Report No. : GEO Report No. 168

Report Title : Correlation between Rainfall and Natural Terrain Landslide Occurrence in Hong Kong (2005), 77 p.

Author : F.W.Y. Ko


A preliminary assessment of the relationship between rainfall and natural terrain landslide occurrence in Hong Kong was carried out by Evans (1997). From the assessment, a plot of cumulative percentage of natural terrain landslides against maximum rolling 24-hour rainfall, based on manual extraction of data from 1985 to 1994, was derived. The assessment was updated in this study using data from 1985 to 2000. Geographic Information System (GIS) and geostatistical techniques were applied, which improve the accuracy of the cumulative plot. However, the plot only gives a causal relationship between the historical natural terrain landslides and the maximum rolling 24-hour rainfall at the landslide locations in the year of landsliding. It is not a rigorous rainfall-natural terrain landslide analysis, and its application to identifying rainfall thresholds and estimating the potential number of natural terrain landslides for a given rainfall condition could be misleading.

To overcome the limitations of the cumulative plot, a detailed assessment was carried out for establishing the rainfall-natural terrain landslide correlation in Hong Kong. The assessment incorporated the methodology that has been applied over the years in deriving rainfall-landslide correlation for man-made slopes in Hong Kong, together with the use of GIS technology and statistical models to further improve the assessment. In the correlation, maximum rolling 24-hour rainfall normalized by location-specific mean annual rainfall was adopted, to give a better representation of the severity of the rainfall conditions at different locations.

Based on spatial analysis of rainfall-natural terrain landslide data from 1985 to 2000, a year-based rainfall-natural terrain landslide correlation was established. Then, the year-based correlation was transformed statistically into a storm-based correlation. The storm-based correlation was tested by comparing the predicted number of natural terrain landslides with the actual number of natural terrain landslides from 1985 to 2000, and was found to be reliable. The correlation enables the assessment of the probability of occurrence of different natural terrain landslide densities under different rainfall scenarios, and is applicable to hazard identification, risk quantification and formulation of natural terrain landslide warning system.

For terrain subject to comparable rainfall intensity, a notable correlation of natural terrain landslide density with terrain susceptibility classes was observed. The year-based correlation has been further developed into correlations that apply to different susceptibility classes. Further work is at hand to improve the terrain susceptibility classification, based on which the rainfall landslide correlation can be enhanced. Other areas of further work were also identified.

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