On Monday I gave a presentation to staff and students at the State Key Laboratory for Geohazards at Chengdu University of Technology - the topic was the lessons that can be learnt about earthquake-induced landslides from the Taiwan (1999) and Kashmir (2005) earthquakes. This is the presentation:
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Friday, March 27, 2009
Earthquake damage in Hanwang, Sichuan Province
This is the third of my five sets of images of the impact of the Wenchuan earthquake in Sichuan, China. The first two are as follows:
Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3 (this part): Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
In this set of images I feature the town of Hanwang, on the edge of the mountains. The location of Hanwang is shown below (click on the image for a better view in a new window):
Hanwang is a smallish town by Chinese standards - before the earthquake it had a population of 60,000 people. It was largely unremarkable, being best known perhaps for a huge steam turbine factory around which the town has grown. Hanwang suffered very strong shaking in the earthquake. In the town centre, in fact immediately in front of the turbine factory, there is a clock tower. The clock stopped in the earthquake and now stands witness to the time that the earthquake struck:
Note the shut up shops on the far side of the tower. In fact, this central part of the settlement has been totally abandoned and is now like a ghost town with just a few people passing through on their way to somewhere else. The newer buildings mostly remained standing but have been so badly damaged that they are not usable. They have been stripped bare and the windows have been removed:
The older buildings and smaller residential properties performed far less well - there are large areas in which the structures have collapsed completely:
The inhabitants of this section of the town have moved out to temporary camps. In the southern part of the town the picture is a little better, with some of the buildings intact, albeit damaged:
Here the streets are lined with temporary buildings in which almost every aspect of life is continuing:
Meanwhile, on the edge of the town the bridge over the river is damaged and under repair, meaning that the traffic has to use a temporary road across the river bed. Note the holes by the side of the road - a huge amount of material is being removed from the rivers to be crushed and used as aggregate during reconstruction.
The other reports in this series to date are as follows:
Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3 (this part): Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
Your comments and corrections are welcome.
Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3 (this part): Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
In this set of images I feature the town of Hanwang, on the edge of the mountains. The location of Hanwang is shown below (click on the image for a better view in a new window):
Hanwang is a smallish town by Chinese standards - before the earthquake it had a population of 60,000 people. It was largely unremarkable, being best known perhaps for a huge steam turbine factory around which the town has grown. Hanwang suffered very strong shaking in the earthquake. In the town centre, in fact immediately in front of the turbine factory, there is a clock tower. The clock stopped in the earthquake and now stands witness to the time that the earthquake struck:
Note the shut up shops on the far side of the tower. In fact, this central part of the settlement has been totally abandoned and is now like a ghost town with just a few people passing through on their way to somewhere else. The newer buildings mostly remained standing but have been so badly damaged that they are not usable. They have been stripped bare and the windows have been removed:
The older buildings and smaller residential properties performed far less well - there are large areas in which the structures have collapsed completely:
The inhabitants of this section of the town have moved out to temporary camps. In the southern part of the town the picture is a little better, with some of the buildings intact, albeit damaged:
Here the streets are lined with temporary buildings in which almost every aspect of life is continuing:
Meanwhile, on the edge of the town the bridge over the river is damaged and under repair, meaning that the traffic has to use a temporary road across the river bed. Note the holes by the side of the road - a huge amount of material is being removed from the rivers to be crushed and used as aggregate during reconstruction.
The other reports in this series to date are as follows:
Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3 (this part): Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
Your comments and corrections are welcome.
Wednesday, March 25, 2009
Tangjiashan - images of a potential disaster that was averted
This is the second of my series of photographic reviews of the earthquake affected area in Sichuan Province. The other sets are as follows:
Part 1: Beichuan town
Part 2 (this part): The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
On Sunday I was lucky enough to be allowed to visit the landslide site at Tangjiashan, thanks again to my friends from Chengdu University of Technology. To remind you, Tangjiashan was the most hazardous of the 40 or so valley-blocking landslides triggered by the earthquake. Over a period of about a month a team battled heroically to drain it - ultimately succeeding. I blogged about these efforts in detail back in May and June.
Since the earthquake the dam site has been closed, so I was exceptionally fortunate to be allowed to go up there. Access is via a track along the river bed. This will inevitably be lost in the rainy season, which starts in May, so I suspect that few other people will make it up there before the autumn.
A good reference point for this is this pair of NASA ASTER images of the Tangjiashan site and the river down to Beichuan. The image on the left is before the earthquake and on the right is an image from after the event (whilst the dam was still intact. I have annotated on the image pair the location of Beichuan and of the Tangjiashan landslide (as always, click on the image for a bigger version in a new window):
The basic geography of the situation should be clear from the above. The Tangjiashan landslide had blocked the river and a lake was forming behind. Downstream are two smaller landslides that had also blocked the river, and then downstream again was the now ruined town of Beichuan, which still had thousands of dead victims trapped under the rubble.
