CHAPTER 8 LANDSLIDES AND OTHER DOWNSLOPE MOVEMENTS: FALLING MOUNTAINS

Correct answers are indicated by an asterisk, both in short answer and multiple choice questions. True or False questions can be easily prepared from multiple choice questions

Web Sites: http://landslides.usgs.gov/ http://landslides.usgs.gov/html_files/landslides/newsinfo.html http://pubs.usgs.gov/of/1994/ofr-94-0615/tvstudy.htm http://Vulcan.wr.usgs.gov/Projects/CalifLandslide/Publications/ReidLaHusen/framework.html http://www.fema.gov/hazards/landslides/landsli.shtm

Videos: Debris-flow dynamics, 1984, U.S. Geol. Survey (23 min.) Geologic Hazards Slide Set: Landslides. National Geophysical Data Center, NOAA, Dept. 930, 325 Broadway, Boulder, CO 80303 Landslide: The 1979 Abbotsford disaster, University of Otago, 1984. Available from the USGS Video Library. Landslides/rock falls: slides in the California Coast Range. Soil creep and soils flows The Rissa Landslide, 1985, Norwegian Geotechnical Institute: (25 min.) Order from Leif Carserud , Sveriges Geologiska Undersokning: Killiangatan 10, 223 50 Lund Sweden. Tel.: 009 46 46 13 14 56. USGS Landslide Prediction efforts: 1987, (120 min.) Avail on loan from USGS Library Spec. Collections, MS 955, 345 Middlefield Road, Menlo Park, CA 94025. (415)329-4009.

References: Crandell, D.R., 1989, Gigantic debris avalanche of Pleistocene age from ancestral Mount Shasta volcano, California, and debris-avalanche hazard zonation: U.S. Geol. Survey Bull., 1861, 32 p. Evans, S.G. and J.J. Clague, 1994, Recent climatic change and catastrophic geomorphic processes in mountain environments: in M. Morisawa, Geomorphology and natural hazards: Vol. 10, no. 1-4, p. 107-128. Hyndman, D.W. and J.J. Brown, 2000, Slump block and debris slide on the Blackfoot River, Montana, U.S.A.: Landslide News, No. 13, p. 15-18. Keefer, D.K., and others, 1987, Real-time landslide warning during heavy rainfall: Science, v. 238, p. 921- 925. Kiersch, G.A., 1991, Modern practice, training, and academic endeavors 1940s to 1980s, p. 51-85: in The Heritage of Engineering Geology; The First Hundred Years, Kiersch, G.A., ed., Geol. Soc. of America Centennial Spec. Vol. 3, 605 p. Lipman, P.W., W.P. Normark, J.G. Moore, J.B. Wilson, and C.E. Gutmacher, 1988, The giant submarine Alika debris slide, Mauna Loa, Hawaii: Journal of Geophysical Research, vol. 93, p. 4279-4299. Marti, J., M. Hurlimann, G.J. Ablay, and A. Gudmundsson, 1997, Vertical and lateral collapses on Tenerife (Canary Islands) and other volcanic ocean islands: Geology, vol. 25, p. 897-882. Melosh, H.J., 1990, Giant rock avalanches: Nature, v. 348, p. 482-483. Montgomery, D.R., K.M. Schmidt, H.M Greenberg, and W.E. Dietrich, 2000, Forest clearing and regional landsliding: Geology, vol. 28, p. 311-314. Moore, J.G., W.B. Bryan, M.H. Beeson, and W.R. Normark, 1995, Giant blocks in the South Kona landslide Hawaii, Geology, vol. 23, p. 125-128. Vallance, J.W. and K.M. Scott, 1997, The Osceola mudflow from Mount Rainier: Sedimentology and hazard implications of a huge clay-rich debris flow: Geol. Soc. America Bull. v. 109, p. 143-163. Plant, N. and G.B. Griggs, 1990, Coastal landslides and the Loma Prieta earthquake: Earth Science, v. 43, p. 12-18. Rose, W.I., J.J. Bommer, D.L. Lopez, M.J. Carr, and J.J. Major, editors, 2004, Natural Hazards in El Salvador: Geological Society of America Special Paper 375, Social Issues, p. 445-480. Schuster, R.L. and R.J. Krizek, editors, 1978, Landslides: analysis and control: National Research Council Transportation Research Board Spec. Rept. No. 176, 234 p. Scott, K.M., J.W. Vallance, and P.T. Pringle, 1995, Sedimentology, behavior, and hazards of debris flows at Mount Rainier, Washington: U.S. Geol. Survey Prof. Paper 1547. Turner, A.K. and R.L. Schuster, editors, Landslides – Investigation and mitigation: National Research Council, Transportation Research Board, Special Report 247, 672 p. Varnes, D.J., 1978, Slope movement types and processes: in Schuster, R.L. and R.J. Krizek, editors, 1978, Landslides: analysis and control: National Research Council Transportation Research Board Spec. Rept. No. 176, p. 11-33. Webb, R.H., T.S. Melis, P.G. Griffiths, J.G. Elliott, T.E. Cerling, R.J. Poreda, T.W. Wise, and J.E. Pizzuto, 1999, Lava Falls Rapid in Grand Canyon: Effects of Late Holocene debris flows on the Colorado River: U.S. Geological Survey Professional Paper 1591, 90 p.

