Nepal’s flash floods show the devastating power of ‘cascading hazards’

A devastating flash flood struck northern Nepal near the Tibetan border on Wednesday, claiming at least 157 lives and leaving hundreds missing, including 34 Australians. The scale of the disaster has raised concerns about "cascading hazards," chain reactions where an initial event triggers a series of subsequent events often leading to a larger disaster.

This particular flood is suspected to have been triggered by a massive glacial collapse high in the Himalayas, which produced an ice-rock avalanche. The avalanche entered the Bhotekoshi river system, temporarily blocking the flow and accumulating water behind it. When the dam failed, it released a surge of water, debris, and sediment downstream, causing widespread destruction in the mountain districts of Rasuwa, Nuwakot, and Dhading.

The flood carried a significant volume of water, raising the Trishuli River by nine meters within 30 minutes at one monitoring point. Early evidence suggests this was not an ordinary rain-driven flood, but a "cascading event" exacerbated by climate change-driven glacier melt in the region. The disaster highlights the difficulty in anticipating, preparing for, and surviving cascading hazards, which can be challenging to manage as they do not fit into conventional hazard categories.

Disaster risk management must consider how individual hazards can trigger or amplify one another, requiring a better understanding of potential hazard chains in local environments to effectively assess risk and plan for natural disasters.