NATURAL TERRAIN IN HONG KONG

LANDSLIDES IN HONG KONG

Natural Terrain Landsliding
Classification
Slumps, Slides, Avalanches and Flows

• Slumps
• Slides
• Debris Avalanches
• Debris Flows
• Ancient Deep Seated Landslides

Natural Terrain Landsliding

• Mostly shallow failures on the middle and upper hill slopes
• Commonly in groups on 30^o to 40^o slopes
• Landslides are concentrated in areas affected by intense rainstorms
• Densities of more than 10 landslides per km² have been recorded
• On average, about 320 natural terrain landslides occurred each year from 1945 to 1994
• Typical volumes range between 50 m³ and 2,000 m³
• Very large landslides of thousands of cubic metres volume are rare

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Classification

• Landslides are classified according to two criteria:
  • Type of material: rock, boulder, or debris
  • Movement mechanism: fall, slide, topple, slump, flow or avalanche
• Names combine the two terms: e.g. rock fall, debris flow and debris avalanche
• Most common types in Hong Kong: rock falls, boulder falls, slumps, debris slides, debris avalanches and channelised debris flows.

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Slumps, Slides, Avalanches and Flows

• Represent a continuum of increasing mobility of generally granular materials on slopes
  

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  Slumps

  • A sliding failure in which the intact displaced mass partially overlies the surface of rupture
  • Small debris slumps are the most common shallow failures on hillslopes
  • Defined by tension cracks or small landslide scarps
  • A number of natural terrain failures triggered by excavations are of this type

  
  

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  Slides

  • Slides are the next stage of mobilisation
  • The displaced material moves beyond the plane of rupture, but remains intact
  • Only represents a small proportion of natural terrain landslides in Hong Kong

  
  

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  Debris Avalanches

  • Debris avalanches are the most common types of natural terrain landslide in Hong Kong
  • Most of the displaced material breaks up and becomes remoulded
  • The debris commonly spreads out as a debris lobe
  • Open slope debris avalanches are usually small with limited runout
  • However, the debris may be funneled into channels
  • This results in a channelised debris avalanche, in which the trail is narrower than the source

  
  

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  Debris Flows

  • A Debris flow occurs when landslide debris is mixed with surface water and slurry flow develops
  • Most commonly occurs when the debris enters a stream and the flowing debris is channelised along the drainage line
  • Channelised debris flows may increase in volume as they erode and entrain loose deposits from the stream bed and banks
  • For example, the 1990 Tsing Shan Debris Flow had an initial volume of about 2,000 m³, but eroded and entrained an additional 18,000 m³ of material containing large boulders
  • As dilution increases the debris slurry will become more mobile until hyperconcentrated stream flow develops
  • A debris flood develops when further dilution causes the mechanism to grade into stream flow, which is an alluvial process

  
  

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  Ancient Deep Seated Landslides

  • Scars of ancient, more deep-seated landslides can be recognised on aerial photographs
  • Many of these larger landscape features, and their related deposits, probably formed during periods of wetter climate, such as between 8,000 to 10,500 years ago
  • The source areas are usually degraded by more recent small landslides and erosion
  • Failure material forms fans and aprons of colluvium, e.g. the Sham Wat debris lobe comprises several large rock and soil failures
  • Provisional dating indicates that the events may have occurred within the last 10,000 years

  
  

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