Lecture 21 - Avalanches Flashcards
What is an avalanche?
rapid flow of snow down a hill or mountainside
At what time of year do most avalanches tend to happen?
In the winter, particularly from December to April. However, avalanche fatalities have been recorded for every month of the year.
How does a snow avalanche start and develop?
It starts when an unstable mass of snow breaks away from a slope. The snow picks up speed as it moves downhill, producing a river of snow and a cloud of icy particles that rises high into the air. The moving mass picks up even more snow as it rushes downhill.
How much can a large, fully developed avalanche weigh?
as much as a million tons
How fast can a large, fully developed avalanche travel?
faster than 320 km/h, i.e. faster than the fastest skier
When do avalanches occur?
when layers in a snowpack shear
What is snowpack?
They are layerings of snow that build up in an area, such as the side of a mountain. In winter, repeated snowfalls build a snowpack dozens of meters thick. The layers vary in thickness and texture.
How do snowpacks shear?
The bonds between the layers of a snowpack may be weak. Melted snow that refreezes may cause a slick coating of ice to form on the surface of a layer. A new snowfall may not stick to this slippery layer, and it may slide off. During spring thaw, melted snow can seep through a snowpack, making the surface of a lower layer slippery. Added weight or vibration can easily send the top layers of a snowpack hurtling downhill.
What are the two main types of avalanche?
loose powder (“sluffs”) slab
When do loose powder (“sluff”) avalanches occur?
They occur when the weak layer of a snowpack is on the top.
What is a sluff avalanche?
A sluff is a small slide of dry, powdery snow that moves as a formless mass.
What is the more dangerous type of snow avalanche?
slab avalanche
When do slab avalanches occur?
They occur when the weak layer lies lower down in a snowpack. This layer is covered with other layers of compressed snow.
What happens during a slab avalanche?
When the avalanche is triggered, the weak layer breaks off, pulling all the layers on top of it down the slope. These layers tumble and fall in a giant block, or slab.
Once a slab avalanche starts, the slab shatters into many separate blocks. These snow blocks break up into ever-smaller pieces. Some of the pieces rise into the air as a moving cloud of icy particles. The cloud races downhill at very high speeds.
How many avalanches are there each year in the mountains of the western United States?
about 100,000 avalanches each year
How many people are killed worldwide each year by avalanches?
more than 150 people - most are snowmobilers, skiers, and snowboarders
In what ways can snowpack on slopes fail due to gravity?
Creep
Fall
Slide
Flow
What slopes does an avalanche path involve?
Starting zone - steepest slope (30-45°)
Track - guided by topography (20-30°)
Run out zone (<20°)
What is the typical mass, run and force of a size 1 avalanche?
relatively harmless to people
mass = 10 tonnes
run = 10 metres
force = 1 kilopascal
What is the typical mass, run and force of a size 2 avalanche?
could bury, injure or kill a person
mass = 100 tonnes
run = 100 metres
force = 10 kilopascals
What is the typical mass, run and force of a size 3 avalanche?
could bury or destroy a car, damage a truck, destroy a wood frame house, or break a few trees
mass = 1,000 tonnes
run = 1,000 metres
force = 100 kilopascals
What is the typical mass, run and force of a size 4 avalanche?
could destroy a railway car, a large truck, several buildings, or up to 4 hectares of forest
mass = 10,000 tonnes
run = 2,000 metres
force = 500 kilopascals
What is the typical mass, run and force of a size 5 avalanche?
mass = 100,000 tonnes run = 3,000 metres force = 1,000 kilopascals
What are the main properties of loose powder avalanches?
Acts like flow
High speed (65-100 km/hr)
Light snow often buries victims largely unharmed
What are the main properties of slab avalanches?
Acts like a transitional slide evolving into flow
Speeds of 30-65 km/hr but heavy snow can cause casualties and destroy buildings and railways
Most dangerous avalanche
How many people lost their lives in the 1910 Roger’s Pass avalanche?
58 men - believed to be Canada’s deadliest avalanche.
Why did so many people lose their lives in the 1910 Roger’s Pass avalanche?
Workers were clearing snow from another avalanche that had buried the Canadian Pacific Railway line when another slide came roaring down the other side of the narrow valley with little warning.
In the previous 26 years, how many people were killed by avalanches in the area of the Rogers Pass avalanche?
more than 200 people
How many metres of track were buried by the 1910 Roger’s Pass avalanche?
400 metres
How many metres had a locomotive and plow been hurled by the 1910 Roger’s Pass avalanche?
15 metres (it landed upside down) and the wooden cars behind the locomotive were crushed
