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.

Thursday, 29 May 2008

Two new photographs of the Beichuan area

I came across these two new images, both from AP, of the Beichuan area.

The first shows the landslide that struck Beichuan itself. The landslide is on the left side of the image. Note the debris piled up around its edges - the landslide has clearly bulldozed all before it.


The second is an alarming image of the work underway on the Tangjiashan landslide dam. The shear scale of the blockage is clear. Of great concern here though is the steepness of the slope. Controlling water flow in a channel on slopes like this will be immensely difficult. The potential of scouring out of the channel, and thus of rapid collapse, is worrying. Looking on the bright side, seepage shouldn't be an imminent problem.

NASA imagery of Sichuan landslides

NASA has now released a pair of ASTER images of the earthquake affected area. I should add that it is good to see some imagery of areas beyond Beichuan. These images really do highlight the magnitude of the landslide problem.

These images are centred on the Min Jiang river, which is about 150 kilometers from the epicentre of the earthquake. For a non-specialist they look a little odd - technically they are "false colour composites", designed to allow classification of land use. Thus, the bright red colour represents areas of vegetation; blue-grey represents earth, stone or brick surfaces - which can mean bare ground, buildings and paved areas; and dark blue represents water. In the interpretation of landslides from a set of before and after images like this we typically look first for vegetated areas (i.e. red colours) in the first image that have become bare ground (i.e. blue-grey) in the second. We then look for the shape and location of the change, because deforestation or construction can also cause forested areas to become bare ground, stone or soil. Most landslides have an arcuate (i.e. curved) back-scar (this is the top of the landslide) for example, and earthquake triggered landslides in mountainous areas often start at the ridge or a break of slope and extend to the river. They are typically (but not exclusively) long and narrow. Land use change for deforestation or construction usually occurs in distinct blocks or along roads.

NASA images collected by the ASTER satellite showing an area around the Min Jiang River in Sichuan. The bottom image was collected on 19th February 2003, the top one on 23rd May 2008. Click on the image for a better view.

Now, as NASA point out, we do need to be a little careful in the interpretation of the images. This is an area that has undergone substantial change in recent years due to economic development and population growth, so some of what we see here might not have come from the earthquake.

What is clear is that there is extraordinary change between the images. Most of this change takes the form of a huge number of long, narrow strips that extend down the slope to the river. In these strips the land use has changed from vegetation (red) to bare ground (blue-grey). In places there are larger features with arcuate back scars.

My interpretation is that most of this change represents shallow (<10 class="blsp-spelling-error" id="SPELLING_ERROR_9">Tachia River area of Taiwan after the 1999 Chi-Chi earthquake. To provide an idea of what this looked like on the ground, this image was taken of this area about two weeks after the earthquake:

Shallow landslides in the Tachia River area of Taiwan after the 1999 Chi-Chi earthquake

In this area the landslides themselves took a dreadful toll. Nearly ten years on the area is still struggling to get back on its feet as the landslides reactivate in every heavy storm. The debris reseased by the landslides has choked the rivers.

Wednesday, 28 May 2008

Sichuan - what does a landslide look like in reality?

Understanding what a landslide is actually like is rather difficult when you can only see the deposit and the scar. Now, all landslides are of course different, but in the case of Sichuan we are dealing with some of the most rapid and dangerous ones. So, I thought it would be constructive to post the following Youtube video, which was taken in Malaysia in 1993. Click on it to view - I promise you won't be disappointed. It remains the very best landslide video that I have ever seen, despite the fairly poor resolution.


If you can't view it then click here.

This landslide is a little bit of an enigma. We know that it occurred in Malaysia on 21st October 1993. The poster describes it as having happened at Pantai (Beach) Remis. The voices that can be heard in the background are speaking Cantonese I think. As far as I can ascertain it occurred in an abandoned tin mine, clearly near the coast given that the sea rushes is at the end. Quite how the person recording it came to be able to get this clip is unknown. It very clearly allows one to understand the threat posed by landslides though.

Tuesday, 27 May 2008

Updated 28th May: Sichuan landslides dam - alarming video

The BBC is carrying some interesting footage of soldiers creating a channel across a landslide deposit. This can be seen here:

http://news.bbc.co.uk/1/hi/world/asia-pacific/7421263.stm

The video is at the bottom of the page. It is worth a look.

The video shows soldiers digging a channel through what is apparently a landslide deposit. I am not sure which landslide this is or even what the channel is to achieve. It could be that this is a small blockage rather than the big one at Beichuan (Update - it is now clear that this is not the main Beichuan landslide as Xinhua says "the water diversion channel won't be ready in place until June 5"). Some of the newswires are carrying this image, which seems to correspond to the one at the start of the video.


Now in an earlier post I produced this illustration of the FORMOSAT-2 image draped over the Google Earth DEM. I have relabeled it to show the major landslides:

Click on the image for a better view

As far as I can tell the landslide in the aerial image is Landslide 3 (i.e. one of the small dams that impounds very little water) not the big, worrying one (Landslide 4) (update: now confirmed - see above). I certainly hope that this is the case because if the team were creating a channel through a major blockage (like Landslide 4) then I would be extremely concerned. In particular, the landslide deposit is a highly fragmented (i.e. broken up) deposit. The channel is quite steep and is completely unlined and unprotected. If this approach was used for a large landslide dam (e.g. Landslide 4 above) then there would be a very serious risk of the channel eroding down (the material cannot have any resistance to erosion), potentially allowing the formation of a major flood.

