27 May 2008

Updated 28th May: Sichuan landslides dam – alarming video

Posted by Dave Petley

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


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.

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