Hyderabad: Satellite imagery released by ISRO’s National Remote Sensing Centre (NRSC) has provided the clearest evidence yet of what may have triggered the devastating flash floods in Nepal’s Rasuwa district, pointing to the collapse of nearly two-thirds of a Himalayan glacier upstream that unleashed a torrent of water, ice and debris downstream.The post-disaster images, captured by ISRO’s Resourcesat-2A AWiFS sensor on Aug 26 at around 10.30 am, show a vast glacier collapse zone, a swollen Bhote Koshi river and extensive inundation along the river corridor. NRSC compared the imagery with pre-event satellite data to assess changes in terrain and river systems after the disaster.The assessment indicates a major disturbance in the high altitude glacier and mountain terrain upstream of the affected region. According to NRSC’s preliminary analysis, an ice rock avalanche or landslide triggered the sequence of events, leading to the temporary blockage of a river channel and the sudden release of impounded water and debris downstream.NRSC estimates that nearly two-thirds of the glacier detached and collapsed. A post-disaster Landsat-9 image (dated Aug 26) identifies the area as the “zone of glacier breakage and collapse” in the high altitude Himalayan terrain upstream of the flood-hit region.According to the agency’s preliminary cause sequence, the collapse sent a massive volume of ice and debris rushing downslope and downstream. This was followed by rapid melting, inundation, and a powerful debris-laden flood surge. NRSC linked the sequence to the sudden flash floods and heavy debris flow along the Trishuli river system.River swelled after collapseBefore and after satellite images show a significant widening of Bhote Koshi river following the event. NRSC has marked several stretches of the river as “swollen”, with floodwater spreading beyond the normal river channel into surrounding valley areas.The affected zone lies in Rasuwa district near the Nepal-Tibet border. Rasuwagadhi, Timure and sections of the Trishuli river system are among the locations identified in the satellite assessment.The imagery also reveals a broad disturbed tract extending from the glacier and ice-covered mountain terrain towards downstream drainage channels. A Sentinel-2 image captured on Aug 24 showed the area as an intact glacier, while a post-event image from Aug 26 identifies the affected section as an “ice rock avalanche”.Debris surge capturedNRSC described the disaster as a sudden flash flood accompanied by a heavy debris surge along the Trishuli river system. Its preliminary assessment suggests that the glacier or ice-rock collapse may have temporarily dammed the river before the accumulated water and debris burst through and surged downstream.While the satellite images provide crucial clues about the trigger, NRSC said they do not quantify the total inundated area, debris volume, water discharge or damage to infrastructure. Parts of the affected region also remain obscured by cloud cover, limiting the extent of visible assessment.NRSC, however, said that its findings are preliminary and said a more detailed assessment will follow.
