Why Is Nepal Vulnerable to Devastating Floods?

Why Nepal Is Increasingly Vulnerable to Flash Floods and Himalayan Disasters

How Climate Change, Avalanches and Unplanned Development Are Raising Nepal’s Flood Risk

The devastating flood that struck Nepal on the morning of August 26 has raised concerns over why the Himalayan nation is becoming increasingly vulnerable to catastrophic flooding. Nepal has long been located in a high-risk Himalayan disaster zone, but the scale of large infrastructure and development projects underway across the country is adding to the risks.

Nepal is particularly vulnerable to Himalayan disasters and sudden, devastating flash floods because of a dangerous combination of geological instability, steep terrain and climate change.

Why Nepal Is So Vulnerable to Flash Floods

A young and fragile mountain range: Nepal lies in the geologically active Himalayan region, where the Indian and Eurasian tectonic plates continue to collide. This makes the region highly unstable, with rocks and slopes that are naturally fractured and vulnerable to sudden movement.

Climate change and melting glaciers: Rising global temperatures are causing rapid warming in high-altitude areas. This is also affecting permafrost, the permanently frozen ground that helps bind mountain slopes together. As the frozen layer weakens, rocks and ice can become increasingly unstable, raising the risk of landslides, rockfalls and avalanches.

Steep slopes and narrow valleys: Nepal's rugged terrain is characterised by steep slopes and narrow valleys. When large volumes of water, ice, rocks or debris move downhill, these valleys can act like funnels, greatly increasing the speed and destructive force of the flow.

Cascading disasters: Landslides, avalanches and sudden river blockages can occur as part of a chain reaction. A landslide or avalanche may temporarily block a river, creating a natural dam. When that unstable barrier eventually collapses, it can release a huge volume of water and debris downstream, making such events extremely difficult to predict and manage.

Human intervention and settlements: Limited flat land in the mountains has encouraged settlements and infrastructure development along valleys and river corridors. Unscientific road construction, poorly planned infrastructure and growing populations close to rivers have further increased exposure to flood-related hazards.

Nepal Has Faced Similar Disasters Before

This is not the first time Nepal's rivers have faced such a disaster. Last year, a Glacial Lake Outburst Flood (GLOF) affected the river system, damaging four major hydropower plants and disrupting around 8% of Nepal's electricity supply.

Such incidents highlight the vulnerability of Nepal's hydropower infrastructure, settlements and transport networks to sudden changes in the Himalayan environment.

How Do Flash Floods Occur?

A flash flood is an extremely rapid and often violent surge of water that can inundate downstream areas within a very short period.

In mountainous regions, flash floods can carry enormous amounts of energy and debris, destroying structures and infrastructure along their path. Because of the speed and force of these floods, communities living downstream may have very little time to receive warnings and move to safer locations.

How Can an Earthquake Trigger an Avalanche?

The peaks of the Himalayas are covered by massive glaciers and fragile rock formations that can remain in a delicate state of equilibrium. An earthquake can disturb this balance, causing rocks, ice, soil and other material to move violently downslope.

Authorities suspect that an earthquake may have triggered an avalanche in the Lende Khola River, a narrow, high-altitude tributary of the Bhote Koshi. The avalanche carried a large volume of ice and rock debris into the river system.

At some point downstream, the debris may have formed a dense and temporary natural dam. In such situations, water can rapidly accumulate behind the unstable barrier until the increasing pressure causes cracks to develop and the dam eventually fails.

The sudden release of this stored water, combined with massive amounts of debris, can produce a powerful flash flood capable of causing widespread destruction downstream.