Mass Movements Flashcards
(59 cards)
What can increase shear stress in a slope?
Heavy rainfall, pore water pressure, erosion of cliff foot (reduction of lateral support), transient stresses
What can cause a reduction in shear strength?
Removal of vegetation, dry weather, pore water pressure, restructuring due to human activity, animals burrowing
How can shear strength be increased?
Afforestation, drainage, restrictions on drilling/quarrying, fences
How can shear stress be reduced?
Drainage, defences at foot of slope, restricted construction on conditionally stable slopes
When did the Venezuelan mudslides take place?
15th-16th December 1999
Why were many people near Caracas at risk?
Shanty towns had grown up steep valley slopes
What caused the 1999 Venezuelan mudslides?
Heavy rainfall; 40-50% above normal during the first two weeks of December
Why was there so much material in the Venezuelan mudslides?
High rainfall and temperatures leads to high rates of chemical weathering and therefore deep soils; 20-30m deep
Define mudslide.
A fast slide of mud, often after heavy rainfall, with high water content and a basal slide plane
The 1999 mudslides were the worst disaster in Venezuela in…
…200 years
Define shear strength.
The internal resistance of a body to movement
Define shear stress.
The force acting on a body that causes movement of the body downslope
What does pore water pressure do to a slope?
Reduces the cohesion of particles and decreases the frictional strength of the slope material
Where do mass movements often occur?
Where there are weak rocks, steep slopes and/or active undercutting
How m at people were made Ho else’s due to the Venezuela mudslides?
Over 150,000
How did most of the victims of the Venezuela mudslides die?
They were buried in mudslides 8-10m deep
What were some of the secondary impacts of the Venezuela mudslides?
Destroyed infrastructure, buried crops, impact on tourist industry
After the mudslides had run their course for a while, they turned into debris flows that included…
…water, sand, gravel, boulders, trees and other materials
Why are the Seward facing slopes in Vargas so steep?
The summit ridge (max 2700m) is just 10km from the sea
What was the geology of the mountains in Venezuela like?
The mountains were mantled in an easily eroded clay which made up the body of material that moved
What effect may widespread deforestation in Venezuela have had on the slopes?
Destabilised as roots increase shear strength
Why was it difficult to get some supplies to Vargas after the mudslides?
Caracas airport temporarily closed, costal highway destroyed, La Guaira seaport flooded
Why did people settle on alluvial fans and what are they?
Fan shaped deposits of debris where landslides have reached and extended into the sea; people settled here due to rapid population growth
How much of Venezuela’s population lives on the coast?
70%; people don’t want to live in the interior of the country, however governments tried to relocate refugees there
