Transcript 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 • 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