# Chapter 7 Study Guide, Version 2: Mass Wasting This version follows the textbook section by section. Review each section's summary, key ideas, examples, and questions before moving to the next section. ## Chapter Learning Objectives - **LO:1:** Describe slope processes and the different types of landslides. - **LO:2:** Analyze the forces that act on slopes and how they affect the stability of a slope. - **LO:3:** Evaluate what geographic regions are at risk from landslides. - **LO:4:** Compare different mass wasting processes and their linkages with other natural hazards. - **LO:5:** Describe the ways in which people can affect the landslide hazard. - **LO:6:** Explain the adjustments people can make to avoid death and damage caused by landslides. - **LO:7:** Summarize the ways in which you can protect yourself and property from the processes of mass wasting. ## 7.1 An Introduction to Landslides (LO:1 and LO:2) ### Section summary **Mass wasting** is any downslope movement of earth materials. In its narrower use in this chapter, it means the rapid downslope movement of rock or soil as a more or less coherent mass. The chapter uses “landslide” broadly for landslides, earthflows, debris flows, rockfalls, and avalanches. Slopes are dynamic, evolving systems. Weathering, erosion, and downslope movement constantly modify them, even when a slope appears stable. Movement rates range from nearly imperceptible creep to avalanches and rockfalls traveling about 160 km/hr or more. ### Slope segments Most natural slopes contain several segments rather than one uniform incline: - **Free face:** A steep or nearly vertical cliff, often made of resistant rock. - **Talus slope:** A pile of broken rock accumulated below a free face. - **Upper convex slope:** A rounded slope segment that curves outward near the top. - **Straight slope:** A relatively uniform inclined segment. - **Lower concave slope:** A curved segment that becomes gentler toward the base. Hard rock and arid conditions favor free faces and talus. Softer rock, thicker soil, and vegetation commonly favor convex, straight, and concave slopes. Local conditions can produce exceptions. ### Types of downslope movement Mass movements are classified by mechanism, earth-material type, amount of water, and rate of movement. | Mechanism | Description | Examples | |-------------|--------------------------------------------------------------------|------------------------------------------| | **Falling** | Material moves freely through the air | Rockfall | | **Sliding** | A coherent mass moves along a planar or curved slip plane | Rock slide, soil slip, slump | | **Flowing** | Particles move within and mix throughout the moving mass | Earthflow, debris flow, creep, avalanche | | **Complex** | Two or more mechanisms occur, or one movement changes into another | Slump that becomes an earthflow | Movement is generally considered rapid when it can be seen with the naked eye. Creep may move only millimeters or centimeters per year. Other movements may advance about 1.5 m per day or 30 m per second. ### Common landslide types - **Rockfall:** Individual blocks of bedrock fall through the air and accumulate as talus. - **Rock slide:** A coherent block of bedrock slides along a planar surface such as bedding, foliation, or a fracture. - **Slump:** Cohesive material rotates and slides along a curved slip plane. Slumps may form benches and tilted blocks. - **Soil slip:** A shallow translational slide of soil or weathered material over bedrock or cohesive sediment. - **Earthflow:** A wet, partly cohesive, internally deformed mass of soil and weathered rock. - **Debris flow:** A water-rich mixture of rock, sand, mud, and debris, intermediate between a landslide and a water flood. Mudflows and lahars are examples. - **Creep:** Very slow downslope movement of soil and rock. It can tilt poles and fences and curve tree trunks. - **Complex landslide:** A combination of movements, such as an upper slump that changes into a lower earthflow. ### Rotational and translational slides - **Rotational slides** move along curved slip surfaces. Slumps are the main example. Rotating blocks may tilt upslope and form small benches. - **Translational slides** move along relatively planar slip surfaces. Soil slips and rock slides are the main examples. Potential slip planes include bedding planes, foliation planes, fractures, faults, old landslide surfaces, and weak clay layers. A **daylighting bed** is a bedding plane that intersects the land surface on a slope and can provide a ready-made slip plane. **Colluvium** is a mixture of weathered rock and debris above bedrock and is commonly involved in shallow soil slips. ### Forces on slopes Gravity is the fundamental driving force. Stability depends on the relationship between: - **Driving force:** The downslope component of the weight of slope material. - **Resisting force:** The force opposing movement, especially the shear strength of the material. - **Shear strength:** Resistance to failure along a potential slip plane. - **Normal force:** The component of weight directed perpendicular to the slip plane. It contributes to frictional resistance. The **safety