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:
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.
I agree wirh Dave, the critical conditon is on flood, specially on the 5 years or above return periods flood which depends on formation of landslide. Actually there are no good solutions to make stronger spillway when surface slope is steeper than 1/10. From my experiences, to use very big rock is better than to use Gabion which shown on News Report in China.
Kung,I agree that the challenges are epic here. I cannot see how gabions (these are steel baskets filled with rocks) can be constructed in time, so I suspect that rip-rap (boulders that slow the flow and armour the bed against erosion) are the only option. Unfortunately obtaining enough boulders of sufficient size is not straightforward at this location.
Dave:I agree with you. The big boulder should be used for bed protection. Another critical situation is development of seepage, so far we do not know anything on seepage.
Dear Kung,I have read the length of proposed channel is about 200yards (183m) which is small compared to similar rockslide dam triggered by heavy rain fall, La Josefina Rockslide dam of Ecuador, 1993 (407m). So we can guess the base length of the dam may be small compare to other dam. The permeability of landslide dam triggered by earthquake may be also very high compared to that triggered by rainfall. From these facts we can not ignore the potential failure by seepage. However, the rapid rise of water level in the lake, probably the landslide dam will be overtopped before seepage failure.
Dear Ripen:yes, indeed the overflow may come first before the seepage development. According to data of Tsaoling Landslide due to Chi-Chi earthquake 1999, the seepage developed 2 months later on three places with small trace, the overflow was only 40 days after impounding of lake. The vlume of Tsaoling landslide lake was around 465 millions cm, the height was around 60 m(pounding depth). The breach of Tsao-Ling landslide lake was 2 years later on 2001 caused by heavy flood from typhoon Taoraji.
Dear Kung,Thanks a lot for the information regarding Tsao-ling landslide dam. In literature, I did not find linkage between Tsao-ling landslide dam and typhoon Toraji. The volume of lake was also just about 46millions cubic meter. Please give me some references about seepage developed in Tsao-ling landslide dam I am interested to know more about it. “The Chingshui River was once again dammed. The landslide material blocked the gorge of the Chingshui River along a distance of 5 km (Water Resources-Agency in Taiwan, 1999). The debris dam had a height of 50 m upstream and 150 m downstream. The Tsaoling Lake, having an estimated capacity larger than 0.046 km3, developed (Cheng, 2000). An emergency spillway was constructed through the plugged section of Chingshui River valley.Finally, overflow of the impounded water commenced on December 22, 1999, without causing damage to the debris dam. Check dams have also been constructed downstream across the Chingshui River, for protection against debris flows due to possible dam failure (which has not happened). The natural dam resulting from the landslide, however, was subject to severe erosion, mainly deep incision of its downstream portion by the Chingshui River.”(Reference: Rou-Fei Chen, Kuo-Jen Chang, Jacques Angelier, Yu-Chang Chan, Benoît Deffontaines, Chyi-Tyi Lee and Ming-Lang Lin: Topographical changes revealed by high-resolution airborne LiDAR data: The 1999 Tsaoling landslide induced by the Chi–Chi earthquake, Engineering Geology, Vol.88, pp.160-172, 2006)
Dear Ripen:Unfortunately, most materials for Tsao-Ling landslide dam was written by Chinese.After landslide happened at Tsao-Ling, the emergency spillway was done as other cases in the world. The giant boulder was used on the crest of spillway. on 1999, the crest was erode about 2 m. on 2000, the crest was erode about 8 m with Typhoon Toraji. The crest was consequently erode through following years to 2004. After 2004, the crest heigh was erode around 20 m and the lake was deposited also by the sediment of floods. Currently, there is no storage on the landslide lake, the river is still reshaping its profiles on the surface of Tsao-Ling deposition area.