The Touzhai rock avalanche

On 23rd September 1991, at 18:10 local time, the catastrophic Touzhai rock avalanche occurred  Zhaotong, Yunnan Province in SW China. This was a large event – the estimated volume of the Touzhai rock avalanche is 15 million cubic metres, and the landslide had a runout distance of over 3.5 km.  After descending a vertical distance of about 950 m the landslide destroyed 202 houses in the village of Touzhai, killing 216 people.  In a paper in the journal Landslides, Xing et al. (2015) (and with a stellar list of authors) have provided a detailed description of this landslide, and have modeled its movement using DAN-W software..

The landslide occurred after a prolonged period of heavy rainfall.  Failure occurred on a steep slope in basalt over an area that was 900 m long (in terms of the slide direction) and up to 700 m, with a thickness of between 5 and 120 m. This is the headscarp are as shown in the paper:-

The headscarp of the Touzhai rock avalanche, from Xing et al. 2015

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The structural control of pre-existing joints is quite clear.  This looks to be a rather complex wedge type failure.  This mass fragmented and transitioned into a rapid flow that travelled down the valley to hit the village below.  The modelling of the landslide suggests that it had a peak velocity of just under 50 m/sec and an average velocity of 21 m/sec, giving a duration of the Touzhai rock avalanche of 175 seconds from the initial rupture event.  This is the image of the landslide itself from the paper:

The headscarp of the Touzhai rock avalanche, from Xing et al. 2015

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This landslide highlights one of the major issues managing the hazard associated with rock avalanches.  The flow initiated in hard rock, but not on a near vertical cliff, over 3 km from the village.  Identifying those areas that might be susceptible to such a failure is really difficult, and forecasting the likely runout speed and distance of the flow is also an enormous challenge.  We still have a lot of work to do in this area, but detailed studies such as this are a great help.

Reference

Xing, A, Wang, G., Yin, Y. Tang, C., Xu, Z., and Li, W. 2015. Investigation and dynamic analysis of a catastrophic rock avalanche on September 23, 1991, Zhaotong, China. Landslides, 13 pages. Published online 11th August 2015.