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Landslides

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.

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Landslides

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  1. Landslides !? !

  2. 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…

  3. 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

  4. 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

  5. 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

  6. Landslide Factors Continued... • Earthquakes and volcanoes • Earthquakes can destabilize slopes • A volcanic eruption can melt glaciers at summit—creating a landslide

  7. “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

  8. Types of Mass Wasting

  9. Flows Natural creep • 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…

  10. 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

  11. 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

  12. 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

  13. Rockfall Individual blocks plummet in a free fall from a cliff or steep mountainside

  14. 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)

  15. 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

  16. 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

  17. Lituya Bay Images • Tsunami inundates 13 km2 of woodland • Sharp line of tree inundation measure of wave height • Wave travels 1.1 km inland

  18. 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

  19. 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

  20. 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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