GLOF: occurrence, causes and effects

A Glacial Lake Outburst Flood (GLOF) is the sudden release of a large volume of water from a glacial lake, usually dammed by a moraine or ice, which produces a destructive flood and debris flow downstream.

How it occurs

1. Glaciers retreat due to climate warming, leaving meltwater in depressions behind end or lateral moraines.
2. The lake grows (e.g. Imja, Tsho Rolpa) and is held back by an unconsolidated moraine dam.
3. A trigger acts and the dam breaches by overtopping, piping or collapse.
4. A flood wave with high discharge, carrying rock and ice debris, rushes down the valley.

 glacier --> [ lake ] |moraine dam|
                      v breach
             ======> flood + debris ===>

Causes

• Climate change and glacier melt, giving rapid lake growth.
• An ice or rock avalanche or a landslide entering the lake, producing waves that overtop the dam.
• Melting of the ice core in the moraine; piping and seepage.
• Earthquake shaking.
• Heavy rainfall or snow melt raising the lake level.
• A weak, steep, unconsolidated moraine dam.
• Failure of a lake upstream (cascade).

Consequences in Nepal

• Loss of life and livestock; the 1985 Dig Tsho GLOF destroyed the nearly completed Namche hydropower project, bridges and farmland.
• Damage to hydropower plants, roads, trails, bridges and settlements; the 1998 Tam Pokhari GLOF and the 2016 Bhote Koshi GLOF (from a lake in Tibet) caused heavy damage.
• Heavy erosion and sedimentation, change of river course, and damage to farmland far downstream, sometimes more than 100 km away.
• Trans-boundary effects (Tibet to Nepal to India).
• Mitigation: monitoring, early warning, artificial lowering of the lake level (Tsho Rolpa), spillways and land-use planning.