Nepal’s disaster is an urgent wake-up call

On 26 August, a huge avalanche and flood swept through Nepal’s Rasuwa district. A bedrock failure on Langtang Lirung mountain released rock and ice, creating one of the largest recorded avalanches worldwide. The flow then travelled through several river valleys.

As the avalanche moved down the Lhende Khola, Bhotekoshi and Trishuli valleys, ice melted and the rushing mass absorbed more water and sediment. This turned the initial collapse into a powerful, debris-filled flood that spread damage more than 100 kilometres from its source.

The human and economic losses were extreme. More than 1,400 people died, over 5,000 were missing and tens of thousands were displaced. Settlements, roads and bridges were washed away, about 10% of Nepal’s hydropower production was knocked out, and reconstruction costs were estimated at around US$5 billion.

A rock-and-ice avalanche occurs when unstable mountain rock collapses and carries overlying glacier ice downhill. It is more than a simple landslide because the moving mass contains rock, ice and often water. Its speed and volume can produce impacts far beyond the original slope.

In Rasuwa, a bedrock failure on Langtang Lirung mountain took the overlying glacier with it. As the avalanche rushed through the Lhende Khola, Bhotekoshi and Trishuli valleys, the ice melted. Extra water and sediment joined the flow, making it a much larger, wetter and more destructive flood.

This process creates a cascade rather than a single hazard. The initial collapse can damage valleys, while the resulting flood carries debris through settlements and infrastructure. The Rasuwa event affected areas more than 100 kilometres from its source, showing why mountain hazards must be assessed together.

The Rasuwa avalanche was enormous, containing around 125 million cubic metres of rock and ice. The article describes it as one of the largest avalanches recorded anywhere in the world. That volume gave the collapse enough energy and material to affect an extensive mountain river system.

The avalanche began on the northern slopes of Langtang Lirung after a failure in the bedrock. It carried the overlying glacier downhill. As the mass travelled through the Lhende Khola, Bhotekoshi and Trishuli valleys, ice melted and additional water and sediment became part of the flow.

Damage extended more than 100 kilometres from the source. The flow destroyed settlements, roads and bridges, displaced tens of thousands of people and disrupted around 10% of Nepal’s hydropower production. Reconstruction costs were estimated at approximately US$5 billion, illustrating how avalanche size can translate into regional consequences.

The avalanche did not remain a dry collapse on a mountain slope. It carried rock and ice into river valleys, where the ice melted and additional water and sediment entered the moving mass. This made the flow deeper, heavier and more destructive downstream.

In Rasuwa, the combined flow travelled through the Lhende Khola, Bhotekoshi and Trishuli valleys. It washed away settlements, roads and bridges, while spreading damage more than 100 kilometres from the source. The mechanism was cascading: a bedrock failure triggered an avalanche, the avalanche fed a flood, and the flood transported sediment and debris into populated valleys.

The consequences were severe. More than 1,400 people died, over 5,000 were missing and tens of thousands were displaced. Around 10% of Nepal’s hydropower production was knocked out. Reconstruction costs were put at around US$5 billion, with trade and tourism also disrupted.

Collapsing glaciers, glacial lakes and mountain slopes can produce linked hazards instead of isolated events. A rockfall may destabilize a glacier, a glacier collapse may release water, and a flood may then carry enormous amounts of sediment. Each step can enlarge the next hazard and extend impacts downstream.

Recent examples show the pattern. In Sikkim in 2023, sediment entering South Lhonak glacier lake triggered an outburst that carried 270 million cubic metres of sediment down the Teesta River. More than 100 people died, about 26,000 buildings were damaged and hydropower structures were destroyed.

The article identifies global warming as a reason further disasters of this magnitude are expected. Communities, roads, energy systems, farms and tourism facilities are often concentrated in mountain valleys. Because these systems are connected, officials need to assess cascading risks rather than treating each avalanche, lake outburst or flood separately.

Historical records cannot reliably show how often the most extreme mountain disasters will occur because these events are exceptionally rare. A short record may contain no comparable collapse, or only one, making estimated frequencies highly uncertain. The article’s recent examples already show several massive events within a few years.

Risk science also treats a changing climate as a moving baseline. When glaciers, slopes and water systems are changing, past conditions may not represent future conditions. This matters for events involving rock, ice, lakes, floods and sediment, because one unusual failure can trigger a larger chain of hazards.

The article does not provide a historical frequency calculation. Instead, it warns that global warming is accelerating and that further disasters of this magnitude are expected. Officials therefore need scenario-based assessments that consider connected hazards, not forecasts based only on how often similar events appeared in old records.

Mountain hazards interact as a connected system. Steep slopes can fail and send rock onto or beneath glaciers. Glaciers and glacial lakes can then release ice and water, while rivers carry the resulting debris downhill. Sediment can fill channels, alter river beds and increase the destructive reach of floods.

The Rasuwa event demonstrates this chain. Bedrock failure on Langtang Lirung carried away the overlying glacier. The avalanche entered the Lhende Khola, Bhotekoshi and Trishuli valleys. Ice melted, and extra water and sediment joined the flow, producing damage more than 100 kilometres from the source.

Similar cascades occurred elsewhere. In Nepal’s Melamchi flood, more than 50 million cubic metres of sediment were washed away and river-bed levels changed by roughly 20 metres in places. In Sikkim, 270 million cubic metres of sediment travelled down the Teesta River. These examples show why each linked process must be assessed together.