CHAPTER 8: End of Chapter Answers

1. The maximum slope at which loose grains will stand depends on many factors. What is the approximate maximum slope for loose, rounded, dry sand grains? * about 30o

2. List the main factors that affect whether a slope will fail in a landslide. * slope angle or steepness * strength of slope material * amount of water * friction or resistance along the slip surface * presence of swelling clays (smectite) * internal surfaces sub-parallel to the slope

3. Why is the friction force on a gently sloping slip surface greater than that on a steeply sloping slip surface? * A larger proportion of the load is pressing against the slip surface on a gentle slope.

4. Why does raising the groundwater level in the ground often lead to slope failure? * That increases the water pressure in the pore spaces between the grains, pushing them farther apart.

5. List several distinctly different ways in which water can be removed from a wet slope that has slide potential. * plant trees and shrubs (increase evapotranspiration) * insert perforated pipes into the slope to help drain it. * dig a trench across a slope and fill it with loose, coarse rock to permit external drainage

6. What causes liquefaction of sediments? Briefly explain the process. * A loosely packed layer of sand grains settles into a more closely packed arrangement, most commonly by the shaking of an earthquake.

7. What circumstances lead to formation of quick clays? List the sequence of events. * Muds are laid down in marine bays or salty lakes. * The muds are raised or the water level falls so salty water no longer fills the pore spaces. * Fresh water rinses out the charged sodium atoms or ions. * The clay is shaken by an earthquake or load is added on top, causing collapse.

8. List several ways in which old landslides are commonly reactivated. * addition of water * steepen the slope by removing material at the lower part of the slope (undercutting the toe) * steepen the slope by adding material at the upper part of the slope (loading the top). * earthquakes

9. Why does the top of a rotational slide tilt back into the slope? Be specific. * The slide mass rotates on a circular curving surface so everything on it rotates back.

10. What can be done to slow or stop movement of a rotational slide? Be specific. * Drain water from the slide by inserting perforated drain pipes * Drain water from the slide by digging a drainage trench and backfilling with gravel. * Pile heavy boulders on the toe of the slide.

11. Why is it dangerous to live on an alluvial fan? Be specific. * Alluvial fans are often built up from debris flows flushing out of a canyon above. * Debris flows move fast and can kill by burial under dense debris or by impact of huge boulders.

12. How can you tell by looking at a canyon that has produced debris flows in the past whether it will produce another within the next few years? * If it has no loose debris in the canyon bottom it is not likely to produce one soon. * If it has abundant loose debris in the canyon bottom it could produce one in any heavy rain or rapid snowmelt.

13. If you see a debris flow or mudflow heading down a valley toward you, what should you do? Be specific. * run to the side of the valley and upslope

14. What parts of the United States are most susceptible to landslides? Be specific as to geographic location. * western California and Oregon (or Coast Ranges of) * western Colorado * Appalachians (or western Carolinas and Virginia, West Virginia, Pennsylvania) 15. Why do people build in landslide-prone areas? * for the views, it is in a scenic area, boulders make for scenic landscaping.

Chapter 8 - Short answer questions:

1. What would permit rounded sand grains to stand at a steeper slope? * a little moisture

2. Why does earthquake shaking of a water-saturated sand often lead to ground settling? * The shaking of a loosely packed sand leads to closer packing of the grains. Since they then take up less space, the ground settles.

3. What would make a pile of rounded sand grains collapse to be almost flat? * too much water – enough to fill all of the pore spaces between the grains

4. What is “cohesion” as applied to slope failure? * The holding together of otherwise loose grains on a slope – generally by moisture or cement.

