The Wenchuan Earthquake - one year on

No words can do justice to the first anniversary of the Wenchuan (Sichuan) earthquake, so I won't even try. Instead I post these pictures as a silent memorial to the victims.

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

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.