Investigating the 7th February, 2021 Landslide Triggered Flash Flood in the Himalayan Region Using Geospatial Techniques

Abstract

On 7th February 2021 just before noon, news reports came in regarding a flash flood in Rishi Ganga/Dhauli Ganga River in Chamoli district of Uttarakhand state, India. This brief report puts forth the probable causes for this flash flood that has originated in the Nanda Devi Biosphere Reserve using geospatial datasets and techniques. Datasets obtained from MODIS, Sentinel-2B, SRTM, ICESat-2 and ERA5 have been effectively utilized to infer the details about this event. Slow drizzle to severe snowfall has been witnessed during 3rd to 6th February 2021 in various parts of the Himalayan region; even the Rishi Ganga witnessed a heavy snowfall during this time. Data acquired on 10th February shows a scar developed due to a landslide on the shoulder of Ronti Mountain that was situated on the western rim of the Nanda Devi sanctuary. There was a gradual rise in temperature on 7th February 2021 at the surroundings of Ronti Mountain that consequently led to a landslide. The landslide perpetuated a movement under the influence of gravity from ~5900 m to ~3900 m with a mass envelope of ~0.290 km² and a velocity of 198 m/s that may have taken ~20 seconds to hit the Ronti bank. Due to the virtue of heat energy generated during this process resulted in contributing huge moraine filled flood water, that has accelerated towards the downstream of Rishi Ganga River and there after Dhauli Ganga River. Elevation profiles from the ICESat-2 and satellite imageries confirm the pre-existing conditions of landslide that is inclusive of weathering and erosion that led to the unstable condition at transportation back-slope of the Ronti Mountain. The triggering factors that influenced this landslide event and related causes were investigated in this study and reported herewith.