhoon Morakot in Taiwan

This afternoon at the Chi-Chi earthquake conference in Taiwan the organiser laid on a special session on the impact of Typhoon Morakot in August. This is of great interest to me, given the impact of the landslides, so I thought I'd give summary of the key points. Apologies for the note form - I have done this Twitter-style!

1. The magnitude of the typhoon
For Taiwan this was an extraordinary event. It appears that in terms of river discharge the floods were the largest since records began - over 200 years ago. For example, Prof. Tsai of the National Cheng Kung University showed that for the Gaoping River the peak discharge was 29,100 cubic metres per second. That is the equivalent of the daily water needs of 150,000 people - each second in a single river! These huge floods were driven by extraordinary rainfall. At Chiayi the statistics are as follows:
Total storm rainfall: 3005 mm
Maximum hourly rainfall: 136 mm
Max 3 hour: 325 mm
Max 6 hour: 548 mm
Max daily: 1623 mm

This is close to but not actually quite, the world record rainfall.

2. Why was it so intense?
Prof. Jou of the Taiwan Meteorological Agency suggested that there were two key reasons why the typhoon was so intense. First, the storm slowed down as it crossed the island. Before it made landform it was moving at 20 km per hour. When it came ashore it moved only about 50 km in 24 hours. This led to very high rainfall accumulations. Second it appears that as it formed the typhoon interacted with a substantive monsoon trough, which drew in moisture from the inter-tropical convergence zone to the southwest. This meant that the typhoon generated huge rainfalls on the south edge of the storm which is where the maximum damage occurred. Interestingly, he admitted that one of their dynamic models run the day before the typhoon struck forecast 1900 mm of rainfall. However they didn't trust the model.

3. Landslide impacts
Very little was presented on the impact of the landslide at Siaolin, but Meei-Lin Ling stated that the death toll was 491 people. She stated that at the moment they have records of 1349 landslides, 46 debris flows and 298 road slope failures. I suspect that that many of the landslides may be classified as debris flows? These landslides cover an area of over 50,000 hectares. Prof. Jenn-Chuan Chern the Deputy CEO of Morakot Post-Disaster Reconstruction Council, suggested that the volume of sediment is 56 million tonnes (I wonder if this is rather low though?). Tainan County had the largest number of failures (515) the Kaohsiung (288) and Chiayi (216). The landslide distribution closely reflects the rainfall distribution. In Sinkai village 32 people were killed by a debris flow. There was also very extensive damage to the highway network - there is some doubt as to whether the road to Alishan (a major tourist area) can be repaired.

4. Other damage and impacts
Coastal flooding was a major impact. The peak of the storm coincided with a high Spring tide, causing major inundation. These flooded areas and those affected by river floods, have had major problems from silt accumulation. In some cases over a metre of slit has had to be removed from houses. This is quite straightforward, but clearing the water and sewage pipe network has been a major headache. Over 500,000 tonnes of wood has been deposited and is having to be removed. This is a substantive task.

5. Human costs
Human impacts are 701 fatalities with another 58 missing. 120,000 houses were flooded, 310,000 houses damaged. 4489 people are being housed in army barracks even now; reconstruction is going to be complex. At present 55 aboriginal tribal villages have been displaced. Government evaluations suggest that 31 of these are permanently unsafe. Therefore resettlement is not simple - the aboriginal communities want to reconstruct their villages in order to maintain their culture (e.g. through targeted schooling), but given the dangers of the sites this is not simple. Government land is being earmarked and NGOs are helping to liaise to find an appropriate solution.

6. The elephant in the room - climate change
None of the speakers wanted to ascribe this event to climate change - there was a real sense of caution about saying anything unwise. This is sensible. The point was made that in recent years the number of landfalling typhoons has been high compared with historic record and also that a dramatic increase in precipitation intensity has been noted. The floods met and even the calculated probable maximum flood of the rivers. Thus, it appears that the rainfall that is now happening is different from the historic record. Is this climate change? They key factor seems to be the interaction of the typhoon and the monsoon. A discussant implied that this could be because the monsoon front is moving. Whether it is climate change or not it does seem sensible to plan for more intense rainfall events. This is going to be very challenging in a place like Taiwan.

