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Landslides. !?. !. 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. - PowerPoint PPT Presentation
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Landslides
!
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 withmass 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
Natural creep
Human 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 sub-
glacial 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
RockfallIndividual 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, real-
time 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
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