Understanding Mass Wasting and Landslides

1. Chapter 7 Mass Wasting

2. Learning Objectives Understand slope processes and the different types of landslides Know the forces that act on slopes and how they affect the stability of a slope Know what geographic regions are at risk from landslides Know the effects of landslides and their linkages with other natural hazards

3. Learning Objectives, cont. Understand how people can affect the landslide hazard Be familiar with adjustments we can make to avoid death and damage caused by landslides

4. Introduction Mass wasting Rapid downslope movement of rock or soil as a coherent mass Include earth flows, rock falls, and avalanches Described collectively as landslides Slope processes All slopes are constantly evolving and materials are always in motion downslope Slopes are composed of different segments: High cliff or free face Talus slope Convex slope Straight slope Concave slope

5. Figure 7.4

6. Types of Landslides—Fall Falling – Free fall of earth material Figure 7.6a Figure 7.5

7. Types of Landslides—Slides Sliding–Movement of material as a coherent block Slumping – Sliding along a curved plane Figure 7.6b Figure 7.6e

8. Types of Landslides—Flows Flow– movement of unconsolidated material Creep – very slow flowage Earth flow, debris flow, and avalanche – rapid flowage Figure 7.6d

9. Table 7.1

10. Forces on Slopes Driving forces move materials downslope Weight of slope material due to things placed on the slope such as Vegetation, fill material, or buildings Resisting forces oppose downslope movement Shear strength of the material Resistance of material to sliding or flowing along slip planes Safety Factor (SF) is ratio of resisting forces to driving forces Stable when >1; unstable <1

11. Forces on Slopes: Earth Material Type Mineral composition Shale or weak volcanic pyroclastic materials failure occurs as creep, earth flows, debris flows, or slumps Rock falls occur when very resistant rock overlies weak rock Degree of consolidation Slumps common in unconsolidated materials Soil slip occurs when unconsolidated materials are over bedrock

12. Figure 7.10

13. Forces on Slopes: Earth Material Type, cont. Presence of zones of weakness Slip planes, natural breaks in consistency of materials Bedding planes in sedimentary rocks, weak clay layers, foliation planes in metamorphic rocks Rotational slides have curved slip surfaces Translational slides have planar slip surfaces Permeability – Ability to transmit water Soil slips occur when layers have contrasts in permeability

14. Figure 7.9

15. Forces on Slopes: Slope and Topography Steepness of slope or incline Steeper the slope, the greater the driving force Steep slopes are associated with rock falls, avalanches, soil slips Moderate slopes are associated with earth flows Gentle slopes are associated with creep Topographic relief or height of hill above land Mass wasting occurs more in high relief areas

16. Forces on Slopes: Climate Amount and timing of water that infiltrates or erodes slope Type and abundance of hillside vegetation Arid regions prone to rock falls, debris flows and soil slips Humid regions prone to complex landslides, earth flows, and creep Figure 7.14b

17. Forces on Slopes: Vegetation Function of climate, soil type, topography, and fire history Vegetation provides protective cover that slows surface erosion Roots add strength and cohesion to slope materials Vegetation adds weight to slopes

18. Forces on Slopes: Water Water saturates soil causing soil slips and debris flows Slumps develop after deep infiltration of water Water erodes base of slope to decrease stability Water can cause spontaneous liquefaction or quick clay Fine grained material that loses strength when disturbed and flows like a liquid

19. Forces on Slopes: Time Forces change with time Driving and resisting forces change with season due to changes in moisture content or water table Chemical erosion occurs slowly over time Carbonic acid from plants dissolves limestones

20. Snow Avalanches Rapid downslope movement of snow and ice Sometimes with rock and vegetation Thousands occur each year in United States and Canada Depend on steepness of slope Angle of repose describes steepest stable slope Snow slopes >25° are unstable, depending on temperature, wetness and shape of snow grains Stability of snowpack Weather

