11/28/2012 1 MASS MOVEMENT Landslides Mass Movement Mass Movement: Movement of earth materials (bedrock, sediment, soil) downslope by the force of gravity. Erosion: Movement of earth materials by a medium such as wind, water or ice. (Differs slightly from book which includes both weathering and mass movement in definition of Erosion) Mass Movement Causes of Slope Instability (leading to Mass Movement) Gravity, Friction, Slope Angle Slope Composition • Solid rock – deformation, weathering, daylighted beds, solubility, jointing, rock cleavage, foliation. • Unconsolidated Materials – angle of repose. • Vegetation. • Water- reduces friction and cohesion, adds weight, saturation. Causes of Slope Instability Relationship to Gravity Mass Movement occurs when gravity that is directed downslope (Gd) overcomes friction and gravity (Gp) that resists downslope movement. A component of friction is weight of the material Steeper the slope, the more instability Cutting away the toe makes slopes steeper
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MASS MOVEMENT - City College of San Francisco · 11/28/2012 1 MASS MOVEMENT Landslides Mass Movement Mass Movement: Movement of earth materials (bedrock, sediment, soil) downslope
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MASS MOVEMENT
Landslides
Mass Movement
Mass Movement: Movement of earth materials (bedrock, sediment, soil) downslope by the force of gravity.
Erosion: Movement of earth materials by a medium such as wind, water or ice.
(Differs slightly from book which includes both weathering and mass movement in definition of Erosion)
Mass Movement
Causes of Slope Instability (leading to Mass
Movement)
Gravity, Friction, Slope Angle
Slope Composition
• Solid rock – deformation, weathering, daylighted beds,
solubility, jointing, rock cleavage, foliation.
• Unconsolidated Materials – angle of repose.
• Vegetation.
• Water- reduces friction and cohesion, adds weight,
saturation.
Causes of Slope Instability
Relationship to Gravity
Mass Movement occurs when gravity that is directed downslope (Gd) overcomes friction and gravity (Gp) that resists downslope movement.
A component of friction is weight of the material
Steeper the slope, the more instability
Cutting away the toe makes slopes steeper
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Causes of Slope Instability
Slope Composition (Cohesion)
Bedrock
Unconsolidated materials
Vegetation
Water
Bed Rock Unconsolidated Materials
Vegetation
• Native vegetation stabilized slope. • Non-native tree destabilized, along with heavy rainfall.
Vegetation
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Water Water
Triggering Mass Movement
Natural Earthquakes
Volcanic eruptions - Lahars
Heavy Rainfall
Erosion of slopes
Human Steepening slopes
Loading
Saturating
Cutting vegetation
Loud Noise
Earthquake Triggered
Madison Mountain, Earthquake Lake; Montana
A large eruption can produce a mudflow known as a lahar. These occur when the volcanic ash and debris become water-saturated.
A lahar developed (Toutle River, WA) when Mt. St. Helens erupted. The volcano contained large amounts of water in the form of snow and glaciers that melted nearly instantaneously during the major eruption.
The resulting debris flow moves down valleys and can be very destructive.
Triggered by Volcanic Eruption
In 1985, 20,000 people were killed in Colombia by a lahar
when Nevado del Ruiz erupted.
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Triggered by Heavy Rainfall Triggered by Erosion of Slope
Triggered by Erosion of Slope Slope Condition – Day-lighted Beds