Report No. : GEO Report No. 169

Report Title : Report on the Debris Flow at Sham Tseng San Tsuen of 23 August 1999, Findings of the Investigation (2005),92 p.

Author : Fugro Maunsell Scott Wilson Joint Venture

Abstract

On 23 August 1999, landslides occurred at the natural hillside above Sham Tseng San Tsuen, giving rise to a debris flow down a stream course. The debris flow demolished a number of squatter dwellings constructed over, and in close proximity to, the stream course in Sham Tseng San Tsuen. One fatality was reported and 13 injured persons were rescued. A comprehensive investigation into the causes of the debris flow was carried out for the Geotechnical Engineering Office during the period August 1999 to March 2000 by Fugro Maunsell Scott Wilson Joint Venture.

The debris flow occurred during a severe rainstorm with a return period of about 49 years. The rainstorm was the most severe in the area in terms of short to medium-duration rainfall since 1984 when the nearest automatic raingauge was installed.

The investigation concluded that the largest of the four landslides (600 m³) within the catchment to the stream course was the primary source of the debris flow. The landslides were probably caused by elevated water pressure within the slope-forming material following the severe rainfall that preceded the failures.

Details of the investigation and its findings are given in this report.

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