FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS

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Landslide risk management

Measures that are taken to mitigate landslide risk to the community are as follows:

v Planning control on land use: It is aimed to reduce landslide losses. It is achieved through removing or converting existing development and /or discouraging or regulating new development in unstable areas. New developments can be prohibited, restricted or regulated in landslide-prone areas. The areas can be used as open space, parks, woodland and recreation. All activities that will make the areas vulnerable to failure, such as irrigation, construction of roads and buildings can be prohibited. For large-scale potential landslides, if there are obvious indications of instability, evacuations must be considered.

v Engineering solution: It involves correction of the underlying unstable slope. This involves modification of the slope geometry by excavation, toe fill, drainage of the surface and ground water, use of retaining structures and internal slope reinforcement (grouting, block bolts, use of lime or chemicals). Also, it may involve controlling of the landslide movement of debris to reduce the initial impact on down slope property. Use mechanical barriers to protect the structures (debris defences, retaining walls or embankment to absorb the kinetic energy/to withstand or deflect the impact. Planting tree where applicable.

v Acceptance: Community may accept the risks based on understanding that the risks from landslide are offset against the benefit which the community will get. Sometimes it is accepted if the risks from landslides are tolerable.

v Monitoring and warning systems: Potentially unstable slopes can be monitored so that residents can be warned and if possible evacuated before landslide can occur. Primary objective of monitoring is to assess the existing conditions, in particular to determine whether or not the landslide is active and where there is sign of movement warning can be issued at the community. Monitoring should cover: Magnitude, rate and direction of deformation by using inclinometers or slope extensometers; pore pressure and piezometric levels by using piezometers and tensiometers; and seismic accelerations.

v Decision-making: Should be based on cost-benefit analysis. It involves the identification and quantification of all desirable and undesirable consequences of a particular mitigation measure. Cost and benefit of each mitigation measure is assessed and compared before reaching the conclusion.