Mass Movements

1. Mass Movements

2. Mass movement- The down slope movement of earth material under the influence of gravity.

3. Figure 10.3 Key role in mass movement is gravity

4. In order for the material to move down slope, it must overcome • Friction –surface resistance to relative motion. • Inertia – tendency of an object at rest to remain at rest.

5. Internal Causes of Slope Failure Inherently weak earth materials

6. 1. Clay – most earth failures in the soil zone are due to clay.

7. Clay layer in a sedimentary rock beach cliff

8. 2. Pyroclastic materials – Light colored tuff from Santorini produced 3500 year ago in Minoan eruption.

9. 3. Shale – sedimentary rock made of clay and silt particles

10. Bedding planes, friend or foe?

11. 54 gal. barrel Internal problems caused by water 1. Adds weight to a slope 8.34 lb/gallon 54 gal. = 457.36 lb!

12. 2. Saturation increases the pore-water pressure Low pore water pressure High pore water pressure

13. 3. Dissolves the cementing material in rock

14. 4. Causes “piping”, the subsurface erosion of earth material

15. The Vaiont Dam Failure Vaiont, Italy 1963 Setting: Dam on river in a glacially carved canyon.

16. Some Factors Leading to Slope Failure • Valley is a syncline • Cavernous weathering of limestone rock • Valley downcutting by river • Slope saturation - water from reservoir and heavy rains.

17. Massive landslide and ~3,000 dead.

18. FallsOccurs when one or more rocks fall down steep slopes.

19. Rockfall, South Island, New Zealand

20. Moonlight Beach, Encinitas

21. Yosemite 1996. 162,000-ton rock mass fell.

22. Yosemite rockfall 1996

23. A common cause of rock fall is frost wedging. Talus cones

24. SlideOccur when a more or less coherent mass moves down slope.

25. Rotational slide- slump • Curved failure plane. • Head moves down, toe moves up and out.

26. Don’t move far or fast

27. In California, triggered by heavy winter rain.Pacific Palisades, CA

28. Another cause of slumps – road cuts.Elks Lodge near Lewiston, Idaho

29. Translational slide – landslide • Mass moves down and out. • Moves far and fast. • Primary causes: heavy rains or earthquakes.

30. Landslide on Big Sur Coast

31. Madison River Canyon Landslide 1959

32. Madison River Canyon from top of slide

33. View from debris dam and visitors center

34. FlowMass loses coherence and moves as a viscous fluid.

35. Earthflow- Downslope movement of a fluid layer of mainly soil and weathered rock material that is saturated with water.

36. Earthflows often begin as slumps. Usually, like slumps, they do not move very fast.

37. Road cut caused slump/earthflow

38. Debris flow- Downslope movement of a fluid mixture of mud and larger rock debris. Venezuela 1999

39. Key differences between earthflows and debris flows: • A debris flow moves faster; meters/second. • A debris flow carries larger rock material.

40. Debris flows can carry large boulders because their density is 1.5 – 2.0 times that of water. http://www.youtube.com/watch?v=8mKC3eID074

41. Causes of debris flows: • Most commonly heavy rains or rapid snow melt. • Volcanic eruptions – a lahar is a type of debris flow. • Earthquakes

42. Debris flow Alps 2004

43. Debris flow after rains in an area previously hit by wildfires in Utah.

44. Debris flow in San Bernardino after 1980 wildfires

45. Amago Creek debris flow after Oct. 2007 fires

46. Debris Avalanche- A very fast moving debris flow occurring on steep slopes.

47. La Conchita 1995 slump/earthflow 2005 debris avalanche