Tangjiashan Lake, Beichuan - so now we wait...

Xinhua is reporting that the spillway at Tangjiashan Lake is now finished and that the soldiers constructing it have withdrawn, leaving just a few hardy and brave souls to raise the alarm.

First, we must recognise the awesome task that has been accomplished here - 135,500 cubic metres of debris have been shifted to create a channel that 475 m long and "up to" 10 m wide. To do this in the circumstances is utterly extraordinary, putting the responses of most other countries to disasters to shame. But where does it leave the situation at Beichuan and downstream?

First, I have tried to collate some images of the channel. All originate from Xinhua. In the first, excavators are shown digging the channel. Now, this is unfinished so great care is needed, but note:

1. The material is very fragmented and soft. There are almost no boulders in the channel bed and in the walls of the channel;
2. The channel is very narrow indeed. It was unfinished when the image was collected, so might not be the final product, but the walls have been well-graded, so it probably is not far from it (?);
   
3. The gradient is quite steep, meaning that water flow is likely to be rapid.
   

In the second image, soldiers are seen working on the channel banks. Rocks are being moved in chains - it appears that this might be to armour the channel bed? Again, the channel is very narrow and steep, and interestingly it also has a very clear bend in it.


The final image shows one end of the channel, which here is broad and quite shallow. Again the material is fine grained, with almost no obvious boulders. The channel gradient is quite low.


There is little more information available to me at present. The channel is expected to start to carry water in the next three to four days. It seems to me (and to one or two other landslide people that I have spoken to ) that the channel will almost certainly erode rapidly once it is carrying water. The steepness of the gradient and the fine grained material appear to almost guarantee this. It seems likely therefore that the authorities have decided that the release of much or all of the water in the lake is inevitable. The key question will be how quickly that erosion occurs, and thus how big the flood wave actually is. The fine-grained nature of the material suggests to me that it could well be rapid, although this could be tempered by the size of the blockage. Note that the blockages downstream will also play a role, allowing the water to pond and then break through, which is likely to increase the maximum size of the flood wave. It seems likely that the flood wave will be very large, but frustratingly it is difficult to say just how big this will be. Evacuating as many people as possible downstream seems to be to be prudent.

Xinhua remains somewhat intriguing in its reporting, saying: "Two other plans require the relocation of 1.2 million people if the half of the lake volume was released, or 1.3 million if the whole landslide blockage is washed away."

I cannot quite understand what this means. It seems to me that the river will in due course cut a channel right through the dam - i.e. all of the water will be released. The key question is how quickly this occurs. Past experience suggests that this could be anything from a small number of hours to months or even years. If I was a betting man I would tend towards sooner rather than later. I hope that the authorities understand that if a catastrophic collapse were to start it would happen in a very short period of time. Thus, the evacuation would need to be very rapid indeed. Hopefully (and probably I guess?) this won't be the case, but...

We must also be very careful to ensure that we understand that this is just one problem of many. We mustn't lose sight of the fact that:
1. There are c.27 other remaining dangerous landslide dams. All need to be dealt with. It would be an extraordinary achievement to mitigate them all, so an unexpected collapse of at least one is surely likely. It worries me that there is so little information about other areas. Given the numbers of landslides that we know have occurred, I wonder if we are even sure that all of the valley blockages have been identified. I guess we have to trust the Chinese on this, and they have some incredibly competent people, but concerns linger.
2. Landslides continue to occur in the aftershocks. These represent a substantial hazard.
3. We are getting closer to the wet season. In heavy rainfall landslides will be a substantial problem, as will the movement of sediment in floods and debris flows. Dealing with the valley blockages is clearly the immediate crisis, but there are many more just around the corner.

Finally, I will be in and out of my office for the next few days but can be contacted by email. I would really like to hear from anyone who can provide more info, and am willing to discuss the above in more detail as and when appropriate. I intend to continue to post on here as much as I can.

Two new photographs of the Beichuan area

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

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


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

NASA imagery of Sichuan landslides

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

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

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

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

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

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

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

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

Sichuan - what does a landslide look like in reality?

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

Updated 28th May: Sichuan landslides dam - alarming video

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

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

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

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


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

Click on the image for a better view

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

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

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

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

Tangjiashan (Beichuan) Lake - update

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

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

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

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

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

The collapse of landslide dams - the challenge in Sichuan

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

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

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

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

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

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

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

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

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

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

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