Monday, July 8, 2013

A scar on Annapurna that had caused the Seti Flood of 2012

I had been going through different articles and papers about the Seti Flash Flood of 2012 and, I wanted to try out something for myself.

On May 5th of 2012, a flash flood full of debris, triggered in the Seti river, had devastated different parts of Pokhara with an estimated 71 people killed. A unique thing about this flood was that, it was probably triggered by an avalanche near the Annapurna range. This triggering of the avalanche was actually captured in tape by a Russian pilot who was flying over the same area at the time of avalanche, and about 40 minutes after the avalanche, the devastating  flood hit Pokhara. One of the most comprehensive reports on the cause of the flood should be the report prepared by Dr. NP Bhandary, Dr. RK Dahal and Prof. M Okamura based on their field visit (Bhandary et al. 2012).

Initial speculations on what might had caused the flood ranged from intensive rainfall-cloudburst flood, possible GLOF (Glacial lake outburst flood) as well as LDOF (Landslide Dam Outburst Flood). But no significant evidences supporting the latter two were found in the field. Also, the flood had occurred way before the monsoon, reducing the possibility of cloudburst flood . Looking at the devastation and magnitude of the flood, initiation of such a flood would require a huge source of water (millions of cubic meters) and these speculations couldn't properly explain the source of such a huge quantity of water.

Now the question was: despite the coincidence in timing of the avalanche on Annapurna IV and the occurrence of the flood, could the avalanche explain the source of such a huge quantity of water for the flood? And, the answer was yes. Prof. Dave Petley of Durham University used Landsat images from NASA and estimated the size of the avalanche to be about 22 million cubic meters. Petley et. al also used global seismic data to estimate the mass of the avalanche debris.

After going through all of these, I wanted to see for myself, if I could find the scar left behind by the avalanche; in fact 22 million cubic meters is indeed a huge amount.

NASA does provide most of its Landsat image data for free. I found two clear Landsat images of the avalanche site: one of 7 April 2010 (i.e. two years before the avalanche) and the other of 10 May 2013 (a year after the avalanche). I compared the two Landsat images, and voila, I could see the difference (the scar). So 22 million cubic meters of avalanche did leave a, somewhat significant, scar on Annapurna that could even be detected from space (I was referring to the fact that Landsat images are actually satellite images). No wonder, other mountains also get their shape from scars like these.

The following figure shows the images that I compared to locate the scar of the avalanche. The coordinate of the scar was at 28°31'24"N - 84°04'38"E.  My estimated horizontal projected area of the scar was about 30 thousand square meters (i.e. the area of the red polygon shown in figure (e) below). Assuming the avalanche mass to be trapezoidal, the height of the mass could have ranged from 1000m to 2000m, to give a net volume of about 22 million cubic meters.

Figure: Different Landsat images of the site where the avalanche, that caused the Seti Flood 2012, was triggered.
(a) unannotated Landsat image of 7 April 2010;
(b) unannotated Landsat image of 10 May 2013;
(c) same Landsat image of 7 April 2010 with annotation. Dark line represents ridge line corresponding to the 2013 image;
(d) same Landsat image of 10 May 2013;
(e) enlarged Landsat image of 7 April 2010 with annotations. Red polygon representing the land mass that was swept by the avalanche.

Following is also a Landsat image of 2010 that I've annotated, showing an overall plan of the Seti basin along with the location of avalanche near Annapurna (indicated in red) and the city of Pokhara (which was affected by the flash flood).

Figure: Seti River Basin

Finally, here is a closeup photo of the aftermath of the avalanche (the scar). Image courtesy goes to Avia Club of Nepal and annotation courtesy goes to Bhandary et al, 2012.

Comparison of pre- and post-failure states of Annapurna IV southwest flank
The Seti Flood of 2012 was indeed a very devastating incident. Despite the nature of the flood, the cause of the flood had interested many scientific community, mainly due to its, somewhat, unique mechanism. Flash floods are very dangerous and there are many mechanisms that can trigger one. The devastated areas of Pokhara might have rehabilitated by now, but the scar left by the avalanche on Annapurna IV will remain there for some time now (may be until another scar overrides it:). For me, this was a good exercise on working with the Landsat data.


Reference

  • Bhandary N. P., Dahal R. K., Okamura M., 2012, Preliminary Understanding of the Seti River Debris-Flood in Pokhara, Nepal, on May 5th, 2012, - A Report based on a Quick Field Visit Program, ISSMGE (International Society of Soil Mechanics and Geotechnical Engineering) Bulletin: Volume 6, Issue 4, pp 8-18. 
  • Shrestha, A. B., Mool, P., Kargel, J., Shrestha, R. B., Bajracharya, S., Bajracharya, S., and Tandukar, D., 2012. Quest to unravel the cause of the Seti flash flood, 5 May 2012, ICIMOD, http://www.icimod.org/?q=7377,

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