I do hope that the soldiers understand the need to line or to protect the channel for the big landslide. Before they release the water from Landslide 4 they will also need to make this spillway much bigger or the water will become trapped again.

Update 27th May: As of 27th May the authorities have reported that the lake level rose 1.6 m on Monday and 1.79 m on Tuesday. The freeboard is now 26 m. At the current level of increase the lake will reach the top in about 14 days, assuming that it doesn't rain, etc. The channel is due to be ready on June 5th (nine days). It is going to be very close!

Update 28th May: CCTV is reporting that "A swollen lake formed by landslides triggered by the Sichuan earthquake in Wenjiaba in Pingwu County has been drained. Soldiers have protected the drain with bamboo poles to prevent further blockages from landslides. Experts say the drain will reduce the risk of the lake bursting and slow rising water levels in the upper reaches of the river. The lake is under constant surveillance to prevent flooding." Perhaps this is the one in the video? The reference to bamboo poles looks to be the same.


Monday, 26 May 2008

Tangjiashan (Beichuan) Lake - update

China Daily is today reporting that real progress is being made with the lake above Beichuan, which is now known as Tangjiashan Lake:

Amid low visibility, a Russian helicopter successfully delivered a large bulldozer and other eight sets of big machineries near the swelling Tangjiashan lake, in order to dig a diverting channel to prevent a flood, said Minister of Water Resources Chen Lei. "I feel relieved this morning as we have reached the area," said Chen, who is in charge of dealing with the emergency situation of dozens of quake lakes in Sichuan. Quake lakes were formed after landslides blocked rivers."All these mean we can officially start to deal with swelling lakes now," said Chen, adding it first time he spoke to the media, as he was not in the mood the previous days to do failures to deal with the possible flooding situation. Other helicopters managed to airdrop materials for the operation by 7:49 am Monday at the top of the quake lake at Tangjiashan in Beichuan County. It was formed by landslides that blocked a local river known as Jianhe after the May 12 earthquake. While military forces are preparing to dynamite the swelling Tangjiashan quake lake, rescue headquarters have urged the expert team to quickly give "risk evaluation" reports on bursting of the barrier. Chen asked the provincial and Mianyang municipal governments to draw "three red lines" on the map until Tuesday noon, with each representing which area will be affected if the barrier bursts by one third, half or to nothing.

This sounds much more promising. The best way to deal with this issue is a combination of a channel constructed using heavy equipment, backed up with an assessment of the areas likely to be inundated by a flood. Blasting the dam should be avoided if at all possible - it is a real last gasp approach. However, great care will be needed in the construction of the channel, in particular in preventing rapid erosion of the channel bed and thus a rapid collapse event.

AFP image entitled: The build-up of water in devastated Beichuan county is a massive concern

This is progress for sure but they are not yet out of the woods. The lake now has a volume of >128 million cubic metres of water.

Sunday, 25 May 2008

The collapse of landslide dams - the challenge in Sichuan

In view of the ongoing deep concern about the stability of the >30 landslide dams in Sichuan (now known in the press as "quake lakes"), and in particular the one upstream at Beichuan, I thought it might be helpful to provide a brief review of what we know about landslide dams.

First, there us a great deal of literature on this topic. For example, Schuster and Costa (1991) detailed 463 examples of landslide dams worldwide. Since then many more have
been described. Most landslide dams ultimately collapse, most commonly through overtopping. Here the water starts to flow over the surface of the dam. In most cases this causes the dam to start to erode, which releases a little more water. Usually this creates a feedback loop that leads to complete release of the water in a short period. Occasionally the dam fails because of seepage or piping through the blockage itself or through porous sediments below the dam or because of a slope failure in the dam wall. These slope failures are sometimes triggered by earthquakes. Finally, failure sometimes occurs because of another landslide entering the lake. failure of the second landslide is often caused by the elevated ground water levels created by the lake, or by an earthquake aftershock. Such failures are generally exceptionally catastrophic.

Failure is not inevitable - and indeed some landslide dams last for hundreds or even thousands of years. These are the exceptions though, usually arising because of specific conditions. Schuster (1995) looked at 187 examples worldwide, finding that 55% of failed within one week of their
formation, and 89% had failed with a year one year.

So why do some dams survive? Usually survival occurs because of some or all of the following factors:
1. The dam consists of large boulders that are not eroded by flow. This is the case so far at least in the Young River landslide dam in New Zealand (Figure 1), which formed in September 2007 and so far has not collapsed.
2. The dam or surrounding slope allow release of water through piping or seepage without eroding.
3. The surface area of the lake is sufficiently large that evaporation balances inflow.
4. It is mitigated by humans (Figure 2).

Figure 1: Stuff.co.nz image of the Young River landslide lake in Mt Aspiring National Park, New Zealand.

Figure 2: Spillway being constructed on the Hattian Bala landslide dam in Pakistan in 2006.


Unfortunately, the conditions required for 1-3 are comparably rare. They do not seem likely in Sichuan given the materials and the rainfall.

Dam collapse events are very dangerous indeed. Collapse usually occurs rapidly, causing a disastrous flood. The magnitude of the flood wave can be very large, which is exacerbated by the fact that the water picks up the debris from the dam, changing from a flood to a debris flow consisting of water, soil, rocks, trees, etc. The statistics are quite frightening - for example the artificial breaching of a landslide dam on the Bairaman River, Papua New Guinea induced debris flow that was 80 m deep immediately downstream of the dam. 39 km downstream the flood wave was still 8 m high. The collapse of the‘La Josefina’ rockslide dam on the Paute River in Ecuador, 26 dams after it was formed, induced massive flooding along a 100 km stretch of the river. The 1786 collapse of a landslide dam on the Dadu River in Sichuan is believed to have killed over 100,000 people. Finally, the failure of landslide dam near Mount Cayley in Canada created a debris flow that travelled at about 35 metres per second. The flow was sufficiently violent to propel wood splinters that became impaled in trees that they hit.

