Yosemite rockfall - seismic signal

Improvements in the quality of seismic monitoring systems, in addition to increases in the numbers of instruments and better processing techniques have meant that in the last few years it has become increasingly common to detect large rockfall events from their seismic signature. A nice example of this occurred this week. On 28th March a large rockfall occurred from Ahwiyah Point near Half Dome in Yosemite. This is a pretty large lump of granite:

(Image from http://wikimapia.org/1881070/Ahwiyah-Point)

There is a very nice image of the rockfall scar and track available on Flicker, but it is not possible to import the file. Take a look. This is a less good image from the National Park Service - you do get a pretty good idea of what was involved:


The National Park Service description of this event is as follows:
"[the rocks] fell roughly [600 m] to the floor of Tenaya Canyon, striking ledges along the way. Debris extended well out into the Canyon, knocking down hundreds of trees and burying the southern portion of the Mirror Lake loop trail." They are also reporting that there was a small air blast, but no-one was killed.

Of course, seismic monitoring in the part of the USA is a bit of a fine art, so it is unsurprising that the event was picked up on multiple instruments. There is a nice image on Seismoblog of the recorded seismic events from a range of instruments located an increasing distance from the site:


Based on the sesmic data the USGS were quickly able to locate the fall - pretty impressive!

Terrific rockfall resource

The Colorado Geological Survey publishes a terrific quarterly newsletter on "all aspects of geology throughout the state of Colorado. The latest edition, which is available online for free from here, focuses on rockfalls. It is a terrific read, with some very interesting articles on how and why rockfalls happen; how they can be detected; mitigation approaches; and a number of case studies. It is illustrated with some great photos as well. My favourite two, showing the range of scales of hazardous rockfalls:


And this "spot the difference" is rather nice too. The images were taken about 120 years apart:

Images from the Sichuan earthquake part 5 - Xingyiu

This is the fifth and last part of my photographic description of damage caused by the Sichuan earthquake. The other parts are as follows:

Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5 (this part): The Xingyiu area

This final section shows images from a visit to the Xingyiu area, which is quite close to the main epicentral region. In this area the main fault rupture appears to sit just out on the plain, being picked out by the line of a river. This is of course bad news for any bridges in the local area. This one has been impacted twice. First, the displacement on the fault has cut through one section of the bridge itself, uplifting one half compared to the other:


The fault runs through the abutment in the foreground, which has been lifted up about 1.5 metres to form this scarp. In addition, because the fault was so close to the fault the bridge was also intensely shaken. As a result, a few of the decks collapsed completely:

Parts of the bridge do show rather nicely the advantages of having rebar in the concrete (these are steel reinforcing rods). Here the concrete has failed completely but the bridge has been prevented from collapsing by the rebar:


Fortunately, at this site rebuilding is well under way – a new bridge is already half constructed (it is visible in the background of the image below). The rush presumably is to get the structure open before the summer rains, when the river bed crossings will become very difficult:


The local authorities have turned the bridge into a memorial and have erected a monument to the event. Note the rockslope failure in the background.



Of course because the fault is in the plain, the mountain front was on the hanging wall block, which means that it was very seriously shaken in the earthquake. This is reflected in the presence of multiple landslides, which I have photographed through the numbers of the monument. Note the damaged town at the bottom of the image, and the temporary structures just in front of them:


In fact the following picture gives a rather more dramatic overview of the full range of landslide impacts:


Or perhaps most usefully, this picture shows the effects of the earthquake rather better. The fault is at the bridge, which has collapsed, severing communications. The town in the middle distance has been badly affected by the earthquake shaking – the damaged buildings and temporary structures are visible – and finally, in the background, lie the earthquake induced landslides:


Another section of the fault runs through one of the small towns in the area:


The tractor is on the trace of the fault (note the ramp and the cracks in the road. The buildings on the fault have collapsed completely, those near to it are intact but so badly damaged that they have had to be abandoned. In the mountains the picture is even worse. Here, whole communities have been devastated and have had to be abandoned. For example, in this settlement every building is very seriously damaged. There is clearly no-one living there now, and rebuilding has yet to start here:


In this area, which is quite beautiful actually, despite the devastation (mind you the weather helped as the pictures show), there was one very large landslide that created a barrier lake. This is a high speed rather complex earthflow, which has a steep rockslope source. The picture below provides quite a reasonable overview of it. The source is a steep rock wall. The main flow has come down a moderately steep gully to enter the main channel, which was duly blocked.


