CS: Mass Movements in Hong Kong

Question 1

What is mass movement?

Answer 1

• large scale downslope movement of material under influence of gravity, caused when stress exerted on slope exceeds internal strength, causing instability

Question 2

General

Answer 2

• 1of the most densely populated urban areas in the world (>7M in 1075km2)
• has dev 1 of the most advanced slope management systems in the world, bc of repeated fatal landslides in 1970s + 80s

Question 3

Nature + causes (1)

Answer 3

• heavy rainfall, in which HK receives >2000mm of rainfall annually, esp during summer monsoon + typhoon season, which saturates soil + dec erosion
  
  • e.g. in June 2008, 290mm of rain fell in 24hrs, triggering >100 landslides, esp in N New Territories
• natural steep terrain, w >60% of land being hilly or Mt, in which upper slopes are steeper than 30°

Question 4

Nature + causes (2)

Answer 4

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

Question 5

Nature + causes (3)

Answer 5

• dense urban dev on islands, inc excavation + building works, trigger landslides, bc clear veg, dec root cohesion + inc SR
  
  • however, some believed that veg inc problems of the 1966 landslides, bc veg held back many smaller landslides, causing larger ones (washout) to occur

Question 6

Impacts on lives + property

Answer 6

• between 1947 + 1997, >470 ppl died
  
  • in June 1966, rainstorms triggered massive landslides, bc heavy rain fell for 1st 10 days, amounting to >300mm (130mm un normal yr). During 11-12th, a further >400mm fell (nearly 1/3 in 1hr), + by the 15th area received >1650mm
  • this led to >700 landslides being recorded that month, which killed 64 ppl, made 2500 ppl homeless + led to a further 8000 ppl being evacuated
• landslides also led to destruction of homes + infrastructure, in which the Sau Mau Ping landslide, in 1976, destroyed 2 13-storey buildings + killed 18 ppl

Question 7

Prediction + monitoring of mass movements

Answer 7

• HK Observatory + GEO use >100 rain gauges + slope sensors (e.g. inclinometers), installed on key slopes, to monitor rainfall + trigger warnings + action protocols when landslide risks are forecasted

Question 8

Hazard mapping

Answer 8

• Slope Safety System, managed by >700 GEO staff for its wide range of activities, maintains detailed records of >60,000 man made slopes
  
  • this has allowed it to create hazard maps, that identify high risk areas, based on continuous research + investigation on causes of sig landslides to improve system

Question 9

Preparedness (1)

Answer 9

• 1 of HK’s gov. interventions is to ensure private owners of slopes take responsibility for slope safety, by resulting in $50,000 fines + up to 1 yr imprisonment if not obeyed
  
  • this has been successful, bc has dec risk by 50% sine 1977

Question 10

Preparedness (2)

Answer 10

• also, in order to maintain slopes, it’s vital to remove excess water from slopes, thus surface drainage systems + protective covers have been used to protect slopes
  
  • however, surface drains are vulnerable to blockages, + so w/o maintenance (many aren’t properly maintained) it makes situation worse
• man-made slopes are 1 of main methods of slope stabilisation, + contain drains to intercept + direct water away
  
  • this protects slope from infiltration + erosive effects of water bc of impermeable hard covers

Question 11

Preparedness (3)

Answer 11

• greening techniques (e.g. mulching system + long-rooting grass), which use natural veg. to dec. risk of mass movements, have also been put in place

Question 12

What is the mulching system?

Answer 12

• provides a protective cover, allowing natural veg. cover to grow on slope + through mat, securing it in place
• this is advantageous bc inc adhesive capacity on steep slopes, resistance to rain erosion + water retaining capacity

Question 13

What is long-rooting grass?

Answer 13

• covers man-made slopes + is applied by drilling planter holes into hard cover, filling w soil mix + fertilisers, then planting grass
• this dec moisture in soil, + so dec loss of cohesion + risk of landslides
• it is advantageous bc is natural + envi friendly, cost effective, fast + easy to install + low maintenance

Question 14

Conclusion/Evaluation

Answer 14

• although rainfall-triggered mass movements have had serious impacts in the past, Hong Kong has effectively dec mass movements + their risks through gov intervention + implementing a variety of techniques
• however, if pop continues to grow, these techniques will become less effective as building inc further up slopes + inc risk of landslides + damage from potential slides, thus new methods should be put in place to remove water from slopes more efficiently