factor** is: `SF = resisting forces / driving forces` - `SF > 1`: Resisting forces exceed driving forces; the slope is considered stable. - `SF < 1`: Driving forces exceed resisting forces; slope failure can be expected. - `SF = 1`: Driving and resisting forces are balanced at the threshold of failure. The forces are not fixed. Changes in water, slope geometry, vegetation, loading, erosion, or weathering can increase or decrease the safety factor. ### Slope angle and gravity Gravity can be divided into a downslope force parallel to the slip plane and a normal force perpendicular to it. As slope angle increases: - the downslope driving force increases; - the normal force decreases; - frictional resistance generally decreases; and - the safety factor decreases. As slope angle decreases, the opposite pattern occurs. ### Variables affecting slope stability The major interacting variables are: - earth-material type; - slope angle and topographic relief; - climate; - vegetation; - water; and - time. #### Earth material Mineral composition, cementation, consolidation, permeability, and zones of weakness affect the type and frequency of landslides. Weaknesses may occur along bedding, foliation, fractures, faults, clay layers, or old landslide surfaces. Weak planes are especially hazardous when they intersect or run parallel to a slope. Permeability controls how easily water enters and moves through a slope. A permeable soil over less-permeable bedrock can trap water, weaken cohesion, and promote a soil slip. #### Slope and topography Steeper slopes have greater downslope driving forces. High-relief mountainous areas have more potential for rockfalls, soil slips, debris flows, and other mass movements. #### Climate Climate includes average temperature and precipitation as well as the type and seasonal pattern of precipitation. Rain, snow, freeze-thaw conditions, rapid warming, and wet years can all affect slope stability. #### Vegetation Vegetation has three competing effects: 1. Leaves and stems cushion rainfall and reduce surface erosion. 2. Roots add strength and cohesion to soil. 3. Vegetation adds weight and can increase infiltration, sometimes lowering stability on shallow, steep slopes. Shallow-rooted ice plant and chaparral can contribute to shallow soil slips in Southern California because they can increase infiltration and add water weight to a slope. #### Water Water is involved directly or indirectly in nearly all landslides: 1. Rain can saturate slopes and trigger soil slips and debris flows. 2. Deep infiltration can contribute to slumps months or years later. 3. Streams, lakes, and waves can erode the toe of a slope, remove support, steepen the slope, and reactivate an older landslide. Water also increases the weight of a slope and raises underground water pressure, reducing effective resistance. Quick or expansive clays may lose shear strength and liquefy when disturbed. #### Time and weathering Seasonal moisture changes alter driving and resisting forces. Chemical weathering gradually weakens rock and reduces cohesion. Increasing creep can be an early sign that a slope is becoming less stable. ### Snow avalanches A snow avalanche is the rapid downslope movement of snow and ice, sometimes mixed with rock, soil, and vegetation. Three factors interact to produce unstable avalanche conditions: - slope steepness; - snowpack stability; and - weather. Avalanches generally occur on slopes steeper than about 25 degrees, with many occurring between 35 and 40 degrees. On slopes steeper than about 50 to 60 degrees, snow tends to fall off continually. Wind can load snow on the leeward side of a ridge, new snowfall can add weight, and rapid warming can make snow wet and unstable. - **Loose-snow avalanches** begin at a point and widen as they move. - **Slab avalanches** begin as cohesive blocks of snow and ice. They are generally more dangerous and damaging and may be triggered by overloading or weakness in the snowpack. Avalanches can travel up to about 100 km/hr and commonly follow established tracks or chutes. Most avalanches that kill people are triggered by victims or members of their party. ### Key terms - **Mass wasting:** Downslope movement driven primarily by gravity. - **Landslide:** Rapid downslope movement of earth material, used broadly in this chapter. - **Slip plane:** A surface of weakness along which material moves. - **Daylighting bed:** A bedding plane that intersects the surface on a slope. - **Driving force:** Force tending to move material downslope. - **Resisting force:** Force opposing downslope movement. - **Safety factor:** Ratio of resisting forces to driving forces. - **Angle of repose:** Steepest angle at which loose material can remain stable. - **Runout:** Distance from the headscarp to the toe or farthest point reached. ### Section 7.1 review questions 1. What are slope segments, and how do the common types differ? 2. What are the three main ways materials on a slope may fail? 3. What is the safety factor, and how is it defined? 4. How do slumps differ from soil slips and rock slides? 5. How and where do debris flows occur? 6. What variables affect driving and resisting forces on slopes, and how are they interrelated? 