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.

Tuesday, 30 September 2008

Further update on situation in Sichuan

The China Daily has today published an article that describes the situation around Beichuan as a result of the current heavy rains. The article is here. The situation is sounding increasingly dangerous. For example:

"As a result of heavy rains and incessant mudslides, the plan to rebuild quake-devastated Beichuan county in northern Sichuan province as an earthquake museum has been put on hold, a local official said."

"A large part of the county is now buried under the mudslides caused by unprecedented downpour since last Wednesday."

"Beichuan ... consists of an old town in the southwest and a new town in the northeast. Half of the old town has already been buried by mudslides. Only two roofs could be seen in the 1 sq km area. In some places, the mud is even more than 40 m deep."

"As if the mudslides were not enough, there's a looming threat of floods as the mudslides from mountains have blocked the upper reaches of the river, Lin said" [Lin Chuan is the head of local culture and tourist bureau].

Monday, 29 September 2008

Grave concerns in the earthquake affected area in Sichuan

Reliefweb is this morning carrying a story highlighting grave concerns about the situation in Sichuan in view of recent weather. The article is here. This article appears to have originated from the Jinde charity, who are active in the earthquake affected areas. In a nutshell they are reporting that:
  • Sichuan Province is suffering the most severe rainstorms for a century. They report that since 21st September the city of Jiangyou has received 129.4 mm and in some districts as much as 338.7 mm has fallen [my comment - I am surprised that this is the worst rain in a century to be honest, although the totals are high];
  • This heavy rainfall has caused extensive disruption in the earthquake affected areas;
  • Communications with many affected areas appear to have broken down;
  • The water level in the Tangjiashan Lake was reported to be rising to an alarming level, due to large landslides blocking the overflow channel. Yuli Village near Beichuan is facing the danger of being flooded again.
Clearly the level of information glowing out is limited, but this is consistent with the impacts seen in other areas in the first heavy rainfall event after a large earthquake.

Sunday, 28 September 2008

Updates on the Cairo and Shanxi landslides

The search for victims in both the Manshiyet Nasser rockslide and Shanxi flowslide continues. The following is the latest news:

Manshiyet Nasser, Cairo

The number of recovered victims is now 107, with many more still believed to be trapped under the rubble. Clearly there are still extraordinary difficulties in getting beneath the large blocks. Interestingly, there is now a Quickbird image of the site, as highlighted by Almasry Alyoum. Below I have compared the before Google Earth image with the after Quickbird (click on the image for a better view - before in the left (Google Earth), after on the right (Quickbird)).

The Almasry article makes two interesting observations:
  1. Since the Quickbird imagery was collected on 18th September, no official body has requested or made use of the images;
  2. "The shots reveal that Muqattam Mountain may be vulnerable to other collapses, especially in its southern areas and in particular, its “Sharei tesa’ah” and “Maidan al-Naforah" areas, said geologist of the National Authority for Space Sciences Muhammad Naguib Hegazy"
There is also a nice article about the human aspect of vulnerability in shanty town here.

Shanxi flowslide

AP image of the landslide debris

There is a good article reviewing the causes of the Shanxi flowslide here. Interestingly it states that:
  1. The number of fatalities is now 265;
  2. The landslide was not associated with a spell of we weather. Indeed, "The whole county had received only a minimum rainfall amounting to 1.5 mm since the beginning of September, according to the Xiangfen bureau. This amount was not enough to sufficiently wet the ground, let alone cause a mudslide, said specialists at the China Meteorological Bureau."

Friday, 26 September 2008

The Yigong Rock Avalanche, Tibet

Occasionally I like to take a look back at a significant landslide. Today I thought I'd write about the amazing Yigong rock avalanche in Tibet. The location of the landslide is shown on Figure 1: it's location is 30°14' N, 94°59' E, located high on the Tibetan Plateau.

Figure 1: Google Earth image showing the location of the Yigong landslide. Click on the image for a better view.

The landslide itself occurred at midday (UT) on 9th April 2000, when a wedge failure at about 5,500 m elevation (3,300 m above the valley floor) detached a volume of rock of over 100 million cubic metres.

the landslide occurred in Zhamu Creek. It took only 10 min to travel down a horizontal distance of 8 km through a vertical elevation difference of 3330 m from its source at 5520 m a.s.l. to its sediment fan at 2190 m a.s.l.(Fig. 3). The trigger for the failure is not clear, but no obvious rainfall or seismic event was recorded. The debris travelled down the valley (Fig. 2) at velocities of up to about 14 m/sec (Shang et al. 2003). Over the next 10 minutes the landslide travelled about 10 km, entraining debris, snow and ice as it flowed to generate a deposit with a volume of about 300 million cubic metres. The landslide came to rest on the valley floor at an elevation of 2190 m, blocking the Yigong River to a height of 60 m over a distance of about 1000 m (Fig. 3).