7. Meteorology
The final interesting comment was a note that classifying typhoon intensity by wind, which is the convention, is meaningless when most of the damage is done by rainfall. They suggested that a new classification is needed that combines both wind and rainfall, allowing better forecasting of landslide and flood impacts. Frustratingly I submitted a grant application to DFID ten years ago to develop a scheme to do exactly this, tied to a terrain classification scheme. They didn't fund it.

All-in-all a fascinating set of presentations that really helped in the understanding of this extraordinary event.

Your questions and thoughts are welcome!

Friday, 18 September 2009

Extraordinary! Using a helicopter sledgehammer to clear loose rock from a cliff!

I thought I had seen everything until I saw this video! It is from Norway, showing an extraordinary method for removing a loose rock pillar from a high, steep cliff. The answer is obvious - just hang a 1.8 tonne block on a cable under the helicopter and bash the offending rock with your new sledgehammer. This must take very skillful flying. The most amazing thing is that it worked - and there is some good footage of the not - insubstantial block descending the cliff.

I really strongly advise that you take look! It is on the following Norwegian news site:

Presentation at the International Conference in Commemoration of the 10th anniversary of the 1999 Chi-Chi earthquake

Below is my presentation from the conference in Taiwan. The file is on authorstream (my authorstream page is here - there are several of my presentations there). You should be able to play or download the presentation below or from the authorstream site. As ever, please acknowledge anything that you use from it.

The paper was a review of the key things that we have learnt from research into landslides triggered by this earthquake. However, at the start there is some more general material on earthquake-triggered landslides worldwide, which might be of some interest.

Feedback welcome of course!

Thursday, 17 September 2009

Presentations on Day 1 of the International Conference in Commemoration of the 10th Anniversary of the 1999 Chi-Chi Earthquake in Taiwan

The Chi-Chi earthquake struck Taiwan on 21st September 1999. To remember that event, a range of organisations in Taiwan have organised a scientific conference aimed at sharing the lessons learnt from the earthquake. I was lucky enough to be invited to speak on landslides triggered by the event - my paper is tomorrow - but I thought I would also post on some of the most interesting presentations as they occur.

This morning there were a number of introductory and keynote speeches. I thought I would focus on two of them. First, Prof. Huang Jong-Tsun, President of the China Medical University, gave an overview of the impacts of the earthquake and the lessons learnt from a governmental perspective. He stated that the final death toll of the earthquake was 2505 people, with damage including the total destruction of 27,273 houses and 293 schools. He highlighted the post-event response with some highly impressive statistics:
  • The national authorities released information on the epicentre location and the magnitude 102 seconds after the earthquake (!);
  • The government mobilised the first army response teams within 13 minutes;
  • At the peak, there were 460,000 people working on rescue and recovery operations.
The demands on the government in the five years recovery period were acute. In total, 17% of GDP was released for rebuilding, which meant that in the first three years 20,000 public engineering projects were required, costing US$3 billion. 10 million cubic metres of solid wastes was produced, about half of which was recycled. Schools were reconstructed using seismic codes that dictated that the should be able to withstand peak ground accelerations of 0.41g, equivalent to the design criteria for a small nuclear power station.

He also touched on some more nebulous but very interesting issues:
  • The government discouraged the desire amongst the people to create multiple landmark memorials. The strong sense was to focus attention on looking forwards not back.
  • Studies showed that the suicide rate amongst those affected by the earthquake increased by 40-50% in the aftermath, but quite quickly returned to normal levels;
  • Full post-traumatic stress disorder affected 9% of the affected population, but reduced to 3% within three years
  • Substantial legal problems arose with redefining property values as the landscape was altered to such a high degree;
  • Landslides and sediment delivery represented major ongoing legacies that in some cases have rendered the reconstruction of infrastructure unviable. In consequence a decision has been made to wait for the landscape to naturally stabilise.
Finally, Prof. Huang highlighted a major problem moving forwards, which is how to make decisions under situations of ambiguity. He gave the example of decisions about the evacuation of landslide-prone villages as typhoons move towards the island. Given the impact of typhoon Morakot, this is an important issue that needs work.