7. What are the three ways vegetation affects slope stability? 8. What is the relationship between downslope force and normal force? 9. What are the two types of snow avalanches, and how do they differ? ## 7.2 Geographic Regions at Risk from Landslides (LO:3) ### Section summary Landslides can occur anywhere significant slopes exist, but mountainous and high-relief areas generally have greater risk than flat or low-relief areas. Risk is controlled by slope angle, topographic relief, geology, climate, water, vegetation, development, and exposure. ### North American regions Major U.S. landslide-risk areas include: - the coastal mountains of California, Oregon, Washington, and British Columbia; - the Rocky Mountains; - the Appalachian Mountains; - Alaska and other mountainous areas of the western United States; - coastal cliffs and bluffs; and - steep areas underlain by weak or fractured materials. The Plains and Midwest generally have lower risk because they have less topographic relief and fewer steep slopes. Parts of central Canada, including Saskatchewan, Manitoba, and eastern Ontario, also have relatively lower risk. ### Future changes in landslide activity Worldwide landslide activity is expected to increase as: 1. urbanization and development expand into landslide-prone areas; 2. tree cutting continues on unstable slopes; and 3. changing climate patterns produce regional increases in precipitation. These changes affect the variables controlling driving and resisting forces. They can increase both the frequency of movement and the number of people exposed to it. Urbanization can decrease slope stability through grading, excavation, added fill, vegetation removal, and altered drainage. It can also increase stability when development follows hazard maps and grading codes and uses carefully designed drainage and slope-support structures. ### Section 7.2 review questions 1. Describe the general distribution of regions at risk from mass wasting. 2. Why is there relatively little risk of mass wasting in the Midwest and Great Plains? 3. How might urbanization increase or decrease slope stability? ## 7.3 Effects of Landslides and Linkages with Other Natural Hazards (LO:4) ### Section summary Most landslides are small and slow, but large or rapid movements can cause substantial loss of life and damage. In the United States, the chapter reports an average of about 25 deaths per year from landslides, increasing to 100 to 150 when collapses of trenches and other excavations are included. Annual damage exceeds $2.8 billion and may be as high as $8.5 billion. ### Direct effects - People can be struck, buried, or carried away by moving debris. - Homes, roads, railroads, bridges, utilities, and other structures can be damaged or destroyed. - Roads and railroads can be blocked for days, weeks, or months. - Shipping lanes and river channels can be blocked by debris. - Soil, vegetation, and the shape of valleys and coastlines can be altered. - Landslide deposits can increase transportation costs and interrupt businesses and resource extraction. ### Indirect effects - A slide can dam a river and cause upstream flooding. - A landslide dam can later fail and produce a destructive flood. - A slide into a lake, reservoir, or bay can displace water and generate a tsunami. - Disturbed fungal spores or contaminated water can contribute to disease. - Blocked roads and rail lines can delay commerce, rescue, and emergency response. The annual cost is large compared with the cost of many individual natural-hazard events, although the chapter does not provide a single ranking against every other hazard. The important point is that landslides create both recurring local damage and occasional catastrophic losses. ### Linkages with other hazards - **Earthquakes** can trigger landslides, rockfalls, soil slips, and debris flows. - **Volcanic eruptions** can produce landslides, debris avalanches, lahars, and mudflows. - **Storms and prolonged rain** saturate slopes and increase pore-water pressure. - **Wildfires** remove vegetation and roots, increasing erosion and later debris flows. - **Stream and wave erosion** can remove the toe of a slope and reactivate an old slide. - **Landslides** can dam streams, produce floods, and generate tsunamis when they enter water. ### Case study: Oso, Washington, 2014 The Oso landslide was the deadliest landslide in U.S. history. It killed 43 people, destroyed 49 homes and other structures, and buried an area about 2.5 km2 several meters deep. Evidence of the hazard existed before the event. Local residents and geologists had referred to the area as “Slide Hill,” earlier landslides were visible, and a 2010 study identified the hillslope as one of the most dangerous in the county. More than twice the normal precipitation fell during the preceding 20 days. The slide began as a slump but became fluidized and moved more than 1,000 m across the Stillaguamish River and a neighborhood. It blocked the river for 10 days, creating upstream flooding concerns and prompting downstream flood warnings. ### Case study: I-70 rockfalls, Colorado Large granite boulders fell onto Interstate 70 in the Rocky Mountains. Heavy rain and ice wedging in cracks helped dislodge the rocks from a free face. One truck driver was struck by a boulder and survived, but the event illustrates how rockfalls can occur with little warning on transportation corridors. ### Case study: Bob Rasely and wildfire debris flows Geologist Bob Rasely assessed debris-flow risk after wildfires. Light burns can increase erosion several times, while heavy burns can increase it by six to ten times because roots and protective vegetation are destroyed. Steep slopes, established water channels, and water-repellent residues from burned vegetation can further increase runoff and debris-flow risk. ### Section 7.3 review questions 1. How can large landslides be related to flooding and tsunamis? 