GSC 1620 Chapter 8

1. GSC 1620 Chapter 8 Mass Movements (Wasting)

2. Mass Wasting • Mass Wasting – downhill movement of Earth materials under the influence of gravity • Annually in the U.S. average mass wasting property damages exceed 4 billion dollars and typically 50 or more people are killed • Landslide is a colloquial term used by most people to describe all types of mass wasting (see slides)

3. Venezuela, 1999 ~30,000 deaths from rain induced debris flows

4. Venezuela, 1999 Note the large boulders moved by the debris flow

5. Mass Wasting • The adjacent photos show the effects of mass wasting in Laguna Beach, CA (6/1/05) • The average price of homes in this community is 1.75 million dollars!

6. Mass Wasting • Gravity – force of attraction between two bodies; the magnitude of this force is dependent only on the masses of the bodies and their separation distance (see slide) • The Earth’s gravity pulls objects toward its center

7. A B Distance Gravity Force = Constant x (Mass A x Mass B) Distance2

8. Background • On sloped surfaces, the force of gravity can be conceptually subdivided into the shear force (stress) and normal force (see slides)

9. 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 waste or not? Shear stress Normal force Gravity

10. Shear strength (friction) Normal force Shear stress Gravity Notice how the shear stress increases and the normal force decreases as the slope angle increases

11. Angle of Repose • Angle of repose – maximum slope angle at which particles remain stable; for loose materials this angle typically varies from 25 – 40 degrees • If the slope material’s angle of repose is exceeded by natural or human forces, portions of the slope should mass waste (see slides)

13. In unconsolidated (loose) slope materials, larger and more angular particles have a higher angle of repose Vertical cliff Solid rock Gravel slope Sand Clayey soil

14. Slopes • Natural slopes should be viewed as landforms that have attained equilibrium with their surrounding conditions; if that equilibrium is disturbed by natural or human forces the slope may mass waste to reestablish its equilibrium

15. Other Mass Wasting Factors • The amount of water in the slope materials and the type and distribution of slope vegetation also affect slope stability (see slides)

16. Role of Water in Slope Stability

17. The more completely the surface is covered by vegetation and the more interlocking and denser the root system, the more stable a slope of unconsolidated materials. The removal of vegetation during land development or fire sets the stage for mass wasting.

18. Mass Wasting Triggers • Most mass wasting events are “triggered” • Human and natural triggers exist • Common human triggers: • Slope oversteepening • Vegetation removal • Addition of excessive weight to slope • Addition of excessive water to slope (see slide)

21. Mass Wasting Triggers • Four typical natural triggers: • 1) Prolonged and/or intense rainfall, or rapid snow melting, that saturates the slope materials

22. Mass Wasting Triggers • 2) Earthquakes – violent ground shaking induces ground failure FOX News (5/17/2008)

23. Mass Wasting Triggers • 3) Volcanic eruptions – remember lahars?

24. Mass Wasting Triggers • 4) Stream or wave undercutting – water erosion removes support for the overlying slope materials

25. Mass Wasting Classification

26. Rate: potentially quite fast; perhaps 100 miles per hour Rate: generally slow; a few feet to perhaps tens of feet per day

28. Rate: potentially quite fast; perhaps 100 miles per hour Rate: generally slow; a few feet to perhaps tens of feet per day

30. 2005 – renewed mass wasting claims ten lives 1995 Californian slumps and flows

31. Rate: quite fast; up to about 125 miles per hour Wave undercutting of slope

33. Rockfall Blocks

34. Wave undercutting

35. Sediment Fall

36. Many subcategories: debris, mud, earth, lahar, snow avalanche, creep Rate: vast range; extremely slow (inches per year) to perhaps 600 miles per hour Chaotic, turbulent internal motion All fast-moving, extensive travel distance (tens of miles) flows possess a high degree of lubrication from water and/or air

37. California Mudflow

38. In 1970, a very powerful earthquake in Peru triggered a debris avalanche that buried most of the towns of Yungay and Ranrachirca and killed about 20,000 people. Estimated maximum speed of flow: 600 miles per hour!

39. Snow Avalanche Why have deaths from snow avalanches in North America been increasing?

40. Mass Wasting Types • Mass wasting movement rates vary from exceedingly slow to exceedingly fast • Don’t be fooled – even the very slow mass wasting processes can threaten human development over long time periods (see slides)

41. Rate: maybe 12 inches per year

43. Mass Wasting Also Occurs Beneath Water Submarine Slumps

44. Recognition of Mass Wasting Hazards • The steeper the slope the higher the risk • The less vegetated the slope the higher the risk • Knowledge of the subsurface geology and local/regional geologic and climatic conditions helpful • Geologically recent mass wasting events are often recognized by slope scars – sections of oversteepened slopes mostly devoid of vegetation (see slides)

45. 1925

46. Recognition of Mass Wasting Hazards CNN.com; 5/26/08 1925, Wyoming

47. Mass wasting slope scar

48. Slope scars

49. Human Intervention • Humans have used various methods to attempt to reduce the risk of mass wasting including: slope retention structures; mass wasting diversion structures, slope drainage; slope reduction; and slope stitching (see slides) • None of these methods is failsafe!

50. Concrete block retention structure