Thread by @WaterSHEDLab

On Feb 7, a torrent of water, debris, & ice cascaded down Ronti Gad, Rishiganga, & Dhauliganga valleys in Uttarakhand, India. Today in @sciencemagazine, @waterSHEDlab & colleagues provide a comprehensive explanation of the event & ensuing disaster 1/n
science.sciencemag.org/lookup/doi/10.…

204 ppl killed or missing. The event was a disaster not b/c it was big but b/c of location of infrastructure, which placed people in harm’s way. Social scientists & policy experts @disastrousComms & @upadhyay_cavita pieced together the tragic human toll. (pic @irfansalroo) 2/n

I first learned of the event in a tweet by @bhambrirakesh. In the days following event, there was a LOT of confusion about what happened (GLOF? “glacier burst”?). I summarized what we knew at the time, which was also included in @davepetley’s @theAGU Landslides Blog. 3/n

A lot was learned from eyewitness videos posted in real time. #AdamEmmer from @tugraz pieced together the timing of the event from many of these videos. 4/n

Our study (tinyurl.com/yxyecawn) combined satellite remote sensing from @planet @Maxar & @cnes & @Airbus to understand how the event unfolded, including geomorphic changes along the river valleys, and damages to infrastructure. 5/n

Initially, the cause was assumed to be a glacial lake outburst flood, or GLOF, but it was quickly determined (primarily from @planet satellite imagery) that it was a landslide, as shown in this block diagram from @cscottwatson 6/n

https://twitter.com/CScottWatson/status/1358477839281102850?s=20

High-resolution DEMs produced by @etienneberthie2 @sgascoin @csar_db @uwTACOlab @RSGshashank helped to determine the volume of the initial landslide, the erosional and depositional patterns, etc, including at the newly formed lake in Rishiganga river (R pic @HindustanTimes). 7/n

We determined that at 4:51 UTC (10:21 IST), ~27 million cubic meters of rock and ice collapsed from Ronti Peak at an elevation of 5,500m, hit the valley bottom about 1,800m below, and flowed downstream very rapidly, transforming into a debris flow as ice melted. 8/n

Numerical inversions by @max_VWDV estimated that ~80% of the failure was bedrock and the remaining 20% glacier ice. This ratio, and the extreme fall height, is important in understanding why the flow was so mobile 9/n

.@JLKavanaugh @jeffreyskargel & #AndyKääb estimated frictional heating during descent supplied enough energy to melt almost all the ice, allowing the flow to transform to mobile debris flow. This was a ‘worst case’ combination of fall height & rock:ice ratio 10/n

Modeling by #MartinMergili & @glacierhazards showed flow changed character near the confluence of Ronti Gad & Rishiganga. On inspecting DEMs, we found massive pile (~8M m3) of sediment had been deposited here, allowing flow to become more fluid. tinyurl.com/yxyecawn 11/n

Field visits by @wadia_institute (@amitkumarwalia), @kmrUniversity (@irfansalroo) & @iitiofficial (azam_mf) documented massive erosion, including destruction of a temple at confluence of Rishiganga and Dhauliganga rivers. 12/n

Important Q: did #climatechange play role in #uttarakhanddisaster. TLDR: hard to know. Experts @MyleneJac @risk_climate suggest that elements such as permafrost thaw & unusual weather may have been contributing factors, but attribution to a single event is v. difficult. 13/n

We DO know that #climatechange is making mountain disasters more common and we need to be working to understand how. @headundrr @ipcc_ch
ipcc.ch/srocc/chapter/…

This study was a massive undertaking, comprising 53 authors from over a dozen countries. In addition to those tagged above @WSchwanghart @glacio_cook @KaushalGnyawali @GlacierResearch @rock_avalanches @yaboobsufi @mattwestoby played important roles. n/n

Finally, a huge thanks to @BrentGrocholski for shepherding this paper through review. (n+1)/n