14 August 2015
Mangdechu in Bhutan – another significant landslide at a hydroelectric power plant site
Posted by Dave Petley
Mangdechu in Bhutan (see also update below)
The Mangdechu hydropower project is a major hydroelectric scheme under construction in Bhutan. A joint Bhutanese – Indian project, it involves the construction of a 56 metre high concrete dam and associated infrastructure. Construction at Mangdechu began in June 2012 and commissioning of the plant is expected to be complete in September 2017.
Yesterday the project suffered a significant landslide, burying five workers at the site. The Bhutanese newspaper Kuensel has been covering the story via twitter and in its online edition. It appears that the landslide occurred when the wall of the construction pit for the dam collapsed. Fifteen workers were in the pit at the time; whilst ten escaped, five were buried. It is highly unlikely that they have survived, although work to recover them has been suspended because of the risk of further collapses.
The Minister for Economic Affairs in Bhutan, Norbu Wangchuk, tweeted that the Mangdechu landslide occurred in overburden (i.e. soil and broken rock) from the top of the slope (as opposed to being a failure in the rock mass itself). This image apparently shows the landslide debris; if correct then this does appear to be predominantly overburden. This is not a large landslide by any means, but it doesn’t take much to kill people:-
Sadly this event is just the latest in a very long line of fatal landslides at construction sites for major hydroelectric projects. Two years ago I gave a presentation at the Vajont 2013 conference in Padua that highlighted the numbers of landslides that are occurring at hydroelectric power plant construction sites, especially in the high mountains of South Asia and China. I also wrote a paper on this topic (Petley 2013) that can be downloaded for free. Since then there have been many more landslides that have caused loss of life at major hydroelectric power plant construction sites. I remain concerned that this indicates that the slope hazards are being inadequately understood and managed. I fear that it is a warning of a much larger accident yet to come.
Interestingly, this week Scientific American has published an article highlighting the risks to these dam projects associated with earthquakes, questioning whether the structures could withstand a large shock. Of course large earthquakes have the potential to cause massive landslides on the reservoir flanks that could cause waves that overtop the dam. Given our limited ability to assess slope behaviour during large landslides, the risk should be taken seriously.
The Tibet Earthquake Twitter account highlighted a BBS article from 15th July 2015 about a blockade of the Mangdechu site by truck drivers. It includes the following quote:
Trucks, ferrying cement from Dungsam Cement Corporation and Phuentshogling, started arriving about five days ago. However, they have not been allowed to unload the cement after a landslide at the dam’s pit…MHPA’s Dam Chief, Karma Chophel said they never instructed Jaiprakash Associates to stop the work. He said the contractor stopped the work to avoid the risks from falling boulders from the landslide at the dam site. Karma Chophel also said Jaiprakash construction is steel-netting the area to arrest the falling boulders.
Petley, D.N. 2013. Global losses from landslides associated with dams and reservoirs. In: Genevois, R. and Prestininzi, A. (eds) International Conference on Vajont – 1963-2013. Thoughts and analyses after 50 since the catastrophic landslide. Italian Journal of Engineering and Environment – Book Series N. 6, pp 63-72.