Nearly 300,000 at risk from Himalayan floods, as flood zone building accelerates

The study measured how many buildings and people may be exposed to flooding across Himalayan areas of Nepal, India and China. It examined places up to 50 kilometers downstream from 1,361 lakes. This matters because development is increasing where floods may reach.

Building footprints within 50 vertical meters of river channels grew from 12.38 to 18.04 square kilometers between 2016 and 2023. That is a 45.75% increase. In areas at high risk from glacial lake outburst floods, footprints grew 77.78%, from 0.72 to 1.28 square kilometers.

Using these building estimates, researchers projected that about 293,800 people could live in exposed buildings in 2026. That equals 18% of the study-area population. Nepal alone may have 108,500 exposed residents, including 22,800 within 10 vertical meters of a river channel. The estimates remain highly uncertain.

A potential flood zone is an area that may be reached by floodwater. In this study, it refers to locations near Himalayan river channels and downstream from lakes where seasonal floods, extreme rainfall or glacial-lake floods could affect buildings. It describes possible exposure, not a guarantee that every location will flood.

The researchers treated land within 50 vertical meters of a river channel as a proxy for potential flood exposure. They then measured the building footprint in those areas. This gave them a consistent way to compare development across Nepal and neighboring parts of India and China.

The study also examined places up to 50 kilometers downstream of 1,361 Himalayan lakes. That wider distance captured buildings potentially connected to lake-related flood hazards. The authors caution that the estimates are uncertain and that better building databases are needed for more accurate risk calculations.

The study found rapid growth in the area occupied by buildings inside potential flood-exposure zones. Near river channels, the estimated building footprint increased by 45.75% between 2016 and 2023. The footprint measures the ground area covered by building bases, not the total floor space inside them.

In exact figures, the footprint rose from 12.38 to 18.04 square kilometers. The increase was larger in locations considered highly exposed to glacial lake outburst floods. There, building-covered area rose from 0.72 to 1.28 square kilometers, a 77.78% increase.

These figures show that construction expanded in places that may face flooding. The researchers used five datasets, checked against manually recorded data, to estimate the footprints. They stress that both building and population estimates are highly uncertain, highlighting the need for better building databases.

Himalayan valleys can make flooding especially destructive because they are steep and narrow. When large amounts of water enter these confined spaces, the landscape can exacerbate flood heights. Water and debris may then affect buildings and people concentrated along river channels.

The study identifies several possible flood triggers, including seasonal monsoons, extreme rainfall and rare but devastating debris flows. Glacial lake outburst floods are another hazard. In steep mountain catchments, these events can send powerful flows downstream through connected valleys.

This setting matters as construction expands near rivers. The study found that building footprints within 50 vertical meters of channels increased substantially from 2016 to 2023. More development in confined valleys can place more people and structures in areas exposed to potentially high floodwater. The authors call for improved building data to calculate risk more accurately.

A glacial lake outburst flood, or GLOF, is a flood that occurs when water held in a glacial lake is suddenly released. The source describes these floods as rare but devastating. They are one of the hazards considered in the Himalayan study.

Once released, the water can move downstream through steep, narrow valleys. The flow may include debris and can affect buildings located along or near river channels. The researchers examined areas up to 50 kilometers downstream from 1,361 Himalayan lakes to estimate potential exposure.

The risk is growing where construction expands near these channels. In areas at high risk of GLOFs, building footprints increased 77.78%, from 0.72 to 1.28 square kilometers between 2016 and 2023. The authors estimated exposure using building data, but they emphasize that the results are uncertain and need better databases.

When homes and other buildings spread into potential flood zones, more people and property occupy places that flooding may reach. That increases the number of residents and structures potentially exposed to floodwater. The study tracks this exposure through building footprints near Himalayan river channels.

For example, the estimated footprint within 50 vertical meters of channels grew 45.75%, from 12.38 to 18.04 square kilometers between 2016 and 2023. In high-risk glacial lake outburst flood areas, it grew 77.78%. These increases show construction expanding in locations associated with potential flood hazards.

Researchers estimated that about 293,800 people may live in exposed buildings in 2026. The population in flood-risk zones is projected to increase by 17,900, or 6%, between 2015 and 2030. Because the estimates are highly uncertain, the authors want more accurate building databases to improve risk calculations.

Climate change is predicted to increase the severity of flooding events in the Himalayas. The study links this concern to several hazards, including seasonal monsoons, extreme rainfall and floods released from glacial lakes. It does not provide separate numerical projections for each hazard.

The article describes glacial lake outburst floods as rare but devastating. If such an event occurs, water can move through steep, narrow valleys, where the terrain can exacerbate flood heights and their effects. More severe events could therefore increase potential exposure in downstream areas.

This concern comes as construction is expanding near rivers and lakes. Building footprints near channels rose 45.75% between 2016 and 2023, while high-risk glacial-lake areas saw a 77.78% increase. The authors say better building databases are needed to measure future risk more accurately.