5. List two materials that often provide cohesion to a slope. * a little water between the grains (not water saturated) * cement between the grains

6. List several common and distinctly different ways that cause the height of water in the ground to rise and thus increase the danger of sliding. * heavy or prolonged rains * leaking pipes * leaking swimming pools * septic drain fields * filling a reservoir behind a dam * remove vegetation

7. Why does filling a reservoir often cause landsliding? Explain clearly. * rise of water in the reservoir raises the water table in adjacent slopes

8. How does evapotranspiration work? List all the main aspects. * Rain falling on leaves or needles evaporates before reaching the ground. * Roots take in water from the soil, convey it to the leaves or needles, and transpire it to the atmosphere.

9. The presence of which clay mineral is most prone to causing landslides? * smectite or swelling clay

10. What conditions make smectite clay extremely weak? * wet conditions; water gets between its layers.

11. What material is subject to liquefaction? * water-saturated sand

12. What typically happens to ground over an area of liquefaction? * It sags and spreads.

13. What typical type of damage occurs to buildings on top of such a layer? Be specific. * building tilts, sags, or breaks up

14. Describe quick clays? * Clay flakes are stacked in a very open arrangement like a house of cards

15. In what parts of the world are quick clays most common? * Northern areas – Alaska, Canada, northern Europe

16. What three methods can be used to minimize damage from rockfalls? * rockbolts * coating with shotcrete * draping with heavy wire mesh

17. What types of internal surfaces are most prone to sliding? * planes sloping at gentler angles than the slope, especially if they “daylight.” * old landslide slip surfaces

18. For a rockfall or debris avalanche, what two factors lead to a greater distance of travel of the moving debris? * greater height of fall of the mass * greater volume of the falling mass

19. What kind of material is most-likely to fail in a rotational slide? * homogeneous, cohesive materials

20. Where the upper part of a rotational slide drops below the original ground surface, where does the missing material go? * As the mass rotates down, its lower end moves out above the original slope, generally collapsing as an incoherent mass at the foot of the slide.

21. Do tree roots hinder movement of a rotational slide? Explain clearly why they do or do not. * The strength of tree roots does not significantly hinder movement because the roots do not penetrate down to the slide surface. * Tree roots do, however, take up water from the ground; that reduces water pressure and increases friction in the ground and would help to hinder movement. 22. Why do some slides move as translational slides rather than rotational? Explain clearly. * They move on a preexisting surface about parallel to the slope of the ground. * This surface may be a weak layer such as shale or soil over bedrock.

23. What are the differences in appearance between debris flow and a mudflow deposits? * A debris flow has coarse boulders concentrated at the top of the flow, distinct bouldery natural levees and boulders at the toe, and often a deep, narrow channel. * A mudflow is dominated by fine-grained material.

24. What upstream change or changes make a canyon more likely to produce a debris flow? * loss of vegetation cover, such as by a fire or major urbanization.

25. What is the best long-term solution to avoiding a debris flow? * Do not live (or build) on an alluvial (debris-flow) fan.

26. If houses are already scattered across a debris-flow fan, list several ways to protect them from a future debris flow. * open pathways for future flows or channel them away from the houses. * slow debris flow debris at check dams or grid dams * trap a debris flow in a debris-flow basin. * monitor rainfall amounts for the season and within individual storms.

27. How can you determine the depth of a debris flow in a canyon it moved through? * bark battered off the upstream sides of trees up to that height * sand embedded in bark and rocks in tree branches up to that height.

28. What is the most common environment to generate mudflows? * ash on the flank of a volcano

29. Given that source, when is the most likely time for generation of such a mudflow? * during a volcanic eruption

30. Which is likely to flow farther down to the flatter, wider part of a valley, a debris flow or a mudflow and why? * a mudflow because the tiny pore spaces between particles keep water from escaping and therefore keep the mudflow mobile.

31. What is a typical rate of movement for a debris flow (either a numerical rate or “as fast as …) ? * 1 meter per second (or) 3.6 km per hour (or) 2 miles per hour (or) rapid walking rate

32. What makes some very large oceanic volcanoes susceptible to catastrophic collapse of one of their flanks? * heavy, solid lava flows over loose, weak, submarine debris * prominent radial rift zones

33. What catastrophic secondary effect often accompanies flank collapse of an oceanic volcano and can kill thousands of people? * major tsunami

34. What major factors control the maximum height of water downstream from failure of a landslide or natural dam? * water height * volume of water * distance downstream from the dam * width of the valley

35. Landslides are more-frequent in concave-upward parts of slopes (in swales) than on ridges. Why is that the case? * Groundwater is closer to the surface there and thus water pressure in the soil is greater.

