Lecture Thirty One - Landslides, Avalanches and Lahars

Question 1

Describe the effects of gravity on geological features.

Answer 1

Sediments are only stable at certain angles - angles of repose.
Varies depending on the nature of sediments - the rounder and smoother the sediments, the lower the angle of repose.
When the angle of repose is exceeded, the only force holding something down is friction.
When friction is overcome (by force, fluid, vibration, gas) potential energy is converted to kinetic energy, and sediments move.
Angle of repose is reduced by the presence of ‘fluids’ (water, mud, gas).

Angle of repose for:
Fine sand = 35 degrees.
Course sand = 40 degrees.
Angular pebbles = 45 degrees.

Question 2

What is the order of highest to lowest velocity regarding mass movements of geological features.

Answer 2

Very fast - Rock/debris fall -> Snow/debris avalanche -> Debris/rock slide -> Mud flow -> Slump -> Solifluction, creep or rock glacier - Very slow.

The velocity of movement is affected by the angle of the slope and the size of what is moving.

Question 3

Describe avalanches.

Answer 3

Cohesive flows of rick, solid, or ice/snow that flow down mountains.
usually triggered by a single event, such as an earthquake, eruption, noise or by too much weight.
Avalanches are air-lubricated, and act as a fluid once they reach the valley floor - they can flow many kms.
Travel at speeds of up to 200km/h and can just over obstructions in their way.
May either be rock (rock avalanche), snow (snow avalanche), soil and rock (debris avalanche) or any combination.

Debris avalanches:
May be triggered by bulging in the sides of volcanoes as magma moves towards the top.
E.g. Mount St Helens.
Volcanic debris avalanches form hummocks when they stop moving.

Question 4

Describe landslides.

Answer 4

Movement of rock, solid and other debris own non-verticle slopes (often repeatedly).
Not air-lubricated, therefore do not flow as quickly as avalanches.
Main cause is gravity, but slopes can be weakened by:
- Erosion.
- Water saturation.
- Earthquakes.
- Excess weight from snow fall or rock falls.
- Human action - ore piles, deforestation.

Can also trigger earthquakes.

Warning signs includes:

• Cracks opening in hillsides.
• Slow downhill movement of rock or solid.
• Tilting of trees, rocks, poles or walls.
• Formation of sags and bumps in slopes.

Question 5

Describe Lahars, Mud Flows and Debris Flows.

Answer 5

Water lubricated rivers of ash, mud, rocks and any mixture of these.
Triggered by rainfall and water loading of sediments.
Flow downhill as a ‘slurry,’ following existing channels.
travel at several ams from source.
Fill and choke channels as they slow and stop.
Carry sediments up to house-sized boletus and can slow at speeds of up to 100 km/h.

Question 6

Can these events be predicted or prevented?

Answer 6

Snow avalanches can be predicted and deliberately triggered to prevent accidental triggers.
Rock avalanches unpreventable, but can sometimes be predicted.
Specific signs before landslides occur, cracks, tilting etc. Prevent deforestation and water pipe leaked upslope.
Lahars can be channelled and redirected past vulnerable areas.