Figure 2: The track of the Yigong landslide. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

Figure 3: The deposit the Yigong landslide. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

The landslide itself is rather well shown in Google Earth, although unfortunately the source area has a slightly lower resolution than does the track (Figure 4). It is clear that the debris travelled down a comparatively linear tributary valley until the main valley was reached. In the latter stages of the movement the landslide was technically a distal debris flow, essentially meaning that it entrained a large volume of water (from snow and ice). This would have been immensely destructive in an inhabited area, but fortunately the area is sparsely populated.

Figure 4: The track the Yigong landslide as shown on Google Earth. Click on the image for a better view.

Of course the landslide did create a major problem in that it blocked the valley. A lake rapidly started to fill, and over the next two months a volume of about 3 billion cubic metres of water accumulated. The authorities sought to mitigate the problem by relocating the 4,000 living in the path of the flood whilst simultaneously constructing a drainage channel across the landslide deposit. This lowered the peak height of the dam to 44 m above the river level. The water broke through the channel on 10th June and the lake drained over a period of two days. 17 km downstream a peak discharge of 120,000 cumecs (cubic metres per second) was recorded, with the river level rising to 32 m above the deck of the bridge across the river. The trimline created by the flood is clearly visible in Fig. 5. The area that had been inundated by the lake is clear on Figure 6.


Figure 5: The trimline created by the flood resulting from the overtopping of the Yigong landslide dam. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.


Figure 6: The area inundated by the landslide dammed lake at Yigong shown on Google Earth. Click on the image for a better view.

The emergency measures were effective in preventing loss of life in Tibet, although there was considerable damage to infrastructure downstream. However, the flood did cause an international incident as the Chinese authorities failed to warn their counterparts in India that the flood was coming. As a result, the people of Arunachal Pradesh were not prepared for the event, leading to about 130 fatalities and 50,000 being rendered homeless (Yin and Wang 2005).

Finally, as an aside, the team that successfully built the drainage channel in this case was kept intact after the event and were thus ready to respond to the landslide dams created by the Wenchuan (Sichuan) earthquake this year. Their experience at Yingong was undoubtedly critical in their success in mitigating the major landslide dammed lakes that were created. It appears that their skills continue to be needed.

References:
Shang, Y. et al. 2003. A super-large landslide in Tibet in 2000: background, occurrence, disaster, and origin. Geomorphology. 54 (3-4), 225-243.
Yin, Y. and Wang, S. 2005. Landslide hazard and reduction strategy in China. In: Hungr et al. Landslide Risk Management. Tayor and Francis, 423-6.
Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

Thursday, 25 September 2008

Debris flows and fatalities

One question that I am quite often asked is why it is that fatalities in landslides occur more at night than in the day. In some ways this is counter-intuitive as one might expect that people would be safer in their houses than they are when they are out and about.

At least a part of the answer lies I think in the destructive potential of comparatively slow landslides. Most people have probably seen the video of the recent Hong Kong debris flow, which I blogged about earlier in the year. Clearly when a landslide is as rapid and destructive as this one then the chances of escape are quite limited, although running sideways out of the path, rather than away from the slide, would give the best chance.

However, many fatal landslides are much slower than this. The following Youtube video shows a debris flow from Clear Creek County in Colorado.



If you cannot see this above, click here.

This is a fairly typical debris flow. They very often occur in a series of pulses, each of which start with the movement of large boulders that are moving quickly but not exceptionally fast. This is clear in the video above. These boulders continually roll-over each other, bulldozing everything in their path. Behind the boulders comes of a flow of much finer and more fluid material, which often moves quite quickly, as in this case. The flow slowly declines before the next pulse arrives.

In many cases people who are awake and alert are able to move out of the way of one of these flows, not least because they are pretty noisy. However, people who are asleep in a house often do not know that the debris flow is coming until it hits the building. Unfortunately very few buildings can withstand the huge forces that the flow can generate, as the USGS image below shows. Even where people do survive the initial event, they are quite often then hit in the dark by the next pulse.

