3.2 Flows/Slides

Question 1

What were the causes of the Vajont Dam

Answer 1

1953 landslide occurred on slopes surrounding reservoir behind the dam

• 260m cm3 of debris slip into the reservoir, displacing 50m cm3 of water with waves up to 250m high, overflowing the dam.
• Reservoir was filled, reinforcing the slope. When filled, water seeped into the rock, increasing pressures and reducing cohesion
• Heavy rainfall resulted in 260m cm3 of rock to flow into the reservoir at high speeds, creating 250m high waves. During the first filling, small landslides occured
• In the second filling, 700kcm3 of material slid into the reservoir in 10 minutes
• Responses weak as unable to be stabilised and continued to fill causing small landslides, so the reservoir level had to be lowered
• Heavy rain added volume and weight to the slopes surrounding the reservoir - small EQs also decreased stability
• Deforestation, overgrazing and burning all increased erosion processes from rain and surface runoff.
• Valley hard, alternating rock of limestone and clay - water usually seeps out limestone into clay but filling prevented this
• Joints, cracks and bedding planes in the rock allowed increase water content, leading to sliding. Porous clay was easily saturated so pore pressure forced particles apart, reducing friction and causing soil movement.

Question 2

Impacts of Vajont:

Answer 2

5 Villages destroyed

• 2600 died
• Air displacement due to huge movement and splash - led to further damage
• Dam left unusable due to sedimentation.
• displacing 50m cm3 of water with waves up to 250m high, overflowing the dam.
• 700kcm3 of material slid into the reservoir in 10 minutes

Question 3

California landlides - causes and impacts

Answer 3

Slides and flows common

• Intense rainfall and storms
• Soft and poorly consolidated rock
• Steep relief
• Road and housing construction on slopes
• Oil and water extraction
• Eargquakes
• Hurricane force gusts
• 4 inches of rain in 2 days
• Climate change increased wildfire requency
• Deforestation from wildfires

Impacts:
23 deaths, 163 injured
-20k power lost
-Damage to 400 homes
-$177m damage
-$43m to clean up
-Damaged roads, railways, pipes, electricity and other infrastructure

Attempts to reduce the impact were made using past landslide features, detailed evaluations to map hazard zoning and mapping.
Rock bolts, nets and shotcrete not possible as slopes too soft and unconsolidated

Question 4

Aberfan causes

Answer 4

1966 mudflow in Wales.
Mining on mountain left mounds of discarded coal and silt which led authorities to believe a coal slurry would occur from this tip however was ignored.
-Phone lines down during disaster so could not warn officials
-Coal tip was on mountain spring meaning the base bacame saturated
-3 weeks of severe rainfall weeks prior turned the tip into slurry - 6.5 inches
-Rainfall increased pore pressure, decreasing shear strengh of the tip and increasing its vulnerability
-Waste tip and slag was 35m high and increased the slope angle. It was built on a natural spring where the water table was at the surface, adding water for mobilisation.

Question 5

Aberfan impacts

Answer 5

144 died - 116 children as junior school destroyed, polluted wider landscape, colliery closed - primary source of income, farmland destroyed

• £500 per family compensation
• Tips removed costing £500k