The second presentation that I will mention briefly is that of Prof. Gordon McBean, who holds a Chair at the University of Western Ontario in Canada but who is also the Chair of the Science Committee of the Integrated Research on Disaster Risk (IRDR) programme, which is an initiative by ICSU. He used some statistics to demonstrate that disaster impacts are increasing and so to justify (quite correctly) the proposed programmes. The programme aims to drive a shift in focus from response - recovery to prevention - mitigation by driving a series of large science programmes covering characterisation of hazard, vulnerability and risk, and effective decision-making, with a planned legacy after ten years of enhanced capacity to address hazards and to make informed decisions. This is of course a highly worthy initiative that I support fully, but I do hope that they but considerable focus on the translation of knowledge from scientists to practitioners and policy-makers. So often the issue is not a lack of knowledge but rather this exchange process. In my view it is this that is the greatest challenge of all at the moment.

Tuesday, 15 September 2009

Interesting landslide footage from La Jolla, California

Image from here

The La Jolla landslide in San Diego California occurred in October 2007, destroying three houses and a road, and leaving many more damaged. The landslide is currently the subject of a lawsuit that was filed by the householders. I am not going to comment further on the slide whilst this is going on, but as part of the evidence in the case some mobile phone footage has emerged of the slide as it occurred. This footage is unusual in that it covers a progressive, non-catastrophic slide, essentially showing the tension cracks opening as the slide occurred. The footage can be viewed here. The complete (8 minute) footage, available on that page, is worth viewing.

Fatal landslides in S. Asia in 2009

The key driver of landslides in S. Asia is the SW (summer) monsoon, which brings prolonged and sometimes heavy rainfall across much of the Indian subcontinent. This year the monsoon has been comparatively weak so far, with only a month or so to go. Indeed, the Indian Institute of Tropical Meteorology maps of the total monsoon rainfall give a picture of considerable rainfall deficit, although it should be noted that heavy rainfall in the last few days may have started to reduce this:So I wondered to what degree this is being reflected in the landslide pattern for S. Asia. The graph below shows the monthly occurrence of rainfall-induced fatal landslides for S. Asia for the period 2002 to 2008, with the 2009 figures up to the end of August included (click on the graph to view a bigger version in a new window):

The black line is the long term average, and red line is 2009. I have used a lighter line for previous years to make the graph clearer. As I have pointed out previously for similar graphs, this graph shows clearly the impact of the monsoon, with a clear, asymmetric summer peak and low levels the rest of the year. Note that there is some inter-annual variation, with 2007 and 2008 varying somewhat from the patterns seen in other years for reasons that are not at all clear.

So what of 2009? Well, perhaps surprisingly 2009 shows a very unexceptional picture. May was somewhat above average, and June below, whilst July and August are almost exactly as per the long term mean. There is some evidence that in previous El Nino years September has been the month with the most intense rainfall in S. Asia, so it will be interesting to see what is to come.

Monday, 14 September 2009

A small collection of interesting landslide images

A quick post today - I thought I'd post a small selection of impressive landslide images that I have come across recently:

Ongoing rockslide in India, from here.

Landslides resulting from deforestation in Taraza, Antioquia, Colombia, from here.

A long runout landslide from Sichuan in China, from here.

After effects of a landslide on the island of Ibiza in Spain, from here.

Sunday, 13 September 2009

On the loss of life in landslides during the 1949 Khait earthquake

Regular readers will know that one of my interests lies in trying to get a better understanding of the loss of life associated with landslides. A key realisation of this work for me has been that earthquake-triggered slides cause a very substantial proportional (probably in fact the majority) of fatalities is mass movement events. Unfortunately our understanding of seismically-driven landslides, and their impacts, remains poor, certainly in comparison with rainfall induced slides. For that reason, work to re-examine past seismically-driven events is very welcome, helping us to get a much better understanding of the range of processes and impacts in these events.

