Lecture 14 Mass Movement

1. Lecture 14 Mass Movement • What is mass movement? • Classification of mass wasting • Type of movement • Causes of slope movement

2. Robert L. Schuster/U.S. Geological Survey

3. Mass Movement • Mass movement (or mass wasting) is the movement of soil or rock mass downslope under the influence of gravity.

4. Venezuelan Mudslides(December, 1999) • Venezuela’s worst natural disaster of the last century. • As much as 30,000 people were killed. • Triggered by torrential rains and floods over Venezuela’s Caribbean central coastal area.

5. 1999 Venezuela Mudslids

6. A mudslide ravaged neighborhood of Los Corales, north of the capital Caracas.

7. Caracas, the capital. A woman tries to gather what she could find in the remains of her destroyed house.

8. Caracas outskirts. A Venezuelan man surveys the destruction of a mudlslide as he leaves his home with a suitcase.

9. A lone statue in a destroyed plaza along Venezuela's Caribbean coast.

10. Landslides • The term "landslide" has been used vaguely by popular media for almost any kind of slope failure.

11. Classification of mass wasting • Mass movements are classified based on the type of materials involved, the type of movement, and the rate of movement. Thus we have rock fall, debris flow, earth slump, etc. • type of material • Slope materials can be divided into bedrock ("rock"), or unconsolidated materials/soil ("debris", "mud", "earth").

12. Type of movement • fall • The mass pieces fall through the air. It is a common form of movement on steep slopes. For example, this is the primary way in which talus slopes are built from frost wedging. • slide • Slides occur when material remains fairly coherent and moves along a weakness surface, such as a joint, a fault, or a bedding plane. • flow • Flows occur when material moves downslope as a viscous fluid. Most flows are saturated with water.

13. Rockfall: individual blocks drop in a free fall from a cliff or steep mountainside.

14. An illustration of frost wedging. (Tarbuck and Lutgens)

15. Rockslide. Large masses of bedrocks move as a unit in a fast downward slide.

16. Flows occur when material moves downslope as a viscous fluid. Debris flow contains material that is coarser than sand and can travel a few km/hour. (Press and Siever)

17. Soil creep • Creep is the slow downslope migration of soil and loose rock fragments. • One cause of creep is frost wedging. Freezing lifts particles at right angles to the slope, and thawing allows the particles to fall back to a slightly lower level. Thus each cycle moves the material a short distance downhill.

18. The repeated expansion and contraction of the surface material (frost wedging) causes a net downslope migration of rock particles -- a process called creep. (Tarbuck and Lutgens)

19. Creep is a slow movement, but its effects are often visible. (Tarbuck and Lutgens)

20. A fence offset by creep in California (T. Amos)

21. Trees that grow in creeping soil gradually develop pronounced curves. (Martin Miller)

23. Slump • similar to sliding movement, but the descending material move along a curved (circular) surface of rupture. Slumps are common in soils or rocks of low shear strength.

24. Slump occurs when material slips downslope along a curved surface of rupture. Earthflows and debris flows often form at the base of the slump. (Tarbuck and Lutgents)

25. Slump is often triggered when slopes become oversteepened by erosional processes such as wave action. Point Fermin, CA (J.S. Shelton)

27. Debris flow • Debris flow contains material that is coarser than sand, generally restricted to channels and commonly result from unusually heavy rainfall. • Debris flows move faster than creeps and slumps

28. Flows occur when material moves downslope as a viscous fluid. Debris flow contains material that is coarser than sand and can travel a few km/hour. (Press and Siever)

29. Debris flows are common on the slopes of some volcanoes (such flows are termed lahars). A lahar along a river northwest of Mt. St. Helens.

30. Mudflow • Mudflow contains materials finer than sand and large quantities of water. Mudflows are generally even faster than debris flow. • Mudflows are rapid and are common in semiarid regions and are confined to channels. Soil and debris accumulate until infrequent rainfall triggers high runoff. Water decreases shear strength, and the wet mass flows rapidly down the channel.

31. Mudflow in Mill Valley, California in Jan 1997, triggered by heavy rains from a powerful Pacific storm. (Tarbuck and Lutgens)

33. Rockslide • Rockslides occur when blocks of bedrock break loose and slide down a slope. Rockslides are among fastest and most destructive mass movements. • Rockslides typically take place where the rock strata are inclined.

34. Rock slides along beding planes. (West, p.303)

35. A massive rockslide along the Madison River of Montana triggered by Aug 17, 1959 earthquake. The rocks blocked the canyon and created Earthquake Lake (J. Montagne)

37. Speeds of mass movement: slow->fast soil creep -> earth slump -> debris flow, mud flow -> rockslide -> rock fall, rock avalanche

39. Causes of slope movement • Slope stability analysis (P.297) The controlling force of mass wasting is gravity. But slope failure often involves many variables. A basic approach to evaluating slope stability is to identify the driving forces and the resisting forces. • factor of safety (FS)= • (total resisting forces)/(total driving forces)

40. Mass Movement • Mass movement (or mass wasting) is the movement of soil or rock mass downslope under the influence of gravity.

41. Influence of water • Water is an important factor in the occurrence of mass movement. Most rapid mass movements occur during or after periods of heavy rainfall. • The addition of water increases pore pressure, decreases the effective normal stress acting in the slope, and thus decrease the shear resistance of the soil according to the Mohr-Coulomb equation: • S = C + se tan(f) • On the other hand, the driving force increases with the weight of water added to the soil.

42. Removal of lateral support • A common way to increase the driving force on a potential failure plane is to remove materials from the base of the slope, by river erosion, previous slope movements, or human modification such as road cuts.

43. A river cuts into the base of a slope, steepening the sides of the valley, making the slopes unstable.

44. Cutting terraces in a hill slope creates a steeper slope. (W.W. Norton)

45. The Gros Ventre landslide, 1925 (near Jackson Hole, Wyoming). River erosion cuts down to the weak shale, which dips parallel to the slope. Rainfall in the weeks before seeped into the ground, weakening the Amsden Shale and making the overlying rock layer (Tensleep Sandstone) heavier. (W.W. Norton)

46. Vegetation • Plants protect against erosion and contribute to the stability of slopes because their root systems bind soil together. Plants also absorb subsurface water. • Where plants are lacking (e.g. removed by fire), mass wasting is enhanced, especially when slopes are steep and water is plentiful.

47. A fire that stripped of vegetation and root systems in Yellowstone National Park caused a weakening of the soil, making it susceptible to erosion and mass movement. (G. Meyer)

48. Vegetation … Several decades ago, near steep slopes of Menton, France, farmers replaced olive trees (with deep roots) with a more profitable carnations (with shallow roots). When the slope failed, the landslide killed 11 people.

49. Earthquakes as triggers • Conditions that favor mass wasting may exist in an area for a long time without movement occurring. Earthquakes are dramatic triggers that can dislodge enormous volumes of earth materials. • In many areas, it is not ground vibrations directly, but landslides and ground subsidence triggered by the vibrations that cause the greatest damage.

50. This 4-km long tongue of rubble was deposited atop a glacier by a rock avalanche after the great 1964 Alaska earthquake. A rock avalanche is a high velocity flow of large masses of broken rocky material.