USGS image of the aftermath of the Vargas debris flows in Venenzuela

Clearly in this case the required mitigation is first to try to prevent these flows from occurring at all - for example by stopping deforestation and by placing catch structures in the tributary valley areas in which the flows start. Second, it is important to identify where flows might occur and then to try to ensure that people are not in the way of them. Finally, in some cases a warning system can be helpful, but this should be seen as a last resort once the above two measures have been implemented as effectively as is reasonably practicable.

Wednesday, 24 September 2008

More trouble at Tangjiashan

Tangjiashan was the most serious of the "quake lakes" (valley-blocking landslides) generated by the Wenchuan earthquake. Back in June I followed the remarkable exercise undertaken by the Chinese authorities to mitigate the problem by draining the lake. Once the lake was drained the concern was what would happen in the wet season.

Today, China.org.cn is reporting that new problems have developed:

Two people were dead and 30 were missing amid torrential rain in one of the counties that was hardest-hit by the May 12 earthquake, local authorities said on Wednesday. More than 300 people sustained injuries and about 6,000 were stranded or in dire need of help, according to the local committee in charge of post-quake restoration. Downpours began to pound Beichuan County in Sichuan Province, southwest China, on Monday night, collapsing more than 1,100 houses since then. Heavy rain, with maximum and average totals of 352.8 mm and 150 mm respectively, triggered mountain torrents, cave-ins and landslides. Local party chiefs were supervising relief work.

In a slightly older report, Xinhua gives slightly more detail:

Fourteen people have gone missing in the wake of landslides triggered by heavy rain in the quake-hit Beichuan County of Sichuan Province in southwest China, a local official said on Wednesday. Some villagers in Leigu town have also been injured, said Zuo Daifu, a county official in charge the reconstruction of Beichuan. The town received 104 mm of rain between 8 a.m. Tuesday and 7 a.m. Wednesday. Heavy rain totaling 194 mm during the period also hit the Tangjiashan area, blocking the sluice of the dangerous Tangjiashan quake lake and raising its water level by five meters.

It goes without saying that a landslide that has blocked the sluice, given its size, must be a fairly large slide. The image below shows water flowing over the channel during release of the water. Note the scale of the channel compared to the bulldozers parked on the dam. Of course the flow will be much lower now.

There can be little doubt that the authorities will be able to deal with this blockage once again, but the challenges ahead in terms of instability are clear.

Monday, 22 September 2008

Another Chinese flowslide? 1st August 2008

Xinhua is today reporting that:

"The State Council, China's Cabinet, has sent a team to investigate the landslide that buried dozens of people in the northern Shanxi Province, but was initially played down by local authorities. The landslide toppled a waste dump of a local iron mine and buried Sigou Village in Loufan County in the suburbs of the provincial capital Taiyuan on Aug. 1."

The landslide was reported to have killed 11 people, but Xinhua notes that the toll may in fact be much higher:

"Eleven dead bodies were found by Aug. 16, and the official death toll ended there. Families complained at least 34 others had been trapped under more than 800,000 cubic meters of rocks and waste from the mine. The emergency rescue headquarters have since dug 122,000 cubic meters of waste to search for more victims, but to no avail. The exact number of people trapped remains unknown because most of them were migrants and were not registered at the local public security bureau, which means their disappearance might not be immediately reported."

The mining industry in China has a very poor safety record, with over 3,000 annual fatalities. However, it appears that there is also a substantial safety issue associated with the spoil tips and tailings dams. To be fair these problems have affected the mining industry worldwide and in each case have required a strong safety management ethos to be engendered as the industry matures. The fact that China is now reporting these events suggests that they are now getting a grip of this issue, but it will take a considerable amount of effort to deal with all of the problems.

Saturday, 20 September 2008

Marriot Hotel bomb, Islamabad, Pakistan

Just a short aside from landslides. In January 2006 I attended a conference organised by the Geological Survey of Pakistan. The theme was to gain an understanding of the lessons learnt from the Kashmir earthquake - the proceedings are available online here. The conference was fascinating but desperately sad as speaker after speaker showed images of the destruction associated with the earthquake.


Today the Marriott hotel was senselessly bombed and destroyed, with dozens of fatalities. IThis is a desperately tragic event both for those involved and for Pakistan. I hope that those injured are able to recover as soon as possible. The reality of this event feels very real to me as we had been due in Pakistan this month to provide hazard mapping training. We were scheduled to stay in the Marriott in Islamabad. We postponed because of issues around timing with respect to Ramadan. I feel very fortunate tonight.