The complexity here is in the volumes, as you can probably see that although the rockslope is large, the amount of material that has detached from it is nothing like enough to have created this enormous deposit at the toe. My interpretation is as follows:
1. The landslide started with a substantial rockfall (or several rockfalls perhaps) from the back wall of the landslide. This is shown quite well in the following image:
 

2. At the toe of the slope was located a large body of old landslide and rockfall material. This was probably holding quite a large amount of water below the surface. The sudden fall destabilised this material, which started to flow. Note that the rock debris that can be seen in the picture above is mostly weathered (there are a few fresh fragments too, but not much), reflecting the fact that most of this material had been sitting on the hillside for a long period. This steep upper track is shown in the image below. Note also the very small volume of material at the toe of the rockslope, even though the face has clearly undergone wide scale rockfalls. This indicates that most of the rockfall material fell onto the landslide below before it moved, and was then carried downhill, supporting the notion that the trigger for the main slide was the rockfalls from the back wall.


3. As the slide moved down the slope it loaded and these entrained material along its track. In the image below you can clearly see how the landslide has eroded and entrained material along its track:


The landslide buried several houses towards the toe of the slope, killing the occupants. Several other houses very narrowly escaped being buried. The pile of debris behind the house is the landslide:


4. Finally, the landslide stopped in the valley allowing the formation of a barrier lake, the last vestiges of which can still be seen:



Note the landslide on the upper left side of the image, and the signs of the previously higher water level left on the rockwalls beside the lake. Of course, this was not the only landslide in this mountainous area. There are many other examples:



Finally, one of the great threats here in the aftermath of the earthquake is wholescale environmental damage. This is shown below, where local people are harvesting trees to be used in rebuilding. I mean no criticism by this (it is a very understandable thing to do), but the impacts could be severe:


That completes my photographic description of damage caused by the Sichuan earthquake. The other parts are as follows:

Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5 (thia part): The Xingyiu area

Of course there is a lot more that I could included - I will return to specific issues over the next few months. There are still some surprises up my sleeve...

Comments, corrections and requests are very welcome. Finally, thanks to my friends at the State Key Laboratory for Geohazards at the Chengdu University of Technology for arranging my visit and looking after me so well.

Images of landslides and other damage from the Sichuan earthquake part 4 - the Mianyuanhe area

This is the fourth part of my photographic description of damage caused by the Sichuan earthquake. The other parts are as follows:

Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4 (this part): The Mianyuanhe area
Part 5: The Xingyiu area

First a location map. The Mianyuanhe area is a fairly large river valley that cuts through the Longmen mountain chain, with its mouth as Hanwang as shown in the Google Earth image (which predates the earthquake) below:

As you may know, it has been shown that the earthquake was associated with movement on two different thrust faults (and an additional strike-slip fault that sits between the two). The Mianyuanhe shows the surface expression of the two thrust faults. I have annotated a perspective Google Earth view of the Mianyuanhe area to show the approximate position of the two faults below:

This image also gives an idea of the terrain in this area. The surface features of these two faults is pretty clear. This is the Jiangyou-Guanxian fault (JGF) - the car has tilted as it crosses the crest of the fault scarp:

The displacement on the Yingyou-Beichuan fault (YBF) is much larger:

The presence of these two faults has meant that this is an area that has been pretty badly affected by the earthquake, with a large amount of building damage and many, many landslides. It is also an area with some active mines (coal mines I think) - for example, this mine, located just on the hanging wall side of the JGF, has been pretty badly damaged by the earthquake:

Coal mining in China has a dreadful environmental and safety record, so one wonders what happened to the miners when the earthquake struck. In fact, the impact that the mine is having on the environment is pretty clear - the red colours here are acid mine drainage, associated with acidic water coming from the underground workings. Note the extensive earthquake triggered shallow rockslides in the background too:

There has also been a huge amount of damage to buildings, such as this school:

Unfortunately, destroyed buildings create a huge volume of waste material that is very hard to dispose of without causing real environmental damage:


The electricity infrastructure was seriously damaged, as this pylon shows:

And of course the roads are in a very sorry state in places:

However, there is a surprising and very impressive amount of rebuilding going on, some of which is now quite advanced:

In this valley the earthquake triggered a vast number of highly destructive landslides, some of which are quite large. This photo, taken just on the hanging wall side of the YBF, shows gives an idea of the number of slides, as well as the level of damage to buildings:

