Avalanches Flashcards
What is an avalanche?
Rapid flow of snow down a slope such as a hill or a mountain
What are the two types of avalanche?
Slab Avalanche
Loose Snow Avalanche
What is a slab avalanche?
- Where large chunks of snow and ice sheer away from the hillside
- move rapidly, covering rocks and trees
- immense power, causing great damage
What is a loose snow avalanche?
- avalanche involving loose, powdery snow, originating from a single point, spreading out like a fan
What are the causes of an avalanche?
- slope angle
- tree removal
- weather
- human factors
How does the slope angle affect the formation of an avalanche?
most avalanches occur on a slope between 30-45 degrees
- the steeper the slope, the more likely it is to avalanche, as the angle affects the stability of the snowpack in terms of gravitational potential
How does tree removal affect the formation of an avalanche?
- removal of trees allows snow to flow unimpeded
- removal of the roots affects the stability of the snowpack
How does aspects of weather affect the formation of an avalanche?
- heavy snowfall
- recent snowfall
- wind direction
- temperature rise
- uneven rates of freezing
- heavy rainfall
How does heavy snowfall affect the formation of an avalanche?
- 24h after a storm has hit is the most critical time
- it overloads the snowpack, causing the gravitational potential to increase
How does recent snowfall affect the formation of an avalanche?
- it puts extra stress on the existing snowpack, especially if the snow does not adequately bond with the pre-existing snow layer
How does uneven rates of freezing affect the formation of an avalanche?
- if new snow falls on frosty snow, the icy layer might cause a new slab of snow to slip
How does wind direction affect the formation of an avalanche?
- can create a cornice (an overhang of snow) that can then become too heavy to support or suddenly shears off
How does temperature rise affect the formation of an avalanche?
- sudden melting makes the snow denser and heavier, increasing gravitational potential
- meltwater may also one layer to unfreeze from another snow layer, causing a slab avalanche
- lubricates the slope, making slippages easier
How does heavy rainfall affect the formation of an avalanche?
- water makes the snowpack denser and less stable
- can also lubricate the slope, making slippages easier
What human factors affect the formation of an avalanche?
- skiing
- snowboarding
- hiking
- tree removal
- interactions
(all add extra stress to the slope)
What strategies can be used to manage avalanches?
- controlled explosions/avalanche cannons
- warning systems
- snow fences
- trenches
- retardant mounds
- tunnels
- avalanche dams
- afforestation
- sloping house roofs
- deflecting barriers
To what extent was the management of Galtür successful?
INITIALLY VERY UNSUCCESSFUL
- not prepared for the scale
- complacent: thought they were safe
- lack of knowledge/understanding
SINCE: LONG TERM PLANNING
- research about avalanches to understand them so they can build appropriate protection/plan the management
- they can model the worst case scenario and plan effectively (although you can never quite know what the WCS is)
EXTENDED ZONE, COLOUR CHANGE IMPACTS:
- development: increased cost
- tourism: more unsafe places
What is a Glacial Outburst Flood? (GLOF)
a powerful flood caused by the sudden discharge of a sub-glacial or ice-dammed lake
What is another name for a GLOF?
jökulhlaups (Icelandic term)
What extreme events can trigger a GLOF?
- earthquakes
- avalanches
- landslides
- erosion of moraine
- increase in meltwater from seasonality, climate change, or volcanic eruptions from ice topped mountains
Why are proglacial lakes prone to GLOFs?
dammed by moraine = WEAK
What are the causes of a GLOF?
- increased flotation of ice as water levels rise
- overflow and melting of an ice dam
- breakdown of ice because of tectonic activity
- enlargement of pre-existing tunnels beneath an ice dam because of increased water pressure
Give an example of an area prone to GLOFs
Iceland - volcanic activity generates meltwater beneath glaciers and triggers ice instability, subsequently releasing more meltwater
e.g. Vatnajökull ice cap, SE Iceland (although it has lost 15% volume in last century)
- 8% country
- 400m thick
Give an example of a GLOF
Grimsvötn volcano
- underneath the ice cap
- melts ice, creates subglacial lake under the caldera
- subglaical lake reaches critical size, and forces itself through subglacial tunnels and drains catastrophically in a few hours
this happens (on average) every 5-6 years
total discharge 0.5-3.5km3
e.g. 1996 - ash 10km in air, melted ice, burst through 6 days after the eruption
- damaged bridges, roads, powerlines
- US $15 million economic loss
Give an example of an area that is a major risk of a GLOF
Mount Rainer
- has 25 glaciers radiating from summit, most in USA bar Alaska
- also a volcano: an eruption would melt a LOT of snow and ice
- could also cause lahars if the meltwater mixes with sediment
Give an example of an area that is a growing risk of a GLOF due to climate change
Himalayas
- CC melting glaciers
- lakes in Nepal and Bhutan becoming dangerously full
Mitigation:
- install sensors and sirens in hazard zone so people have an early warning and enough time to evacute
e.g. Dig Tsho, Nepal, 1985
- 14 bridges down
- US $1.5 million damage to HEP plant downstream
