Flood Warning Systems Failed In Nepal. Can Satellites Strengthen Them?

When a towering wall of sludge swept through settlements along the Bhote Koshi river on August 26, communities had little warning of what was coming. The flash floods, triggered by the collapse of an ice-rock formation along the Nepal-Tibet border, killed more than 1,000 people.

The warning reached some communities only after the glacier collapse had already triggered the flood, according to Nepali media reports. With the threat of similar events looming across the Himalayan region, scientists and geospatial experts are now examining whether satellite-based radar monitoring could help detect warning signs earlier and strengthen early-warning systems.

An NDTV Datafy analysis of Sentinel-1 Synthetic Aperture Radar (SAR) data suggests that the ice-rock formation that eventually collapsed had been moving downhill well before the disaster. The analysis tracks changes in the position of ice and rock across individual pixels.

Blue pixels indicate potential downslope movement, while red pixels indicate accumulation of ice or mud. The map below shows potential sinking or slipping of the glacial rock system. The movement appears to have been more concentrated in the section that eventually broke away.

SAR satellites are among the most useful tools for remotely monitoring glaciers and unstable terrain. A SAR satellite sends microwave signals towards the Earth's surface and measures the echoes that return. Unlike optical satellites, which rely on visible or infrared light, SAR can collect observations during both day and night and is less affected by cloud cover.

That makes it particularly useful in remote and frequently cloud-covered mountain regions. By comparing radar observations of the same glacier taken at different times, researchers can detect changes in the position of the ice or surrounding terrain, sometimes down to millimetre-scale measurements.

One technique, known as Interferometric Synthetic Aperture Radar, or InSAR, uses differences between radar observations to measure surface deformation. It can help identify whether a glacier or slope is relatively stable, accelerating or slowing down.

For our analysis, we used Line-of-Sight (LOS) velocity, which measures movement towards or away from the satellite along its viewing direction. There is, however, an important limitation. A satellite does not measure the full three-dimensional movement of a glacier in a single LOS measurement.

Movement that occurs perpendicular to the satellite's viewing direction may therefore be underestimated or missed. Analyses by researchers in India, China and the US have also pointed to movement in the area before the collapse, although estimates of the speed and scale of that movement vary.

Manoochehr Shirzaei, a professor at Virginia Tech, said on X that the break-off glacier had been moving downslope at an average monthly rate of around 10 mm since January this year. A researcher at China's Chang'an University, analysing data from NISAR, the NASA-ISRO Synthetic Aperture Radar satellite mission, said the cumulative motion in the glacier's break-off area was on a "meter-scale".