Mitigating landslide dams
Mitigating the threats associated with landslide dams is by no means simple. The following approaches are usually adopted:
1. Monitoring: the most immediate response is usually to put in place monitoring, often in the early days consisting of an observer with a suitable communication system. This is often accompanied by closure and evacuation of potentially-inundated areas. This monitoring is often then automated to create a warning system, but this requires that people in affected areas are aware of the hazards, receive the warning when it occurs, and know how to react. Achieving this is far from trivial.
2. Syphons and pumps: In some smaller lakes short term management of the problem can be achieved using syphons and / or pumps to draw down the water level. This is not a suitable approach for long term management due to cost and reliability, but it can be effective in the short term.
3. Tunnels and spillways: The most common mitigation measure is to create a spillway - i.e. a channel across the top of the dam (at the lowest pint obviously) that will allow prevent overtopping. This is usually done by digging a channel from the downstream side to the lake. The channel must be armoured to prevent erosion. In some cases a tunnel is created for the same purpose. This is clearly more expensive. Great care is needed to prevent catastrophic release of the water.
4. Stabilising the dam and adjacent slopes: sometimes the dam is stabilised to ensure that it does not collapse due to slope failure. This is usually undertaken by reinforcing and reprofiling the dam surface, sometimes with an added layer to reduce seepage.
5. Removal: Occasionally it is necessary to remove all or part of a landslide dam, for example to reopen a river for navigation. Clearly this is not usually practicable for very large landslides. Enormous care is needed when undertaking this type of exercise to ensure that a catastrophic collapse does not occur.
6. Flow management: Sometimes attempts are made to manage the flow when collapse occurs. This is a hazardous activity as the flows can be exceptionally large and energetic.

Mitigation of landslide dams in Sichuan
So, where does this leave Sichuan? Well, the situation there is an absolute crisis, with bthe lake filling rapidly and rain forecast. Xinhua today reports that:
"About 1,800 armed police officers and People's Liberation Army (PLA) soldiers were hiking on Sunday toward an expanding "quake lake" in southwest Sichuan Province, hoping to blast away its landslide barrier before it bursts and causes a flood. The rescuers have 10 kilograms of dynamite each and are expected to arrive at the site on Sunday night," a PLA spokesman told Xinhua early on Sunday. The team were about two kilometers from Beichuan county seat at 5 p.m. Their trekking would be long and hard, with high mountains to climb and potential landslides. "We've found a local guide in Beichuan County who is willing to take us there," said Gong Juncang, an officer. "We tried once before, but couldn't make it to the top after 12 hours." The local meteorological bureau forecast high winds and thunderstorms for the area on Sunday and Monday. The Tangjiashan quake lake, which is in danger of bursting as water builds up in it, is one of the more than 30 such lakes in rivers blocked by landslides from the earthquake and thousands of aftershocks. Its barrier is in danger of bursting as the water level rose by nearly 2 meters on Saturday to 723 meters, only 29 meters below the lowest part of the barrier, which measured 752 meters high."

There can be little doubt that this lake needs mitigation very quickly and one has to sympathise with the plight of the Chinese authorities, who cannot get access to it at the moment due to the weather and the amount of debris, However, blasting the dam with dynamite would be a grave risk. I suspect that the view is that triggering a collapse now, catastrophic though that might be, is preferable to allowing the lake to overtop and collapse with a much larger volume. It is an appalling dilemma.

Friday, 23 May 2008

New images of Beichuan and its lakes

CCTV has posted the following image of the lake at Beichuan:

Meanwhile Reuters have published this image, captioned: "An aerial view of the roofs of houses in earthquake-hit Beichuan, Sichuan province May 20, 2008. The houses, which were originally built next to a reservoir, were uprooted and floated downriver after water levels started to rise." I suspect that by reservoir they mean river?


AP has this image of the landslides in Beichuan, captioned "The ruins of collapsed buildings lay among those still standing following last week's earthquake, in Beichuan, in China's southwest Sichuan province Thursday, May 22, 2008."

Thursday, 22 May 2008

Beichuan landslide-dammed lake - new imagery

NASA has now released new images collected by the FORMOSAT-2 satellite of one of the lakes forming near to Beichuan in Sichuan behind the landslide dam shown in this post (click on the image for a better view).

The first image shows a reach of the river upstream of the dam after the earthquake (on May 14th) but before the lake really started to form. The second image shows the lake a day later. Note that now the lake has started to fill, threatening the two villages built on the river banks. The river has overtopped its banks and is starting to inundate the farm land. In the third image, collected on 19th May, the lake is well formed. River flow has more or less stopped and a large pool has formed. Both villages and the farmland have been inundated. The only sign of them is a large amount of debris floating on the surface. This may well be all that is left of people's houses.

Note that the volume of water behind the dam is large and that the lake is filling quickly. Although the authorities appear to be responding quickly and appropriately, this situation remains abolsutely critical. One can only hope that the reports in the Times yesterday, which said the following, are incorrect:

One plan is to build up earth barriers halfway across the Beichuan river, at several points downstream from the reservoir. These barriers would divert the water, slowing its flow and reducing the danger to communities in its path. Once these were ready, some or all of the water could be released over the next day or two, possibly by blasting away part of the landslide.

