30 May 2012

More information about the Seti River landslide and debris flood in Nepal

Posted by dr-dave

Over the last few days additional information has come to light about the Seti landslide in Nepal, which killed between 30 and 72 people (I remain confused about the actual number of deaths in this case).  I will summarise these below:

NASA imagery of the landslide and the track of the flow down the Seti River

Our colleagues at NASA have been very proactive in using their full range of satellite tools to collect imagery of this event.  Their latest post examines the track of the landslide debris and reinterprets the event.  The magnitude of the impact of the debris and water wave is quite stark in their latest image:

An eyewitness report of the aftermath

Meanwhile, Robin Marston, a resident of Nepal, contacted me to let me know about the damage to Kharapani, which is shown in the image above.  In his email he notes that:

“The Kadoorie built suspension bridge near the top of the valley is at least 70 mts long and the valley (gorge) at that point is some 90 ms wide. The bridge lies some 30 -35 ms horizontally above the normal level of the river. There is flood debris lying on the bridge and side netting was damaged”

This image shows the bridge and the damage that it has received:

This is the Kharapani area before the debris flood:

And this is the same place afterwards:

An ICIMOD report on the landslide

Meanwhile, ICIMOD also sent a team into the field and they have been working on satellite images of the area in question.  They have correctly identified a landslide in the upper reaches of the Seti River that progressively developed prior to the major collapse event, and speculate that it was responsible for the debris flood.  This is a nice piece of work, but none of the imagery that I have seen indicates that a barrier lake was present (though to be fair detecting such a feature in this terrain may be very challenging).  The coincidence in timing of the large landslide on Annapurna IV (timed using the seismic data and from the aerial images) and flood means that this is almost certainly the cause.  However, this other landslide may well have contributed material and water to the debris flood, intensifying its impacts.

An alternative view of the event

Many of you will have seen the comments on my earlier post from Dr Ranjan Dahal, who strongly indicated that in his opinion the interpretation that Colin Stark and I had made was incorrect.  I exchanged emails with Dr Dahal, who I consider to be a friend, and he has now posted online a report and interpretation.  I will not comment on this interpretation, or indeed on the apparent references to our work, except to say that I stand by the interpretation that we have previously posted.  I would also note that I am not sure why there is a reference to seismic triggering of the landslide – I have seen no suggestion that this was a mechanism for the initiation of the failure.  Colin’s work is based upon the seismic signature that the landslide generated, and this does not imply a seismic trigger (indeed quite the opposite).

I will point out two really interesting aspects of Dr Dahal’s report though.  First, he provides a photograph of the landslide site that shows the source of the rockfall that initiated the sequence.  This location is entirely consistent with the satellite images that show a section of missing ridge after the event, although I am not sure that I would describe this event as small.  Second, he also provides an image of the canyons down which the landslide debris travelled to reach the Seti river.

Mainstream media and the landslide

Finally, the Himalayan Times in Nepal have posted an article on the Seti River debris flood that heavily quotes the work that Colin and I have been undertaking on this event.  It is nice to be quoted, even if the nationality of both of us has been changed!