Key Takeaways:
- Massive landslide and glacial debris triggered Budhi Gandaki flood surges
- Largest flood wave reached 20 to 25 metres and hit Salleri at 6:30am
- Repeated river blockages amplified destructive downstream flooding
- Landslide released energy comparable to a magnitude 3.5 earthquake
A massive landslide involving snow, glacier debris and water high above the Budhi Gandaki basin triggered the destructive flood surges that swept through parts of Gorkha on October 11, according to a technical assessment by the National Disaster Risk Reduction and Management Authority.
The landslide occurred at an altitude of around 5,500 to 6,000 metres in the upper reaches of Ungdilung Khola between Chamar Himal and Syerangi Himal. The report said the slope consisted of relatively weak rock formations found in the Tibetan Tethys Himalaya. It said that a snow avalanche and a major collapse involving glacial debris generated powerful flows that moved downstream, repeatedly blocking and releasing water at key river confluences.
The most destructive surge reached Salleri at around 6:30am, rising an estimated 20 to 25 metres and sweeping away a hydropower camp. The report identifies this wave as the primary cause of damage to structures downstream, including buildings in Jagat and concrete bridges over the Soti Khola and Budhi Gandaki River.
Investigators found that large volumes of debris accumulated at the confluence of Ungdilung Khola and Syar Khola, creating temporary blockages that repeatedly released floodwaters. Similar processes may also have occurred further downstream, intensifying the flood surges.
Unlike typical flash floods, the flow contained unusually high amounts of soil and fine sediment, giving it the characteristics of a dense mud flow. According to a preliminary assessment by the Department of Mines and Geology, the landslide released energy equivalent to a magnitude 3.5 earthquake.