The second largest mass movement triggered by the earthquake, the Wenjiagou landslide, occurred in this area. It is located between the two faults. This is an overview of this large and complex slide:


The source is a dip-slope failure high up on the left side of the image. The picture below shows the source area more clearly:

Note that on the steep scarp on the centre-left side of the image dust being blown up from ongoing rockfalls is clearly visible. The landslide slide down the dip-slope, turned to the left and travelled down the valley, and then turned to the right before running out. The movement rate must have been very high as the landslide super-elevated (banked up) as it went around the second turn, as this image clearly shows:

About 45 people were killed by the landslide. There is now a problem here with ongoing debris flow activity - more on this below.

There were also some very impressive valley blocking landslides. The most visually dramatic is this one, which has created a large barrier lake:

The slide mass is evident in the gap between the cliffs on the upper centre right of the image. These barrier lakes are causing considerable difficulties (although they are not dangerous now). For example, they are causing flooding of properties and roads:

Even the natural vegetation is being killed:

There are several of these very large valley blocking landslides, such as this one (which is different from the one above):

Note the multiple landslides in the background.

One of the most interesting slides lies on a tributary valley to the main one. This is a c.1 km long rockslide that has created a very tall but restricted debris pile:

This slide appears to have originated as a wedge failure controlled mainly by a bedding plane that has then changed direction and slid into the valley. In the foreground is a school - it is lucky that the slide did not spread. The bedding plane is pretty clear on this image:

The landslide blocked the valley to a height of over 50 m. A lake built up behind the blockage, but in this case drainage has not been necessary as water flowed through the rock dam. The muddy sediments laid down in the lake show this very clearly:

And there are natural markings and pieces of debris showing the old lake levels on the valley sides:

Finally one of the major problems that lies ahead is that of ongoing landslide and debris flow activity, and the resultant impact on the river systems. There are already plenty of signs that these problems are very serious indeed:


Your comments, thoughts and corrections, and indeed you general impressions, are very welcome. Please feel free to use the pictures in lectures and seminars, but please do acknowledge me. I retain copyright on the images.

Finally, just a reminder that this is the fourth part of my photographic description of damage caused by the Sichuan earthquake. The other parts are as follows:

Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4 (this part): The Mianyuanhe area
Part 5: The Xingyiu area

A Powerpoint presentation on earthquake-induced landslides

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:


Uploaded on authorSTREAM by Dr_Dave

I ask only that you acknowledge me in any use that you might make of it.

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.

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.

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.

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.

Norwegian landslide - is this a quick clay slide?

Most people would probably not think of Norway as being hazard prone, but it does have two particular landslide issues. The first is that occasional large-scale failures occur on the walls of fjords. Given the height of these rock walls, these failures can be large and energetic, and sometimes trigger small scale tsunamis. The other is a strange type of failure known as a quick clay slide. Perhaps the best way to explain a quick clay slide is to direct readers towards the original landslide viseo, which shows the Rissa slide. This is described at the site below, which also contains the video in two parts:

http://www.ce.washington.edu/~geotech/courses/cee522/RissaLandslide/rissa.html

Do take a look - the film is amazing. quick clay slides occur in marine clays that have an unstable structure. Disturbance of that structure can cause a massive reduction in strength, which then destabilises the slope allowing failure on even low slope angles. In the case of Rissa, a small excavation for the foundations of a barn was enough to trigger the failure.

Today, reports have emerged of a landslide at Namsos in Norway that has destroyed ten or so houses. Fortunately there have been no fatalities. Various pictures of the landslide are available - these two particularly caught my eye (from here and here - click on the image for a better view in a new window. The people are homeowners I think):




Familiar? It is clear that this is a landslide occurred in clay on a pretty low angle slope on the edge of a lake. What's more, the newspaper reports (e.g. this one) suggest that "It happened near a road construction site, and NRK said it could be related to blasting or excavation." I don't know for certain that this is a quick clay landslide, but I have my suspicions, especially when I look at this image of the site (from here):


There are a some other pretty good images around too, mostly focusing on the aftermath. For example, this one (from here) shows the displaced houses:

Whilst this one (from here) shows just what a low angle slope the failure occurred on. Presumably the road works in the foreground are the proposed cause of the problems:


Quick clay landslides are pretty rare (I have never seen one) if rather dramatic.

