CS Landslides in Hong Kong

Question 1

Key facts

Answer 1

• 1 of the most densely populated urban areas in the world (>7M in 1075km2)
• natural steep terrain (upper slopes steeper than 30°)

Question 2

Causes

Answer 2

• high rainfall, steep slopes + dense human dev. on islands
  
  • w urban dev. landslides triggered by excavation + building works
• 1966 landslides: some believe veg. inc. problem
  
  • trees held back many smaller landslides causing larger ones (washout) to occur

Question 3

Causes - geology

Answer 3

• geology mainly constructed of sedimentary rocks, granites + volcanic rocks
  
  • sedimentary rocks usually form lowlands
  • granites + volcanic rocks situated on higher ground + prone to failure
    
    • both seriously weathered but granites > volcanic rocks (more resilient + less prone to slope failure)

Question 4

Consequences/examples

Answer 4

• 1947-1997: >470 ppl died
• June 1966: rainstorms triggered massive landslides (killed 64 ppl)
  
  • 2500 ppl made homeless + further 8000 evacuated

Due to:
- high rainfall for 1st 10 days: >300mm (130mm in normal yr)
- 11-12th: >400mm fell (nearly 1/3 in 1hr)
- by 15th area received >1650mm
- >700 landslides recoded that month

Question 5

Solutions: managing landslides (SSS)

Answer 5

• Slope Safety System managed by (GEO)
  
  • has >700 staff for its wide range of activities
  • maintains slope safety: investigating + researching causes of sig. + serious landslides to improve system

Question 6

Solutions: managing landslides (gov. action)

Answer 6

• 1 of HK’s gov. interventions: ensures private owners of slopes take responsibility for slope safety
  
  • if not obeyed, $50,000 fine + imprisonment up to 1 yr
  • successful: risk dec. by 50% since 1977

Question 7

Solutions: maintenance of slopes

Answer 7

• vital to remove excess water from slopes: surface draining systems + protective covers used to protect slopes
  
  • surface drains vulnerable to blockages: w/o maintenance makes situation worse (many not properly maintained)
• man-made slopes 1 of main methods of slope stabilisation: contain drains to intercept + direct water away
  
  • slope usually protected from infiltration + erosive effects of water by impermeable hard covers

Question 8

Maintenance of slopes: greening techniques

Answer 8

• use natural veg. to dec. risk of mass movements
• mulching system, long-rooting grass + fibre reinforced soil system

Question 9

What is the mulching system?

Answer 9

• provides protective cover allowing natural veg. to grow on slope
• a natural veg. cover is able to grow through mat, securing it in place

Question 10

What is long-rooting grass?

Answer 10

• covers man-made slopes
• applied by drilling planter holes into hard cover, filling w soil mix + fertilisers then planting grass
• dec. moisture in soil. so dec. loss of cohesion dec. risk of landslides

Question 11

Advantages of mulching system

Answer 11

• higher adhesive capacity on steep slopes
• high resistance to rain erosion
• high water-retaining capacity

Question 12

Advantages of planting long-root grass

Answer 12

• natural + environmentally friendly
• cost-effective
• fast + easy installation
• low maintenance

Question 13

What is worsening the situation?

Answer 13

• inc. pop growth therefore inc. building further up slopes: inc. risk from landslides + inc. damages from potential slide