HONG KONG

Question 1

Why is Hong Kong vulnerable to mass movements?

Answer 1

1. Geology - Due to volcanic past, colluvium is found - unconsolidated volcanic ash and lava - silty or sandy soils with 30% clay content. This allows rapid saturation and pore-water pressure build up, lubrication thereby reduction of friction therefore, creating instability of slopes.
2. Climate - 22° North of the Equator - Experiences heavy rainfall as part of its tropical climate and Hong Kong has a mean annual rainfall of about 2,200 millimetres.
3. Human factors - population growth and urbanisation has led to a high population density, with a population of 7.4 million.

Question 2

How have human activities weakened slopes in Hong Kong?

Answer 2

1 Urbanisation - to expand urban area, cutting and filling has occurred which has undercut rock faces

2. Buildings have been constructed upslope due to the lack of space - extra weight is loaded onto the slope which increases shear stress on the slope whilst the strength has been reduced by the cutting - this reduces the slope safety factor and can lead to slope failure.
3. Deforestation to make way for further urbanisation has reduced vegetation cover meaning that there is less tree cover to bind soil together and to help water drainage

Question 3

Management: Prediction/Monitoring

Answer 3

• Hong Kong established the Geotechnical Engineering Office (GEO) which has over 700 staff who oversee slope management in Hong Kong. The Office maintains a ‘catalogue of slopes’ which contains information on around 60,000 slopes in Hong Kong. The office then uses this information to identify high risk areas which they then use to limit human development in those areas. The government overcame problems with ensuring landowners undertake the necessary steps to increase slop safety by fining slope owners HK$50,000 if they do not comply with regulation, as well as potential imprisonment for a year.
• MUST be partnered with good governance and dissemination of information - Only 10% of 1,800 people interviewed had even heard of the landslide potential index.
• Only tries to MITIGATE the events, not PREVENT the landslides themselves.
• Another limitation is that the current warning system tells people to stay home if rainfall reaches 70mm but doesn’t advise any other actions.

Question 4

Management: Hard Engineering

Answer 4

• Methods such as masonry wall trees and surface drainage systems.
• Around 1,275 stone wall trees (ie trees that are grown from stone retaining walls) are situated on 504 masonry stone walls. This reduces mass movement as it provides support to prevent the slope from sliding in the first place.
• Drainage systems - e.g. Hong Kong West Drainage Tunnel in 2012. These horizontal drainage systems have been installed by the GEO in 87 locations and is especially effective in reducing mass movements in Hong Kong as it receives over 2000 mm of rainfall annually, so helps to drain the water from the slope which lowers the shear stress of the slope and also reduces lubrication of the soil.
• H/E it is expensive and therefore, not accessible to LICs. Even Hong Kong themselves, who are in the top 10 of world economies in terms of GDP per capita, admitted that it was too expensive to implement the surface drainage systems throughout Hong Kong on a larger scale.

Question 5

Management: Soft Engineering

Answer 5

• Cheaper and more environmentally friendly than hard engineering. H/E not always as effective.
• Mulching system provides a protective cover that makes it possible for natural vegetation to grow on the slope, which increases the stability of the slope. While it may stabilise the slope, it may be that it increases chance of mass movements as it adds to the shear stress of the slope.
• Planting long rooting grass is the quickest and most cost-effective way to cover man-made slopes. Holes are drilled into the slope which is then filled with soil mix and fertilisers to help consolidate the soil and increase stability.
• Soil is reinforced with fibre so capable of resisting soil erosion, which in turn prevents the build-up of unconsolidated soils, so chance of mass movements is reduced.

Question 6

Conclusion

Answer 6

• Overall, managing mass movements in Hong Kong has been relatively successful.
• Since mid-1980s, there has been a reduction in mass movements and since 2008, there hasn’t been any deaths associated with mass movements.
• Also, there were only 250 mass movement events in 2018, compared with 300 the year before.