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MASS WASTING

MASS WASTING

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MASS WASTING

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  1. MASS WASTING

  2. SURFICIAL PROCESSES • Erosion, Transportation, Deposition on the Earth’s Surface • Landscapes created and destroyed • Involves atmosphere, water, gravity • Agents: • Mass wasting (gravity), Running water (streams), glaciers (ice), wind, water waves, ground water

  3. MASS WASTING • Masses of debris (mud, sand, gravel) or bedrock moving downhill • Landslides and slower movements • Driven by GRAVITY

  4. Classification of Mass Wasting • RATE of MOVEMENT • Extremely slow (~1mm/year) to very rapid (>100 km/hour) • MATERIAL • Bedrock • Debris- (“soil”, sediment)

  5. Classification of Mass Wasting • TYPE OF MOVEMENT • Flow • Slide • Translational slide • Rotational slide (Slump) • Fall

  6. Classification of Mass Wasting • TYPE OF MOVEMENT • Flow • Slide • Translational slide • Rotational slide (Slump) • Fall

  7. Controlling Factors • 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

  8. Controlling Factors • Climatic controls • Ice- above freezing vs freeze & thaw • Water in soil- film around grain vs saturation • Precipitation- frequent but light vs periods of drought and heavy rainfall • Vegetation- heavily vegetated vs light or no vegetation • Gravity • Shear force- parallel to slope, block’s ability to move • Normal force- perpendicular to slope, block’s ability to stay in place due to friction • Shear strength- resistance to movement or deformation of debris

  9. The Effect of Slope & Gravity G=gravity S=shear F=friction N=normal F S G N

  10. Controlling Factors • Water • adds weight • increased pore pressure in saturated debris decreases shear strength • surface tension in unsaturated debris increases shear strength • Triggering Mechanisms • Overloading • Undercutting • Earthquakes

  11. Common types of mass wasting • CREEP • 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

  12. Solifluction & Permafrost • Solifluction: • Flow of water saturated debris over impermeable material • Permafrost: • Ground that remains frozen for many years

  13. Common types of mass wasting • DEBRIS FLOW • Motion taking place throughout moving mass • Includes • Earthflow • Mudflow • Debris Avalanche

  14. Earthflow • Primarily flow of debris • may involve rotational sliding • Scarp above • Hummocky surface in lower part • May be slow or fast • Solifluction • role of Permafrost in cold climates

  15. Mudflow • Flow of watery debris • Occurs where lack of vegetation: • Dry climates • Volcanoes • After forest fires

  16. Debris Avalanche • Very rapid, turbulent flow of debris • mud-boulders • >150 km/hr • Triggered by • volcanic eruptions- Mt. St. Helens 1980; Nevado del Ruiz 1985 • intense rainstorms- Venezuela 1999 • earthquakes- Japan 2000

  17. Rockfalls and Rockslides • Rockfall • 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)

  18. Debris Slides and Debris Falls • Debris fall • Free-falling mass of debris • Debris slide • Debris moving along a well-defined surface

  19. The St. Francis Dam The dam stood 180 feet high and 600 feet long Curved Concrete Structure

  20. On March 12, 1928, after its reservoir reached full capacity for the first time, the St. Francis Dam began to leak. At 11:57 PM, the dam collapsed, sending 12 billion gallons of water raging through the narrow San Francisquito Canyon into the Santa Clara Valley. Designed and built two years earlier by William Mulholland to store water brought by the Los Angeles Aqueduct from Owens Valley. Its failure resulted in a flood which killed over 450 people and destroyed buildings, bridges, railroads, and farms. The St. Francis was only one of 19 dams that Mulholland had constructed to store Los Angeles' water supplies.