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
Glacial Lake Outburst Floods (GLOFs)
Hazards
Such outburst events carry significant amounts of debris downstream, altering the geomorphology of river basins, and can often transport large rocks and boulders for considerable distances.
Glacial Lake Outburst Floods (GLOFs)
Destruction to property and infrastructure
Loss of essential power supplies due to failure of HEP schemes
Glacial Lake Outburst Floods (GLOFs)
Death and injury
Some of the most deadly moraine-dammed outburst floods have occurred in the Peruvian Andes where local communities and cities exist in relative close proximity to potentially dangerous glacial lakes.
The most serious occurred in 1941 when an outburst flood originating from Lake Palcacocha destroyed a third of the city and killed 5000 people.
Glacial Lake Outburst Floods (GLOFs)
Sabai Tsho GLOF
1998 warm wet monsoon increased lake depth, and increased velocity and destabilised glacier.
Then three small earthquakes, caused an ice avalanche into the lake, caused the displacement wave, collapse of moraine in 5-10 minutes, 25 million m3 water and lowered lake surface by 50m
Glacial Floods - Mitigation
Hard measures (engineering)
At the lake site:
Drainage by syphon or pump
Cut drainage channels
Tunneling through bedrock
Early warning system at GLOF ste and along the downstream river channel
Glacial Floods - Mitigation
Hard measures (engineering)
Downstream:
Early warning audio system adjacent to population centres
Flood control structures
Building reinforcement
Glacial Floods - Mitigation
Soft measures (engineering)
Mapping of hazards and vulnerabilities
Computer modelling of potential GLOFs
Geophysical analysis of lake dames to determine structure and ice content
Observation and forecasting systems
Ground penetrating radar
Increase in community awareness and participation
Promotion of afforestation and conservation
Planning
Tsho Rolpa, Nepal (Rana et al., 2000)
Largest moraine-dammed in Himalayas (ice –cored moraine)
Ground penetrating radar (GPR) studies of the moraine to locate ice-core
Tried to use syphon’s to lower lake level
Then cut a channel, installed a structure to regulate flow
Array of sensors and audio warnings
What is an iceberg?
Anicebergis a large piece of freshwater ice that has broken off a glacier or an ice shelf and is floating freely in open water.
14th April 1912 Titanic, loss of 1517 lives – since then no iceberg impacts have come near the disaster in severity.
Icebergs – Hazards
Direct impact with ships or installations – 3539 fatalities since 1800’s
Sea bed scour
Disruption of ecological systems
Icebergs – Mitigation
Visual and satellite reconnaissance RADARSAT -1 1995 and RADARSAT-2 in 2007 Immobile structures (e.g. oil rigs) need to divert the iceberg Towing Water cannons
Glacier retreat
Increased risk of GLOFs due to glacier retreat creating more moraine dammed lakes which could fail
Debris flows are also a well-recorded consequence of glacier retreat (Ballantyne and Benn, 1994).
Existing moraine-dams may experience accelerated degradation through melting of ice cores
Lakes may increase in volume due to greater melt water input.
Steep hanging glaciers that are partially or entirely frozen to their beds may become prone to ice avalanching as their snouts become warm based
Continued glacier mass-balance losses will result in
Water shortages after an initial period of increased discharge.
The potential loss of glaciers as water resources is a greater source of anxiety to the people and industries that rely on glacial melt during dry seasons.
e.g.
HEP
A further widespread problem that may develop with global warming is a shift in hazard zones
Glacial and related hazards may begin to affect areas with no history of such events, where regulatory and social aspects of preparedness will not exist.
Glacial retreat leading to increased volcanic activity.
Sea Level Rise
Currently small glaciers and ice caps contributing most to sea level rise
In future Greenland and Antarctic have the potential to contribute large volumes
Glaciers and Climate Change
Iceberg flux
Albedo
Ocean circulation
Relationship between slope and volume can predict how far ice will travel
Tanα = 1.111 – 0.118 log (V)
α is the average slope, and V the volume of the ice avalanche in cubic metres.
