Mass Wasting Chapter

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Mass Wasting Chapter

• Prepared by Iggy Isiorho for
• Dr. Isiorho
• Mass Wasting
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Mass Wasting

• Mass Wasting Movement, caused by gravity, in
  which bedrock, rock debris, or soil
  moves downslope in bulk.
• Landslides The general term for a slowly to
  very rapidly descending mass of rock
  or debris.
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Classification of Mass Wasting

• The classification system used here and
  summarized in Table 9.1 is based on (1) rate of
  movement, (2) type of material, and (3) nature of
  the movement.
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Rate of Movement

• A landslide like the one in Peru clearly involves
  rapid movement. Just as clearly, movement of soil
  at a rate of less than a centimeter a year is
  slow movement. Between these extremes is a wide
  range of velocities.
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Type of Material

• Mass wasting processes are usually distinguished
  on the basis of whether the descending mass
  started as bedrock (as in a rockslide) or as
  debris. The term debris, as applied to mass
  wasting processes, means any unconsolidated
  material at the earths surface, such as soil and
  rock fragments of any size.
• The amount of water (or ice and snow) in a
  descending mass strongly influences the rate and
  type of movement.
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Type of Movement

• In general, the type of movement in mass wasting
  can be classified as mainly flow, slide, or fall
  (Fig. 9.1). A flow implies that the descending
  mass is moving downslope as a viscous fluid.
  Slide means the descending mass remains
  relatively coherent, moving along one or more
  well-defined surfaces. A fall occurs when
  material free-falls or bounces down a cliff.
• Two kinds of slip are shown in Fig. 9.1. In a
  translational slide, the descending mass moves
  along a plane approximately parallel to the slope
  of the surface. A rotational slide involves
  movement along a curved surface, the upper part
  moving downward while the lower part moves
  outward.
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Controlling Factors in Mass Wasting

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Gravity

• Shear force In mass wasting, component of
  gravitational force that is parallel to an
  inclined surface.
• Shear strength In mass wasting, the resistance
  to movement or deformation of
  material.
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Water

• Water is a critical factor in mass wasting. When
  debris is saturated with water (as from heavy
  rain or melting snow), it becomes heavier and is
  more likely to flow downslope. The added
  gravitational shear force from the increased
  weight, however, is probably less important than
  the reduction in shear strength. This is due to
  increased pore pressure in which water forces
  grains of debris apart.
• Paradoxically, a small amount of water in soil
  can actually prevent downslope movement. When
  water does not completely fill the pore spaces
  between the grains of soil, it forms a thin film
  around each grain (Fig. 9.3). Loose grains adhere
  to one another because of the surface tension
  created by the the film of water and shear
  strength increases.
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Common Types of Mass Wasting

• Creep
• Debris Flow
• Rockfalls and Rockslides
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Creep

• Creep is very slow, downslope movement of soil or
  unconsolidated debris. Shear forces, over time,
  are only slightly greater than shear strengths.
• Two factors that contribute significantly to
  creep are water in the soil and daily cycles of
  freezing and thawing. As we have said,
  water-saturated ground facilitates movement of
  soil downhill. What keeps downslope movement from
  becoming more rapid in most areas is the presence
  of abundant grass or other plants that anchor the
  soil.
• Although creep does take place in year-round warm
  climates, the process is more active where the
  soil freezes and thaws during part of the year.
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Debris Flow

• The general term debris flow is used for mass
  wasting in which motion is taking place
  throughout the moving mass (flow). The common
  varieties Earthflow, Mudflow, and Debris
  avalanche are described in this section.
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Earthflow

• In an earthflow, debris moves downslope as a
  viscous fluid the process can be slow or rapid.
  Earthflows usually occur on hillsides that have a
  thick cover of debris, often after heavy rains
  have saturated the soil.
• A landslide may be entirely an earthflow, as in
  Fig. 9.7A, with debris particles moving past one
  another roughly parallel to the slope. Commonly,
  however, rotational sliding takes place above the
  earthflow as in Fig. 9.7B and Fig. 9.8. This
  examples is a debris slide (upper part) and an
  earthflow (lower part). In such cases, debris
  remains in a relatively coherent block or blocks
  that rotate downward and outward, forcing the
  debris below to flow.

Debris Flow ?
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Solifluction and Permafrost

• Solifluction Flow of water-saturated debris
  over impermeable material.
• Permafrost Ground that remains permanently
  frozen for many years.
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Mudflow

• Mudflow A flowing mixture of debris and water,
  usually moving down a channel.
• Mudflows are most likely to occur in places where
  debris is not protected by a vegetative cover.
  For this reason, mudflows are more likely to
  occur in arid regions than in wet climates. A
  hillside in a desert environment, where it may
  not have rained for many years, may be covered
  with a blanket of loose material. With sparse
  desert vegetation offering little protection, a
  sudden thunderstorm with drenching rain can
  rapidly saturate the loose debris and create a
  mudflow in minutes.
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Debris Avalanche

• The fastest variety of debris flow is a debris
  avalanche, a very rapidly moving, turbulent mass
  of debris, air, and water.
• Some geologists have suggested that in very
  rapidly moving rock avalanches, air trapped under
  the rock mass creates an air cushion that reduces
  friction. This could explain why some landslides
  reach speeds of several hundred kilometers per
  hour. But other geologists have contended that
  the rock mass is too turbulent to permit such an
  air cushion to form.
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Rockfalls and Rockslides

• Rockfall Rock falling freely or bouncing down a
  cliff.
• Talus An accumulation of broken rock at the
  base of a cliff.
• Rockslide Rapid sliding of a mass of bedrock
  along an inclined surface of weakness.
• Rock avalanche A very rapidly moving, turbulent
  mass of broken-up bedrock.
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