Why 1255 flash flood in the Seti River? - a brief report
Ranjan Kumar Dahal PhD, Department of Geology, Tribhuvan University, Ghantaghar, Kathmandu, Nepal
Netra Prakash Bhandary PhD, Department of Civil and Environmental Engineering, Ehime University, Matsuyama, Japan
Mistu Okamura PhD, Department of Civil and Environmental Engineering, Ehime University, Matsuyama, Japan
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Dahal R.K., Bhandary N.P and Okamura M., 2012, Why 1255 flash flood in the Seti River? - a brief report published in www.ranjan.net.np.
(Updated for triggering mechanism on 2012/5/27)
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On May 5 2012, a devastating flash flood in the Seti River killed more than 40 persons and 30 persons are still missing. We named this flood as "1255 flash flood in the Seti River". Our team (Prof. Dr. Mitsu Okamura, Dr. Netra Prakash Bhandary and Dr. Ranjan Kumar Dahal) has visited the site on May 18-19th 2012. After field survey and aerial survey (flying over the source area), we found following facts about the 1255 flash flood in the Seti River:
Based on the available pre- and post-disaster photos, we assume that the story of a massive avalanche in "Annapurna Greater Depression" is true and eyewitness is there, but it was not only a snow avalanche but also a debris-mixed snow avalanche. Falling off elevation of about 6,000 meters, the slid snow (ice pieces) and debris (Fig 1), which might have been close to melting point produced frictional heat leading to sudden slurrification, probably within 5-10 minutes. In the transportation zone of avalanche (about 3500 m), the avalanche also hit huge moraine and glacial flour (rock flour) deposit with ice-capped peaks inAnnapurna Greater Depression (Fig 2), some of which can still be seen from the sky, which also enhance to make slurry-debris. Finally, the the slurry-debris fell into the gorge (Fig 3). We can see destroyed tree trunks in mass near the confluence of Seti river and its tributary (southern part of Annapurna Greater Depression) at an altitude of 2,800 m (Fig 4). We suppose that it happened because the debris slurry hit the standing trees. We also felt that millions of cubic meters of debris slurry was transported in this event, but by the time it arrived in Pokhara city area, we could only see muddy flood water mixed with fine silt, which is found as similar to glacial flour deposit at the source of Seti River itself (Fig 5). Our photos indicate that the material in the Seti River source point and its downstream near Pokhara is the same.
Fig 1. Source of avalanche and transportation zone. Huge glacial flour deposit and its eroded peak are also seen on the photograph. Probable small rock fall from Annapurna IV range might be responsible to trigger snow avalanche. Although the scarp seems fresh than other place, it is hard to confirm the rock fall because small peak situated on right side of same ridge also has fresh scar.
Fig 2, Ice-capped peak of rock flour or glacial flour. Yellow lines are tentative boundary of ice and glacial flour deposit.
Fig 3, The Seti gorge where the snow slurry-debris fell and flash flood originated. View from south to north.
Fig 4. View of narrow Seti gorge and debris accumulated area. View from north to south.
Fig 5, Similarity in color of debris in source and in deposit at Kharpani.
There are no traces of landslide damming in the upstream, but the gorge is so deep that nothing can be seen from the sky and it is possible that the damming occurred in the gorge itself due to debris deposit that came from the avalanche source. However, the time difference between occurrence of the avalanche and arrival of the debris flood in Kharpani area, the place where a village was buried and a few foreigners were also killed, is very evident that no landslide damming and its bursting could be so devastating in merely 40 minutes. It is noticed that the avalanche occurred around 9:00AM (we can watch the video availed by Avia Club Nepal, which seems to have been taken by the Russian pilot a little before 9:00AM, 5 May 2012), and a photo taken by picnicking college boys (Fig 5) at Tatopani area (Kharapani area) at the time the debris flood arrived here indicates that it was taken at 9:38AM. It is more than evident that the debris flood traveled about 20 km (elevation difference: about 1,800m) in about 35 minutes, which is a very justifiable speed of a debris flow.
Fig 5, Before and after situation at Kharpani, Tatopani (Photo courtesy: A student who was planning a picnic at Khapani with his friend, we are not aware of his name)
Some international researchers are also mentioning that it is rock fall and it blocked the river to store a huge amount of water that later converted into a massive debris flow. We could not notice this event in our aerial survey. The aerial survey we conducted over the Seti River source area, where we could see various amazing things, for about 30 minutes revealed that there are no traces of rock mass failure, mainly because if it did take place, we could have seen a few rock debris (boulders) in the area. All that 2000 m rock slope (probably thousands of cubic meter in volume) cannot immediately turn into pieces and dust. What we could also see on the slope were traces of debris movement, and the dust mentioned in image provided by NASA, is probably transportation zone of snow avalanche over glacial flour. Our image analysis (photographs as well as video) shows that there was no massive rock mass failure deposit. We have received the photo from Avia Club Nepal and found that there is no rock debris in the area immediate after avalanche (Fig 6). Yes, there is some rock fall like scar but when we compare before and after photo, we feel that the slope might have failed much before than the 5th May disaster event. If this is a huge rock fall, where did such rock debris gone? Meantime, if we check Fig 1, small rock fall from Annapurna IV ridge is possible and it might be responsible to trigger snow avalanche on down slope.
Fig 6, The site immediately after the avalanche. We should notice rock boulders on slope if it is rock fall event (Photo courtesy:Avia Club Nepal)
We are sure that a rockslide or massive landslide in downstream cannot be the cause of this disaster. The interpretation made by ICIMOD (Fig 7) is also wrong because the point identified by them as a landslide and landslide dam is not a recent landslide. Most interestingly, Seti River does not have much water (on 18 May 2012, we estimated at Kharapani by thumb rule that it had some 7-10 cumec water flow, which was obviously not more than this on the day of disaster, and it might have been much less than this in the upstream). So, we cannot expect so much of water to have been stored by a landslide dam in just 5-10 minutes or say 15 minutes.
Fig 7, There is no landslide dam. Previously a report distributed by ICIMOD mentioned that this landslide built a dam in seti river and the flash flood is due to failure of landslide dam.
The reason why there were 7-8 waves of flood may be damming up by derbis-mixed ice mass and gradual melting or occurrence of repeated avalanches for a few hours which were accumulated in very narrow gorge of the Seti River.
The chances of seismic cause for triggering the avalanche cannot be totally denied, but we know the avalanche was a regular process and this time it triggered a massive failure which was prepared by long-term deterioration of slope surface material. We also agree that a tiny earthquake might have also been a triggering factor of snow or debris avalanche. But there is no announcement from National Seismological Center of Nepal.
We prepared this report from our field observation and we always welcome new findings and evidence if different than this. We believed that we know just a tiny part of our natural processes although we claim our self as experts and this event is also a great mystery to us. New findings are always welcomed and we will be happy to revise this report as and when required.
Our greatest appreciations go to Avia Club Nepal for the help they have provided to us in this field visit, not only in relation with the business hospitality but also in making us understand the real situation of the Seti disasters. The whole team in Avia deserves our greatest appreciations. The cooperating pilots of Avia Club Nepal as well as their inner will to help us find the fact have truly impressed us.
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