36. Why do highway departments sometimes pile huge boulders near the base of a roadcut? What are they trying to prevent and how does it work? * They are trying to prevent slumping of the roadcut by loading the toe of the potential slump.

37. What is the difference between a slump and a translational slide? * a slump fails as a relatively coherent rotational mass; a translational slide moves parallel to the surface of the slope.

38. Why do some slopes fail in a rotational slump rather than a translational slide? * slumps occur in relatively homogeneous material; translational slides occur where weak material lies in a zone parallel to the ground surface.

39. The disastrous Vaiont landslide in Italy involved what combination of circumstances? * Weak rock layers dipping parallel to a slope, filling a new reservoir at the base of the slope, and prolonged heavy rainfall.

40. Numerous big rocks spread across the surface of an alluvial fan indicate what process of deposition? (assume there glaciers have not been present in the region) * debris flow

41. How can you recognize evidence for debris-flow activity in a narrow canyon? List two pieces of evidence, excluding characteristics of the deposits. * bark battered off the upstream sides of trees; sand and rocks embedded in bark; rocks lodged in tree branches.

42. What can you use to protect homes on an alluvial fan from debris flow damage? * excavate a debris flow collection basin; place a strong, grid of steel rods or pipes across the debris flow channel to stop the coarser debris but let the water pass through.

43. What can you use to recognize the action of soil creep? * tree trunks that bend out from a slope before curving up; fences or walls that tilt down slope.

44. What is the arrangement of clay grains before their re-arrangement during quick-clay movement? * Random orientation or “house of cards” arrangement.

45. What is the arrangement of clay grains after their re-arrangement during quick-clay movement? * The flakes lie flat, parallel to one another – like a deck of cards dropped on a table.

46. What can be done to stabilize a steep, high, dangerous rock cliff or roadcut to keep it from collapsing onto a highway or railroad track? * insert rockbolts (spreading bolts drilled into the rock to keep the fractured pieces together)

47. What is a daylighted surface or layer? * a weak layer sloping down towards a roadcut or hillside at a somewhat gentler slope than the hillside so that it becomes exposed at the surface.

48. What orientation of rock layers or fractures in a hillside or roadcut is particularly dangerous? Why? * layers dipping at a gentler angle towards a road or manmade structure than the hillslope, that is daylighted surfaces.

49. List 3 factors that affect whether a slope will landslide. * load, slope angle, material strength, frictional resistance, water content

50. List three ways in which human activity can increase the danger of landsliding: * adding water, loading the top of a slope, undercutting the slope, removing vegetation

51. Explain why removal of vegetation from a slope increases the danger of landsliding? * It reduces evapotranspiration by vegetation and so increases the water in the ground.

52. What factors lead to the occasional collapse of giant slices of Hawaiian volcanoes? * Heavy basalt lava flows above sea level overlie weak, rubbly basalt debris below sea level. Big fractures in the basalt lava flows permit a big slice to slide into the deep sea. 53. Aside from being atop the collapsing slab, what other major hazard or hazards can accompany collapse of a giant slab of a Hawaiian volcano? * a giant tsunami and a major earthquake.

Chapter 8 - Multiple choice questions:

1. The main factors that affect the chance that a slope will fail in a landslide do not include. a. slope angle or steepness b. strength of slope material c. amount of water d. * age of the slope e. internal surfaces sub-parallel to the slope

2. Why does raising the groundwater level often lead to slope failure? a. That lubricates the ground, making it slide easier b. That creates a layer of water that the slope slides on. c. * That increases the water pressure in the pore spaces between the grains, pushing them farther apart d. When the ground freezes, the water turns to ice, making it slide easily e. The water seeps out of the ground and the slope above slides on it

3. One way in which water can not be removed from a wet slope that may slide is the following: a. plant trees and shrubs to increase evapotranspiration b. insert perforated pipes into the slope to help drain it. c. dig a trench across a slope and fill it with loose, coarse rock to permit drainage d. pump water out from a well e. * place a load on the slope to squeeze water out of it.

4. Why does earthquake shaking of a water-saturated sand often lead to ground settling? a. * The shaking of sand leads to closer packing of the grains b. The shaking wears off the edges of the grains making them smaller c. The shaking makes the grains settle in the water d. It doesn’t. Settling only occurs if you pump water out of the ground e. Shaking rearranges the atoms in the grains allowing them to fit closer together.

