AVALANCHESAVALANCHESASASHAZARDSHAZARDS

WHAT IS AN AVALANCHE?WHAT IS AN AVALANCHE?• An avalanche is a rapid flow of snow

down a slope.• Can be naturally triggered or a consequence

of a human activity. • It occurs in the mountains.

• An avalanche is a mix of water and snow. • Powerful avalanches: ice, rocks, trees.

ROCKSLIDESROCKSLIDES

Slides of rocks or debris, behaving in a similar way to snow, are also referred to as

avalanches (rockslide).

AFTER INITIATIONAFTER INITIATION

After initiation, avalanches usually accelerate rapidly and grow in mass and

volume as they entrain more snow.

If the avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type

of gravity current.

SLAB AVALANCHESSLAB AVALANCHES

Slab avalanches form frequently in snow that has been deposited, or redeposited

by wind. They have the characteristic appearance of a block (slab) of snow cut out

from its surroundings by fractures. Slab avalanches account for around 90% of

avalanche-related fatalities.

POWDER SNOW AVALANCHESPOWDER SNOW AVALANCHES

The largest avalanches form turbulent suspension currents known as powder snow avalanches or mixed avalanches. These consist of a powder cloud, which overlies a dense avalanche. They can

exceed speeds of 300 km/h, and masses of 10,000,000 tonnes.

WET SNOW AVALANCHESWET SNOW AVALANCHES

In contrast to powder snow avalanches, wet snow avalanches are a low velocity suspension of snow and water, with the flow confined to the track surface. The low

speed of travel is due to the friction between the sliding surface of the track

and the water saturated flow.

BIG AVALANCHES IN HISTORYBIG AVALANCHES IN HISTORY

WORLD WAR 1 - ALPSWORLD WAR 1 - ALPS

During World War I, an estimated 40,000 to 80,000 soldiers died as a result of

avalanches during the mountain campaign in the Alps at the Austrian-Italian front, many

of which were caused by artillery fire.

THE WINTER OF TERRORTHE WINTER OF TERROR

In the northern hemisphere winter of 1950–1951 approximately 649 avalanches were

recorded in a three-month period throughout the Alps in Austria, France, Switzerland, Italy

and Germany. This series of avalanches killed around 265 people and was termed

the Winter of Terror.

THE GALTUR AVALANCHETHE GALTUR AVALANCHE

The Austrian village of Galtür was hit by the avalanche in 1999. The village was thought to be in a safe zone but the avalanche was

exceptionally large and flowed into the village. Thirty-one people died.