2. How large are the annual deaths and costs associated with landslides in the United States? 3. Explain how earthquakes, volcanoes, storms, fires, floods, and erosion can interact with landslides. 4. Use Oso or the I-70 rockfalls to explain why past events and site history matter. ## 7.4 Natural Service Functions of Landslides (supports LO:4) ### Section summary Landslides are hazardous, but they can also provide limited natural services. Like fire, mass wasting is an ecological disturbance that can create new habitats and increase biological diversity. ### Natural services - Landslides create new habitats in forests and aquatic ecosystems. - Landslide-dammed lakes can create habitat for fish and other organisms. - Mass wasting exposes new rock and soil surfaces. - Landslide deposits can transport and concentrate mineral grains. - Heavy minerals such as gold and diamonds may accumulate at the base of slopes or in adjacent streams. These benefits do not remove the danger. A landslide-dammed lake may provide habitat but can also fail and produce a flood. ### Section 7.4 review questions 1. What natural service functions can landslides provide? 2. How do mass wasting and erosion help expose, transport, and concentrate valuable earth materials such as gold and diamonds? ## 7.5 Human Interaction with Landslides (LO:5) ### Section summary Landslides occur naturally, but human activities can increase or decrease their number, frequency, and severity. The effect may be insignificant on stable ground and very significant on weak or already unstable slopes. ### Timber harvesting and roads Clear-cutting removes roots and protective cover. Logging and road building may not significantly increase landslide erosion on geologically stable land, but logging on weak, unstable slopes can increase landslide erosion several times. Roads can: - interrupt surface drainage; - alter subsurface water movement; - cut into the toe of a slope; - add or remove material through cut-and-fill grading; and - concentrate runoff. ### Urbanization and grading Urban development commonly places roads and homes on steep slopes. Grading can cut benches or pads into hillsides, remove support from one location, and add fill weight elsewhere. Irrigation, leaking pools, septic systems, and cesspools can add water to a slope. Construction can increase landslide risk by: - cutting into a potential slip plane; - removing material from the toe of a slope; - loading the top of a slope with fill or buildings; - removing vegetation and root strength; and - changing surface or subsurface drainage. Grading codes and engineering-geology studies have reduced damage in some areas, but codes do not make every hillside safe. ### Reservoirs and altered water conditions Filling a large reservoir can change groundwater conditions along its shores and contribute to slope failure. Conversely, planned drainage, retaining structures, and careful grading can stabilize sensitive slopes. ### Case study: Portuguese Bend, California Portuguese Bend is part of an ancient landslide that was reactivated by road construction, fill, and altered drainage. The slide damaged or destroyed more than 150 homes, a pier, and a swim club. From 1956 to 1978 it moved about 0.3 to 1.3 cm per day, accelerating during wet periods. Wells were drilled into the slide in 1980 to remove groundwater. By 1985 the slide appeared to be stabilized, but future precipitation and groundwater conditions remained important. The case demonstrates that drainage can increase resistance, while a “stabilized” slide may not be permanently safe. ### Case study: Rio de Janeiro, Brazil Rio de Janeiro combines steep fractured granite, thin soil, intense rainfall, deforestation, roads, cut-and-fill construction, and dense hillside development. In 1988, intense rainfall caused flooding and debris flows that killed at least 90 people and left about 3,000 homeless. The case shows how physical conditions and human decisions combine to create a serious urban landslide hazard. ### Section 7.5 review questions 1. How does clear-cutting affect landslide occurrence? 2. How does road construction increase landslide occurrence? 3. How has human activity in Rio de Janeiro contributed to devastating landslides? 