Such an approach would be unconventional at the very least. Blasting the landslides, rather than building a spillway, is not an experiment that I would recommend. The idea of controlling the resultant flow with barriers is at best optimistic.

Tuesday, 20 May 2008

The problem of cracks in mountains after earthquakes

Xinhua is today reporting the following:

Nearly 9,000 people in a quake zone were evacuated on Tuesday for fear that huge cracks on a mountain could lead to further disasters.
Many crevices, measuring up to 1,500 meters long, 250 m high and 50 centimeters wide, have been spotted on the Shiziliang Mountain in the Qingchuan County seat, Guangyuan City, since Sunday, threatening about 50,000 people and quake-relief soldiers. Part of the mountain, distorted in the 8.0-magnitude earthquake and many aftershocks over the past week, has sunk about 1 meter and caused many road works to cave in. A house at the foot of the mountain was damaged by a falling boulder. Any new aftershock or heavy rainfall in the area could trigger serious landslides and cause casualties, according to the quake relief headquarters at the site. The local government on Tuesday launched an emergency evacuation of 9,000 residents near the mountain. The area has been cordoned off and is under round-the-clock monitoring.

Reading this gave me a very strong sense of deja vu. One of the greatest challenges in Pakistan after the 2005 earthquake was trying to interpret the meaning of similar crack networks, which we found all over the hillsides. The following two images show what these looked like.

A cracked slope near to Muzaffarabad in Kashmir after the 2005 earthquake (click for a bigger image). Cracked slopes like this occurred throughout the earthquake zone. To date very few have led to a slope failure, but we remain very concerned by them.

A further cracked slope near to Muzaffarabad in Kashmir after the 2005 earthquake (click for a bigger image). Note that in this case the cracks are very close to a steep cliff. Nonetheless, this slope has still not failed.

In Pakistan we assumed that these were incipient landslides and worked with the authorities to move people off the worst affected areas before the monsoon. We also put instruments to measure four of the most notable sites. However, this was only an assumption. As it turned out, almost none of these slopes have to date failed. We don't know why. Are they in fact not landslides at all - if so, what are they? If they are landslides, are they in fact sufficiently stable that they can only move in a large earthquake? Or are they incipient landslides, but they have not failed yet as the rainfall associated with the two monsoons (2006 and 2007) since the earthquake has not been intense in this area (which is the case)? I must admit that I err on the latter, and worry greatly about what will happen in Pakistan in the next really strong monsoon.

In Sichuan, I would be willing to bet that close inspection will reveal that these cracks also occur very extensively across the landscape. Once again the same conundrum will occur. In the case described by Xinhua above the authorities are absolutely right to be cautious, especially if it is actually moving. Evacuation and monitoring appears to be the correct approach. I suspect that this drama will play out in many other places. In Kashmir essentially no resource was dedicated to finding out what these cracks actually meant. I hope very much that this is not repeated in Sichuan.

Beichuan landslides from Google Earth

The FORMOSAT-2 image of the Beichuan area is now available on Google Earth (see here). The image is interesting as it shows that the landslides were incredibly spatially variable - for example, in the Beichuan area (which is junction of the rivers on the middle right of the image - the Google label is a little way from the main part of the town) the occurrence of landslides is very high and the river is blocked in at least three places. Elsewhere, the occurrence of landslides is much lower.

FORMOSAT-2 image of the Beichuan area, from Google Earth. Click for a larger view.

This allows a first attempt at perspective view of the Beichuan area using the Google Earth rather basic terrain model. Note that the imagery and the model aren't quite aligned correctly, so the picture is a bit confused, but it is a good starting place. I have annotated the image below to indicate the major landslides and places where the river is blocked. As usual, click on the image to get a decent view (it will open in a new window).


Annotated FORMOSAT-2 image of the Beichuan area, using the Google Earth terrain model to give a perspective view. Click for a larger view.

Finally, as mentioned above elsewhere the density of landslides is lower. However, close inspection shows that there are a large number of landslides in the landscape. As the image below shows, these display the typical features of mass movements triggered by earthquakes, which is that they start right at the top of the slope near the ridge, then travel most or all of the way down the slope. This is because the maximum shaking occurs at the ridges due to a process known as topographic amplification. This is consistent with the observations of witnesses, who said things like (please excuse the link to the Sun): “The mountains just seemed to explode as if they’d been detonated with dynamite."

FORMOSAT-2 image of an area near Zicheng, using the Google Earth terrain model to give a perspective view. Note the landslides in the middle of the image start from the ridge and extend to the valley floor. Click for a larger view.

Monday, 19 May 2008

Sichuan - further images of giant landslides

Two further images have been sent to me of giant landslides from the Sichuan earthquake. At the moment I cannot find out where they are from, although they clearly both show the same giant landslide. Note that the valley is completely filled (which means that a lake if forming somewhere upslope) and the amazing flow structures in the landslide mass. I wonder if this is the same landslide as is featured in my earlier post.


Updated: Sichuan Earthquake aftermath - landslides are taking a terrible toll

In previous posts I have warned that landslides will wreak a terrible toll in the aftermath of the earthquake. Sadly, this appears to be all too true. Xinhua is today carrying this report:

More than 200 relief workers, engaged in repairing the broken roads in the quake-stricken Sichuan Province, had been buried in mud flows over the past three days, said Dai Dongchang, an official with the Ministry of Transport on Monday. Two construction machines and six vehicles were also buried in the mud flows, he said.