Thursday, 18 September 2008

Global warming and landslide occurrence

One of the most vexed questions in landslide science at the moment is that of the potential link between climate change and mass movement occurrence. I have yet to meet a landslide researcher who does not believe in the reality of anthropogenic global warming, so we are all deeply interested in how our particular systems are likely to respond. Unfortunately this is not an easy question to answer for three reasons:
  1. Landslides respond to changes in pore pressure (i.e. groundwater level). Groundwater level is controlled by precipitation input and by evapotranspiration outputs. So, to know how groundwater will respond requires quantification of both of these parameters. It might be expected that in a warmer world on average precipitation will increase (see below), but evapotranspiration will also increase. Understanding the balance between these parameters is at best a challenge.
  2. Landslides are localised phenomena, usually sited in upland areas in which rainfall patterns are complex and variable. Unfortunately, global climate models work at much larger spatial scales (typically 1 or 1.5 degrees of latitude and longitude). This makes it difficult to scale the outputs to an individual landslide.
  3. In many parts of the world, precipitation is controlled by large-scale weather systems, such as ENSO and the Asian SW monsoon. Climate models are struggling to model these systems adequately.
In the last month an interesting paper has been published that starts to take us in the right direction. The paper is this one:

Allan, R.P. and B.J. Soden, 2008. Atmospheric warming and the amplification of precipitation extremes. Science, 321 (5895), 1481-1483. You can download a copy from Brian Soden's website.

This paper is interesting because it looks at extreme precipitation in the context of climate change. In the last couple of years it has become clear that many of the most damaging landslide events tend to occur as a result of precipitation extremes - i.e. comparatively short duration, high intensity rainfall (the type associated with a particular storm or front) rather than long duration, lower intensity events. Understanding how extreme precipitation will change is thus very helpful.


Allen and Soden have started from the observation that the GCMs all forecast that extreme precipitation events will become more common as the climate warms. They have used a combination of measurements of daily precipitation over the tropical ocean using a NASA satellite instrument called SSM/I and the outputs from global climate models to look at the response of tropical precipitation events to natural changes in surface temperature and atmospheric moisture content. In the context of landslides, a very clear link was observed. In periods when temperatures were high the number of observed extreme rainfall events increased, and vice-versa. What is surprising though is that the response of the natural system seems to be more extreme than that of the climate models - i.e. the climate models are too conservative in terms of their forecasts of these extreme events.

The relevance of these findings for landslides should be quite clear. Increases in the occurrence of extreme precipitation intensities might well be expected to increase the occurrence of landslides. It should be noted though that there is some way to go to really establish this link. For example, this paper is essentially based on a dataset collected over the ocean. There is a need to see whether the same applies on land. However, it is good to see papers being produced that start to answer the questions that we are asking.

Wednesday, 17 September 2008

August 2008 fatal landslide map

Better late than never (just a tad busy at the moment...), here is the fatal landslide map for August 2008. First the statistics:
Number of recorded fatal landslides: 61
Number of recorded fatalities: 537

As such August was substantially above the average for the last five years (212 fatalities). This is interesting as regular readers will know that until the end of July most months have been substantially below the five year average. July was particularly low. Therefore, this sudden increase in activity in August is slightly surprising and will be worthy of detailed analysis in due course.

So, first the map for August 2008. As ever, you will need to click on the map to get a decent view of it:

A few things to note:
  1. The very strong cluster along the Himalayan Arc is now very evident. Nepal in particular is suffering an intense period of landslides.
  2. There is also a clear cluster in the Caribbean as the Hurricane season starts to take its toll.
  3. China is also experiencing a few events, but with no clear pattern.
The map of the year to date is shown below. Again, click on the map for a decent view. The normal clusters are now well developed. Compare this map with that from 2007 (see here). The similarity between the two patterns is now very clear.

I welcome any comments, corrections and clarifications.

Cairo landslide - the fall out continues

The Manshiet Nasser landslide continues to cause a huge political row in Cairo. Perhaps most importantly the population of the affected area remain deeply concerned about the nature of the response by the authorities, and their subsequent treatment. I suspect that this will rumble on for a long time. The rescue and recovery efforts have now ceased - to be honest this is probably the right decision as trying to excavate those boulders would be a terribly dangerous task, although one must feel deep sympathy for those whose relatives remain buried. Unfortunately, there is also a big political argument over the number of entombed victims - Almasry Alyoum reports that:

"Official papers said 92 corpses had been pulled out of the rubbles and only 12 corpses remained under the rocks. The Secretary General of the ruling National Democratic Party (NDP) Sheikhdom in Manshyat Nasser,however, said there are still 180 corpses under the rocks. This was confirmed by some Deweiqa inhabitants"

To me the higher figure would seem more likely, but who knows? I guess if the victims cannot be recovered then there is a need to try to ascertain a correct figure.

