Unit 5 Case Study: Hong Kong Flashcards
its not even facts
Why is Hong Kong so vulnerable to landslides
It is hilly with thousands of steep slopes on which are constructed tall buildings and high-density residential complexes. The fast rate of urbanisation and building construction along with topographic relief has led to many disastrous landslides during rainy season. Landslide hazard has become the major natural hazard in the Kong Kong region
The relationship between mass movements, rock type, gradient and vegetation
Overall, vegetation had the greatest influence on mass movements with woodland areas having the most related to human activity and the most in general. Rock type has the least impact on mass movements with granite accounting for 70 due to human activity and 17 natural. A slope gradient greater than 1:2 has 60 due to human activity and 93 natural
Most common mass movement
Lanslips
The difference between a washout and a landslip
Washouts are more heavily influenced by rainfall than landslip’s. A washout is the sudden erosion of soft soil by a gush of water during a heavy downpour or flooding. A landslip is the continuous slow movement over a long period of time
Which type of rock was most affected by washouts and landslips
Granite has the most landslips with 75 total followed by volcanic with 50 total and then sedimentary with 2 total. Volcanic was most affected by washouts with 14 total then granite with 12 and sedimentary with 8
What is the relationship between gradient and mass movement
Landslips are the mass movement mainly affected by gradient. The steepest gradients of more than 1:2 had a total of 95 landslips and only 8 washouts. This is because the downslope force is greatest on these slopes compared to the sticking force. The gradient of 1:5-1:3 also had a large number of mass movements with 100 total whereas the gradients of less than 1:5 and over 1:3-1:2 only has 41 and 37 total respectively
Impact of vegetation on the type and number of mass movements
In general, afforested and grassland areas had the least number of mass movements with only 10 and 20 landslip and 0 and 8 washouts respectively. Woodland had the most mass movements with a total of 145. This is because roots stabilise the ground and binds the soil but can also move slowly so landslips are more common
The impact of human activity on mass movements
For all of the mass movements, the movements related to human activity were much greater in number than the natural movements. Most human related movements are in vegetation and the least prevalent human activities is on rock type. 181 mass movements related to human activities occurred in woodland whereas only 7 occured due to human activities on sedimentary rocks
Why are landslides a hazard
Hong Kong is very densely populated especially in areas such as North Point (0866) and Causeway Bay (0765). This means that any movement is likely to cause structural damage to the foundations of buildings, damaging the stability of them, especially high rise buildings which there are a lot of due to the dense population. There are also steep slopes in Hong Kong shown by the contour lines on the map so the gradient is higher meaning that stability is compromised and there is greater gravity forces acting on the material
How population growth contributes to the landslide hazard
Population growth will increase the population density of regions so more buildings and infrastructure will have to be constructed. Foundations will compromise slope stability even on flat slopes. There will also be more pressure on resources so more areas may have to be deforested to provide wood or access to land. Removing the trees will remove the roots that help to bind the soil so slope stability will decrease
Methods of management
The slope safety system is managed by the geotechnical engineering office (GEO) of the Civil Engineering Development Department (CEDD). The GEO has over 700 staff. It maintains its slope safety through investigation and researching the causes of significant and serious landslides to improve the slope safety systems. It is continuously updating, maintaining and disclosing the Catalogue of slopes which contains information of 57000 man-made slopes. One government intervention is the ensure the private sector owners of slopes take responsibility for slope safety. If an owner doesn’t comply with regulations, prosecution of HK$50000 and 1 year. The risk of landslides has been reduced by 50% since 1977 but due to population growth, developers increasingly build further up the slopes. Risks therefore increases and damages from a potential alide become greater
Geology of Hong Kong
Constructed mainly from sedimentary, granite and volcanic rocks. Sedimentary generally form the lowlands. Granite and volcanic are on higher ground and prone to failure. Granite tend to be weathered more deeply than volcanic. Volcanic are more resilient and less prone to weathering therefore less prone to slope failure
Maintenance of slopes
Heavy rainfall and surface runoff contribute to slope failure so it is vital to remove excess water. Surface drainage systems and protective cover sprotect sloes. Surface drains are very vulnerable to blockage. Without proper maintenance, landslides become more common than without drains. Due to confusion over responsibility, most aren’t maintained. Man-made slopes are used for stabilization. They have drains t intercept and direct water away. The slope is usually protect from infiltration and their erosive effect of water by impermeable hard covers
Greening techniques
The use of natural vegetation to reduce the risk of mass movements. The mulching system provides a protective cover so natural vegetation can grow. A natural vegetative cover and grow through the mat, securing it. Long-rooting grass is a fast and cost-effective system to cover man-made sloes. Planter holes are drilled into a hardcover. The drill hole is filled with soil mix and fertilisers and the long-rooting grass is planted within. The fibre reinforced soil systems mixes polyester fibre into sandy soils. The resists tension
Advantages of mulching
Higher adhesive capacity on steep slopes
Higher resistance to rain erosion
Higher water retaining capacity
Long lasting fertilisers
Adaptable to rough surfaces