5. What are quick clays? a. Clays that form quickly when acid mine drainage contaminates a lake b. Clays that form quickly when air-fall volcanic ash falls in seawater c. Clays which flow quickly in a volcanic mudflow d. * Clays with randomly-oriented “house-of-cards” flakes that can collapse when shaken e. Swelling clays that expand when they get wet.

6. Old landslides are not normally reactivated by: a. adding water b. earthquakes c. steepening the slope by removing material at the lower part of the slope d. steepening the slope by adding material at the upper part of the slope. e. * piling heavy rocks on the lower part of the slope. 7. Why does the top of a rotational slide tilt back into the slope? a. As the slide moves away from its head scarp, material slides back into the gap. b. * The slide mass rotates on a circular curving surface so everything on it rotates backward. c. The inertial pull of the fast-moving slide leaves a gap behind at the top. d. Since the slide mass pulls away from the head scarp, it leaves a gap that tilts backward. e. Since the top of the mass is pulled forward, the bottom tilts back, just as you would if you slip on ice.

8. It is dangerous to live on an alluvial fan because they a. are extremely windy places. b. form on the inside bends of fast-moving streams – a dangerous place in high water. c. * are often built up from debris flows flushing out of a canyon above. d. are areas of common subsidence and sinkholes. e. are formed at the mouths of river where deep floods are common.

9. If you see a debris flow or mudflow heading down a valley toward you, what should you do? a. * run to the side of the valley and upslope b. Watch carefully so a boulder doesn’t hit you. c. Run down the valley as fast as possible. d. Stand behind a big tree. e. stand at the edge of the channel and watch it go by, since they don’t move very fast.

10. What parts of the United States are not susceptible to landslides? a. Florida b. western Colorado c. Appalachian Mountains d. Pennsylvania e. * Texas. 11. What is “cohesion” as applied to slope failure? a. the condition of a slope that has not yet begun to slide b. loose grains in a slope that are angular rather than rounded so they don’t slip past one another easily. c. * the holding together of otherwise loose grains on a slope, often by a little moisture d. grains in a rock that have interlocked as a result of metamorphism e. grains in a granite or similar rock that don’t separate easily

12. Which of the following would not cause the height of water in the ground to rise and thus increase the danger of sliding? a. leaking pipes b. leaking swimming pools c. septic drain fields d. * planting trees e. filling a reservoir behind a dam

13. Which of the following would not help reduce the movement of a rotational landslide? a. * the roots of trees b. evapotranspiration c. installing trenched drains into the slope d. inserting perforated horizontal pipes into the base of the slide e. adding boulders to the base of the slope.

14. Why does a swelling clay swell? a. landsliding pushes the edges of the grains so they buckle and thicken b. freezing causes the water in them to swell c. * water gets into the clay layers spreading them apart d. drying the clay makes it swell like cooking a cake e. deformation of grains during sliding interleaves clay flakes making them thicker

15. What material is subject to liquefaction? a. swelling clays b. smectite clay c. clay flakes stacked like a “house of cards” d. * water-saturated sand e. dry sand like dune sand with well rounded grains .

16. Where are quick clays most common? a. * Northern areas – Alaska, Canada, northern Europe b. Tropical areas – Central America, Brazil, Africa c. Texas Gulf Coast d. Coastal cliffs of California e. Colorado 17. What types of internal surfaces are least prone to sliding? a. * planes sloping at somewhat steeper angles than the slope b. planes sloping at somewhat gentler angles than the slope c. old landslide slip surfaces d. daylighted surfaces e. planes cemented by smectite.

18. For a rockfall or debris avalanche, what two factors lead to a greater distance of travel of the moving debris? a. more water in the mass and release promoted by freezing b. greater height of fall and a block that is not too angular. c. greater size of the falling mass and smaller particles d. * greater height of fall of the mass and greater volume of the falling mass. e. greater height of fall and higher clay content

19. What kind of material is most-likely to fail in a rotational slide? a. * homogeneous, cohesive materials b. rounded sand grains in the slope c. elongate boulders with smoothly rounded sides d. loose, dry soils e. talus slopes.

20. Which slope material is most likely to move as a translational slide? a. homogeneous cohesive material b. a raised terrace of old beach sediments c. moist soil 300 meters thick d. * 3-meter-thick soil over bedrock. e. deeply weathered granite rich in clays.