Given that the summer rains have not really started as yet, the problems that this area faces are all too clear.

Update: the report has now been changed to:
Five vehicles were buried over the weekend by mud flows as they attempted to leave quake-ravaged regions of Sichuan Province, and an estimated 158 people were known to have been on board, a transport official told Xinhua on Monday. The death toll was not confirmed as rescue work was still underway, said Director of Road Traffic Dai Dongchang with the Ministry of Transport in an interview. Dai earlier told the central government website (www.gov.cn) that more than 158 relief workers with the Sichuan transportation department were trapped by mud flows. The identities of the victims were yet to be confirmed, he said. The official confirmed two construction machines were buried by mud flows but the drivers escaped in time, with no injuries. The official admitted that mud flows and aftershocks following the deadly quake and blocked rivers swollen by heavy rain had hampered restoration of roads and rescue work in the quake zone.

Beichuan - what happens when a massive rockslide hits a town

I am not normally a fan of Chicken Noodle News (CNN), but they have an interesting report detailing the damage in Beichuan resulting from the impact of the rockslide. It is worth a view. I cannot find a way to link to the original report, but there is a good quality version of it here.

AP image captioned: Rescue workers search collapsed buildings in Beichuan, China's southwest Sichuan province, Sunday May 18, 2008.

Sunday, 18 May 2008

Landslides after the Sichuan Earthquake - the major threats

I thought it would be useful to summarise the key issues / threats associated with landslides in the aftermath of the Sichuan earthquake that I can see at the moment:
  1. Delayed landslides from the main earthquake: there is some evidence that there can be a substantial delay between the earthquake and occurrence of large-scale landslides. This is because: i) in some cases pore pressures in slopes take some days to respond to the earthquake shaking; and ii) there is a mechanism of landsliding called progressive failure, which means that the landslide evolves over time. There remains a serious risk of large-scale slope failures in the earthquake zone, notwithstanding other triggers (see below);
  2. Landslides triggered by aftershocks: there is also a great deal of evidence that landslides are being triggered by aftershock events, some of which are substantial earthquakes in their own right (i.e. magnitude M>6). These landslides will continue to pose a real threat in the earthquake affected areas, although this will diminish with time;
  3. Landslides triggered by rainfall. Unfortunately, rains are expected to return to this region on 20th May. If this rainfall is heavy then we are likely to see very substantial amounts of landslide activity. This situation will become increasingly critical as the monsoon develops. The threats posed by this must not be under-estimated. In the worst affected areas in the 1999 Chi-Chi earthquake in Taiwan, the area of landsliding increased three-fold in the next rainy season!
  4. Failure of landslide dams: this threat is well-reported. For example, Xinhua reports that "Massive landslides following Monday's 7.8-magnitude earthquake have dammed rivers and lakes in several counties of southwest China's Sichuan Province, posing threats to downstream areas, the water conservancy authority said on Sunday. Twenty-one landslide blockages had been reported in the counties of Beichuan and Anxian counties in the city of Mianyang, Qingchuan county of Guangyuan, Mianzhu and Shifang cities of Deyang, and Pengzhou city near the provincial capital Chengdu." Fortunately, so far this appears to have been well-handled by the Chinese authorities, for which they deserve great credit. However, the ones that have failed to date will be the smaller blockages. It is the large ones that pose the real threat. I have included a little more detail on this below. However, there is also a substantial threat from very large landslide dams that might have formed in the high mountain areas, which are uninhabited. These can be absolutely catastrophic - for example, Dai et al. 2005 report a landslide dam in Sichuan that formed during the 1786 M=7.75 earthquake . This collapsed ten days later, killing over 100,000 people. There is an urgent need to survey the high mountains to find any similar landslide dams.
  5. Sediment production: One of the lessons of the 1999 Chi-Chi earthquake in Taiwan was that the sediment released by the landslides represents an appalling threat in its own right. This sediment is mobilised by the rains, causing massively damaging debris flows. This affects infrastructure downstream. For example, in the aftermath of the Taiwan earthquake six hydroelectric stations in the Tachia River Valley were put of commission long term by this problem. In places the sediment choked the river system, causing the river bed to rise by as much as 30 metres. The impact on communities and roads close to the river was terrible - last December I calculated a post-earthquake cost in this area alone of £384 million ($770 million).
All of these issues need energency attention. Hopefully the authoroties, with help from the international community, are being able to act appropriately.

Dam break landslides
Thanks to my friends Lynn Highland and Edwin Harp at the USGS for comments on this point. Lynn in particular has provided some very useful translations of Chinese articles. Two of the smaller blockages are at Beichuan. These blockages, which are considered to be very dangerous, are shown very well in this image:

This image and associated article is from here. The translation states that:

After the Wenchuan earthquake [Note: aka East Sichuan Earthquake] the Ministry of Land and Natural Resources immediately send remote sensing aircraft to carryout out remote sensing and photography in the disaster zone to study the disaster and geological disasters caused by the earthquake. The Ministry of Land and Natural Resources provided the information to the expert group on May 15 for their interpretation. They interpreted 69 images covering between 500 square meters and 10,000 square meters. In those images they found that in 19 places rockslides from the mountains with a total volume of 100,000 cubic meters had collapsed or slid off the mountainsides. Roads or bridges were damaged in 38 places, for a total stretch of road damaged of 5.39 kilometers. The debris slide blocked the Jianjiang River in Beichuan County. If the there is a heavy rainfall soon, this could result in the break of this natural dams. Moreover there are also six places where debris slides of 500,000 to 1,000,000 cubic meters could create a new disaster if there were to be an aftershock or heavy rains. The Ministry of Land and Natural Resources has asked that the Land and Natural Resources bureaus of disaster-affected provinces such as Sichuan, Gansu, and Shaanxi take full advantage of technologies to save lives and so organize work on preventing geological disasters. With respect to areas where serious geological disasters may occur, suggest to the local government a plan for evacuating the local population in order to avoid deaths from secondary disasters caused by debris slides. Moreover, it is important to strengthen work on the analysis of damage to the transportation network, determine their causes and to send technical personnel to the scene to assist in and provide guidance in repairing the transportation network. (article from the China News Network by Ruan Yulin)

These images clearly cover only a small part of the affected area. There is a great deal of work to do.

