Threat of Natural Hazards
- Active and relict glacial landscapes are inherently dangerous:
- Avalanches
- Rock falls
- Debris slides
- Flooding
- Increased populations, developments, and tourism put people at risk within these areas
Avalanches
- Avalanches are rapid movements of snow down a slope, and are common in mountainous areas
- Avalanches occur when shear stress exceeds the shear strength of a mass of snow on a slope, which is linked to the snow's density and temperature
- Two types of pack snow failure:
- Loose snow - small amounts of snow move and roll down the slope
- Slab avalanches - large 'slabs' of snow break away from underlying snow
- This type of avalanche is the most dangerous
- A large, fully-developed avalanche can weigh up to a million tonnes
- Created when higher temperatures are followed by a freeze, creating an 'ice crust' on the surface, which is unstable
- Avalanches start with a glide and then accelerate to speeds of 320km per hour (200m per hour), picking up even more snow as it rushes downhill
- Occur most frequently on slopes over 22°
- Also, occur on north-facing slopes where the lack of snow limits snow stability (doesn’t have time to bind with partial melting)
- Three types of avalanches are:
- New snow can fall off older snow (dry avalanche)
- Or partially melted snow moves (wet avalanche)
- Mostly from slab avalanches (powder avalanches)
- A powder cloud forms with a large quantity of snow being suspended in the air
- These are the most dangerous of avalanches
- All avalanches are a significant hazard and kill approximately 200 people a year
Lahars
- A lahar is a large mud and debris flow as a result of volcanic activity
- Nevado del Ruiz, Colombia is known locally as the ‘sleeping lion’ and has not erupted for nearly 150 years
- In the early hours of November 13th, 1985 it erupted
- A combination of seismic activity, glacial instability, and extreme temperatures all combined to form one of the deadliest mudflows in history
- Occurred during the night when most of the 27000 residents of Armero in Tolima were asleep
- Armero was virtually destroyed by the 3-8m of mud and rubble that swept onto it and killed an estimated 23,000 people
Glacial outburst floods (GOFs)
- Powerful floods caused by the sudden release of meltwater from a subglacial or ice moraine-dammed lake
- Can be trigged in six ways:
- Overflow and melting of the ice dam
- Breakdown of ice dam due to tectonic activity
- Increased rise in water levels
- Mass movement creating an overtopping of the moraine dam through displacement
- Catastrophic failure of moraine dam by slow ice melt within the moraine
- Increased water pressure enlarging pre-existing tunnels beneath the ice dam
- GOFs can be seasonal and dams can reform each year
- These outbursts are sudden and pose a threat to property and people around the world
- Meltwater can travel hundreds of kilometres from its sources and areas such as the Alps, Iceland, the Andes, and the Himalayas have a long history of GOFs
- Periglacial areas are also seeing increased ground subsidence due to ice melt and flooding, risking new areas to sudden flooding