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.

How Radar Satellites Track Glacier Movement

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.

What Other Satellite Analyses Found

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".

"The 3-D views further highlight how the deformation was concentrated on the steep source slope," the researcher wrote.

Spatial Sense, a Chennai-based geospatial intelligence company, estimated that the area in question sank by nearly 51 mm per year between April and August this year, compared with around 18 mm per year during the 2025 snow-melting season.

The estimates differ because the studies use different datasets, methods, time periods and measures of movement. But they point to the same broad question: whether persistent deformation at vulnerable Himalayan sites can be detected early enough to trigger action downstream.

Can Satellites Help Prevent Future Tragedies?

The apparent movement of the glacier before its collapse has renewed discussion within the geospatial community about integrating satellite observations into early-warning systems.

But detecting movement is not the same as predicting a collapse.

Satellite monitoring can identify deformation, changes in glaciers and lakes, and other signs of instability. Turning those observations into a reliable warning requires them to be combined with information from the ground and an understanding of how a particular slope or glacier behaves.

Dr Nithiyanandam Y, head of the Geospatial Research Program at Bengaluru-based Takshashila Institution, said geospatial intelligence could strengthen early-warning mechanisms for flash floods of the kind seen in Nepal.

"A combination of optical satellite imagery, radar data, InSAR and LiDAR, river flow measurements, seismic records, and high-altitude weather observations can all assist in tracking slope movement, glacier and lake changes, debris movement, and downstream exposure," he wrote in a newspaper editorial.

Experts caution that satellite monitoring cannot predict every landslide or glacier collapse. The priority, they say, should be identifying locations where instability could have severe consequences downstream and monitoring them continuously.

Raghav Asawa, an IIT Madras alumnus and founder of Spatial Sense, said the practical approach would be to focus monitoring on critical locations.

"The pragmatic approach would be to monitor critical locations like glaciers on the edge of mountains or those lodged at steep slopes," he said.