4. How can human activities sometimes reduce landslide risk? ## 7.6 Minimizing the Landslide Hazard (LO:6) ### Section summary Landslide-risk reduction requires identification, prevention, correction, warning, evacuation, and appropriate land-use planning. The most preferable and least expensive adjustment is to avoid development where landslides are occurring or likely to occur. ### Identifying potential landslides Geologists look for sensitive earth materials such as clay and shale and for surface evidence including: - hummocky or irregular ground at the base of a slope; - crescent-shaped cracks or terraces on a hillside; - tongue-shaped bare soil or rock; - large boulders or talus piles below cliffs; - a linear strip of cleared or disturbed vegetation down a slope; - exposed bedding parallel to the slope; - tongue-shaped sediment deposits at the base of a slope or valley; - cracks in the ground, walls, foundations, or roads; - leaning structures, tilted trees, or fences; and - unusually wet or green areas and small springs. ### Landslide maps - **Landslide inventory map:** Shows landslides that have occurred and may show their age or activity. - **Landslide hazard map:** Shows areas likely to experience landslides and may recommend land uses. - **Landslide risk map:** Combines the probability of landsliding with an assessment of potential losses. Maps are valuable for planning but do not replace a detailed, site-specific geologic evaluation. ### Avoidance and prevention Avoid or carefully evaluate: - building on active slides or old slide deposits; - loading the top of a slope; - cutting into sensitive slopes or their toes; - placing fill on unstable slopes; and - changing water conditions without a geologic evaluation. Critical facilities such as hospitals, schools, and police stations should be located away from slide paths, unstable slopes, and areas directly below hazardous hillsides. ### Drainage control Because increased water pressure is a common cause of failure, drainage is often the most effective stabilization method: - divert surface water around the slope; - use drains at the head of a slide to reduce infiltration; - install subsurface drains or wells to remove groundwater; - cover a slope with an impermeable layer when appropriate; and - maintain drains so they do not become blocked. ### Grading and slope support Cut-and-fill grading can reduce the overall slope angle, remove weight from the upper slope, and place material at the toe to increase resistance. On high or steep slopes, benches or steps can reduce steepness, improve drainage, and catch falling rock and small slides. Support structures include retaining walls, piles, gabions, and other barriers. Retaining walls should be anchored into stable material, backfilled with permeable material, and provided with drain holes to reduce water pressure. ### Landslide correction After movement begins, the best strategy is to attack the initiating process. In many cases that means lowering groundwater pressure with surface drains, subsurface drainpipes, or wells. Correction may be expensive and cannot guarantee that a large natural slide will stop. ### Warning and monitoring Warning systems do not prevent landslides, but they can provide time to evacuate people, move possessions, stop trains, or reroute traffic. Methods include: - visual surveillance and field inspections; - rain gauges and precipitation thresholds; - shallow wells that monitor water in an existing slide; - tiltmeters and other deformation instruments; - geophones that detect vibrations from moving rock or debris; - slide fences connected to railroad signals; and - acoustic-flow monitors that detect debris flows and lahars. At Mount Rainier, automated debris-flow warning systems were designed to provide communities with roughly 40 minutes to three hours of warning, depending on the event and location. ### Case study: Thistle, Utah, 1983 The Thistle landslide reactivated an older slide, blocked a canyon and the Spanish Fork River, and created a natural dam about 60 m high. The resulting lake flooded the community of Thistle, a railroad, and two major highways. Total direct and indirect losses exceeded $400 million. Earlier recognition and a network of surface and subsurface drains might have prevented the slide. The estimated prevention cost was about $300,000 to $500,000, illustrating the potential economic value of prevention. ### Section 7.6 review questions 1. What surface features are associated with landslides? 2. What are the main steps for preventing landslides? 3. How can a cut-and-fill operation increase a slope's safety factor? 4. How do landslide inventory, hazard, and risk maps differ? 