Insights behind the unexpected flooding in the Budhi Gandaki River, Gorkha, Nepal

Insights behind the unexpected flooding in the Budhi Gandaki River, Gorkha, Nepal - ICIMOD

Key messages

• On 21 April 2024, a flood from the glacial lake Birendra Tal occurred. This was triggered by a massive ice avalanche after the breaking of ice chunks (known as calving) from the snout (end) of the Manaslu Glacier into the lake water. Manaslu is the eighth-highest mountain in the world at 8,163 metres above sea level (masl), located in west-central Nepal.
• The displacement wave caused the sudden release of water from the lake outlet into the Budhi Gandaki River, in Gorkha district.
• This flood event was not a typical glacial lake outburst flood (GLOF), and the overflows did not breach the moraine dam. There was no significant impact on the dam observed in the post-event images.
• The threat remains. The analysis found significant risk of debris flow and ice/snow avalanches from the left adjacent valley (considering the direction of flow of the lake water), and, together with anticipated temperature rises and glacier retreat, indicate future occurrences could exceed the moderate impact witnessed in the current event. Vigilance from relevant agencies and roles, including Disaster Risk Management Officers, community leaders, national and local governments, is recommended.
• A detailed understanding of glacier dynamics, crevasse formation, and ice detachment processes is crucial to assess the potential risks posed by glacier avalanches and debris flow into the lake. Remote sensing techniques and field-based monitoring, such as GB-InSAR and time-lapse cameras, can provide this information.
• Continuous lake monitoring via satellite imagery and time-lapse cameras, coupled with in-situ sensor installation for lake-level monitoring and establishment of a flood early warning system, is recommended for relevant agencies, in order to mitigate risks and enhance preparedness.
• Weakening glaciers in the region pose risks from various hazards while the countries in the region remain ill-prepared to cope with the rapid pace of these changes. Addressing this issue requires urgent and strong political action to implement effective risk mitigation strategies.