Tangjiashan - here we go...

Xinhua has reported in the last 20 minutes that:
"Water level of China's main quake lake Tangjiashan climbed to the critical point of 740 meters on a sluice channel at 0:00 a.m. Saturday, but the long-awaited drainage hasn't started yet."

Given that we know that it has been raining in the area this afternoon, over-topping must be about to start. Hold onto your hats...!

And can I just say good luck to everyone in the path. I hope to wake up tomorrow to hear that this hasn't even made the news. If so, the Chinese authorities will have achieved the almost impossible.

Tangjiashan - overtopping is imminent

Xinhua is reporting that over-topping of the Tangjiashan dam is now absolutely imminent. They Chinese media have reported that the water level in the dam rose to 739.52 metres above sea level at 4 a.m. GMT today. This is 48 centimetres from the bottom of the channel. It appears that further evacuations are now underway.

More as and when information becomes available.

Updated: Tangjiashan (Beichuan) dam - a summary of what we know

In the next 24-48 hours the water level at Tangjiashan should reach the spillway and flow should start. I thought therefore that it would be useful to summarise what we know about the site:

• The landslide dam is 124 metres high;
• At the time that the over topping begins the lake will have a volume of about 205 million cubic metres of water (UPDATED - the volume is now estimates as 211.6 million cubic metres as at 2 pm (loca) Wednesday. The lake level had 1.37 m to go before reaching the spillway)
  
• The channel that has been cut appears to be about 5 (now known to be 7 m) wide at the bed (though note that the film with the journalist in the bed suggested less than this) and about 15 m at the top. It is 12 m deep The bed is unlined and unprotected. It is formed in silty materials with a few clasts (stones);
• Failure can occur through any of three mechanisms:
• The water can reach the spillway and start to flow over the dam;
• The two known seepage points (total reported flow = 10 cubic metres per second) in the dam could cause collapse;
• A landslide into the lake could trigger a wave that would overtop the dam. A dangerous slope has been identified at Xuanping township.
  
• Downstream there are three further (smaller) landslide dams in the Beichuan area;
• The Chinese government has relocated about 250,000 people. In total, about 1.3 million would be threatened by total collapse.

Reuters image of the top of the dam showing the lake and the spillway under construction

These are the best and worst case scenarios for the dam as I see them:
Best case: water flows over the spillway and erodes it slowly. The lake level reduces gradually over a few weeks;
Worst case: a large landslide into the lake triggers a wave that overtops the dam, causing complete collapse. The entire volume of the lake is released very rapidly. The resultant flood pools behind and then breaks through the downstream dams, creating a devastating debris flow.

The most likely scenario at this stage is that water will flow over the spillway. A ball park flow rate appears to be about 100 cubic metres per second when there is no rainfall. This channel will not be able to cope with this as far as I can see, leading to rapid erosion and therefore collapse of the dam. I suspect that most of the lake volume will be released in a few days at most. The water is likely to pool briefly behind the landslide dams downstream, which will probably increase the size of the flood wave once released.

Where will the water go? I have tried to produce a quick Google Earth summary image below of the anticipated flow path (click on the image for a better view):-


The flood is likely to be large, fast and very damaging in the mountains, but should spread at least a little and slow on the plains. We can only hope for the best.

I am travelling to Nepal this afternoon, but will do what I can to keep this updated from there.

Tangjiashan - CCTV footage of the channel

There is a very good film (in Chinese) of the Tangjiashan landslide. It is available at:

http://video.sina.com.cn/news/c/v/2008-06-03/084017059.shtml

The first 6 minutes or so are worth a look. Early in the video they fly a helicopter over Beichuan, which shows the damage caused by landslides there. They then land the helicopter on the landslide mass and the reporter goes into the channel. It appears to be about 5 m wide at the base with steep walls. The material is silt by the look of it, with just a few clasts.

The Chinese goivernment has done a fine job to build a channel at all, but this is not going to be able to contain the flow.

Tangjiashan - is the dam leaking?

Quite alarming reports started to emerge yesterday that the dam at Tangjiashan might now be "leaking". For example, a correspondent in Japan, Hideo Ototake, sent me a link to this website, which suggests that a leak started at the bottom of the dam in the afternoon of 1st June. Reuters today is carrying this report, which states that:

"The largest "quake lake" formed by China's most devastating earthquake in decades is not expected to start draining until Thursday due to a lack of rain, state media said. The water level at Tangjiashan was 735.78 meters on Monday, only 1.24 meters higher than a day earlier, because of a lack of rain and multiple leaks on the natural dam made of mud and rock, the newspaper said."

Multiple leaks?

In addition, the Hideo Ototake also pointed me to a new and rather splendid aerial image of the lake that is available here (click on the image for a better view).

This image shows the lake and the dam, with the dry river valley below. As far as I can tell this image predates the construction of the channel. What is slightly alarming is the area at the toe of the dam that I have enlarged here:

This image might suggest that at the toe of the dam there is what appears to be a damp area? This is very unclear, but this is one interpretation. Is this the start of seepage?

There are strong reasons for concern here. If the dam is showing signs of seepage then there is a risk that the dam will fail through this mechanism. This is a worse case scenario as collapse under this mechanism can be very rapid indeed. Failure is not necessarily catastrophic, but there is a reasonable chance. So if the flow rate starts to increase through these seeps then the authorities will need to move all of the population in the affected areas without delay. Note also that if seepage has started then this suggests that the core of the dam is more permeable than I had anticipated. This is not good news at all when the overspill channel starts to erode.

Assuming that seepage failure doesn't occur the spillway is expected to start handling flow on Thursday at the latest. It will be an interesting couple of days.

May 2008 Landslide map

The map below shows the distribution of fatal landslides for May 2008. At the moment I am not including the landslides associated with the Sichuan earthquake as I cannot yet estimate the impacts of them. Clearly the numbers of landslides and the numbers of fatalities will dwarf the numbers below. I will try to ascertain the magnitudes in due course, though this is very challenging. I have included reported rainfall-triggered landslides in the aftermath of the earthquake, though again this is likely to be an underestimate.

The statistics are:
Number of fatal landslides (excluding those triggered by the Sichuan earthquake): 21
Number of fatalities: 280

May was once again very substantially below the average number of fatalities for may in 2003-2007, which is 336 deaths . In my post on the April figures I said that May "could be an interesting month!" I was right, but for the wrong reasons. The distribution is again fairly well-scattered, but as we come into the wet season it is increasingly focussed on Asia. The red triangle represents Sichuan.

May 2008 fatal landslide locations (click for a larger version)

The patterns that were starting to emerge in the annual map last month are now strengthening. In particular the inevitable clusters in the Himalayas, western Central America, the Philippines and Indonesia are now clear. The cluster in the Himalayas is still focused on the west side at the moment as this area gets heavy rain in the early part of the year. The monsoon rains have now started in the very southern part of the Indian subcontinent and will now start moving northwards. This will drive the growth of this cluster. There are certainly more landslides in Central America than is normal.

2008 fatal landslide locations to the end of May (click for a larger version)

So what of the month ahead? We, expect to see the number of fatal landslides increase markedly, especially in Asia. As the rains start in Central China landslides seem inevitable. As the La Nina conditions fade, we might expect to see fewer landslides in South America, but more in Asia.

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.