Meanwhile, The BBC is reporting that: "Prime Minister Ahmed Nazif will oversee a review of some 13 unauthorised suburbs of the city, with special focus on areas close to unstable land."

This seems reasonable, but such an approach will need to be undertaken with great care. Whilst people are undoubtedly at risk in some of these locations, making people homeless or putting them in refugee camps also carries very high risks. The key is to ascertain the actual nature of the hazard associated with the rockfalls. Whoever gets this task might like to look at some work that we undertook in Gibraltar to determine the hazards associated with rockfalls in residential areas:

Massey, C., Hodgson, I. and Petley, D.N. 2006. A rockfall simulation study for housing development in Gibraltar. Proceedings of the 10th IAEG Congress. Paper number 377. 9 pp.
Download the paper from here.


Tuesday, 16 September 2008

Loss of life in the China and Egypt landslidesp

Just a brief update on the latest estimate of the loss of life in the landslides last week in Egypt and China:


Taoshi (Shanxi): as of 16th September the official death toll is 259 people. Searches continue (see above AFP image).


Manshiet Nasser (Cairo): as of 16th September the official death toll is 92 people. It appears that recovery actions have now ceased as the site is considered to be too dangerous. There are likely to be many victims still buried. It is difficult to see how recovery operations can be safely undertaken under blocks such as the above (Reuters image).

River bank erosion in Taiwan during Typhoon Sinlaku

Over the weekend Typhoon Sinlaku slowly tracked across the northern part of Taiwan (Fig. 1). Whilst attention was diverted towards the landfall of Hurricane Ike in Texas, this typhoon wreaked considerable havoc across Taiwan. The steep, weak mountains of Taiwan are amongst the most landslide prone environments on Earth. Typhoons in Taiwan deposit extraordinary amounts of rain. Typhoon Sinlaku was no exception - parts of Taichung County in central Taiwan recorded over 1600 mm of rainfall during the passage of the Typhoon - that is about double the annual rainfall of Durham.

Fig. 1: The passage of Typhoon Sinlaku (image from Tropical Storms Worldwide).

Unsurprisingly the typhoon has triggered extensive landslides; at the time of writing at least four landslide related fatalities have been recorded, and there is the possibility of more victims in a landslide event that buried a tunnel in the Central Mountains. Probably the most spectacular slope related event has however occurred as a result of extensive river bank erosion in the hot springs resort of Lushan in Nantou County (Fig. 2). In particular, the Kimei Hot Springs Hotel has had rather an unfortunate time.

Fig. 2: Google Earth image of Lushan

Before (Expedia):

After (Taipei Times):

Sunday, 14 September 2008

Loss of life in the Manshiet Nasser (Cairo) and Taoshi township (Shaanxi) landslides

Xinhua is now reporting that the death toll in the Shaanxi landslide has now reached 254. They also report that the recovery teams are yet to search two channels in which local people believe more bodies are buried

AFP is reporting that the number of fatalities in the Manshiet Nasser (Cairo) landslide is now 82 people, with at least another week of recovery work to follow. Interestingly. they also report that "Residents have blamed the rock slide on work that had been going on for several weeks on the Moqattam hill overlooking the shantytown". It would be interesting to find out more about this work.

Friday, 12 September 2008

The location of the Cairo landslide

Thanks to posters over at the EgyptSearch forum, and in particular poster AntonD who finally identified the place, I can finally identify what I think is the location of the Cairo landslide. The point is at: 30 degrees 2.728 minutes N, 31 degrees 17.261 minutes E. This is the Google Earth image of this point:


The cliff from which the failure occurred is clear, as are the houses below. According to various people there has been considerable development in this area since the image was collected.

Helpfully, the following image is online of the cliff before the failure. The image is on a Panoramio site belonging to Hazem Mamdouh:
It appears to me that this image was taken from on top of the railway embankment. Compare that with this image, taken from Ghafari's Picassa site, of the aftermath:

I will try to post an analysis of the two images by Sunday.

In the meantime, there is a good analysis of the background to the landslide here. The official death toll is now 62, whilst the recovery operation looks increasingly difficult due to the size of the boulders.

