Nepal's flash flood teaches the value of geospatial data

The devastating flash flood in Nepal on 26 August 2026 was an unprecedented natural disaster. Natural hazards are generally predictable, with earthquakes being a major exception. For predictable events, their progression can often be monitored and their potential impacts assessed. 

Cyclones, for example, can be tracked long before landfall, while annual river floods can usually be detected and forecast with considerable lead time. Flash floods and landslides may appear sudden, but they are generally associated with heavy rainfall that can be observed.

Nepal's recent flash flood was different. Its mechanism was so unique and multidimensional that there was virtually no lead time to prepare for the impact. Nepal already has a good flash-flood warning system, but it is primarily centred on rainfall events. A glacier collapse leading to a flash flood was not anticipated. Understanding this event highlights both the complexity of natural hazards and the importance of geospatial information in disaster management.

A flash flood generally requires two conditions: heavy rainfall in an upstream area and higher elevation, which creates steep slopes. Compared with flat land, water moves rapidly down slopes and can quickly reach downstream areas. If the underlying terrain contains porous rock and has limited vegetation cover, landslides can also occur. 

The speed at which a flash flood and landslide-induced debris flow reach downstream areas depends on river morphology. A straight, narrow river can accelerate the flow, while a wide, meandering river can absorb the shock and reduce its speed. In short, the characteristics of flash floods are strongly controlled by the geology and topography of the affected area.

Nepal's event was particularly unusual because it was a cascading event — a "domino effect" in which one event triggered another, creating a progressively more destructive chain of events.

The sequence began with an avalanche in the Himalayas at around 5 kilometres above sea level in northern Nepal. Because Nepal lies within the Himalayan terrain, elevation rises northward as the Indian and Eurasian tectonic plates collide. At these higher elevations, colder temperatures cause precipitation to remain stored as ice and glaciers rather than immediately flowing downstream.

The freshwater stored in Himalayan glaciers is the lifeline of downstream rivers during dry periods, which is why the Himalayas are often called the "Third Pole", after the Arctic and Antarctic.

Glacier melting is particularly relevant to this disaster. Normally, glacier melting is gradual and does not produce large floods. However, if meltwater is blocked by debris, a temporary lake can form. If that lake suddenly bursts, it can generate a Glacial Lake Outburst Flood, or GLOF. It is similar to a high dam suddenly collapsing and releasing a huge volume of water. When such water rushes down steep slopes and through narrow, relatively straight valleys, the impact can become much more severe.

Nepal's case was even more unusual because a large volume of meltwater was generated suddenly by the collapse of a glacier. There was no rainfall that day, so the huge volume of water involved cannot be explained by rainfall alone.

In the event of a similar disaster in Bangladesh or elsewhere, decision-makers would need answers to three basic questions: where, when and how? These questions are fundamentally connected to time and space, making location-based information crucial during disasters.

The event began early in the morning in the Langtang Lirung area of northern Nepal, where a glacier plunges about a kilometre into a valley. The immense friction and kinetic energy generated by the fall rapidly shattered the ice and turned it into meltwater. While earthquakes are generally responsible for such collapses, that does not appear to have been the case here. One possible explanation is the failure of the rock base supporting the glacier.

The collapse was so powerful that it generated an earthquake measuring 5.2 in magnitude. The associated energy may have dislodged large amounts of material through the resulting shockwave. The meltwater and debris rapidly formed a temporary lake. The lake existed only briefly before collapsing under the pressure of the large volume of water and the steep terrain.

When the lake burst, it carried rocks and boulders downstream at speeds of about 45 metres per second, or 160 kilometres per hour. As the flow moved downstream, it gathered additional sediment, gravel and boulders, becoming an increasingly heavy and destructive debris flow. A flow containing both water and solid material can cause far greater damage than water alone, much like the greater impact of hail compared with ordinary rain.

The result was a rapid sequence of interconnected events:

Glacier collapses into a deep valley — kinetic energy shatters the ice — shockwave dislodges rocks — debris forms a temporary lake — lake outburst caused by water volume and steep slopes — debris flow gains enormous momentum — downstream flash flood.

The entire sequence unfolded within a very short period. Because there was no associated rainfall event, people had little warning and were unprepared for the speed and scale of the disaster.

This brings us to the importance of geospatial data in managing natural hazards. In the event of a similar disaster in Bangladesh or elsewhere, decision-makers would need answers to three basic questions: where, when and how? These questions are fundamentally connected to time and space, making location-based information crucial during disasters.

Geospatial data provides this information by allowing large amounts of spatial information to be plotted on maps. Such maps support informed decision-making. We already use location-based information in everyday life. For example, people in Dhaka routinely use Google traffic maps to decide which roads to take. In the same way, geospatial data can act as a decision-support system during disasters. Without it, decisions are more likely to be flawed.

In Nepal's case, decision-makers first need a detailed map of the affected area showing the location and extent of the hazard, damage to infrastructure, emergency relief routes, search-and-rescue points and health centres. For this particular flash flood, another important feature would be the location of hydroelectric projects along the Trishuli River. The suitability of placing such projects in this area also deserves scrutiny.

Geospatial techniques, heavily supported by satellite data, are particularly useful for creating such maps. Satellite data can provide information across political boundaries and allow almost any part of the world to be analysed. Its applications extend across disaster management, security, geostrategy, climate change, crop productivity, deforestation and sea-level changes.

Satellite imagery can detect changes by comparing older and newer images, providing quantitative and context-specific information for damage assessment. Terrain characteristics such as elevation, slope and ruggedness can also be derived from satellite data, helping recreate the conditions that shaped a disaster.

Such tools are already widely used to monitor glaciers in the Himalayan region. Monitoring shows that Himalayan glaciers are currently receding at an accelerated rate. Climate change-driven glacier recession may have contributed to Nepal's glacier collapse, although this requires further field-based confirmation.

Geospatial data is also essential before large infrastructure projects are built. Dams, power plants, bridges, tunnels and other strategic structures require careful assessment of whether a site is suitable. Geological information helps identify both opportunities and constraints. Areas vulnerable to earthquakes, flash floods and landslides should generally be avoided, while simulation and modelling based on geospatial data can help assess project vulnerability.

Nepal can also adopt new geospatial technologies for future hazard monitoring. It already has institutions with strong capabilities in geospatial data management, including the International Centre for Integrated Mountain Development (ICIMOD), an intergovernmental organisation founded in 1983 and comprising eight Himalayan countries, including Nepal and Bangladesh. ICIMOD is highly active in monitoring the Himalayan cryosphere and associated hazards.

Yet the question remains: why did such an event go undetected?

The answer lies in the word "unprecedented." An unprecedented event requires new monitoring and management tools. The 2004 Indian Ocean tsunami offers a useful example: the disaster later led to a major overhaul of the Indian Ocean's tsunami warning system.

Nepal therefore needs to rethink how hazards are defined, monitored and analysed. It needs new tools, methods and specialised sensors suited to the monsoon-dominated Himalayan region. More broadly, disaster management should shift from traditional intervention-based approaches towards geospatial technology- and forecasting-based monitoring and management.

Without such a change in perspective, disaster management will not be sustainable. For Bangladesh, the limitations of past structural approaches are evident in the experience of coastal polders and other flood-management interventions. Similarly, large infrastructure projects must account for local geological opportunities and constraints. If such information is ignored, the fate of the hydroelectric projects along the Trishuli River may serve as an important reminder.

Md Mahfuzul Haque is a professor in the Department of Geological Sciences at Jahangirnagar University.

Disclaimer: The views and opinions expressed in this article are those of the author and do not necessarily reflect the opinions and views of The Business Standard.