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Natural Disasters

Natural Disasters. Mass Wasting: Landslides http://www.youtube.com/watch?v=BKTG58Bpziw http://www.youtube.com/watch?v=H6Ma0SVjMHA http://www.youtube.com/watch?NR=1&v=R__3DYQCVnA. What is a Mass Wasting?. Downslope movement of soil or rock . Anatomy of a Landslide.

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Natural Disasters

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  1. Natural Disasters Mass Wasting: Landslides http://www.youtube.com/watch?v=BKTG58Bpziw http://www.youtube.com/watch?v=H6Ma0SVjMHA http://www.youtube.com/watch?NR=1&v=R__3DYQCVnA

  2. What is a Mass Wasting? • Downslope movement of soil or rock

  3. Anatomy of a Landslide • Scarp is crescent-shaped cliff at upslope end • Bulge is usually at toe where soil accumulates • Tensional crack at top

  4. Mass Wasting Hazards

  5. Mitigation - Identification

  6. Factors that Influence Slope Stability • Gravity – main force responsible for mass wasting • Pulls object toward the center of the Earth

  7. Gravity • Force of gravity can be broken into two components: • Force acting perpendicular (gp) • Helps hold object in place on slope • Tangential component (gt) • Shear stress pulling object down-slope

  8. Shear Strength • Shear Strength – forces resistingmovement down slope • Two components: • Frictional resistance • Cohesion • When shear stress becomes greater than shear strength, slope failure will occur Fs = shear strength/shear stress If Fs < 1, slope failure is expected

  9. Example Problem • If a body of rock has a shear stress of 35 and a shear strength of 25, what is the force? 255 35 7 Strength Stress F = F = = • Will slope failure occur? Yes

  10. Factors Controlling Mass Wasting Slope Angle Pore Water Material Orientation

  11. Slope Angle • Angle of Repose • The angle at which loose material sits at rest The Angle of Repose increases with size of grain

  12. Pore Water • Water held between pore spaces • Slightly wet may provide surface tension • When saturated, adds weight and provides lubrication – grains move freely

  13. Material • Clays are expansive and hydrocompacted • Water enters crystal structure and volume increases • Loss of water causes volume to decrease (shrink)

  14. Material • Salts that hold clay structure may dissolve when water is added causing clays to compact

  15. Orientation • When layered rocks dip in the same direction as slope, failure is likely • Most stable slopes are when rock layers dip upslope

  16. Types of Mass Wasting Slow Movement Creep Rapid Movement Landslides Slumps Debris flows Rock falls

  17. Creep • Continuous gradual downhill movement of slope material

  18. Landslides • Blocks of rock and solid move down along a well-defined plane

  19. Slumps • A mass of material sliding along a curved, rotational surface (shaped like a spoon

  20. http://www.youtube.com/watch?v=mknStAMia0Q

  21. Debris Flows • Downslope movement of a viscous (thick) fluid

  22. Rock Falls • Free fall of detached pieces of material of any size

  23. Triggering Events Shaking Modification of Slopes Undercutting of Slopes Changes in Hydrologic Cycles Volcanic Eruptions

  24. Shaking • Sudden movements may trigger instability • Friction is lost when water gets between grains • Solid ground acts like a fluid

  25. Yungay, Peru • Nevados de Huascarán - Peruvian Andes Mtns. • 7.7M earthquake • 45 seconds of shaking • Large block failed producing a debris avalanche • 18,000 people buried • 600+ died on opposite side of slope

  26. Modification of Slope • By humans or natural causes, may change slope angle • Mass wasting event can restore stability of slope

  27. Elm, Switzerland • Large demand of slate for blackboards • Mining for slate began • Mass of rock collapsed and bounced uphill • 115 people died

  28. Changes in Hydrologic Cycles • Heavy rains, rapid snow melt, changes in groundwater can saturate grounds • Increase fluid pressure

  29. Vaiont Reservoir, Italy • Dam was built with steeply dipping clay layers • Heavy rain added weight to rocks above dam • Large block fell into reservoir displacing large amounts of water • Waves killed nearly 3000 people

  30. Volcanic Eruptions • Eruptions may cause rapid snow melt; thus enhancing the probability of a debris flow

  31. Nevado del Ruiz, Colombia • Town of Armero end of valley at base of volcano • Small eruption triggered rapid snow melt • 23,000 people died

  32. Prediction & Hazard Assessment • Past evidence may leave distinctive clues of previous landslides • Hazard maps help locate areas prone to mass wasting • Can be used to make decisions for land use policies

  33. Mitigation - Prevention • Steep slopes can be graded into gradual slopes • Retaining walls can be built • Drain pipes to reduce pore water

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