Reference
Dai, F.C., Lee, C.F., Deng, J.H. and Tham, L.G. 2005. The 1786 earthquake-triggered landslide dam and subsequent dam-break flood on the Dadu River, southwestern China. Geomorphology, 65 (3-4), 205-221.

Landslides at Donghekou, Ciban and Kangle in the Sichuan Earthquake

Reports and images are now emerging of the complete burial of three villages, Donghekou, Ciban and Kangle, located on the Qingzhu river in Sichuan Provinc as a result of (separate) landslides in the Sichuan Earthquake. In particular, images are now available of Doghekou, which is one of the landslides that has blocked the river upstream of Beichuan. These three landslides have together entombed an estimated 300 families, which would indicate a death toll of about 1000 people based upon an average household size of 3.32.

The Donghekou landslide appears to be very large. AP have released the following images. The caption describes this as a mountain collapse, which would seem to be appropriate for a slide of this size. Note that in at least one case the rising water levels behind the landslide dam can be seen. The hazards of this should not be under-estimated. The landslide appears to consist of very disaggregated and fragmented material, not the bouldery surface that we usually see for rock avalanches.

Caption: Survivors, bottom left and right, gather near the aftermath of a mountain collapse that swallowed up Donghekou village and two other village near Qingchuan, southwestern China's Sichuan province, Saturday, May 17, 2008. Around a hundred households and hundreds of residents were completely buried after Monday's quake.

Caption: Residents stand on the highway and view the aftermath of a mountain collapse that swallowed up Donghekou village and two other villages near Qingchuan, southwestern China's Sichuan province, Saturday, May 17, 2008. Around a hundred households and some hundreds of residents are lost, completely buried after Monday's quake.


Caption: A man is dwarfed by the aftermath of a mountain collapse landslide triggered by an earthquake that swallowed up Donghekou village and two other villages near Qingchuan, southwestern China's Sichuan province, Saturday, May 17, 2008. Around a hundred household and hundreds of residents are completely buried after Monday's quake. (AP Photo/Ng Han Guan)

Saturday, 17 May 2008

Satellite imagery of the landslides in the Sichuan earthquake

Now that there is a brief weather window, satellite imagery is starting to emerge of the Sichuan Earthquake zone. At present most of these images are too low resolution to allow detection of the real damage to the buildings (which is notoriously difficult to achieve), but they are excellent for looking at landslides.

The best to date that I have seen has come from the Taiwanese Formosat-2 instrument, which is a moderately high resolution imaging system that passes over Taiwan (and thus China) on a twice daily basis (10 am and 10 pm). They have helpfully released before and after images of the Beichuan area:

(Annoted from the BBC - click for a better resolution version)

The town in the centre of the image located around the river is Beichuan. The landslides are the brown areas on the left hand image - it is clear from this that about 20% of the land mass in this area has slipped. The southern edge of the town has been hit by a big landslide (see here) that appears from this image to have spread over a large area. Worryingly, some of the rivers also appear to have been blocked in two locations by landslides. I hope that urgent works are underway to resolve this as there is a real danger of a secondary disaster when the blockages are breached. This is an absolute crisis that needs very urgent action.

The Formosat-2 image has also been enlarged for Beichuan, again in before and after format. This is available here. This the before image:

This is the after image annotated to show the big landslide:

Note that the river appears to be blocked in this image too. There is no evidence of flow. Note also that the location of the very damaging landslide does not appear exceptional in the "before" image - this is a forested slope that looks the same as the surrounding slopes.

Finally, there are now ALOS images (both optical and radar) available. Radar adds little as the data are very difficult to interpret without processing. The optical image is better, and indeed has been annotated to indicate the location of landslides. To my eye there are many landslides that have not been annotated. Unfortunately the resolution is too low to be really helpful in this form. This is the image (click for a bigger view - warning this is a big file):

Updated: The Beichuan landslides in the Sichuan earthquake

Some information is now becoming available about the impact of landslides in Beichuan. I have blogged before about this. However, a report in the Chinese Economic Observer online is now beginning to highlight just how bad the problem actually has been. This report states the following:

In fact, landslides triggered by the May 12 earthquake had exacerbated the destruction in Beichuan. Take Maoba Middle School for instance, all traces of its existence – save a basketball hoop in the far end of a field – had "vanished" under fallen boulders from the hills. Of the 400 or more students and teachers there, merely one to two percent survived. What's more disturbing was that the landslide had blocked the flows of the White River, creating an artificial lake in its upstream, near the Kuzhuba hydrolic dam. If the water pressure built up and threatened to burst the dam, the cost of life and property would be dreadful. The risks involved had added urgency and hazards to the rescue work that was racing against time.