Climate Change Vulnerability Mapping for Southeast Asia

A very interesting report was released last week by the Economy and Environment Program for Southeast Asia (EEPSEA) in which an attempt has been made to map the pattern of vulnerability across the region to climate change. The aim of the project, for which the report is available online, was to "identify which regions in Southeast Asia are the most vulnerable to climate change." This is a fairly bold thing to try to do, so lets take a look at what they have achieved and how they have done so.

The researchers have taken as a starting point maps of five hazards - cyclones, drought, floods, landslides and coastal inundation (sea level change). In each case they have used an external dataset to indicate the hazard associated with each of these events - so, for example, for landslides they have used the NGI landslide hazard maps produced in the World Bank Natural Disaster Hotspots project (Fig. 1), whereas for sea level change they have modelled the inundation associated with a 5 m increase in sea level.

Fig 1: NGI landslide hazard map used as an input into the Climate Change Vulnerability Map.

For each hazard a scale of 1-10 has been used, with 10 being highest hazard and 1 the lowest. For each cell on the map, the average score across the five hazards was then taken as an indication of the overall hazard. I will return to this below. The outcome of this analysis is termed the "Multiple Climate Hazard Index". The resultant map is shown in Fig. 2 below.

Fig 2: The Multiple Climate Hazard Map.

This map has then been combined with data for population density (incorporating areas that are ecologically protected) and "adaptive capability" (defined as "the degree to which adjustments in practices, processes, or structures can moderate or offset potential damage or take advantage of opportunities (from climate change)." The latter has used expert judgement to create an index based upon data on education, poverty, income inequality and suchlike. These three indicators have then been averaged to determine the level of vulnerability to climate change (Fig. 3).

Fig 3: The Climate Change Vulnerability Map.

Hmmmm! First, let me say that this is a brave thing to do - such exercises are really challenging given the complexity of the dataset. Such exercises are important and useful given the need to prioritise. I am hesitant to be too critical. I would however like to point out four things that are worth thinking about.

1. Perhaps most importantly, I don't see how this is a map of climate change vulnerability. An argument can be made that this is a map of vulnerability to meteorologically driven hazards, but most of the parameters do not appear to consider a changing climate. The data for floods, droughts and cyclones used historic data of occurrence. This does not consider change. The only parameter that considered climate change was the sea level inundation, but this used a terribly simplistic model (a binary switch at 5 m sea level rise).
2. The decision to average across the five hazards is strange. The problem can be illustrated with an extreme example. Imagine you live on a flat, tropical plain 20 cm above sea level. Most of the hazards are likely to be low - no landslides, no river floods, no droughts, no tropical cyclones. However, a comparatively small rise in sea level wipes you out. In the system used here, your hazard comes out low whereas actually it is very high. It might be more rationale to take the highest value of hazard, or a more subtle measure.
3. The decision to weight the parameters equally is also interesting and surprising. Given the vastly different impact of the hazards, it might be worth weighting the hazards appropriately.
4. The decision to average the hazard, the population and adaptive capability is also odd. I would have thought that these parameters should be combined so that they interact (r.g. through multiplication and/or division). Clearly, the case where the level of hazard is high, the population is high and the adaptive capacity is low is exactly where really serious disasters occur.

I suspect that this map needs another iteration or two, perhaps backed up with a sensitivity analysis, but as a first step the authors deserve praise.

USGS educational video on landslides in the San Francisco Bay Area

Thanks to the Geology blog In Terra Veritas for highlighting a USGS video on landslides in the San Francisco Bay area. The video is a useful teaching / background resource, especially in terms of the way that it highlights the impacts of landslides on home owners, which is easily forgotten by both students and their teachers. The video compares two different slides; first, the Love Creek Heights landslide in the Santa Cruz Mountains, which was a large (600 m long x 250 m wide; 500,000 cubic metres) slide triggered by a heavy rainstorm on 5th January 1982. It killed 10 people.

The second is the La Honda landslide, which is a slow moving failure, also in the Santa Cruz mountains. This slide was the subject of one of my very first posts in December 2007.

Overall, this video is a useful resource that helps to understand landslides themselves and also their impact.

The loss of a geotechnical great - Sir Alan Muir-Wood

The Daily Telegraph is today carrying an obituary of Sir Alan Muir-Wood. Whilst Sir Alan will be best remembered as one of the modern-day fathers of tunnelling, his early work was focused on erosion on the south coast of England. For example, he was responsible for developing an understanding of the Folkestone Warren landslide in Kent in SE England (see image below), and for many other coastal slopes beside.