The landslide scar is clearly shown in the image above. Unfortunately I struggled to get a good image of it as it was very hazy and we were looking into the sun, and am not very adept at improving images with photoshop, so the below is the best that I could manage:
The landslide occurred on a slope that is structurally rather complex, but at least in places it is clear that the landslide slipped on natural joints or bedding planes inclined out of the slope. If these planes are persistent then there is a risk of a further failure. This needs investigation. The image below shows a set of these planes located in the central part of the slope:
The landslide body is large. This image gives an idea of the scale. It was taken from the top of the landslide mass looking down into the channel. If you look very carefully you will see that there is a full-sized, green-blue back hoe excavator working in the channel in the centre of the image:
Similarly, this image was taken from the upstream side of the dam looking down the channel onto the landslide. Again, if you look very carefully you can see two people standing on top of the landslide body:
And here is an image looking upstream onto the landslide body. For scale, note the road up the mass and, if you look very carefully, you can just see a building on top:
When the landslide came down the slope it hit the opposite valley wall. A zoom in on the above image shows a very sharp contact between the valley wall and the landslide body. There is no evidence of there having been an air blast:
Of course after the channel was constructed, half of the dam eroded away. However, there is still a small lake on the upstream side, terminated by a fairly steep bar:
Again, if you look carefully at the above you will see that there is a back hoe in the channel trying to widen it - the scale of this feature becomes clear once you get your eye in to this digger. The channel itself is still quite steep but the bed is mantled with some large blocks of rock:
However, the narrowness of the channel is causing real concern given that the rainy season is only two months away. As the image below shows, the contractors are working very hard indeed at trying to widen the channel - this is a massive effort:
Downstream of the dam the channel is quite wide. The flood plain deposits left by the flood are clear to see. Note the multiple slope failures on the valley walls and the debris flow deposits from the September 2008 rains in the valley mouths (there is a large fan at the far end of the valley floor that must post-date the flood from Tangjiashan). There is a huge volume of sediment waiting to be transported in the tributary valleys and gullies:
The road up the valley to the dam is quite clear on this image - clearly this will not survive the wet season, so the contractors are trying to open / rebuild the old valley side road (this road can be seen on the "before" satellite image):
However, as the above image shows, in opening the road the team are undercutting the slope and leaving it unsupported. Unless some support is emplaced I doubt that this will survive the wet season.
The flood from Tangjiashan swept downstream, impacting the small hydro plant shown on the satellite images above. The central sluice gates of the dam were swept away, although the ones on the edge of the channel survived:
Finally the flood passed through Beichuan itself, fortunately without causing too much damage. This bridge shows the force of the water. It is difficult to imagine what the flood would have been like if the dam had overtopped naturally:
So, that is the current state of affairs at Tangjiashan. As ever, your comments are very welcome. I hope that these images are helpful and interesting.
This is the second of my series of photographic reviews of the earthquake affected area in Sichuan Province. The other sets are as follows:
Part 1: Beichuan town
Part 2 (this part): The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
Part 1: Beichuan town
Part 2 (this part): The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
On Sunday I was lucky enough to be allowed to visit the landslide site at Tangjiashan, thanks again to my friends from Chengdu University of Technology. To remind you, Tangjiashan was the most hazardous of the 40 or so valley-blocking landslides triggered by the earthquake. Over a period of about a month a team battled heroically to drain it - ultimately succeeding. I blogged about these efforts in detail back in May and June.
Since the earthquake the dam site has been closed, so I was exceptionally fortunate to be allowed to go up there. Access is via a track along the river bed. This will inevitably be lost in the rainy season, which starts in May, so I suspect that few other people will make it up there before the autumn.