5. How can warning systems reduce landslide losses even though they do not stop movement? ## 7.7 Perception of and Adjustment to the Landslide Hazard (LO:7) ### Section summary People often perceive landslide risk as minimal unless they have experienced a slide. Large slides are infrequent, evidence of past movement may be hidden, and residents may think that a slide could happen on another hillside but not their own. Technical information alone may not persuade people to avoid attractive hillside property. ### Best adjustment The best adjustment is to avoid building or buying in landslide-prone areas. If development already exists, risk reduction may combine critical-facility siting, drainage, slope correction, warning systems, evacuation planning, and land-use regulation. ### Before buying or building on a slope - Have a professional geologist evaluate the property and surrounding slopes. - Consult city or county engineering departments. - Review landslide inventory, hazard, and risk maps. - Avoid homes at the mouth of a valley or canyon where debris flows may originate upstream. - Look for hummocky ground, old slide deposits, bare or tongue-shaped areas, cracks, tilted trees or fences, springs, and unusually wet areas. - Inspect retaining walls for leaning or cracking. - Check walls, foundations, roads, and ground surfaces for cracks. - Treat sticking doors, sticking windows, and uneven floors as possible warning signs, while remembering they can have other causes. - Look for leaking pools or septic systems and utility wires that are unusually taut or sagging. - Do not assume that an old or quiet landslide is permanently stable. One sign alone does not prove that a landslide is present or imminent. Multiple signs or unexplained movement warrant professional investigation. ### Personal safety during landslide conditions - Keep water away from unstable slopes and maintain drainage systems. - Do not cut into, load, or add fill to a slope without professional evaluation. - Stay out of valleys, channels, chutes, and areas below unstable slopes during heavy rain or warnings. - Follow evacuation orders and do not return until officials determine that the area is safe. - In avalanche country, obtain forecasts, evaluate snow conditions, travel with trained people, and learn avalanche rescue procedures. ### Applying the fundamental concepts: La Conchita, California La Conchita illustrates the chapter's five fundamental concepts: 1. **Science helps predict hazards:** Geologic studies and the history of repeated slides showed that landsliding was common along the coast. 2. **Risk information can support decisions:** Residents and officials knew about earlier slides and the steep, unstable coastal setting, but many continued to value the location. 3. **Hazards are linked:** Active tectonic uplift, earthquakes, fractured bedrock, heavy rainfall, coastal erosion, landslides, debris flows, and flooding interacted. 4. **Humans can turn disasters into catastrophes:** Development in a desirable hazard zone, inadequate drainage, and the decision to remain increased exposure and vulnerability. 5. **Consequences can be minimized:** Research, drainage, retaining structures, warnings, land-use restrictions, and evacuation can reduce losses, although limited measures cannot guarantee safety. ### La Conchita events - A large landslide occurred in 1995 after unusually heavy rain. Cracks had appeared on the upper slope beforehand. - A retaining wall and limited drainage were installed after the 1995 slide, but these measures did not stabilize the entire slope. - On January 10, 2005, a water-saturated earthflow moved rapidly into the community, destroying or damaging about 30 homes and killing 10 people. - The 2005 movement remobilized material from the earlier slide and locally overtopped or destroyed the retaining wall. The lesson is that an old or partially stabilized slide remains a hazard when its water conditions, geology, and exposure continue to change. ### Section 7.7 review questions 1. List five signs that could indicate a potential landslide when inspecting property. 2. Except for buying property on low-relief ground, what is the best way to minimize the probability of a landslide on your land? 3. Why may people resist technical information about landslide risk? 4. What precautions should people take when traveling in avalanche country? ## Connections among the Sections - **7.1 to 7.2:** Slope materials, geometry, climate, water, and vegetation help explain the geographic distribution of landslides. - **7.1 to 7.3:** The type and speed of movement determine direct damage and the potential for linked floods or tsunamis. - **7.3 to 7.4:** Landslides can destroy communities but can also create ecological habitats and concentrate minerals. - **7.1 to 7.5:** Human changes to slope geometry, vegetation, drainage, and loading alter the forces controlling stability. - **7.5 to 7.6:** Recognizing how development increases risk allows engineers and planners to select drainage, grading, supports, and warning systems. - **7.6 to 7.7:** Technical identification and mitigation reduce risk only when people understand the hazard and make appropriate decisions. ## Chapter-Wide Comparison Questions - How do free faces, talus slopes, upper convex slopes, straight slopes, and lower concave slopes differ? - How do falling, sliding, flowing, and complex movements differ? - How do rockfalls, rock slides, slumps, soil slips, earthflows, debris flows, creep, and avalanches differ? - How do rotational and translational slides differ? - How do driving force, resisting force, shear strength, normal force, and safety factor relate? - How does increasing slope angle affect driving force, normal force, and stability? - How do earth material, permeability, daylighting beds, climate, vegetation, water, and time affect slope failure? - How