One significant but until now slightly elusive such event has been the 1949 Khait earthquake. This was a Mw=7.4 event on 10th July 1949 in the Tien Shan mountains of what is now Tajikistan, but was then the Soviet Union. The timing and location of this event, soon after the war in an area about which the Soviet Union was very secretive, has meant that it has been very difficult to determine any details about the landslides that were triggered in the earthquake. However, some rather speculative reports have suggested that the impacts were very large - for example this article in Mountain Research and Development reported a huge landslide at a site that it termed Borgulchak Rock. This slide was reported to have travelled 12 km. Some reports, such as the Wikipedia article on this landslide, have suggested a death toll as high as 28,000 people, although to be fair this may well be something of a misinterpretation of the original source.

An article in press in the journal Engineering Geology, by Steve Evans and colleagues (Evans et al. 2009 in press) seeks to re-examine the landslides triggered by this earthquake. Unusually for a science paper the article is a ripping-good read. The paper re-evaluates the Khait landslide, and the other large slide that was triggered in the earthquake, providing a rational analysis of the likely impacts of the mass movements.

First, they look at the Khait landslide (termed in the paper as a rockslide / loess flow), which is still clearly visible in the landscape, even on the low resolution Google Earth imagery available for this area:

For the Khait landslide they conclude that the volume was probably rather lower than earlier estimates have suggested. The other landslide considered is a very large and complex flowslide that swept down the Yasman Valley, covering about 20 km. This slide had multiple source areas on the southern side of the valley:

They conclude that this is a rather destructive loess flow slide with a volume of about 245 million cubic metres. Remarkably it travelled over a slope with an angle of just two degrees!

Evans et al. (2009) then consider the fatalities caused by the landslides. By looking at contempory reports of the population of the Khait area and census data on settlement size and population density they reject earlier estimates of the loss of life. For the Khait landslide itself they conclude that about 800 fatalities is probably a reasonable estimate - note that this very considerably less than earlier estimates. For the Yasman Valley flowslide they estimate about 4,000 fatalities, and they determine that there were probably a further 2,400 deaths. This gives a total fatality count of about 7,200 - far lower than previous counts, but still substantial of course.

In conclusion, this is a very important contribution, filling in another gap in our understanding of previous landslide impacts. Steve and his colleagues have also just published a similar paper (Evans et al. 2009) re-examining the 1960 Huascaran rock avalanche in Peru. This will be the topic of an upcoming post.


Evans, S., Roberts, N., Ischuk, A., Delaney, K., Morozova, G., & Tutubalina, O. (2009 in press). Landslides triggered by the 1949 Khait Earthquake, Tajikistan, and associated loss of life Engineering Geology DOI: 10.1016/j.enggeo.2009.08.007

Evans, S., Bishop, N., Fidel Smoll, L., Valderrama Murillo, P., Delaney, K., & Oliver-Smith, A. (2009). A re-examination of the mechanism and human impact of catastrophic mass flows originating on Nevado Huascarán, Cordillera Blanca, Peru in 1962 and 1970 Engineering Geology, 108 (1-2), 96-118 DOI: 10.1016/j.enggeo.2009.06.020

Saturday, 12 September 2009

Coastal erosion - the last groyne problem

One of the most emotive topics for local authorities in the UK is that of coastal erosion. Over the last few years in Britain we have increasingly taken a view that coasts should only be defended where absolutely necessary - elsewhere erosion should be allowed to occur at a "natural" rate. In most cases the view has been taken that protection of coasts in which erosion will affect domestic properties (i.e. peoples' houses) does not qualify as necessary. This is particularly harsh for those whose properties are affected by erosion - in the UK domestic insurance does not cover erosion and there is currently no compensation scheme for people whose houses are affected by erosion. Even worse, when the house becomes dangerous the local authority will often take possession of the hose and demolish it. Guess who gets the bill for the demolition...

One of the key reasons for choosing not to protect coasts is the so-called "last groyne" problem. This occurs at the point of the last (down drift) part of the coast that is protected. Here, the coastal protection updrift typically starves the coast of sediment that would normally provide some protection against erosion, leading to greatly increased erosion rates at this point. I was playing with the historic imagery feature on Google Earth this week and came across a very clear example of this problem, from the east coast of Britain.

