Case Study

Question 1

Key facts about Hong Kong

Answer 1

• Hong Kong is one of the most densely populated areas in the world - 1075 km^2 with a population of 7 million
• most areas of Hong Kong have naturally steep terrain - upper slopes steeper than 30°
• most of the population is concentrated in less steep areas on both sides of Victoria harbour

Question 2

Causes of landslides in Hong Kong

Answer 2

• a combination of high rainfall (wet season May to sept), steep slopes and dense human developments on the islands
• the geology of Hong Kong is constructed mainly from 3 rock types: sedimentary rocks, granites, and volcanic rocks
• the sedimentary rock forms the lowlands
• the granites + volcanic rocks are situated on higher grounds + are prone to failure -> both are seriously weathered, although granite rocks tend to be weathered more deeply
• volcanic rocks are more resilient + less prone to weathering + therefore less prone to slope failure
• vegetation is believed to have increased the problem in the 1966 landslides - trees held back many smaller landslides + allowed larger ones (washouts) to occur
• other forms of landslides involve debris valances and rockslides

Question 3

Examples of landslides

Answer 3

• 1947-1997, more than 470 people died as a result of landslides
• June 1996 - rainstorms triggered massive landslides

Question 4

Cause of June 1996 landslide

Answer 4

• rainfall had been high for the first 10 days in June - over 300mm had fallen, compared with 130mm in a normal year
• on 11 + 12 June over 400mm fell - nearly 1/3 of this happened in one hour
• 15 June - the area had received over 1650mm of rainfall
• over 700 landslides were recorded in Hong Kong that month

Question 5

Managing landslides in Hong Kong: GEO

Answer 5

• the slope safety system is managed by the Geotechnical Engineering Office (GEO) - has staff establishment of over 700
• the GEO maintains its slope safety through investigating + researching the causes of significant + serious landslides to improve the slope safety systems
• the GEO is continuously updating + maintaining the Catalogue of Slopes which contains info of some 57,000 sizeable man-made slopes in Hong Kong

Question 6

Managing landslides: govt

Answer 6

• govt. intervention to ensure that private owners of slopes take responsibility for slope safety - if a slope owner doesn’t comply with the regulation, prosecution will lead to a HK$50,000 fine + imprisonment for up to one year
• this has had successful results - risks from landslides has been reduced by 50% since 1977
• however, as a result of continued pop. growth, developers increasingly building up the slopes - the risk of landslides increases

Question 7

Managing landslides: drainage

Answer 7

• vital to remove excess water from slopes - surface drainage systems + protective covers are two methods used to protect slopes
• surface drains are very vulnerable to blockage - without proper drain maintenance, landslides are more common than on slopes without drains - due to confusion over responsibility many are not maintained in Hong Kong
• man made slopes are used in Hong Kong for slope stabilisation - these contain drains to intercept + direct water away from slopes
• the slope is usually protected from infiltration + the erosive effect of water by impermeable hard covers

Question 8

Managing slopes: greening technique

Answer 8

3 main types used in Hong Kong:
- mulching system -> provides a protective cover that makes it impossible for natural vegetation to grow on the slope; a natural vegetation cover is able to grow through the mat, securing it in place
- long-rooting grass - fast + cost-effective -> it’s applied by drilling planter holes into a hard cider. The drilled hole is filled with soil mix and fertiliser + the long rooting grass is planted within
- fibre reinforced soil system -> constructed by mixing polyester fibre into sandy soils = capable of resisting tension

Question 9

Adv of mulching system

Answer 9

• higher adhesive capacity on steep slopes
• higher resistance to rain erosion
• higher water-retaining capacity
• long-lasting fertilisers
• adaptable to rough surfaces

Question 10

Adv of planting long rooting grass

Answer 10

• reduces moisture from soil, decreasing the risk of loss of cohesion
• natural + environmentally friendly
• cost-effective
• fast + easy installation
• can be applied on steep slopes
• low maintenance

Question 11

Adv of fibre reinforced soil system

Answer 11

• self-sustained vegetation system with low maintenance
• fibre strengthens soil particles to prevent erosion
• visual improvements of slope with various plant species
• restoration of natural habitats on the slope

Question 12

What is mulching

Answer 12

• protective mat is placed on top of slope to form a protective barrier
• it protects the soil from the direct impact of rainfall + reduces the speed + volume of run off = major causes of erosion on slopes
• helps retain soil moisture = improving soil health + promoting plant growth which further stabilises the slopee by strengthening the soil structure
• increasing water infiltration reduces the force of water flowing down the slop, minimising landslides + mudflows

Question 13

What is long rooted grass

Answer 13

• root systems bind soil particles, increasing cohesion + reducing erosion
• absorbs rainwater, increasing the infiltration capacity of the soil = decreasing pressure of soil saturation = increasing slope strength
• also provides a protective surface layer shielding the soil from direct impact of rainfall + wind

Question 14

What is fibre reinforced soil

Answer 14

• fibres such as polyester mixed with soil increases the frictional resistance of the soil by increasing the cohesion
  significantly improves soil slope stability by increasing shear strength + reducing erosion