Lecture 13: Glacial Hazards in a warming world Flashcards
The cryosphere comprises
Snow River/lake/sea ice Glaciers Ice shelves and ice sheets Frozen ground
Ice sheet
A mass of land ice of continental size, and thick enough to cover the underlying bedrock topography. Its shape is mainly determined by the dynamics of its outward flow.
Ice shelf
A thick, floating slab of freshwater ice extending from the coast, nourished by land ice.
Glacier
A mass of surface-ice on land which flows downhill under gravity and is constrained by internal stress and friction at the base and sides.
Ice cap
Dome-shaped ice mass with radial flow, usually covering the underlying topography.
A glacial hazard
Is any glacier or glacier-related feature or process that adversely affects human activities
Ice avalanche
Sudden mass movement of ice down slope. Starting areas can be classified as ramp- or cliff-type.
Rock avalanche
High velocity (~90-350 kph) transport of fractured rock mass, often starting as a rock fall or slide on step slopes. Can be triggered by freeze-thaw cycles (permafrost degradation), seismicity, or stress release following glacier retreat.
Debris flow (Aluvión)
Typically a slurry of mixed grade soil and water. Commonly triggered by ice/rock avalanches and/or glacier/glacial lake floods.
Glacier Mass Movements – Mitigation
Monitoring
Hazard map
Glacier outburst
Catastrophic discharge of water from the subglacial and/or englacial system.
Often triggered by geothermal heating (jökulhlaup), opening of englacial channels at the start of the ablation season (spring events) or intense rainfall.
Glacial Lake Outburst Flood (GLOF)
Catastrophic flood from the breaching of a moraine dam. Term used in Himalayan regions (synonymous with the French term dêbacle).
Ice-dammed lake outburst
Catastrophic flood from the failure of an ice dam. Often occurs periodically from the same lake/glacier.
Glacier outburst
The type of flood associated with a sub-glacial volcanic eruption.
There are three recorded mechanisms by which glacier outbursts occur:
The rupture of an internal water pocket
The progressive enlargement of internal drainage channels
Catastrophic glacier buoyancy, or `jacking’, with sub-glacial discharge.
If meltwater within the glacier is able to build up pressure to such a point that the hydrostatic pressure exceeds the constraining cryostatic pressure then…
A catastrophic burst through the ice may occur.
Water may drain into the existing sub-glacial drainage from where it can discharge to the
glacier snout.
The flood wave is dangerous as
it often quickly leaving little warning for communities downstream
Eyjafjallajökull
Glaciated volcano in southern Iceland Eruption on 13th April 2010 Ice cap melted 700 people evacuated Destroyed main road, 20 farms destroyed
Grimsvötn, Iceland – 1996
Eruption of subglacial volcano
Melting of the ice cap and draining of Lake Grimsvötn
3.2 km3 drained in 40 hours
Flow as high as 40,000 m3s-1
Was widely predicted and monitored in detail
No loss of life but damage to infrastructure
Glacial Lake Outburst Floods (GLOFs)
Most destructive glacier flood events tend to be those associated with unexpected releases of stored glacier melt water and surface run off
Formation and subsequent failure of ice- and moraine-dammed lakes.
Ice-dammed lakes
Developed when sub-glacial, en-glacial, supra-glacial or ice-marginal water bodies are impounded by ice.
Such outburst or drainage events can occur regularly, on a near seasonal basis caused by changes
in ice thickness, lake bathymetry, sub-glacial water pressure, changes in meltwater production and =– water bodies impounded by terminal and lateral moraines left behind by retreating glacier snouts.
Trigger Mechanisms
Avalanche Displacement wave
Ice avalanches Glacier calving Collapse of hanging glaciers Valley side rock falls Debris flows
Capacity of lakes to store water is reduced
Melting of ice-core within moraine
Seismic activity
60% of known GLOFs in the Himalayas caused by Avalanche Displacement Waves