Report No. : GEO Report No. 128
Report Title : Comparative Risk Indicators & Possible Climate Change Trends in Hong Kong and Implications for the Slope Safety System (2002), 50 p.
Author : H.W. Sun & N.C. Evans
This GEO Report comprises a Special Project Report and a Technical Note on two separate research and development projects carried out by the Special Projects Division in 2001.
They are presented in two separate sections in this Report.
Section 1 : Comparative Risk Indicators
This Report reviews the use of risk indicators as part of the risk assessment process. It examines the possible implications of using different risk indicators for various types of risk assessment.
Risk assessment involves a combination of analytical methods and consideration of social factors, and is best regarded as a framework within which informed decisions can be made. Quantitative Risk Assessment (QRA), comprising the calculation of risk numerically and comparison with various criteria, can form part of this process but it is not necessarily the most important part.
Risk assessments using calculated risk are usually based on probability of death. However, studies on the public perception of risk have highlighted its multi-dimensional and subjective social value aspects.
Risk, especially calculated risk, is a measure of the effect of a hazard on a susceptible population. Very few risks are distributed evenly. This can be referred to as risk "clustering". The implications of clustering need to be considered when comparing different types of risk.
Risk acceptance criteria are controversial. Decisions made based solely on these criteria are placing very high reliance on both the reliability of the risk analysis and the suitability of the risk indicator for the required purpose. There is no clear consensus on how such criteria could be applied in different situations. The issue of cumulative risk does not appear to be satisfactorily addressed anywhere.
Risk assessment should be iterative and consultative. Attempts should be made to integrate social perceptions with calculation in the risk assessment process. When calculating risk, the risk indicator chosen, and the way in which it is calculated, should be determined by the nature of the decision which is to be made. Decisions should not, however, be made solely on the basis of calculated risk or cost/benefit analyses.
Social perceptions of risk do not necessarily agree with objectively assessed risk. In Hong Kong, social perception of risk appears to be governed by a reasonably well-informed opinion of the probability of an individual's exposure to that risk.
Section 2 : Possible Climate Change Trends in Hong Kong and Implications for the Slope Safety System
The Slope Safety Technical Review Board recommended in 2000 that GEO examine rainfall data to see whether any possible climate change related trends were visible and, if so, what the implications might be in terms of landslide risk and slope safety.
The current scientific consensus on climate change is that "global warming" is a real phenomenon, and that noticeable regional climate changes in the next 50 to 100 years are probable. In addition, there is an increasing acceptance that development often results in local climate change.
Analysis of 50 years of annual rainfall data from Hong Kong shows that average annual rainfall has increased slightly over this period. However, this increase has not been uniform, and has been concentrated in the central (developed) parts of Hong Kong, particularly the northern parts of Hong Kong Island, Kwai Chung, Tsing Yi, Shatin, Tai Po and Tsuen Wan, where annual increases of several hundred millimetres appear to have occurred.
It seems probable that these changes are related to the development of an 'urban heat-island'. The fifty-year trend in other climatological parameters appears to support this hypothesis.
These apparently local changes might continue together with Hong Kong's on-going development. In addition, it is possible that global climate change in the next 50 to 100 years might be significant enough to have a local impact.
Past records should not be automatically assumed to be a sound basis for predicting future events. It would be prudent to review regularly those rainfall parameters used for slope stability analysis, slope design, landslide risk assessment and landslide emergency preparedness planning.