Here is an image collected in 1999 of the site in question. As ever, click on the image for a better view in a new window:

Here, longshore drift is from the northwest to the southeast, such that the groyne (sea defence) in the centre of the image is the last of the chain. Most of these sea defences have been in place for half a century. What should be immediately obvious is the way that the coast steps back at the point of the last groyne, indicating the much higher erosion rate at this point.

Let's now zoom forward just seven years to an image that was taken in 2006:

The change in this period is quite startling when you look closely. First, you will see that the area previously affected by enhanced erosion has retreated further. The sea defence has broken down and three houses have been lost. It appears that some rock armour has been dumped at this point to try to reduce erosion. The sea has also breached and destroyed the coastal defence just to the NW of this point, and as a result the coast has started to erode. In consequence a whole series of properties have been lost along this frontage.

The town in question is called Happisburgh. The residents have a campaign to try to have sea defences constructed, with an excellent website.

Perhaps most startling is this image, from a sequence of aerial photographs, which shows the same location in 1986:

At this point the groynes were intact all the way down the coast, and the current step back is not present. Therefore, the big step back in the coast that is seen in the first image above occurred in just 13 years.

Friday, 11 September 2009

Real time landslide monitoring in New Zealand - the Utiku landslide

I have posted previously about the near real time landslide monitoring project that is being undertaken on the Taihape landslide by the GEONET team in New Zealand. The most amazing aspect of this project is that the monitoring data is streamed onto a web server and can then be viewed using a rather neat online graphing package (available here).

The great news is that the team have now instrumented another landslide on North Island, this time on a slightly more active slide at Utiku, which is not far from Taihape. Once again the data is being put online in near real time, and once again it can be viewed using the graphing package. This is available here.

This is how Geonet describes Utiku:
"The Utiku landslide has been classified as a deep-seated translational block-slide earth-flow. This classification refers to the characteristics of the landslide. Deep-seated refers to the depth of movement (depth to the landslide slip-plane); the slip plane of the Utiku landslide has been recorded at 20 m below ground level at the toe (bottom) and increasing to 65 m towards the back scarp (top). The term translational refers to the movement style of the landslide, where it moves as a relatively intact mass (raft) of material, along a planar zone of weakness. In this case, the zone of weakness corresponds to a thin clay layer thought to represent a bedding plane within the local sandstone . The term block-slide and earth-flow describe the landslide structure and movement mechanisms."

This is quite an old slide (at least 1,800 years, and possibly much older), but it remains active. The road that crosses the slide shows some minor signs of deformation that has required patching:

It will be very interesting to see how this landslide behaves as groundwater levels increase.

Thursday, 10 September 2009

Successful anticipation of the Rest and Be Thankful landslide in Scotland

Image from The Times

The Scotsman is reporting today that engineers anticipated the occurrence of the Rest and Be Thankful slide this week, shutting down the road before the slide happened. The report suggests that the intense rainfall prompted an inspection of the hillside by engineers because of the previous occurrence of landslides at this site. Once there, the engineers noted that the water in the burn (stream) was discoloured and the slope showed signs of movement. As a result they used existing traffic lights to close the road, whereupon 800 tonnes of material came down.

This is a rare and significant example of successful short-term anticipation of a landslide event. The engineers deserve praise for their actions. The damage to the road was also minimised by existing protection on the road.

Tuesday, 8 September 2009

New landslide at the Rest and Be Thankful in Scotland

Probably the most infamous landslide event in recent years in the UK occurred in Scotland in August 2004 when a series of landslides occurred, including one on the A83 at the rather delightfully named "Rest and Be Thankful". The same area was affected again by landslides in 2007. The images from the time were quite spectacular, as shown by the BBC at this web page, which is the 2007 slide:

The slide here, and several other slides on the same day in 2004, prompted quite a major study into landslides on the Scottish Highway system. The report is online here.

The BBC is this afternoon reporting that the road is once again blocked by landslides.:

"A long stretch of the A83 has been shut after a landslide at the Rest and be Thankful road, west of Loch Lomond. Police said westbound traffic was being diverted at Arrochar while eastbound traffic was being diverted at Inverary. According to reports, some heavy vehicles are stuck in mud which has come down from the hillside."

For those interested there is a nice background to the 2004 events here, whilst the BGS have information on the 2007 landslide here.

