Report No. : GEO Report No. 57

Report Title : Study of Rainfall Induced Landslides on Natural Slopes in the Vicinity of Tung Chung New Town, Lantau Island (1998), 102 p. plus 3 drgs. (Reprinted, 1999)

Author : C.A.M. Franks

Abstract

This report describes the methodology, results and conclusions of a study of some recent and relict rainfall induced landslides on natural slopes overlooking the Tung Chung New Town Development Area.

The study was carried out to document the characteristics of these landslides and to define the criteria that may be appropriate to develop a hazard assessment and zoning method. The study was focused on landslides that occurred during heavy rainfalls in 17 July 1992 and 5 November 1993, when over 80 landslides occurred in the study area. The study also examined the evidence for previous landslides primarily from examination of aerial photography. The study examined the geomorphological, geological and other factors that may influence the generation of rainfall induced landslides. The study also proposes a number of landslide models to describe the characteristic landslides observed.

The results of the study indicate that channelised debris flow landslides are potentially the most hazardous as they result in larger volumes of mobilised debris with long debris trails. The debris from these long trails may impact on developable land. The maximum magnitude landslide, estimated from field observation, comprised of approximately 2000 m³ of debris and was a channelised debris flow that occurred during the November 1993 rainstorm. This landslide had a total trail length of 450 m from the scarp at 180 mPD (35° slope) and deposited debris on a development platform at 24 mPD.

The limited data set indicates that for channelised debris flows the lower limit of slope angles on which deposition takes place (deposition point) is 8° whilst for non-channelised landslides the lower limit of slope angles on which deposition of debris takes place is 10°. The average runout distance from the deposition point is 28 m and 47 m for non-channelised and channelised debris flows respectively. However, the range of runout distance is quite large for both types of landslide.

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