The death toll in the Shaanxi tailings dam disaster is now listed as 151 people, with more victims to be recovered.

Thursday, 11 September 2008

Updates on the Cairo and Shaanxi landslides

It is unusual to have two major stories running simultaneously about landslides so here is a combined update:

1. The Manshiet Nasser landslide in Cairo

The death toll in the Cairo landslide disaster continues to rise - it is now officially 69 people. There seems to be considerable confusion about how many people are left trapped - but is fair to say that there is now no hope of rescuing anyone alive. The magnitude of the task in terms of recovery is shown well by the picture above.

There is some discussion on the newswires about the causes, with several suggestions that sewage in the joints may have played a role. To me this is far from clear. There are also now suggestions that quarrying may have occurred on the rock face - this would be the first thing that I would look at if I was investigating the slide.

Meanwhile, there appears to be considerable political fall-out over both the government's failure to recognise the threat posed by this slope (see here) and the slowness of the response. This has led the government to seal off the site. One wonders whether we will ever know the true toll of this event.

2. The Taoshi township flowslide in Shaaxi



The pictures emerging from the Taoshi township site continue to horrify. The official death toll is now 128, but there can be no hope of anyone else being found alive. Officials are now admitting that the number of missing people is "several hundred". The debris flow was reportedly three stories high and 600 m wide when it hit an office building, a market and several houses. At the time the market was just starting and there was a meeting, attended by over 100 people, in the offices of the mine company. Only three people survived the meeting.

Again, the difficulties of recovering victims here may make it difficult to estimate the true impact of this event.


Wednesday, 10 September 2008

China tailings dam disaster death toll

Reuters image of the search for victims

Reuters is now reporting that the number of fatalities in the China tailings dam disaster (landslide / debris flow) is now 128 people. A unknown number remain buried in the debris. Interestingly, Xinhua appears to have ceased reporting on the event (corrected: they do now have a report), amongst suggestions that journalists are being kept away.

The difficulties facing the rescuers are clear from the above image. Recovering anyone alive from within that deposit is very unlikely.

Tuesday, 9 September 2008

Updated: A flow slide disaster in China

UPDATED: New image of the site added
UPDATED: Google Earth image of location added.



Update: This image from http://aboutxinjiang.com/ shows the site of the failure. This confirms both the magnitude of the event and the type.

This has been a terrible week for landslide disasters. Sadly, news emerged yesterday of yet another, this time in Taoshi township, Linfen County in Shaanxi province. I think that I have managed to identify the location - see Google Earth image below (location is 35°54' N, 111°30' E). Initial reports suggested that 26 people had been killed; this has now been increased to 34, but it seems likely that the death toll is much higher.

Details are a little sketchy at the moment, but the suggestion appear to be that heavy rain caused a tailings dam to fail, releasing a large amount of material that swept as a debris flow down the valley. Xinhua has a limited commentary but a rather interesting set of images of the disaster:

The Xinhua report states that "The mud-rock flow ... destroyed a three-story office building, a market and some villagers' houses in the valley. Witness said the flow roared down the valley and washed away the market and the houses in a few minutes."

The description and images of the landslide suggest that this was what is termed a flowslide - this is a large volume debris flow consisting of soil, rock and water. They are able to move exceptionally fast and to travel large distances. Sadly the mining industry has a terrible record of flowslides originating from spoil heaps or the failure of tailings dams. Examples include:

There is a full if rather horrifying list of these events here. In most more developed countries the accidents at Aberfan, Buffalo Creek and Stava have meant that mine dumps are very tightly regulated, meaning that the accident rate is now low. The impact of not doing so are graphically illustrated by this pair of images (see this web page) of the Stava event in Italy (click on the image for a better view):

Here, 200,000 cubic metres of fluorite tailings flowed over 4 km downstream at over 90 km/h.

Monday, 8 September 2008

Catastrophic rockfalls in the Middle East

The terrible landslide at El Doweiqa in the Manshiet Nasser slum in Cairo (see earlier posts here and here) continues to shock. The death toll has now reached 45 people, with many more still buried in the debris. News reports suggest that there is growing dissatisfaction with the response of the authorities in Egypt. To be fair, relatives in disasters such as this are often very frustrated at the speed of the response - for understandable reasons. Excavating blocks as large as this whilst keeping the rescuers safe is a terribly difficult task.