21. Which of the following features would not be typical of a mudflow deposit? a. * distinct bouldery natural levees b. dominantly fine-grained material. c. erodes a narrow channel d. boulders mixed in with fine-grained material e. nicely defined internal layers.

22. What upstream change or changes make a canyon more likely to produce a debris flow? a. major urbanization. b. deforestation by wildfire c. long-time accumulation of loose rock debris in a steep channel d. an earthquake shakes loose large amounts of shallow landslides e. * undercutting the toe of the canyon by either natural or artificial means. 23. If houses are already scattered across a debris-flow fan, one way you cannot protect them from a future debris flow is to a. open channels for future flows or channel them away from the houses. b. slow debris flow debris at check dams or grid dams c. trap a debris flow in a debris-flow basin. d. monitor rainfall amounts for the season and within individual storms e. * place large storm drains at the upper end of the fan to collect the water as it enters the fan

24. How can you not determine the depth of a debris flow in a canyon it moved through? a. bark battered off the upstream sides of trees up to that height b. sand embedded in bark up to that height c. rocks lodged in tree branches to that height. d. * the maximum height of sand eroded from the valley sides. e. the highest rocks on natural levees deposited by the flow

25. What is the most common environment to generate mudflows? a. erosion of the fine-grained sediments from a soil-covered slope. b. * erosion of ash from the flank of a volcano c. erosion of the muddy bottom of a glacial lake d. erosion of soft shales in a coastal terrace e. erosion of mud from a former bay, now raised as a coastal terrace

26. What is a typical rate of movement for a debris flow? a. 3 meters per hour b. 30 meters per hour c. * 3 km per hour (or) rapid walking rate d. 60 km per hour (speed limit on through street in city) e. 110 km per hour (speed limit on many freeways)

27. What catastrophic secondary effect often accompanies flank collapse of an oceanic volcano and can kill thousands of people? a. eruption of voluminous basalt lava flows b. * a major tsunami c. magnitude 5 earthquakes d. release of huge volumes of carbon dioxide e. widespread forest fires.

28. What major factor does not control the maximum height of water downstream from failure of a dam? a. water height behind the dam b. volume of water c. distance downstream from the dam d. width of the valley e. * length of the reservoir upstream of the dam.

29. Why are landslides more-frequent in concave (outward from slope) parts of slopes (in swales) than on ridges. a. * Groundwater is closer to the surface there. b. There may be less soil on the ridge that would not slide as easily. c. Water seeping up into the soil from the valley bottom can’t reach as far as the ridge. d. Wildfires more commonly burn ridges, making them less prone to sliding. e. It is too great a distance from the ridge to the valley bottom to landslide.

30. Why do highway departments sometimes pile huge boulders near the base of a sloping roadcut? a. To prevent gullying of the sloping roadcut b. To keep vegetation from growing on the sloping roadcut c. To prevent water from getting into the slope d. * To prevent slumping of the roadcut by loading the toe of the potential slump e. To prevent damage to the roadcut from collisions with erratic drivers. 31. Big rocks spread across the smooth surface of an alluvial fan indicate deposition by what process? a. glaciers b. * debris flows c. slumps d. streams e. lakes

32. Debris flows in a narrow canyon would not leave which of the following?. a. unsorted particle sizes in deposits b. natural levees made of coarser debris c. * thin layers in the deposits d. rocks stranded in tree branches. e. largest boulders at the surface of deposits

33. What can you not use to protect homes on an alluvial fan from debris flow damage? a. excavate a debris flow collection basin at the head of the fan b. place a strong grid of steel rods or pipes across the debris flow channel to stop the coarser debris c. * drop some trees across the channel to stop the debris flow d. place a concrete-lined channel from the head of the fan to the base to carry debris flows through e. build a levee from the head of the fan to channel the flows to one side away from the houses.

34. What can you not use to recognize the action of soil creep? a. tree trunks that bend out from a slope before curving up b. fences that lean down slope c. walls that lean down slope d. * fragments of rocks that clearly came from farther upslope e. bedrock layers that curve downslope as they approach the surface

35. What is a daylighted surface or layer? a. * a weak layer sloping down towards a roadcut or hillside at a somewhat gentler slope than the hillside b. a part of the bedrock that is exposed at the surface c. a layer that becomes exposed by excavation of a roadcut d. a layer that becomes exposed at the surface by artificial removal of soil over it. e. a layer exposed at the surface by landsliding.

36. The angle of repose for dry sand is: a. 10o-15o b. * 30o-35o c. 45o-50o 75o-80o