Lecture 21 - Avalanches

Question 1

What is an avalanche?

Answer 1

rapid flow of snow down a hill or mountainside

Question 2

At what time of year do most avalanches tend to happen?

Answer 2

In the winter, particularly from December to April. However, avalanche fatalities have been recorded for every month of the year.

Question 3

How does a snow avalanche start and develop?

Answer 3

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.

Question 4

How much can a large, fully developed avalanche weigh?

Answer 4

as much as a million tons

Question 5

How fast can a large, fully developed avalanche travel?

Answer 5

faster than 320 km/h, i.e. faster than the fastest skier

Question 6

When do avalanches occur?

Answer 6

when layers in a snowpack shear

Question 7

What is snowpack?

Answer 7

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.

Question 8

How do snowpacks shear?

Answer 8

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.

Question 9

What are the two main types of avalanche?

Answer 9

loose powder (“sluffs”)
slab

Question 10

When do loose powder (“sluff”) avalanches occur?

Answer 10

They occur when the weak layer of a snowpack is on the top.

Question 11

What is a sluff avalanche?

Answer 11

A sluff is a small slide of dry, powdery snow that moves as a formless mass.

Question 12

What is the more dangerous type of snow avalanche?

Answer 12

slab avalanche

Question 13

When do slab avalanches occur?

Answer 13

They occur when the weak layer lies lower down in a snowpack. This layer is covered with other layers of compressed snow.

Question 14

What happens during a slab avalanche?

Answer 14

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.

Question 15

How many avalanches are there each year in the mountains of the western United States?

Answer 15

about 100,000 avalanches each year

Question 16

How many people are killed worldwide each year by avalanches?

Answer 16

more than 150 people - most are snowmobilers, skiers, and snowboarders

Question 17

In what ways can snowpack on slopes fail due to gravity?

Answer 17

Creep
Fall
Slide
Flow

Question 18

What slopes does an avalanche path involve?

Answer 18

Starting zone - steepest slope (30-45°)
Track - guided by topography (20-30°)
Run out zone (<20°)

Question 19

What is the typical mass, run and force of a size 1 avalanche?

Answer 19

relatively harmless to people
mass = 10 tonnes
run = 10 metres
force = 1 kilopascal

Question 20

What is the typical mass, run and force of a size 2 avalanche?

Answer 20

could bury, injure or kill a person
mass = 100 tonnes
run = 100 metres
force = 10 kilopascals

Question 21

What is the typical mass, run and force of a size 3 avalanche?

Answer 21

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

Question 22

What is the typical mass, run and force of a size 4 avalanche?

Answer 22

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

Question 23

What is the typical mass, run and force of a size 5 avalanche?

Answer 23

mass = 100,000 tonnes
run = 3,000 metres
force = 1,000 kilopascals

Question 24

What are the main properties of loose powder avalanches?

Answer 24

Acts like flow
High speed (65-100 km/hr)
Light snow often buries victims largely unharmed

Question 25

What are the main properties of slab avalanches?

Answer 25

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

Question 26

How many people lost their lives in the 1910 Roger’s Pass avalanche?

Answer 26

58 men - believed to be Canada’s deadliest avalanche.

Question 27

Why did so many people lose their lives in the 1910 Roger’s Pass avalanche?

Answer 27

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.

Question 28

In the previous 26 years, how many people were killed by avalanches in the area of the Rogers Pass avalanche?

Answer 28

more than 200 people

Question 29

How many metres of track were buried by the 1910 Roger’s Pass avalanche?

Answer 29

400 metres

Question 30

How many metres had a locomotive and plow been hurled by the 1910 Roger’s Pass avalanche?

Answer 30

15 metres (it landed upside down) and the wooden cars behind the locomotive were crushed