Slope systems mass wasting with notes

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  • 1. Slope Systems – Mass Wasting
  • 2.
    • The downslope transfer of material through the direct action of gravity
    • Component of erosion and transport of sediment
    • Follows weathering, which weakens and breaks the rock
    Mass Wasting
  • 3. Controls on Mass Wasting
    • Requires weathering of rock or unconsolidated material
    • Primary driving force is GRAVITY
    • Force mechanics: For the rock to move downslope; force (A) has to be greater than the resistance force ( R ) holding it up, e.g. friction.
    • Slope angle influences force ( A ); the greater the angle the greater the force
    A A R R
  • 4. Secondary Controls
    • Factors that either weaken cohesion forces or increase downslope force
      • Saturation of the material
      • Over-steepening of the slope
      • Slope Modification
      • Ground vibrations
      • Expansion/contraction cycles of soil/regolith
  • 5. Saturation of Material
    • Addition of water
    • Lubricates the material (decreases cohesion)
    • Adds weight (increases downslope force)
    • Increases pore pressure (increases downslope force and decreases cohesion)
  • 6. Over-steepening of the Slope
    • Can be human-induced or by natural processes – increases the downslope force.
    • Stream undercutting a valley wall (headward erosion, bank erosion, etc.).
    • Waves cutting cliffs on a shoreline.
    • Construction of roads, buildings, homes etc.
  • 7. Angle of Repose
    • The steepest angle material can create yet remain stable
    • Function of
      • Particle size
      • Particle shape
      • Moisture Content
  • 8. Slope Modification
    • Removal of Vegetation
      • Roots of plants and trees hold regolith together
      • Plants and trees remove water from the soil
      • Removal decrease cohesive force
    • Building of structures
      • Depending upon the structure and the location, either a decrease in cohesive force &/or increase downslope force due to added weight will occur
  • 9. Ground Vibrations
    • Earthquakes – decrease cohesion forces
      • Initiates movement (it is easier for a particle to move than it is to begin to move)
      • Liquefaction induced slides
    • Human induced – blasting for construction, large equipment, etc.
  • 10. Expansion/Contraction Cycles
    • Movement of material due to
      • Wetting and drying cycles
      • Freeze-thaw cycles
  • 11. Daily Assignment Identify five concepts/characteristics and place them appropriately on the Venn Diagram. See next slide for list of concepts/characteristics
  • 12. Concepts/characteristics
  • 13. Classification of Mass Wasting
    • Type of Material
      • Bedrock - Rock
      • Unconsolidated material - Debris
        • Soil
        • Regolith
        • Sediment
    • Rate of movement
      • mm/yr
      • km/hr
    • Type of Motion
      • Fall – free-fall of detached particles, slope steep enough that material falls to base
      • Slide – material remains cohesive and moves along a well-defined surface
      • Flow – material moves downslope as a viscous flow (most are saturated with water)
  • 14.
    • Slow downslope movement – mm/yr
    • Process so slow one cannot observe it in action
    • Expansion/contraction cycles play a key role
    • Enhanced by burrowing organisms, periods of prolonged rains or snow,
  • 15.  
  • 16. Solifaction
    • Special type of creep
    • Occurs in regions underlain by permafrost (permanently frozen, water-bearing ground)
    • During warm periods top portion thaws and becomes saturated
    • Saturated zone flows over frozen layers
  • 17. Debris Flow
    • Mixture of rocks, mud, & water
    • Moves as a viscous fluid
    • Common after heavy rains
    • Rapid movement – up to 50 km/hr, the more water present the faster the rate of movement
    • Common in semi-arid regions and along volcanoes (lahars)
  • 18.  
  • 19. Earthflow
    • A type of debris flow, generally move slower
    • Found on hill sides
    • Can remain active over periods of years
  • 20. Slump
    • Slow to rapid movement
    • Material moves as a coherent unit along a curved surface (spoon-shaped)
    • Blocks of material rotate
    • Debris flows commonly associated with sump
  • 21.  
  • 22.
    • Movement of material along a defined slippage plane
    • Sudden and rapid movement
    • Large blocks of rock detach along bedding planes, joints, fractures, etc.
    • Occur on steep slopes
    • Can be triggered by rain falls or ground vibration
    • Fastest and most destructive type of mass wasting
    Land-Rock Slide
  • 23.  
  • 24. Rock Fall (Rock Avalanche)
    • Sudden and rapid free fall movement of rock
    • Occur on steep slopes
    • Forms talus piles
    • Can grade into a rockslide if material greater a slope greater than the angle of repose
  • 25.  
  • 26. Subaqueous Slope Failure
    • Mass failure in which the material movement occurs below water
  • 27. Dynamics of a slope system