Construction boom pushes Nepal’s mountain settlements into flood-prone areas

A glacier lake outburst flood, or GLOF, happens when water stored in a glacier lake suddenly escapes. The released water can rush downhill through steep mountain valleys and river channels. This matters because settlements may face a flood originating far above them, not just from the nearby river.

Nepal’s researchers simulated possible GLOFs to estimate how far floodwaters could travel and which buildings could be hit. The study examined structures within 50 meters above existing river levels and buildings located directly in possible inundation paths. A recent Rasuwa-Bhotekoshi flood reached settlements 82 meters above the river.

Glacier lakes are a growing concern as Himalayan temperatures rise and glaciers melt faster. Nepal identified 21 potentially dangerous glacier lakes six years ago, and the article says their number and size may have increased. New construction along river corridors therefore increases the number of people and buildings exposed to sudden, powerful floods.

At least 108,500 people in Nepal are exposed to potential flooding in the high mountain study region. More than 23,000 live less than 10 meters above riverbeds, where exposure is especially high. The researchers also found that more than 80 percent of structures exposed across Nepal, China, and India are in Nepal.

Construction has expanded quickly. Between 2016 and 2023, the number of buildings directly in possible GLOF inundation paths increased by 78 percent. The total area occupied by buildings within 50 meters of riverbeds grew by 46 percent during the same period.

The figures show that risk is not only a natural hazard problem. New buildings are being added where floodwaters may travel. The wider study region contains about 300,000 people exposed to potential flooding, but Nepal carries the largest share of exposed structures. Continued building could increase future losses unless development avoids these corridors.

Mountain development is concentrating people and buildings near rivers. Hydropower projects need access to powerful waterways and require roads, tunnels, bridges, and workers. Border and trade routes also commonly follow river corridors. Hotels, lodges, and expanding towns add more structures in floodplains.

The mechanism is practical but dangerous. Roads are often built beside rivers because construction materials are readily available there. Once roads, bridges, hospitals, and other infrastructure arrive, people move in for work and services. The study says flood risks were not adequately assessed as this construction expanded. Hydropower development also changed livelihoods and settlement patterns.

This leaves communities closer to possible inundation routes. The study found a 78 percent increase in buildings directly in potential GLOF paths from 2016 to 2023. Unregulated tourism and rapid urbanisation add pressure. Without risk-sensitive decisions, infrastructure intended to connect or develop mountain areas can also increase the population exposed to floods.

When floodwater reaches settlements along river corridors, it can inundate homes, roads, bridges, hospitals, hotels, and hydropower infrastructure. People may be killed, displaced, or cut off from services and transport. The danger is especially severe when buildings stand directly in possible flood paths or only a few meters above riverbeds.

The article describes the Rasuwa-Bhotekoshi flood as a devastating example. Its waters reached settlements as high as 82 meters above river level. The reported toll was 1,455 dead and 5,285 unaccounted for. The article also describes Tamang communities being uprooted by hydropower development and pushed toward river-based labour.

These effects can compound when a sudden flood meets dense construction. Roads and bridges may fail while homes and workplaces are swept away, increasing isolation and delaying help. As buildings in potential GLOF paths rose 78 percent between 2016 and 2023, the number of people and assets exposed to such consequences also grew.

Nepal’s mountain settlements are vulnerable because steep terrain connects high glaciers and lakes to lower river valleys. A small change in temperature or rainfall can alter conditions in the mountains and send larger or faster flows downstream. Settlements built along those channels then face hazards originating far above them.

The study notes that Himalayan temperatures rose by 3 degrees Celsius in individual winter months, compared with global average warming of 1.4 degrees Celsius since preindustrial times. Glaciers in the Hindu Kush are melting at twice the rate. These changes can increase water stored in glacier lakes or alter river flows, raising the possibility of sudden flooding.

Nepal had identified 21 potentially dangerous glacier lakes across the Gandaki, Koshi, and Karnali basins six years ago. The article says their number and size may have grown. As roads, towns, and infrastructure expand downstream, more people are positioned in the path of changing mountain water systems.

Risk-sensitive land-use planning means using flood maps and hazard information before approving settlements, roads, hospitals, hotels, or hydropower facilities. It can reduce exposure by steering new construction away from riverbeds, low-lying floodplains, and simulated GLOF paths. It can also protect safer existing areas and guide infrastructure toward less dangerous sites.

The study used building data, population data, and GLOF simulations to identify exposed places. That information could support decisions about where construction should not expand. The article reports that more than 23,000 people live less than 10 meters above riverbeds, showing why location matters.

Nepal has struggled because mountain roads often follow river corridors, where materials are available. Basanta Raj Adhikari said flood risks were not adequately assessed as bridges, hospitals, and other infrastructure were built. Hydropower, internal migration, changing livelihoods, tourism, and unplanned urbanisation then reinforced settlement in risky areas. Applying planning earlier and consistently could slow further exposure.

A glacier is a store of frozen water that can melt and contribute water to a glacial lake. That lake sits within a drainage network leading to a river. The river basin gathers and channels water through connected valleys. This links a high mountain event to communities far downstream.

In Nepal, the study considered potential GLOFs across the Gandaki, Koshi, and Karnali river basins. Researchers simulated how floodwaters from glacier lakes might move and identified buildings in their possible inundation paths. Because mountain rivers descend steeply, a sudden release can travel quickly along the same corridors where roads, hotels, and settlements have expanded.

The chain creates downstream risk even when a settlement is not beside a glacier or lake. Rising temperatures and faster glacier melt may increase concern about dangerous glacier lakes. Nepal identified 21 such lakes six years ago, while the article says their number and size may have increased. More construction along connected river corridors means more exposure across the basin.