Report No. : GEO Report No. 73

Report Title : The Natural Terrain Landslide Study Phases I and II (1999), 128 p. plus 2 drgs.

Author : N.C. Evans, S.W. Huang & J.P. King

Abstract

The Natural Terrain Landslide Study (NTLS) is part of a series of integrated GEO studies of the risks associated with natural terrain in Hong Kong. The aims of the NTLS are to investigate the distribution, nature and probable causes of landsliding on natural terrain in Hong Kong and to assess the hazard from such events.

Phase I of the Study, which began in September 1995, comprised the establishment of an inventory of landslides on natural terrain, covering the whole of Hong Kong, based on the interpretation of high-level aerial photographs taken during the past 50 years. Digitisation of this inventory and the compilation and digitisation of other regional datasets was also completed during Phase I.

Phase II of the Study comprised the incorporation of the landslide inventory and the other datasets into a Geographic Information System (GIS), the analysis of these data to determine correlations between landslide distribution and possible causal factors, and a preliminary assessment of hazard. Additional work was carried out to determine correlations between rainfall and natural terrain landsliding, review methods for predicting landslide debris run-out distances and examine suitable types of landslide mitigation techniques.

This report describes the work undertaken during Phases I and II of the Study, presents some conclusions and discusses the implications for future work.

All natural terrain in Hong Kong has been surveyed using aerial photographs taken between 1945 and 1994, and data on over 25,000 visible natural terrain landslides have been collected. These data, together with information on geology, slope angle, geomorphology and vegetation have been entered into a Geographic Information System (GIS) in order to examine the spatial distribution of landslides with respect to these factors.

It appears that the underlying geology and the angle of slope are the most important parameters for determining natural terrain landslide susceptibility at the regional scale. Geological strata which appear to be particularly susceptible include rhyolitic and dacitic lavas, jointed tuffs, layered sequences of volcaniclastic rocks and lavas, and layered sedimentary sequences. The most susceptible slopes are generally those with angles of approximately 35º to 40º. The shape and aspect of a particular slope may also be useful in assessing susceptibility.

The approximate runout distance has been determined for nearly 9,000 landslides (the more recent events). Of these, 2.3% (204) have plan runout distances of more than 150 m and 0.3% (34) have plan runout distances of more than 300 m.

Data on natural terrain landslides for the period 1985 to 1994 have been compared with rainstorm intensity on a yearly basis. This has allowed the assessment of rainfall intensity thresholds for the onset of natural terrain landsliding. It appears that a rainfall event affecting perhaps 20% to 50% of Hong Kong and with the potential to trigger a high density (more than ten per square kilometre) of natural terrain landsliding in susceptible areas, can be expected, on average, every two years.

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