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Friends,
Since I'm very dissatisfied with most media reports on the geophysical events that generated the Rishiganga-Dhauliganga flood in the Garhwal Himalaya, Uttarakhand, India, on 7 Feb 2021, I'd like to use this thread to share my tentative understanding of the situation:
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Since I'm very dissatisfied with most media reports on the geophysical events that generated the Rishiganga-Dhauliganga flood in the Garhwal Himalaya, Uttarakhand, India, on 7 Feb 2021, I'd like to use this thread to share my tentative understanding of the situation:
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Following a snowstorm on 4-5 Feb and a sunny 6 Feb, the morning of 7 Feb saw the en bloc failure of a steep, partly ice-crowned, heavily snow-loaded periglacial rockslope on the north face of Ronti, just NE of its summit (NNE of Nanda Ghunti, NW of Trisul).
See @WaterSHEDLab.
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See @WaterSHEDLab.
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The ensuing rock+ice+snow avalanche descended northwards, crashing on the already thickly snow-clad bed of the supraglacial meltwater stream? (englacial in segments) that flows over (and through) a decaying glacier that descends N/NWwards from b/w Ronti Saddle and Trisul. 2/9
The above glacier gives rise to a north-flowing left-bank tributary of the Rishiganga River. The said crashed mass of snow+ice+rock debris from the detached Ronti rockslope, perhaps along with large quantities of water from the impacted valley snow and decaying ice... 3/9
... (and from any snow-covered supraglacial/englacial meltwater pools within it) rushed NW, then northwards along the glacial trough, transitioning from an avalanche into a viscous flood as it made its way into the NW-flowing Rishiganga River via the said left-bank tributary. 4/9
Speedily travelling northwestwards along the Rishiganga River to its confluence with the westward-flowing Dhauliganga River at Reni, the catastrophic avalanche-turned-flood hit the settlement at Reni as well as the small Rishiganga Hydropower Project there. 5/9
Beyond Reni, the flood followed the westward-flowing Dhauliganga River to hit Tapovan, where it also damaged the 520MW run-of-the-river Vishnugad Hydropower Plant. Beyond Tapovan, the Dhauliganga flows NNW to join the Alaknanda River at Vishnuprayag below Joshimath. 6/9
When my body is stronger and the winter snow thaws, I will try to trek up the Rishiganga and carefully survey the Ronti-Trisul-Mrigthuni section of its high-altitude catchment, especially the area around the failed rockslope on the north face of Ronti. 7/9
@WaterSHEDLab, @davepetley, @CScottWatson present strong evidence for the rockslope failure + avalanche situation. Note that there's absolutely NOTHING to suggest that a glacial lake outburst flood took place, nor any specific evidence of a glacier outburst around Nanda Devi. 8/9
I think the intense, extremely unusual SW monsoon-like tropical (warm) storm that arrived in the area from the Indian Ocean in the first week of Jan (and interacted with a usual extratropical 'western disturbance') also needs scrutiny for any role in slope destabilisation. 9/9