A good reference point for this is this pair of NASA ASTER images of the Tangjiashan site and the river down to Beichuan. The image on the left is before the earthquake and on the right is an image from after the event (whilst the dam was still intact. I have annotated on the image pair the location of Beichuan and of the Tangjiashan landslide (as always, click on the image for a bigger version in a new window):
The basic geography of the situation should be clear from the above. The Tangjiashan landslide had blocked the river and a lake was forming behind. Downstream are two smaller landslides that had also blocked the river, and then downstream again was the now ruined town of Beichuan, which still had thousands of dead victims trapped under the rubble.The landslide scar is clearly shown in the image above. Unfortunately I struggled to get a good image of it as it was very hazy and we were looking into the sun, and am not very adept at improving images with photoshop, so the below is the best that I could manage:
The landslide occurred on a slope that is structurally rather complex, but at least in places it is clear that the landslide slipped on natural joints or bedding planes inclined out of the slope. If these planes are persistent then there is a risk of a further failure. This needs investigation. The image below shows a set of these planes located in the central part of the slope:
The landslide body is large. This image gives an idea of the scale. It was taken from the top of the landslide mass looking down into the channel. If you look very carefully you will see that there is a full-sized, green-blue back hoe excavator working in the channel in the centre of the image:
Similarly, this image was taken from the upstream side of the dam looking down the channel onto the landslide. Again, if you look very carefully you can see two people standing on top of the landslide body:
And here is an image looking upstream onto the landslide body. For scale, note the road up the mass and, if you look very carefully, you can just see a building on top:
When the landslide came down the slope it hit the opposite valley wall. A zoom in on the above image shows a very sharp contact between the valley wall and the landslide body. There is no evidence of there having been an air blast:
Of course after the channel was constructed, half of the dam eroded away. However, there is still a small lake on the upstream side, terminated by a fairly steep bar:
Again, if you look carefully at the above you will see that there is a back hoe in the channel trying to widen it - the scale of this feature becomes clear once you get your eye in to this digger. The channel itself is still quite steep but the bed is mantled with some large blocks of rock:
However, the narrowness of the channel is causing real concern given that the rainy season is only two months away. As the image below shows, the contractors are working very hard indeed at trying to widen the channel - this is a massive effort:
Downstream of the dam the channel is quite wide. The flood plain deposits left by the flood are clear to see. Note the multiple slope failures on the valley walls and the debris flow deposits from the September 2008 rains in the valley mouths (there is a large fan at the far end of the valley floor that must post-date the flood from Tangjiashan). There is a huge volume of sediment waiting to be transported in the tributary valleys and gullies:
The road up the valley to the dam is quite clear on this image - clearly this will not survive the wet season, so the contractors are trying to open / rebuild the old valley side road (this road can be seen on the "before" satellite image):
However, as the above image shows, in opening the road the team are undercutting the slope and leaving it unsupported. Unless some support is emplaced I doubt that this will survive the wet season.The flood from Tangjiashan swept downstream, impacting the small hydro plant shown on the satellite images above. The central sluice gates of the dam were swept away, although the ones on the edge of the channel survived:
Finally the flood passed through Beichuan itself, fortunately without causing too much damage. This bridge shows the force of the water. It is difficult to imagine what the flood would have been like if the dam had overtopped naturally:
So, that is the current state of affairs at Tangjiashan. As ever, your comments are very welcome. I hope that these images are helpful and interesting.This is the second of my series of photographic reviews of the earthquake affected area in Sichuan Province. The other sets are as follows:
Part 1: Beichuan town
Part 2 (this part): The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
Beichuan - photos of the aftermath of a natural catastrophe
This is the first of my series of photographic reviews of the earthquake affected area in Sichuan Province. The other sets are as follows:Part 1 (this part): Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
Thanks to my friends in the State Key Laboratory for Geohazards at the Chengdu University of Technology I have spent the last few days in Sichuan Province visiting some of the key sites destroyed in the 12th May 2008 earthquake with the aim of developing some collaborative research projects. I have been fortunate to be able to visit places such as Beichuan and Tangjiashan. Over the next few days I will try to post a photographic summary of various locations.
An appropriate though tragic starting point is inevitably Beichuan, the town that was so devastated by both the earthquake shaking and by the effects of landslides. I am one of the few outsiders to be able to visit the town. It was a very emotional experience - I hope that below I can portray the state of the place, which has now been permanently abandoned with the intention of transforming it into a permanent memorial to the earthquake and its victims. The toppled lions below seem to symbolise the fall of the town:
The Google Earth image below shows the location of Beichuan within China. Its location in terms of lat and long is 31°50'2.0"N 104°27'30.5"E. Click on the image for a better view - this is the case with all the images in this post.
I managed to find two pictures of Beichuan before the earthquake. I guess that the place was not architecturally much to write home about, but the location is / was very beautiful, such that the town had many tourist hotels within it. Before the earthquake the town had about 20,000 residents.
Beichuan's misfortune was to be relocated right on the fault that ruptured to cause the earthquake. The fault runs through the middle of the newer part of the town - I have annotated the approximate surface trace of the fault on this image below, whilst the following photograph shows the surface expression of it:
The intensity of the earthquake shaking at Beichuan was clearly very high. A good indication of this is the rear jib of a tower crane that was standing in the town - the intensity of the shaking has caused the jib to bend as a result of the forces acting on the rear counterweight, despite the bracing.
