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Landslides
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Mass Wasting
• Downslope movement
of earth materials
• Generally gravity driven
• Generally result from
undercutting of a slope
– Either natural or human
induced
• Landslides
– General term for all
types of mass wasting
Not to be confused with
mass wasted…
Landslide Factors
• Steepness of slope
– Steep slopes are
generally unstable
• Vegetation
– Roots hold soil together
and absorb water
– Vegetated slopes
generally more unstable
than non-vegetated
Landslide Factors Continued...
• Water
– Sandcastle analogy
• Nature of
unconsolidated stuff
– Angle of repose—
maximum slope at which
loose material remains
stable
– Higher for angular rocks
Landslide Factors Continued...
• Type of rock and
orientation of rock
layers
– Sedimentary rocks
dipping in same
direction of slope = bad
– Sedimentary rocks
dipping in the opposite
direction of slope = good
Landslide Factors Continued...
• Earthquakes and
volcanoes
– Earthquakes can
destabilize slopes
– A volcanic eruption can
melt glaciers at
summit—creating a
landslide
“You’re just my type.”
• Flow
– Loose, unconsolidated sediment/soil moves in a
fluid-like way
• Slide
– Movement of a coherent block of material along a
fracture
• Fall
– Rapid, free-fall motion
Types of Mass Wasting
Flows
• Creep
– Slow downhill flow of
rock or soil under the
influence of gravity
– Very slow—1cm/yr
– Shallow stuff moves
more quickly than
deeper stuff
– Pistol butt trees and
leaning fences
Human creep…
Natural creep
Flows: mudflows
• A flow composed entirely of fine-grained
sediment
• Lobe shaped deposits
• Can be wicked fast—60mph
• Two types
– Lahar—volcanic ash mobilized by water
– Jokulhlaup—large release of water from a subglacial lake
The Armero Tragedy
• Armero, Columbia: 1985
• Small eruption from nearby volcano, Nevado del
Ruiz melted the glacier on top of the volcano
• Water from the glacier mixed with volcanic ash
creating a lahar
• “People can evacuate if they feel like it…”
• ~29,000 people killed
• 80 yrs previous, town buried by mud flows
Slides
• Slump
– Occurs when blocks of
material slide downhill
over a curved fracture
– Rotated trees
– Jumbled, hummocky front
• Rockslide ( aka rock
avalanche
– Bedrock slides downslope
over a fracture plane
– Breaks up—jumbled
chaotic mess of rock
– Fast 500 km/hr
Rockfall
Individual blocks
plummet in a free fall
from a cliff or steep
mountainside
Lituya Bay, Alaska:
Rock Fall Example
• July 9, 1958
– 8.0 earthquake on
Fairweather Fault
– Rock fall (slide?) from
face of Gilbert Inlet
slams into Lituya
Glacier removing
1,300 ft (1km fall)
• Total volume = 30 million m3
– Wave was generated 1720’ tall (525m) which
swashed back and forth due to the Bay’s shape
– 100 ft (30m) tall when reaches inlet
Out of 6 boaters in the
bay, 2 killed, 2 tossed
over the spit into the
Pacific, two completely
unharmed
8X greater than
any tsunami
Speed of wave
97-130 mph
Lituya Bay Images
• Tsunami inundates 13 km2 of woodland
– Sharp line of tree inundation measure of wave
height
– Wave travels 1.1 km inland
Landslide Prevention: Intro
• Consequences of
construction
– Land more susceptible to
mass wasting
• Undercutting—removing
support for upper part of
slope
• Vegetation removal
• Weight of buildings
• Irrigation—adding water
Landslide Prevention 1
• Preventative measures
– Retaining walls with
drain pipes
– Terraces
– Planting fast growing
grasses and shrubs
– Sloping “sheds” or
tunnels
– Building roads in low
landslide risk areas
Landslide Prevention 2
• Radio-transmitted, realtime monitoring of areas
that are prone to
landslides
– Especially places where
roads might be affected
• Bedrock “stitching”
– Basically drilling holes into
bedrock and reinforcing
with concrete and steel
cable
• Education