do loose-snow and slab avalanches differ? - Why are mountainous regions and coastal cliffs more vulnerable than low-relief plains? - How can earthquakes, volcanoes, storms, fires, and erosion trigger landslides? - How can a landslide cause flooding or a tsunami? - How can logging, roads, grading, fill, reservoirs, drainage, and urbanization affect landslide risk? - How do inventory, hazard, and risk maps differ? - What surface features can reveal past or potential landslides? - How do drainage, grading, retaining walls, warnings, and land-use planning reduce risk? - Why is avoiding development usually preferable to stabilizing a large slide? - What should be investigated before buying property on a slope? - How do La Conchita, Oso, Portuguese Bend, Thistle, Rio de Janeiro, and Mount Rainier illustrate the interaction of natural processes and human decisions? ## Practice Questions 1. Define mass wasting and explain how it differs from the chapter's narrower use of landslide. 2. Describe the five common slope segments and the conditions that favor each. 3. Compare falling, sliding, flowing, and complex mass movements. 4. Compare rockfalls, slumps, soil slips, rock slides, earthflows, debris flows, creep, and avalanches. 5. Explain the difference between rotational and translational slides. 6. Describe the variables used to classify downslope movements. 7. Explain the difference between driving and resisting forces on a slope. 8. Calculate or interpret a safety factor and explain what values above or below 1 mean. 9. Explain how slope angle changes downslope and normal components of gravity. 10. Describe how rock type, bedding, foliation, fractures, clay, and permeability affect stability. 11. Explain the three ways vegetation can affect slope stability. 12. Describe the three major ways water contributes to landslides. 13. Compare loose-snow and slab avalanches and identify conditions that make avalanches more likely. 14. Identify major geographic regions at risk from landslides in the United States, Canada, and worldwide. 15. Describe direct and indirect effects of landslides. 16. Trace linkages among landslides, earthquakes, volcanoes, storms, fires, floods, and tsunamis. 17. List the natural service functions of landslides. 18. Explain how clear-cutting, road construction, grading, fill, reservoirs, and urbanization affect landslide risk. 19. What evidence should a geologist look for when mapping potential landslides? 20. Distinguish a landslide inventory map, hazard map, and risk map. 21. Describe warning methods for landslides, debris flows, rockfalls, and avalanches. 22. Explain why drainage control is central to landslide prevention and correction. 23. Evaluate the advantages and limits of grading, benches, retaining walls, piles, gabions, and debris basins. 24. Explain why avoiding development on an unstable slope is usually the best adjustment. 25. List signs and questions to investigate before buying a house on a slope. 26. Describe what people should do before and during heavy rain or a landslide warning. 27. Explain avalanche safety practices. 28. Use La Conchita, Oso, Portuguese Bend, Thistle, Rio de Janeiro, or Mount Rainier to explain how science and human decisions affect landslide losses. ## Quick Review Checklist Before the quiz or test, make sure you can: - define mass wasting, landslide, fall, slide, flow, complex movement, rockfall, slump, soil slip, rock slide, earthflow, debris flow, creep, and avalanche; - identify the five common slope segments; - classify movements by mechanism, material, water content, and rate; - distinguish rotational from translational slides; - explain gravity, driving force, resisting force, shear strength, normal force, and safety factor; - calculate and interpret `SF = resisting forces / driving forces`; - explain how slope angle changes downslope and normal forces; - explain the roles of earth material, permeability, bedding, foliation, fractures, clay, climate, vegetation, water, and time; - explain the three ways vegetation affects slope stability; - explain the three ways water contributes to landslides; - compare loose-snow and slab avalanches; - identify major landslide-risk regions; - describe direct, indirect, and cascading effects of landslides; - connect landslides with earthquakes, volcanoes, storms, fires, floods, and tsunamis; - describe the ecological and mineral-resource services of landslides; - explain how logging, roads, grading, reservoirs, drainage, and urbanization alter risk; - identify surface evidence of past or potential landslides; - distinguish landslide inventory, hazard, and risk maps; - describe surveillance, rain gauges, wells, tiltmeters, geophones, slide fences, and debris-flow monitors; - explain why avoidance, drainage control, grading, slope supports, and warning systems reduce risk; - evaluate the limits of engineering corrections; - describe how people perceive landslide risk; - identify what to inspect before purchasing property on a slope; - explain avalanche safety practices; and - use the La Conchita, Oso, Portuguese Bend, Thistle, Rio de Janeiro, I-70, and Mount Rainier examples to explain landslide hazards and adjustments.