Sunday, 6 September 2009

August 2009 fatal landslide map

Here is the fatal landslide map for August 2009. As usual each yellow dot represents a single landslide that killed one or more people:

Click on the map for a better (downloadable) view in a new window.

The statistics are as follows:
Number of recorded fatal landslides: 53
Number of fatalities: 782

In terms of fatalities this places August well above the 2002-2008 average (347 fatalities). Of course this is dominated by Typhoon Morakot in Taiwan, and in particular the landslide at Hsiaolin. However, the map shows clearly the impact of the SW monsoon along the southern edge of the Himalayas and also the impact of typhoons in E. Asia. Elsewhere there is the normal smattering of landslides here and there. The lack of slides in S. America is perhaps surprising. The paucity of hurricanes in the Caribbean means that this area is not figuring at the moment. I wonder if we will see a large, late season event.

Oh dear, more scientific hyperbole about climate change and natural disasters

My Alma Mater, University College London, is this week hosting an interesting colloquium on the theme of "Climate Forcing of Geological and Geomorphological Hazards". This is a really interesting topic and the organisers should be applauded for attracting some really interesting talks. If it wasn't for the fact that I am heading out to Asia on Wednesday I would attend for sure. However, the Observer, which is the Sunday version of the UK broadsheet newspaper The Guardian, has today run a two page story about the conference. Again, this is not unwelcome - it is important that articles are run about the multi-faceted implications of climate change - but unfortunately the tone of the headline and lead material is an example of scientific hyperbole has left me speechless:

Climate change: melting ice will trigger wave of natural disasters
Scientists at a London conference next week will warn of earthquakes, avalanches and volcanic eruptions as the atmosphere heats up and geology is altered. Even Britain could face being struck by tsunamis.

Scientists are to outline dramatic evidence that global warming threatens the planet in a new and unexpected way – by triggering earthquakes, tsunamis, avalanches and volcanic eruptions.

Reports by international groups of researchers – to be presented at a London conference next week – will show that climate change, caused by rising outputs of carbon dioxide from vehicles, factories and power stations, will not only affect the atmosphere and the sea but will alter the geology of the Earth.

Melting glaciers will set off avalanches, floods and mud flows in the Alps and other mountain ranges; torrential rainfall in the UK is likely to cause widespread erosion; while disappearing Greenland and Antarctic ice sheets threaten to let loose underwater landslides, triggering tsunamis that could even strike the seas around Britain.

At the same time the disappearance of ice caps will change the pressures acting on the Earth's crust and set off volcanic eruptions across the globe. Life on Earth faces a warm future – and a fiery one.

Now, there is little doubt that there is a possible link between climate change and geophysical hazards, and that this is a topic that requires study. But to present the topic in this way is ridiculous given our current state of knowledge. Some elements of the quote above are probably untrue (melting glaciers will set of avalanches for example), and some of the remainder is speculative at best (e.g. widespread erosion in the UK, underwater landslides from the loss of ice sheets). Much of the rest has sensationalised climate impacts by presenting end member (i.e. large but unlikely) events as having a far great likelihood than is the reality - e.g. the UK being affected by tsunamis generated by underwater landslides caused by Arctic melting. This is possible, but is very, very unlikely, and there is little if any evidence that such events have occurred in the past.

But, unfortunately it gets worse. Bill McGuire, the Director of the Benfield Hazards Research Centre at UCL, is quoted as saying the following:

'"Not only are the oceans and atmosphere conspiring against us, bringing baking temperatures, more powerful storms and floods, but the crust beneath our feet seems likely to join in too," said Professor Bill McGuire, director of the Benfield Hazard Research Centre, at University College London (UCL)."Maybe the Earth is trying to tell us something,"'.

Now I like and admire Bill, I consider to be a friend, and I think that he has done a lot of good stuff. But this type of quote is really unhelpful. In my view there is no place for scientists to state things sthat the the oceans and atmosphere are "conspiring against us" - they are responding to the forcing that we are causing. And what can one say about a scientist stating that "Maybe the Earth is trying to tell us something"?

The remainder of the article is rather more measured, with some not unreasonable quotes from some good scientists. However, the damage is done in the first part of the article, and of course in the headline.