Fortunately the crane did not collapse, unlike an adjacent one:
The impact of the shaking was first to cause many buildings to crumple. A substantial number underwent so-called "soft storey" collapse, where the bottom few floors pancake whilst the upper floors remain intact (the smaller rooms on upper floors mean that the building is often stronger at higher levels, whilst the larger open spaces on the lower floors (for reception areas, shops, restaurants, etc) mean that the building is weak at this level). Of course soft storey failures are particularly serious when the earthquake strikes during the day as the lower floors tend to be more densely occupied then. The following building underwent a soft storey failure - note how the lower floors have almost completely vanished:
Many other buildings partially or completely collapsed, many creating essentially impenetrable piles of debris that killed, injured and trapped thousands of people:
One of the great fears in the aftermath of earthquakes is what the insurance industry call "fire following", when the damaged buildings ignite and burn as a result of ruptured gas mains, burst gas cylinders, overturned candles and fires, and suchlike. Fire following does not seem to have been a big problem in Beichuan, although one or two of the buildings do show some fire damage:
Unfortunately, worse was to come for the people of Beichuan. In the new part of the town the Middle School was located at the toe of a large rock slope almost directly on the fault trace. The earthquake immediately triggered a massive landslide on the slope that crashed down directly onto the school and adjacent buildings.
The landslide was particularly damaging as the slope collapse took the form of massive boulders (each several metres across) that bulldozed everything in their path. The image below shows the location of the school - all that was left was the flag pole and a solitary basketball hoop. About 600 people died in this landslide, including almost all of the children in the school:
According to local people, about ten minutes after the main landslide a second massive slope failure occurred, this time above the old town on the other side of the river. Of course the landslide slid into and over buildings that were mostly already greatly weakened or even collapsed from the earthquake itself.
This landslide appears to have been extremely rapid, pushing an air blast ahead of it that destroyed almost all of the remaining buildings in the old part of the town. The devastation is almost total:
1,600 people died beneath this second landslide, bringing the death toll in the town as a whole to an estimated 12,000 people. The victims are remembered in a simple but very emotive memorial that is located in the centre of the town:
Unfortunately, for the people of Beichuan and for their rescuers the troubles were not over as upstream of the town the river valley was blocked by the massive Tangjiashan landslide. Those people who read the blog back in May and June will remember my multiple posts on this issue as we followed the ultimately successful attempts to drain the lake. I will post here about my visit to Tangjiashan in the next day or so, but for now this is a photograph of the channel upstream from Beichuan through which the flood wave from Tangjiashan travelled. The multiple landslides that are shown here were primarily triggered by the flood waters undercutting the slope toe:
One of the great problems in earthquake-affected mountainous areas is that the huge amounts of sediment released by landslides make the area very susceptible to debris flows. The final ignominy for Beichuan was that in September the area suffered from exceptionally heavy rainfall that triggered extensive debris flows. Many of the remaining buildings were buried up to and sometimes beyond the second storey. Parts of the old town were covered in 10 metres of sediment:
This is of course why the decision of the government to relocate the town of Beichuan is quite right. I just hope that after all that they have been through the people of Beichuan can find some stability.
A final footnote - I suspect that this may not be the first major slope failure at Beichuan - this huge boulder apparently predates the earthquake (there are mature bushes growing on it). It is located about 100 m from the toe of the slope - I wonder how it got there?
Other parts of this series are:This is the first of my series of photographic reviews of the earthquake affected area in Sichuan Province. The other sets are as follows:
Part 1 (this part): Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5: The Xingyiu area
Your comments and corrections are as every very welcome.
Thursday, March 19, 2009
Coastal landslides and tsunami hazards (and the wonder that is Google Earth)
Richard Tieuw and several of my former colleagues at the Geohazard Reserach Centre at Portsmouth University in southern England have published a nice, short article in EOS looking at issues associated with the generation of tsunamis by coastal landslides. EOS is subscription only, but the article has been covered quite well by New Scientist (available online here), although they look at a particular aspect (see below).
First, let's take a look at the article itself. They highlight the hazards associated with a 1 million tonne (actually ton, but let not quibble) block that they have identified on the flanks of the Morne Aux Diables volcanic edifice in the north of the island. New Scientist have provided an image of the feature:
They suggest that failure of this block (marked A above) could trigger tsunami waves 2.8 m high (annoyingly they don't specify whether these heights are proximal or at some distance from the collapse, which is fairly critical). They also hypothesise that the failure could trigger further, potentially larger events that of course have further to travel and so could generate a large wave. They note that the beaches of Guadeloupe are a few tens of kilometres to the north, although they do not specify how much the wave height would have dissipated by the time it got there (one feels intuitively quite a lot, unless you subscribe to the Ward and Day model).