21. Snow Avalanches, cont. Loose-snow avalanches Widen as move downslope Slab avalanches Move as cohesive block More dangerous and damaging Triggered by overloading slope or zones of weakness in the snowpack Avalanches move down chutes Avoiding these areas can minimize hazard

22. Figure 7.18

23. Geographic Regions at Risk from Landslides Anywhere that have significant slopes and mountains are at risk Factors expected to increase landslide activity: Urbanization and development of landslide-prone areas Tree cutting in landslide-prone areas Changing global climate patterns

24. Figure 7.19

25. Effects of Landslides Can do significant damage In United States 25 people are killed each year, damages >$1 billion People hit with or buried in falling debris Slides may damage homes, roads, and utilities Slides may block roads, impeding travel or block streams causing flooding Disease

26. Links to Other Natural Hazards Linked to almost all other hazards Earthquakes, volcanoes, storms, and fires may cause landslides Landslides may cause flooding or tsunamis

27. Natural Service Functions of Landslides Produce deposits that become mineral resources Such as gold or diamonds Creation of new habitats in forests and aquatic ecosystems Increases plant and animal diversity

28. Human Interaction with Landslides Landslides are a natural phenomenon Humans increase through: Expansion of urban areas, transportation networks, and natural resource use Humans can decrease incidence through: Grading of land surfaces can increase instability of surfaces Building stabilizing structures and improving drainage can decrease landslide incidents

29. Human Interaction with Landslides, cont. 1 Timber harvesting Clearcutting Removing all of the trees from a defined area Road building Building of network of logging roads Increases erosion (soil slips) on geologically unstable land Interrupts surface drainage, alters subsurface movement of water, and changes distribution of earth materials

30. Human Interaction with Landslides, cont. 2 Urbanization increases landslide activity Removal of anchoring vegetation Construction of roads Building at higher elevations Grading Benches are cut into slopes for home sites Placing fill materials on slopes

31. Identifying Potential Landslides Crescent-shaped crack or terraces on hillside Tongue-shaped area of bare soil or rock on hillside Large boulders or talus piles at base of cliff Linear path of cleared vegetation extending down a hill Exposed bedrock with layering parallel to slope

32. Identifying Potential Landslides, cont. Tongue-shaped masses of sediment at base of slope An irregular land surface at the base of a slope Information can be used to create slope stability map or landslide hazards map

33. Figure 7.22

34. Prevention of Landslides Drainage control Keeps water from infiltrating a slope Drains can divert water Impermeable layers can keep water from infiltrating Figure 7.23

35. Prevention of Landslides, cont. Grading can increase slope stability Material from upper slope can be moved to base Slope Supports Retaining walls – concrete or filled wire baskets Figure 7.25

36. Landslide Warning Systems Provide time for people to evacuate Human monitoring for changes or small rockfalls Electrical systems, tiltmeters, and geophones USGS installed acoustic flow monitors in Washington state to detect lahars Wells can indicate dangerous amounts of water

37. Adjustment to Hazard, Community Preparations Movement of critical facilities Hospitals, schools and police stations should be placed in safe locations Landslide correction Landslide may be stopped after it starts using drains

38. Adjustments to the Hazard, Personal Preparations Get a geologic evaluation of property Avoid homes at mouth of valley or canyon Consult local agencies Monitor small landslides on property Look for cracks in house walls, leaning retaining walls, doors or windows that stick

39. Adjustments to the Hazard, Personal Preparations, cont. Be wary of leaks in swimming pools or septic tanks, trees or fences that tilt, or sagging or taut utility wires Be wary of small springs Look for surface features Don’t buy a home that has a landslide hazard

40. Things to be Aware of in Avalanches Most avalanches are triggered by victims or others in their party Obtain forecasts from nearest avalanche center before you go out Most people who survive avalanches are rescued by the other members of their party Learn avalanche safety procedures and how to evaluate snow conditions before traveling to avalanche-prone area

41. End Mass Wasting Chapter 7