Take a look at the comments on the Guardian web page. Unsurprisingly, the denialist community has jumped on this to undermine the research that is being undertaken on climate change. This is a great shame - anthropogenic climate change is a huge issue based on good science. Unfortunately, articles like this, based on speculation and exaggeration, are really unhelpful to those trying to do good science and to persuade society of the importance of this issue. If there is one thing that I have learnt in the last couple of years is that as scientists we need to be measured and realistic about what we write and say. The organisers of this conference would be wise to remember this.

Friday, 4 September 2009

The Cikangkareng village landslide, Indonesia

As I reported a couple of days ago, the Mw=7.0 earthquake in Indonesia on Wednesday triggered a substantive landslide in the village of Cikangkareng in West Java. This landslide buried ten houses, a mosque and an amusement (video game) arcade with the loss of about 57 lives. AP have today released images of the landslide that are quite interesting. In particular this one:

Why is this interesting? Well, first lets note that we should ignore the red colour on the face of the scarp - this is a mantle of tropical soil that have come down from the crown of the slide (you can see the red soil at the very top of the landslide - this is of course typical of a tropical area). More important is the structure behind the mantle of soil debris. Here it is clear that the rocks are horizontally-bedded (or at least nearly so). Such a large failure in horizontally-bedded rocks is certainly not unprecedented, but is slightly surprising. The debris is very coarse-grained and has travelled quite a long way, which is also interesting.

Often, failures like this are associated with some process that has caused undercutting of the toe - for example wave erosion. Clearly there are no waves here - I wonder if there had been any activity to quarry stone from the slope, perhaps as a building material?

I am reminded of the Manshiet Nasser landslide in Cairo a year ago:

In that case the key cause was probably quarrying at the foot of the slope.

The geological structure of the Hsiaolin slide

Thanks to reader Chingying Tsou, who has answered my request for information about the geological structure of the Hsiaolin landslide. He has provided a link to the website of the Sino-Geotechnics Research and Development Foundation, which provides the key information. The page is in Chinese, but the diagrams are really helpful, as is the Google translation.

So here is a topographic map of the landslide site, with the major faults and of course the landslide itself marked on:

You will note two cross-section lines on the map, one of which (A-A') runs down the axis of the landslide. This cross-section is reproduced below:

It is clear from this that the landslide is a dipslope failure - i.e. the slide has occurred on beds that are orientated parallel to the slope, and thus facilitate failure. The cross-section indicates that the rocks are a mixture of sandstone and shale. This can often cause problems as the shale is weak, impermeable and prone to weathering, whereas the sandstone is often stronger but allows the accumulation of water (i.e. pore pressure generation). The presence of the fault is an additional factor - it may well be that the movement on the fault has caused the beds to be disrupted and thus weakened. It should also be noted that this cross-section is probably only indicative. It would not surprise me to find that the river has actually eroded out the lower portions of these beds, then filled in the spaces with the terrace deposits upon which the village was built, further weakening the slope.

Thursday, 3 September 2009

The Hsiaolin landslide slope before failure

In an earlier post I highlighted a satellite image of the Hsiaolin landslide site. I have trimmed this a little below:

This image is rather helpful as it starts to allow the site of the landslide before failure to be examined using Google Earth, which has good quality imagery of this area. This is, as close as I can get it, the same slope prior to failure:

Click on the image for a better view in a new window.

There are a couple of things to note here. First, the slide ran out straight across the village, removing all trace as the earlier photographs showed. Second, the rivers clearly underwent huge amounts of flooding.

A perspective view of the site is a little more helpful:

I have annotated the image below to locate the approximate boundaries of the landslide, using the satellite image above plus the photographs of the site that are now available (see this post)

You may need to click on the image to be able to see the boundaries properly. These boundaries are at the moment very much indicative, but they give the general idea. The landslide is intriguing because the slope was not showing obvious signs of instability as far as I can see, bar a depression in the head scarp area the could be a tension crack? The river has clearly undercut the toe of the slope, which could have been a factor? It would be interesting to know more about the underlying bedrock, and in particular the dip direction. Can anyone provide any more information?

Comments welcome.