MAss wasting

• Mass Wasting


  All processes by which rocks, soil, and debris move downhill under the influence of gravity
• Mass Wasting


  Wasting on a massive scale of the outer crust of the earth, in a constant state of movement towards the lower heights
• How to Trigger Mass Wasting


  1. Volcanic eruption

  2. Earthquakes

  3. Heavy rains

  4. Weathering and erosion

  5. Human activities

  6. Gravity
• Factors Affecting Mass Wasting

  • Cohesiveness of the soil or rock strength
  • Climate condition
  • Geological features
  • Vegetation
• Cohesiveness of the slope


  The cohesiveness of the slope resists the force of gravity. Mass wasting occurs when the force of gravity exceeds the strength of cohesion.
• Factors affecting cohesiveness


  • Nature of slope material
  • Amount of water in slope material
  • Steepness of the slope
• Oversteepened slopes


  Created when a stream undercuts a valley wall or when waves started to pound on a base of a cliff
• Stable slope angle (Angle of Repose)


  The steepest angle at which the pile of unconsolidated, coarser particles remain stable and controlled by the frictional force and minor electric charge attraction between grains

  
• Angle of Repose


  The higher the angle of repose, the more likely mass wasting will occur since friction will not be able to counter gravitational force
• Angle of repose for dry materials

  Increases with the size of the grains
• Slightly wet unconsolidated materials


  Exhibit a very high angle of repose due to the surface tension of water as it holds the grains together
• Water


  Adds weight to the material which may introduce faster flow or downward movement
• Climate condition


  The rate of precipitation affects the water content of the soil. Mountains and hills that experience more rain are more susceptible to mass wasting.
• Geological feature


  The type of rock present in the area may affect the debris flow. The presence of joints, fractures, and bedding planes may affect the occurrence of mass wasting.
• Vegetation


  Plants have extensive root systems which hold the soil and regolith together, preventing mass wasting or soil erosion. Plants modify the water content of their environment by transpiration.
• Other Triggering Events


  • Earthquakes
  • Volcanic eruptions
  • Minor shocks (from heavy vehicles or man-made explosions)
  • Excessive Rainfall
  • Human Actions (building infrastructure, undercutting slopes)
• Mass Wasting


  Can be classified according to the rate of movement (slow, slow-fast, fast), type of material involved (rocks, soil, debris), and the way of movement (falling, sliding, or flowing)
• Creep


  • The slow but inexorable movement of material down any slope due to repeated expansion and contraction caused by freezing, thermal expansion, or seasonal water saturation
• Creep


  • Bedding deformation, bent trees, disturbed telephone poles and monuments, and hummocky terrain
• Solifluction


  Happens when water-saturated soil moves downslope, particularly common in regions where permafrost prevents water from percolating downward
• Slump


  A mass movement process of slope failure, in which a mass of rock or unconsolidated material drops along a concave slip surface
• Falls


  Abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs
• Rockfall


  • Rocks fall from steep surfaces to form talus, may result from undercutting, frost wedging, or chemical weathering
• Debris Fall


  • Just like rockfall but with unconsolidated regolith instead of rocks
• Flows


  Mass movements in which material flows fluidly, i.e. it loses all traces of its original structure. These are very dangerous since they move very quickly, can transport very large objects, and cover great distances.
• Mudflows


  • The most fluid of flows, consisting of at least 50% silt and clay and up to 30% water
• Debris Flows


  • Fast, transport larger particles and less water consequently are less rapid and typically are not confined to existing drainages
• Avalanches


  • A special case of flows in which falling material is mixed with large volumes of air and glides across a layer of trapped air, can move at tremendous speeds (up to 200 mph)
• Slide


  Movement of coherent blocks or a few intact materials along fractures, faster than flow but may still take time for the debris to move completely
• Rockslide


  • If the moving material is rocky
• Debris Slide


  • If the moving material is unconsolidated regolith
• Mitigating Measures


  • Hazard maps
  • Engineering measures
  • Soft mitigating
• Nowhere is it written that a mass wasting event can't combine more than one of these processes. The event that destroyed the town of Yungay began as an ice fall, became an avalanche, and ended up as a combined avalanche and mudflow.