So let's take a quick look at the feature in question, using Google Earth of course. Here is is:
Based on this I would agree that there is a potentially unstable slope here - it's a pretty good catch by Tieuw et al - although it seems to me that the distance to any major vulnerable population would imply that the direct risk is not too high.
The New Scientist article picks up on the more important aspect of the piece though. In the second part of the article Tieuw et al. note just how powerful Google Earth is proving to be in these kinds of studies. "Conventional" techniques, such as Lidar and high resolution satellite data are ruinously expensive and generate datasets that are immensely difficult to manage with high levels of skill and good quality hardware and software. This high res Google Earth imagery is free and can be handled very easily. In my opinion Google Earth has been the most important development in geohazard work in the last five years. It is thrilling to think that it is likely to get better and better as time goes on.
Reference
Tieuw, R., Rust, M., Solana, C. and Dewdney, C. 2009. Large Coastal Landslides and Tsunami Hazard in the Caribbean. Eos, 90 (10) 81-82.
First, let's take a look at the article itself. They highlight the hazards associated with a 1 million tonne (actually ton, but let not quibble) block that they have identified on the flanks of the Morne Aux Diables volcanic edifice in the north of the island. New Scientist have provided an image of the feature:
They suggest that failure of this block (marked A above) could trigger tsunami waves 2.8 m high (annoyingly they don't specify whether these heights are proximal or at some distance from the collapse, which is fairly critical). They also hypothesise that the failure could trigger further, potentially larger events that of course have further to travel and so could generate a large wave. They note that the beaches of Guadeloupe are a few tens of kilometres to the north, although they do not specify how much the wave height would have dissipated by the time it got there (one feels intuitively quite a lot, unless you subscribe to the Ward and Day model).So let's take a quick look at the feature in question, using Google Earth of course. Here is is:
Based on this I would agree that there is a potentially unstable slope here - it's a pretty good catch by Tieuw et al - although it seems to me that the distance to any major vulnerable population would imply that the direct risk is not too high.The New Scientist article picks up on the more important aspect of the piece though. In the second part of the article Tieuw et al. note just how powerful Google Earth is proving to be in these kinds of studies. "Conventional" techniques, such as Lidar and high resolution satellite data are ruinously expensive and generate datasets that are immensely difficult to manage with high levels of skill and good quality hardware and software. This high res Google Earth imagery is free and can be handled very easily. In my opinion Google Earth has been the most important development in geohazard work in the last five years. It is thrilling to think that it is likely to get better and better as time goes on.
Reference
Tieuw, R., Rust, M., Solana, C. and Dewdney, C. 2009. Large Coastal Landslides and Tsunami Hazard in the Caribbean. Eos, 90 (10) 81-82.
Wednesday, March 18, 2009
38 years ago today - the Chungar landslide in Peru
Today is the 38th anniversary of a notable landslide - the Chungar rock avalanche in Peru. This landslide occurred on the banks of Lake Yanahuani (sometime spelt Lake Yanahuin), about 120 km north-east of Lima (see image below):
The landslide, which had a volume of about 100,000 cubic metres, is shown on the image below. Unfortunately the Google Earth image resolution is low in this area, but you can see enough to get an idea. I have annotated the image to show the main features (click on the image for a better view):
The rockslide descended a vertical distance of about 400 m before entering the lake, whereupon it created a displacement wave that crossed the body of water at high speed. The wave struck a mining camp located on the other side of the lake, running up a vertical distance of 30 m and erasing all traces of the settlement. Between 400 and 600 people were killed. The landslide occurred on highly-fractured limestone rocks on a slope that had been over-steepened. The slope remains highly dangerous. Unfortunately the landslide is poorly documented - for example, the trigger is really not at all clear.
The landslide, which had a volume of about 100,000 cubic metres, is shown on the image below. Unfortunately the Google Earth image resolution is low in this area, but you can see enough to get an idea. I have annotated the image to show the main features (click on the image for a better view):
The rockslide descended a vertical distance of about 400 m before entering the lake, whereupon it created a displacement wave that crossed the body of water at high speed. The wave struck a mining camp located on the other side of the lake, running up a vertical distance of 30 m and erasing all traces of the settlement. Between 400 and 600 people were killed. The landslide occurred on highly-fractured limestone rocks on a slope that had been over-steepened. The slope remains highly dangerous. Unfortunately the landslide is poorly documented - for example, the trigger is really not at all clear.
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