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Mass Wasting (Landslides)
Everything eventually gives in to gravity
What is Mass Wasting?
The process that
transports materials (e.g., bedrock, loose sediment,
and soil) down slopes by
the pull of gravity.
Laguna Hills, CA, 2005
Not always fast
China March 2010 & June 2010
Landslides occur when:
Driving forces > Resisting forces
So what controls these two forces?
(Gravity) (Friction)
Factors that Affect
Slope Stability (Increase driving force or decrease resisting force)
1. Steepness of Slope
2. Composition of Slope
3. Presence of Water
4. Presence of
Vegetation
El Salvador, 2001
1. Steepness
Steeper Slopes = Less Stability
Force of Gravity
Flat Surface
Vertical Cliff
What Makes Steep Slopes?
Volcanoes - Mt. Rainier
Faults - Mercury
What
Makes
Steep
Slopes?
Erosion:
Rivers and Waves
2a. Composition: Weight
Heavier Weight = Less Stability
N. of Seattle (1997)
2b. Composition: Cohesion
Less Cohesive = Less Stability
2c. Composition: Layering
Layering Parallel to Slope = Less Stability
Slope and Composition: Angle of Repose
Angle of Repose: the maximum angle at which a pile of unconsolidated
particles can rest
3. Water
More Water = Less Stability
4. Vegetation
Plants will:
• Add cohesion (roots)
• Removed water
4. Vegetation
Less Vegetation = Less Stability
Summary of Slope Stability
Factor Less Stability If:
1. Steepness Steeper
2a. Comp.-Weight Heavier
2b. Comp.-
Cohesion
Lower Cohesion
(Sediments)
2c. Comp. - Layers Parallel to Slope
3. Water More Water
4. Vegetation Less Vegetation
Human Impacts on Slope Stability
How do we impact?
1. Steepness
2. Composition
3. Water
4. Vegetation
Human Impacts Factor What humans do?
1. Steepness Cut off toes of slope
Human Impacts Factor What humans do?
2. Composition Build on slopes / add
weight
Human Impacts Factor What humans do?
3. Water Change slope drainage
Human Impacts Factor What humans do?
4. Vegetation Remove Vegetation
Lewis County,
WA
2007
Human Impacts on Slope Stability
Factor What humans do?
1. Steepness Cut off toes of slope
2. Composition Build on slopes / add
weight
3. Water Change slope
drainage
4. Vegetation Remove Vegetation
Landslide “Triggers”
Now that you have an “unstable slope” where:
Driving forces = Resisting forces
What does it take to upset the balance
and “trigger” an event?
Landslide “Triggers”
Natural triggers • Rainfall - Storms,
Hurricanes
• Floods
• Earthquakes
• Volcanic eruptions
Landslide “Triggers”
Human triggers • Poor land management
• Steepening &
overloading slopes
• Loud sounds and
construction vibrations
Landslide Triggers in the News
“…it is clear that just
above the top of the
slide, on the right side, the slope has
been regraded, and
indeed there is clearly
some fill in the
exposed slope” 23 May 2010
Landslide Triggers in the News
“On Tuesday evening
at 11:45 pm Peru
suffered a Mw=6.0 earthquake …”
20 May 2010
Landslide Triggers in the News
“Landslide at factory
as region is
by storms” Manchester Evening News -
18 Nov 2009
Landslide Triggers in the News
Norway:
Report release 19 Nov 2008
• Blasting of the cliff during the
construction of the apartment block was responsible for the
failure • This blasting, which was intended
to create the platform for the
building, weakened the hillslope
Landslide Triggers in the News “Southern California's fire-ravaged
areas prepare for possible mudslides” LA Times - 25 Nov 2008
Washington Landslides The Perfect Storm
1996
1. Steep slopes cut by waves and rivers
Washington Landslides The Perfect Storm
2. Wet winters
Washington
Landslides The Perfect Storm
3. “Right” composition
of sediments
Landslide Anatomy
Scarp
Toe
Rupture
Surface or
Plane
Head
Types of Mass Wasting Range of Speeds
Slow
(Creep)
Very Fast
(Rock Fall)
mm-cm’s
per year
10-100’s m
per second
China, 24 Nov 2008
Types of Mass Wasting
What is Moving?
Rock
Debris
Earth (Land)
Mud
How is it Moving?
Fall
Slide
Flow
Slump - Type of slide where material
moves along a curved plane
Avalanche -
A type of rock/debris flow
Creep - A very slow form of flow
Rockfall • Falls
• Slides
• Slumps • Flows
Rt 20, Washington, 2003
Yosemite NP
Talus (Nevada)
• Falls
• Slides
• Slumps • Flows
Rockslide
• Falls
• Slides
• Slumps • Flows
Rockslide
• Falls
• Slides
• Slumps • Flows
Oregon, 2006
Chuckanut Rockslide, 2008
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