How Indian and Global Satellites Helped Reconstruct Nepal's Deadly Flash Flood

After a devastating flash flood struck Nepal's Rasuwa district on Aug. 26, India's National Remote Sensing Centre (NRSC), part of ISRO, began comparing satellite images taken before and after the event to reconstruct what happened in the high-altitude terrain along the Bhote Koshi-Trishuli river system.

NRSC's preliminary assessment compared imagery from Europe's Sentinel-2 satellite on Aug. 24 with post-disaster imagery from ISRO's Resourcesat-2A AWiFS sensor on Aug. 26. The post-event image identified what NRSC describes as an ice-rock avalanche in the upper reaches, suggesting the avalanche or a landslide may have temporarily blocked the river before a sudden release of impounded water and debris. A separate comparison of Resourcesat-2A images from April 4 and Aug. 26 showed the Bhote Koshi considerably swollen, with flooding extending beyond its usual channel.

Seismic data added another layer of evidence. The U.S. Geological Survey said seismic waves from the event pointed to a glaciated mountain cliff on the north side of Langtang Lirung, estimating that the initial glacial collapse released seismic energy equivalent to a magnitude 5.2 earthquake, with the resulting debris flow and flooding travelling nearly 100 kilometres downstream. A second seismic event, equivalent to a magnitude 4.2 earthquake, was detected about three hours later.

The International Centre for Integrated Mountain Development (ICIMOD) reported that the Trishuli River at Galchhi rose by as much as 9 metres in 30 minutes, while water levels at Malekhu rose 7 metres over a similar period, damaging or washing away several monitoring stations. Both NRSC and ICIMOD stressed that their assessments remain preliminary and that the precise trigger is still under investigation.

Early reports had raised the possibility that an earthquake triggered the disaster; the seismic signals are now being examined as evidence generated by the glacial collapse itself rather than necessarily its cause.