Gully Erosion, Sand Dunes, and Mass Movement

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Gully Erosion, Sand Dunes, andMass Movement

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4.1 Introduction

• Gullies are severe stage of water erosion.
• Sand dunes are the results of severe wind erosion
  and deposition.
• Mass movement is occurred on steep land which is
  mainly caused by gravity agent.

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4.1 Introduction

• Poor land use practices and poorly planned
  development activities can cause these problems.
• The greatest economic loss from gully erosion,
  wind erosion, and mass movement frequently occur
  in downstream and down wind, or down slope from
  the problem area.

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4.2 Gully Erosion

• Gully erosion is the detachment and removal of
  soil by concentrations of flowing water,
  sufficient to cause the formation of channels
  that could not be passed by normal cultivation
  tools.
• The flow in gully is almost ephemeral
• Gully often occur on area that have a low density
  vegetation cover and highly erodible soils.

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4.2 Gully Erosion

• The causes of gully erosion
• In engineering terms the cause of gully erosion
  is the breakdown of a state of metastable
  equilibrium in the stream or watercourse.
• Increasing the floods
• Decreasing channel capacity

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Classification of gully erosion
Class Total length of erosion gullies (km/km2) Zachar Total length of erosion gullies (km/km2) China
Insignificant lt0.1 lt0.2
Slight 0.1-0.5 0.2-0.4
Moderate 0.5-1.0 0.4-0.8
Severe 1.0-2.0 0.8-1.6
Very severe 2.0-3.0 1.6-3.2
Catastrophic gt3.0 gt3.2
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4.2 Gully Erosion
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4.2 Gully Erosion
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4.2 Gully Erosion
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4.2 Gully Erosion

• Control of gully erosion
• Principles of gully control
• Control by vegetation
• In gully control a bag of fertilizer is more
  effective than a bag of cement
• Structures for gully control
• Temporary structures
• Permanent structures

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4.3 Sand Dunes

• Sand dune is mounds of erosion materials that
  exist independently of any fixed surface feature
  and are capable of movement from place to place.

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4.4 Mass Movement

• Mass movement is the rapid down slope movement of
  finite masses of soils, rock, and debris that
  driven by gravity.
• Falls / Slumps ??
• Land slides / Avalanches ??
• Debris torrents ???
• Earthflows ??
• Creep ??

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4.4 Mass Movement

• The stability of the soil mass on a hillslope in
  respect of mass movement can be assessed by a
  safety factor (F)
• F s/?
• The slope is stable if F gt 1
• The slope will failure if F lt1
• F c (? zcos2? - u) tan?/ ?z sin? cos?

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4.4 Mass Movement

• F can be defined by Skempton and Hutchinson,
  1969
• F c (? zcos2? - u) tan?/ ?z sin? cos?
• Where c the effective cohesion of the soil, ?
  the unit weight of soil, z the vertical depth
  to the failure plane, ? the slope angle and ?
  the effective angle of internal friction of soil.