Mass wasting 2

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  • 1. Mass wasting refers to several processes that have the following in common: 1. Down slope movement of rock or weathered material 2. Movement is due to pull of gravity 3. There is no flowing medium (water, air or ice) that carries the material *Generally, mass wasting occurs when gravitational forces exceed frictional or shear forces (strength) of material.
  • 2.  Shear force (stress) – apparent force exerting a pdowns2lope ull on the object  Normal force – apparent force exerting a pull perpendicular (normal) to the slope  Shear strength (friction) – frictional resistance of the slope material to down slope motion
  • 3. Gravity Shear stress Normal force Shear strength (friction) If the shear stress acting on the slope exceeds the slope’s normal force plus shear strength, what should happen to the slope materials? Mass Wasting Variables
  • 4. Classification of Mass Wasting Type of Material(composition) •Bedrock – Rock •Unconsolidated material – Debris Soil Regolith Sediment Rate of movement(speed) •mm/yr •km/hr Water content (lubrication) •adds weight •increased pore pressure in saturated debris decreases shear strength •surface tension in unsaturated debris increases shear strength Type of Motion (mechanism of movement) 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)
  • 5. Slope angle- gentle vs steep Local relief- low vs high Thickness of debris over bedrock- slight vs great Planes of weakness ( in bedrock) bedding planes; foliation; joints planes at right angle to slope vs parallel to slope most dangerous
  • 6. gentle slopes vegetation slows movement very slow flow (< 1 cm/year) facilitated by water in soil or by freeze-thaw in colder climates Indicators of creep ‘pistol butt’ trees leaning tombstones, walls, posts
  • 7. Rock fall Bedrock breaking loose on cliffs Talus at base of cliffs Rockslide Bedrock involved Sliding along planes of weakness parallel to slope Bedding planes; foliation planes; fractures in rock (joints)
  • 8. 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
  • 9. 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
  • 10. Flow of watery debris Occurs where lack of vegetation: Dry climates Volcanoes After forest fires
  • 11. 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)
  • 12. 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
  • 13. Solifluction: Flow of water saturated debris over impermeable material Permafrost: Ground that remains frozen for many years
  • 14. common to Arctic regions CIIII Arctic regions
  • 15. TypeofMovement Classification Material Velocity Creep Debris Imperceptibly Slow Earth Flow Debris Slope and Material Dependent <5 km/hrMudflow Saturated Debris Avalanche Debris or Rock Very Fast 100 km/hr Rotational Slide Debris Slow-mod. (short) Rock Slide Bedrock Fast Debris Fall Debris Fast FlowSlideFall Rockfall Bedrock Fast