The Taiwanese Formosat-2 satellite has collected an image that shows this landslide. This is shown below, annotated to show the landslide:

It appears from this image that the school is not the only structure to have been hit by this landslide. This is also apparent from these newswire photographs, which show the site:


Caption: Rescuers looks at a Chinese flag and a basketball loop among the rubble of a school after it was flattened by falling rocks following Monday's 7.8 magnitude earthquake in Beichuan county, Sichuan province, China, Friday, May 16, 2008.

Caption: A Chinese flag and a basketball loop are seen among the rubble of a school after it was flattened by falling rocks following Monday's 7.8 magnitude earthquake in Beichuan county, Sichuan province, China, Friday, May 16, 2008.

Friday, 16 May 2008

Updated 18th May: Reported landslides from the 2008 Sichuan Earthquake

I am starting a second thread here to document landslides reported in the Sichuan earthquake. Background information about landslides and the earthquake is provided in the post below. I will update this list as new information becomes available. I would really welcome any information that can be provided - please email me ( d.n.petley@durham.ac.uk ) or post a comment.

General observation: as feared, landslides have taken a terrible toll in this earthquake. Reports from almost every town in the mountains suggest that landslides and rockfalls have killed large numbers of people. For example, Xinhua report:

The road from Dujiangyan, a city northwest of the provincial capital Chengdu, to Wenchuan, the epicenter, was blocked by rocks and mud slides, holding up rescue, medical and other disaster relief teams.

These landslides continue to cause serious disruption to the relief efforts. Again, Xinhua have said:

The official said the vice governors treked into Yingxiu after many hardships and several close brushes with death. Rocks were still rolling down from the mountains on Wednesday. The highway linking Yingxiu and Dujiangyan City was totally paralyzed and a bridge on the highway had collapsed, hampering the arrival of rescuers.

A few photographs are starting to emerge of the landslide damage. For example, Xinhua again have this image, which shows extensive fresh landslides:

Update: 18th May
Location: Donghekou, Ciban and Kangle villages
Report: International Herald Tribune
A mountain sheared off by China's massive earthquake swallowed the village whole, entombing an unknown number of people inside a huge mound of brown earth. The road to the village ends in a tangled twist of metal and tar. The landscape, eerie and still, shows few signs of human life — a soiled green floral scarf, a rubber pipe, a log. Wen Xiaoying, 32, held up a hand as she ticked off the family members buried in the muck before her — her father, her mother, her sister and her brother-in-law. The landslide blocked the valley's Qingzhu river and a lake was swelling behind the wall of debris, posing the threat that it could break its banks and send torrents cascading into villages downstream. Locals said two other villages further upstream from Donghekou — Ciban and Kangle — suffered the same fate as Donghekou. The three villages were home to about 300 families, locals said. "When I saw them the last time, we had a good time together," said Wen, a glimmer of a smile showing through as she remembered happier days with her family. "I didn't expect it would be the last time I saw them."

Update: 17th May
Location: Bai He river upstream from Beichuan
Report: The Independent
Mountains separate Hanwang from Wenchuan county, the epicentre of the quake, which is still cut off after Monday's 7.9 magnitude quake and where many thousands are feared dead. Soldiers who parachuted in to the county said four villages had been overwhelmed by landslides after the quake.

Update: 16th May
Location: Bai He river upstream from Beichuan
Report: Monster and Critics

A lake had formed on the Bai He river above Beichuan town after a landslide caused by the earthquake blocked the river, an official from the Sichuan Seismological Bureau told Deutsche Presse-Agentur dpa by telephone. Because of the high risk of flooding, troops evacuated everyone to higher ground before returning to continue their rescue work in Beichuan, which was home to about 30,000 people, state media said. The Sichuan official declined to respond when asked how the government planned to tackle the blockage on the Bai He.

Update: 14th May @ 07:00
Location: Huixian county, Longnan, Gansu
Report: Reuters

The flow of the Jialing River has been effectively blocked in Huixian county, in south-eastern Gansu's Longnan region, by landslides triggered by the quake, the Xinhua news agency said. Rubble created a 6-meter tall, 30-meter wide and 100-meter long dam, holding back 600,000 cubic meters of water. Commentary: valley blocking landslides presenta very serious threat in earthquake affected areas. Unless mitigated, this dam is liable to coollapse, which would create a devastating flood.

Update: 13th May @16:00
Location: Beichuan, Sichuan - see more detailed update here
Report: People's Daily Online and Xinhua
The county seat of Beichuan, about 160 km northeast of the Wenchuan, was almost leveled. As of early Tuesday, up to 5,000 people were said to be dead there, having been killed not in collapsed buildings but by landslides. The county seat was surrounded by hills 500 to 1,000 meters high. "When the quake struck, rocks rolled down the mountains and buried houses. People had no time to run out, and there was nowhere to run, as roads were blocked," said a survivor surnamed Zhang.


This is a Google Earth image of Beichuan (click on it for a better view). The site is indeed surrounded by steep mountains. Note the deep "bite" out of the mountain above the town - could this be an old landslide source zone? The town has not been completely buried by landslides, as this Xinhua image shows:

But it is likely that at least some of the damage has been the result of landslides and rockfalls.

This image appears to show that landslides have been a serious cause of loss of life in Beichuan:


Original list:
Location: Maoxian County, Aba Prefecture
Report: AFP
At least 37 tourists have been killed and about 15 British tourists were reported missing following the earthquake in China’s Sichuan province, emergency management office said Tuesday. “A tourist coach was buried by a landslide in Maoxian County, Aba Prefecture, leaving 37 people dead,” a Sichuan provincial emergency management spokesman said.