However, of interest is the similarities between this disaster and one that occurred in Yemen on 28th December 2005. This landslide occurred at the village of al-Dhafir, about 50 km from the capital Sanaa. As with Manshiet Nasser, the landslide occurred as a result of the collapse of massive bedrock blocks from an escarpment. Again, as in Cairo, houses were utterly flattened by the landslide. This disaster killed 65 people. The images below of the disaster are from AFP via the BBC. The similarities to the Cairo event are clear:


I have very rarely seen landslides like this from other parts of the world. What is unusual is the size of the blocks (huge) and the mechanism of failure (toppling). This combination is not seen very frequently. So why does the Middle East suffer this type of unusual failure?

I suspect that the answer lies in an unusual juxtaposition of geology and climate. The geology provides a strong rock with a simple joint set in which the spaces between the joints are large. This allows rare detachments of very large blocks. If the rock was weaker or more closely jointed then the blocks that fall off would be smaller - i.e. there would be more falls, each of a lower volume. The climate element comes from the aridity, which means that failure is not triggered by the pore pressure effects seen elsewhere. An additional factor is probably that the dry climate means that the rate of weathering is low. So a wetter climate would probably trigger more failures, each of a smaller volume.

The reports in Cairo suggest that the local people feel that water (or sewage) leaking into the rock has been a factor. This may be the case, although the mechanism of failure is not obvious. I would be interested to know if the slope has been quarried either officially or unofficially. For example, have pieces been hacked out to be used for building stone? Or have pieces been detached to create space for more buildings? This is so often the cause of rock slope collapses in inhabited areas.

Sunday, 7 September 2008

The Cairo rockslide foreseen?


EPA/MIKE NELSON image showing the displaced blocks at El Doweiqa

The death toll in the rock slide at El Doweiqa in the Manshiet Nasser slum of east Cairo is slowly creeping upwards. At the time of writing it has exceeded 30 people, but unfortunately as the image below shows the likelihood is that it will be much higher once these huge blocks have been shifted (if ever). As an aside, the news reports tend to refer to these blocks as weighing hundreds of tonnes. In fact they are probably much heavier than this - a 10 x 10 x 10 m block typically weighs over 2300 tonnes. Some of these blocks may be substantially larger than this, suggesting that blasting will be the only option if the victims' remains are to be recovered.

EPA/MIKE NELSON image showing the displaced blocks at El Doweiqa

There are interesting sets of amateur images here and here.

Most interesting however is an article from the "Egyptian Gazette" dated 23rd July 2006. I have reproduced the article below. Compare the photograph with the image at the top of this posting - it is clearly the same location.

The text of this article says the following:

"NOTHING can be more difficult for a human being than to wait for the death that's drawing nigh. How can someone live in a house at the bottom of a hill while a huge rock is poised precariously at the top, ready to roll down and smash that house to smithereens? Gomaa Abdel-Bari, whose family live along with 2,800 others in el-Doweiqa, have experienced rocks falling on them from the hills above, damaging the walls of their homes.

"I've made numerous complaints to official bodies, including the local administration and human rights organisations, but in vain," Abdel-Bari told October weekly magazine. To prevent more rock falls, local inhabitants say they want to demolish the rocks that threaten them, with the help of machinery belonging to the local council. "We've already broken up some of the rocks, at a cost of LE4,000, money collected from poor residents," explains Ashraf Abdel-Tawwab.

But we cannot afford to demolish the rest on our own. Besides, it's dangerous work. If one rock comes loose, it can cause a domino effect, resulting in a disastrous avalanche," he says bitterly. Ahmed Hosni, a worker, explains that sewage leaks through the rocks, dislodging them and causing them to come tumbling down on their homes.

Meanwhile, a man called Farid Abdel-Tawwab complains of the snakes and dangerous insects that have killed many people in this randomly built area. He too says that the rock falls are to be blamed on sewage leaks. El-Doweiqa forms part of Manshiyet Nasser, the biggest shanty town in Cairo and a big headache for successive governments. It occupies around 850 feddans of land and is home to 1.3 million people, who live in appalling conditions, deprived of clean water, adequate sewerage and even fresh air, due to the huge piles of garbage collected and sorted there by the garbage collectors from all over the capital.

At the end of the last century the Government started to replan Manshiyet Nasser, including el-Doweiqa and other districts. The dea was to replace the old, unplanned districts with new, modern ones. Germany and Abu Dhabi supported the ambitious project. However, no-one knows exactly when the development project will reach the Khazan area of el-Doweiqa and whether it will be in time to prevent another tragedy like that in el-Moqattam in 1992, when a huge rock crushed dozens of citizens to death."