His legacy lives on, partly in the form of Halcrow, for whom he worked until retirement in 1984, and also in the form of his sons - David Muir-Wood is Professor of Civil Engineering at the University of Bristol, whilst another, Robert Muir-Wood, is the chief research officer for the Catastrophe Modelling company RMS. The world of geotechnics is a little poorer for his loss.

Rockslope stability - lessons from rock balancing

One of things that students often struggle to understand is the ability of slopes that appear to be extraordinarily unstable to remain standing for long periods. Take for example this site, taken from this site:


You might (quite reasonably) ask how it is that some of the blocks on this slope can possibly remain in place through strong winds, heavy rain, snow, ice and rapid temperature changes. However, the combination of rock friction and interlocking between blocks can provide remarkable amounts of resistance to movement. Perhaps the best demonstration of this comes in the form of the rock balancing artwork produced by Bill Dan, who is an artist who specialises in this extraordinary work. Bill runs a blog (here) in which he features rock balancing from around the world. I think my favourite example to illustrate rock slope stability is this one, which can be found on a flicker site here:


I think I might run a student practical in the future to describe the ways that rock slopes work, based on the above image. Add it to this one (from here), and a huge amount about rockslopes becomes clear:

The role of landslides in global warming

A rather extraordinary paper has just been published in Geophysical Research Letters about landslides triggered by the Wenchuan (Sichuan) earthquake. Why is it extraordinary - well, let me quote from the abstract. The paper suggests that the landslides caused destruction of vegetation such that "the cumulative CO₂ release to the atmosphere over the coming decades is comparable to that caused by hurricane Katrina 2005 (~105 Tg) and equivalent to ~2% of current annual carbon emissions from global fossil fuel combustion."

Wow! In case you are struggling to decode the above, this suggests that the landslides triggered by the earthquake caused a massive loss of vegetation that will now decay. In decaying it will release CO2, which will add to the effects of global warming. This is a pretty interesting result - and it has already been picked up by the mainstream media.

So, how do the authors reach these remarkable conclusions, and are they valid? Well, I am afraid that I have some serious doubts about this study, which seem to be based on some misunderstandings of earthquake-induced landslides. Lets base the analysis on Fig 2 of the paper, reproduced below, in which the authors highlight one of the landslides that blocked the valley on the river upstream of Beichuan:


So why do I object so strongly to the paper? Well first, the use of terminology is inexcusably weak. For example, the authors describe the landslides thus (referring to Fig 2a): " (a) Living carbon scars left by mudslides, which indicates the geographical locations of the landslides for this region." NO - these are not mudslides - these are clearly shallow rockslides - a very different beast. Of Figure 2b they say "A quake surface wave triggered basal sliding that initiated the movement through liquefying the top ~2 m slab." NO. Failure was not due to liquefaction, and even the most cursory view of the image shows that more than 2 m of material was displaced. Finally, they say of Fig. 2d "An aerial photo taken on May 26, 2008, showing the landslide mud that formed the Tangjiashan quake lake". No again - this is most definitely not mud (see image below) - it is bouldery / fragmented debris (if it was mud then the problems would have been far less serious). They say that there model suggests that "The material reaches a maximum speed of 5 m s⁻¹ but only briefly because the resistance stress is strong for the still coherent sliding material. " Again, this is poppycock. 5 m/sec is 18 km/hour - there is no way that this failure was as slow as that - look at how the debris fragmented and at how it spread across the valley (see image of the landslide deposit being excavated for the drainage channel below):

This is not a deposit that was emplaced at 5 m/sec, and nor is it mud. Pretty poor stuff, frankly. Note finally that figs 2b and 2d are supposed to represent the same area. However, in 2d the debris is clearly in the valley floor, with the source being the slopes above. In 2d the debris is above the 1155 m contour line. There is no debris between 1100 m and 1155 m - so the deposit areas are completely different.

So now lets turn to the modelling. The paper is ridiculously short of proper detail of what they have actually done - I cannot understand how the editors/referees let this through. It states that they have used an "advanced modeling tool—a scalable and extensible geo-fluid model—that explicitly accounts for soil mechanics, vegetation transpiration and root mechanical reinforcement, and relevant hydrological processes. The model considers non-local dynamic balance of the three dimensional topography, soil thickness profile, basal conditions, and vegetation coverage ... in determining the prognostic fields of the driving and resistive forces, and describes the flow fields and the dynamic evolution of thickness profiles of the medium considered, be it granular or plastic."

