A catastrophic flash flood has struck northern Nepal on the Tibetan border, killing at least 157 people and leaving hundreds missing, including at least 34 Australians . The death toll is expected to rise.
Authors
- Milad Haghani
Associate Professor and Principal Fellow in Urban Risk and Resilience, The University of Melbourne
- Zahra Shahhoseini
Lecturer – Disaster Resilience Infrastructure, RMIT University
Foreign tourists are among those feared dead or missing. Nepal’s tourism ministry reported 403 tourists unaccounted for, including 341 foreign nationals .
Footage shows torrents of mud and debris sweeping through settlements and carrying buildings and vehicles with them.
Early evidence suggests this was not an ordinary rain-driven flash flood, but what disaster science describes as a “cascading event”. So what do we know so far?
Not an ordinary flood
Investigators are examining a chain of events high in the Himalayas where one natural hazard seems to have triggered another.
Such phenomena are known as “cascading hazards” – chain reactions where an initial event directly triggers a sequence of subsequent events, often creating a much larger disaster.
Climate change is known to be driving faster glacier melt in the Hindu Kush Himalaya region, but scientists say it is too soon to pinpoint its role in these floods.
We know the flooding struck the steep, fast-flowing Bhotekoshi river system near Nepal’s border with Tibet at around 9am local time on Wednesday morning. It travelled downstream through the mountain district of Rasuwa and into neighbouring districts, including Nuwakot and Dhading.
The flooding carried a huge volume of water. It’s reported the Trishuli River rose by as much as nine metres within 30 minutes at one monitoring point downstream.
In parts of the historic hilltop town of Nuwakot, local officials report scores or hundreds of houses being swept away. Nepal’s Department of Roads says at least 19 motorable bridges and around 40 kilometres of roads have also been damaged or washed away. A 42km road connecting Betrawati with the Rasuwagadhi border crossing between Nepal and China’s Tibet autonomous region has been destroyed at multiple points.
What caused such a massive flood?
Rainfall does not appear to explain the scale of the flood . No heavy rainfall was recorded in the affected area beforehand.
The strongest evidence so far points to a massive glacial collapse high in the mountains. Satellite images show a large section of glacier breaking away at an altitude of around 5,200 metres and falling more than 1,200m into the valley below.
This appears to have produced an enormous ice-rock avalanche , large enough to generate a seismic signal.
The key question is how that avalanche produced such a huge volume of fast-moving water and debris. The most compelling explanation so far comes from Nepal’s hydrology department. Satellite images appear to show the avalanche blocking the river with ice and debris about 20km upstream of the Nepal-Tibet border. It’s been suggested that water accumulated behind this temporary dam before it failed and sent a sudden surge downstream.
When one hazard triggers another
The disaster in Nepal shows why cascading events can be particularly difficult to anticipate, prepare for and survive.
The Nepal flood may have involved several links in a cascading chain. An ice and rock collapse produced an avalanche. The avalanche entered a river, where it may have blocked the flow and temporarily stored water. Its failure then produced or intensified the flood downstream.
Early accounts show the avalanche may also have picked up water , rock and sediment as it travelled, producing an increasingly destructive debris-rich flow.
Cascading hazards are different from “compound hazards”, where multiple hazards combine or occur together without one necessarily triggering another. In a cascade, it is the causal links between the hazards that matter.
These chains can also amplify as they progress. A hazard beginning in a remote location can trigger a much more destructive hazard affecting people far away.
As in the case of the catastrophe in Nepal, the hazard itself can also change form. What starts as an ice and rock collapse can ultimately reach communities as a debris-laden flood.
Beyond individual hazards
The Nepal disaster is still an unfolding event, and the exact sequence that produced the flood will become clearer in the coming days.
But it already points to a broader challenge for disaster risk management. We tend to classify and assess hazards separately, such as fires and floods.
But cascading hazards do not fit into conventional categories. Managing them requires understanding where individual hazards can occur, but also how one can trigger or amplify another.
Australia’s National Emergency Management Agency has warned that the growing trend of concurrent and cascading disasters is expected to continue and, in many cases, increase.
We need to better understand what these chains of hazards could look like in our own local environments, such as the intense storms and flooding that hit the Otways in Victoria in January. This will help us assess risk and plan for natural disasters.