Location: Beichuan County
Report: Xinhua
Beichuan County was by far the worst-hit area in Monday's quake. Up to 7,500 people were said to have died in Beichuan, and it is believed that the final death toll will be higher. "When the quake hit, the buildings rocked and the mountains collapsed," said a survivor surnamed Zhang. "Many buildings at the foot of the mountains were buried and the people inside had no chance to escape." He said he feared the quake could have killed at least half of the 18,000 people in the county seat. "I saw many people running out of buildings, only to be buried in landslides," said Lei Xiaoying, another survivor. "The landslides never stopped because of the continuous aftershocks."

Location: Yingxiu Town, Wenchuan County
Report: Xinhua
A village was immediately engulfed by landslide and water level in a reservoir rose rapidly, recalled an earthquake survivor who is still haunted by terrible memories on Tuesday. The man named Yang Yang told He Biao, deputy secretary-general of Aba prefectural government that he was on a bus in Yingxiu Town, Wenchuan County of Sichuan Province, the epicenter of the earthquake that claimed at least 9,000 lives. "I saw a village on the mountainside disappear in a landslide triggered by the earthquake. Many vehicles on the road were swept away or hit by huge rocks," Yang was quoted by He Biao as saying.The water level in Zipingpu Reservoir also rose quickly, Yang said.

Location: The road from Dujiangyan to Wenchuan, Sichuan
Report: Xinhua

The road from Dujiangyan, a city northwest of the provincial capital Chengdu, to Wenchuan, the epicenter, was blocked by rocks and mud slides, holding up rescue, medical and other disaster relief teams.

Location: Jushui town, northern Sichuan
Report: Xinhua

"We were passing by the Jushui town, and we saw a huge landslide," a man from Mianzhu City, about 60 kilometers to quake center Wenchuan, told Xinhua over the phone.
"Rocks buried many houses down at the foot of the hill, and some of the vehicles on the road were buried too. I saw the rear of a car and the wheels of another. Several people were crying for help and some were bleeding," he said.

Location: 312 national highway near the Wenchuan county, Sichuan
Report: Xinhua

A driver from the Sichuan provincial seismological bureau had been on the 312 national highway near the Wenchuan county when the tremor occurred. "I heard someone calling 'earthquake' and felt my car swaying. Rocks rolled off the hills and dust darkened the sky," he recalled.

Location:
Dujiangyan city
Report: Xinhua
Xinhua reporters had attempted to go to Wenchuan, but were stopped at Dujiangyan city 100 kilometers away, where roads were blocked by rocks.

Location: Various railway tracks near Chengdu (15 reported landslides)
Report: Xinhua
Monday's strong earthquake in southwest China's Sichuan Province have caused multiple landslides and collapses along railway lines near the provincial capital Chengdu, leaving 180 trains stranded on the rails. Thirty-one passenger and 149 cargo trains were stranded on the Baoji-Chengdu line, the Chengdu-Kunming line, the Chengdu-Chongqing line and their branch lines linking Chengdu with the rest of the country. At least 15 cases of landslides and collapses had so far been reported along rail tracks.

Updated 16th May (twice): Sichuan Earthquake landslide images

I thought a thread showing some news images of landslides triggered by the Sichuan earthquake would be useful. The first image shows vividly the likely impact of landslides in this area.

AP photo entitled "Buildings are destroyed by landslides following Monday's powerful magnitude 7.9 earthquake in Beichuan in Mianyang city in southwest China's Sichuan Province Tuesday, May 13, 2008. A signboard atop the building at left reads: Beichuan Hotel"

Xinhua image entitled: An aerial photo taken on May 14 shows the road from Dujiangyan city to Wenchuan County in southwest China's Sichuan Province is destroyed by landslide caused by Monday's earthquake. The serious devastation of the road affected the transportation of rescue teams and relief materials to the disaster areas


Reuters image entitled: "A damaged highway is seen after an earthquake in Beichuan, Sichuan province May 13, 2008. "

Xinhua image entitled "An aerial view shows the earthquake-hit town of Yingxiu in Wenchuan County, Sichuan province May 14, 2008"

Reuters image entitled "Vehicles are stranded on an earthquake-affected highway linking Dujiangyan to Wenchuan, May 15, 2008. China poured more troops into the earthquake-ravaged province of Sichuan on Wednesday to speed up the search for survivors as time ran out for thousands of people buried under rubble and mud."

Reuters image entitled "Vehicles are stranded on an earthquake-affected highway linking Dujiangyan to Wenchuan May 14, 2008."

Reuters image entitled "Soldiers use explosives to clear a road leading to an earthquake-hit village in Dujiangyan, Sichuan province, May 14, 2008."

Xinhua image entitled "a volunteer helps a woman walking out of the earthquake-stricken area in Mianzhu city of southwest China's Sichuan Province, Wednesday, May 14, 2008."

Xinhua image entitled "A soldier from the People's Liberation Army makes his way to the quake-stricken Yingxiu town of Wenchuan county, Sichuan province, May 14, 2008"

Reuters image entitled: "A general view shows the earthquake-hit Beichuan county, about 160 km (99 miles) northeast of the epicenter of Wenchuan county, Sichuan province, May 14, 2008."

Reuters / Stringer image entitled "A local resident walks among the debris of collapsed buildings at the earthquake-hit Beichuan county, about 160 km (99 miles) northeast of the epicentre of Wenchuan county, Sichuan province, May 14, 2008."

Reuters image entitled "A view of the earthquake-affected Beichuan county, Sichuan province, May 13, 2008."