Thus, this two year old article appears to foresee the event that occurred this weekend. One wonders if the interviewees, and their families, survived. Tragic.




Saturday, 6 September 2008

Early reports of a rockslide disaster in Egypt

The newswires are providing early reports of a terrible rockslide at Deweka (also reported as Bekheit and Manshiyet Nasron) on the outskirts of Cairo in Egypt. The slide appears to be a rather strange failure of a rock slope, releasing huge boulders that have buried several (and possibly many) buildings.

This Al Jazeera post contains a very useful report and also has this image of the site:


They are reporting that "Officials said at least eight rocks, some measuring 30m high, had buried more than 50 homes in the poor district of Manshiyet Nasron on Saturday. At least 18 people have been declared dead and 35 injured. Some estimates put the number of buried at 500."

Early reports like this often over-estimate the losses in an event such as this, but if these boulders have hit residential buildings then the toll could be terrible. Al Jazeera also note that "
In a survey carried out by UN Habitat, a human settlement programme, Manshiyet Nasron is described as "the largest squatter/informal area in Cairo. There are 350,000 persons living in this area on about 850 acres with a gross residential density more than 400 persons/acre".

"The area is suffering from poor living qualities, inadequate services, lack of infrastructure, and deteriorated environmental conditions. The site is characterised by sharp contour variations ranging between 56 and 200m," the survey said."

The final point to note is that Al Jazeera claim that "Manshiyet Nasron residents had informed the authorities a year ago that there was a split between the rocks, a potential danger to the homes below." This would make sense as the image above seems to show dry conditions and there are no reports of an earthquake. Thus, the landslide is most likely to have been caused either by progressive failure (i.e. a slow loss of strength through time) or slope cutting.

I will post again when I have more.







Friday, 5 September 2008

Hurricane landslides in Haiti

A perennial landslide story at this time of year is the triggering by a tropical cyclone of landslides in Haiti. This year the hurricane season has been particularly cruel, with three large events in a month. Hanna, the most recent, appears to have stalled close to Haiti for a day or so, causing torrential rainfall. At the moment reports suggest that 136 people have been killed in flash floods and landslides as a result of Hanna, but that figure may well rise as the picture becomes clearer. Unfortunately, there is another, very intense "Cape Verde" type hurricane (called Ike) lurking to the east. Current forecasts suggest that the track will swing north of Haiti (in fact Florida could get a direct hit), but at this stage such forecasts are at best tentative. A direct hit from a hurricane as large and intense as this, on the back of Hanna, could be disastrous for Haiti.

So why is Haiti so vulnerable to hurricanes? Basically, Haiti is the most extreme illustration of the impact of deforestation on landslides and flash floods. Haiti is the poorest country in the Caribbean - over half the population live on less that $1 per day ($1 is the recognised mark of extreme poverty) and over 75% live on less that $2. More that 60% of the working population do not have formal employment. The consequence of this has been extreme deforestation, primarily for firewood to create charcoal.

This is rather well illustrated by this Google Earth image:

The border between Haiti and the Dominican Republic is the river that runs across the centre of the image. On the west (leftish) side is Haiti, to the east (right) is the Dominican Republic. The contrast between the two is striking - in the Dominican Republic deforestation has been limited, in Haiti the loss of forest is almost total.

The result is that during hurricanes the landscape has little capacity to intercept and store water, and once flows across the surface begin the landscape rapidly erodes. This is perhaps best illustrated by the following Google Earth image of the city of Gonaives in the east of the country. Gonaives has been very seriously affected by erosion and landslides again in the most recent floods. The image shows the hills on the outskirts of the edge, with the suburbs clearly visible. The hills are clearly suffering from extreme deforestation. In September 2004, Hurricane Jeanne triggered mudslides and debris flows from these hills that killed over 3000 people in the city. Bearing in mind the fact that Haiti should be densely vegetated with tropical forest, the disaster that is the landscape in this country is all to clear to see. Unfortunately, posts about landslide disasters in Haiti will be a feature of this blog in the late summer for years to come.

Thursday, 4 September 2008

The physics of landslides...

This is a nice illustration that the physical rules that control landslides are the same the world over (or that journalists play games occasionally...). Below is the well-known USGS image of the 1995 La Conchita landslide in California:

Compare the above image with this Telegraph Calcutta picture of a landslide at St Antony’s Colony in Munnar, India.

Almost a mirror image! Hmmm.......!