Hmmm! Not sure what this means really. However, they do state that "we need to use the finest possible digital elevation model (DEM) and soil profile data". However, they have actually used the SRTM data-set, which has a spatial resolution of 30 metres at best, and possibly 90 metres (!). I cannot believe that this is anything like good enough. Where velocity exceeded 1m/s in their model they assume that vegetation is destroyed. They have used this to determine the total amount of vegetation lost, and then calculated the contribution of the CO2 to the atmosphere.

There are several problems with this. First, the landslide model appears to be erroneous, as described above. Second, they seem to omit to include the uptake of CO2 by vegetation as it re-establishes on the slide scars, which will in the long term balance that emitted. Finally, note that they say 2% of CO2 emitted by burning fossil fuels, not 2% of all anthropogenic sources. This makes the contribution sound larger than it actually is. Indeed, 2% of annual anthropogenic emissions spread over a substantial period (it doesn't say how long) indicates a comparatively minor annual total.

In my view the basis of the paper is iffy, although it would have helped if the methodology had been properly outlined. Unfortunately, the work is already being picked up the climate change denier community. Read this and weep. The logic used by Paul Fuhr in this opinion piece makes no sense at all to me, but the fact that he can use this paper in this way is deeply unfortunate, providing yet more ammunition for the pseudo-science community of climate change deniers.

Reference
Diandong Ren, Jiahu Wang, Rong Fu, David J. Karoly, Yang Hong, Lance M. Leslie, Congbin Fu, Gang Huang (2009). Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008 Geophysical Research Letters, 36 (5) DOI: 10.1029/2008GL036702

Karst landslide in China

Xinhua is reporting a landslide in Jinjiling Hill in Guilin, Guangxi yesterday that buried several houses and killed four people. Whilst in global terms the event was not huge, the images that Xinhua have published are quite interesting:


Xinhua describes the event thus: "...several hundred tonnes of rocks and dirt gushed down from the mountain at about 9:20 p.m. Wednesday. Four workers in charge of a cave-digging project in the hill, the project contractor and his wife were in the houses when the accident took place, the city's public security bureau said. Jiang Mingyi, head of the city's geological environment monitoring station, said ongoing rain over the past few days triggered the landslide and the local karst-landform is prone to mountain collapse in the rainy season."

It would be easy to ascribe this collapse to the caving digging work that was going on at the toe of the slope. However, the failure appears to have occurred on an unfavourably orientated joint high up on the slope. The initial collapse was possibly quite small, but the failure has accrued debris on the way down to create quite a large failure. Thus, the interpretation that the failure was rainfall-induced looks spot on.

Of course, what is not clear is the degree to which the work at the toe of the slope disturbed the joints upslope. If blasting was being used this is of course quite possible.

Landslide buries a gold mining village in Peru

Reuters is reporting that at least 10 people have been killed and a further 30 are missing after a landslide struck the gold mining village of Huanchumay, which is situated in a remote part of Carabaya province. They quote a civil defence official, Victor Ibanez, as having said:

"We have 10 dead so far and five wounded. We have no accurate data for the missing but are talking about 30 to 35 missing people. The landslide took place early on Monday morning and was due to heavy rain in the area.'

The image below shows the general location of the mine in Ayapata. This is pretty inhospitable terrain.

A useful tailings dam failure resource

Image of the Merriespruit Tailings Dam failure in South Africa (image from Tailings.org)

In the last year I have twice posted extensively about particular tailings dams failures and the resulting flowslides. The events in question were the 8th September 2008 Taoshi failure in Shaanxi, China and the 22nd December 2008 Tennessee Valley Authority failure in at the Kingston fossil plant in Harriman, Tennessee, USA.

The WISE Uranium project has some very interesting pages here on the safety of tailings dams. I would like to bring to your attention two particularly useful elements of this:

1. The site contains a novel narrated presentation on tailings dam stability.
2. Perhaps most usefully, the site also contains a chronology of major tailings dam failures from 1961 to the present.

The latter notes that it is not complete due to lack of data, but most of the largest events are probably recorded. It is depressing to note that the trend in occurrence through time has remained remarkably constant (see Fig. 1 below), suggesting that there is still much to do to reduce the occurrence of this type of disaster.

Fig. 1: Trend in recorded instances of tailings dam failures as recorded on the WISE Uranium project website.