Report No. : GEO Report No. 240

Report Title : Detailed Study of the 20 August 2005 Debris Flood at Route Twisk, Tsuen Wan (2009), 77 p.

Author : Maunsell Geotechnical Services Limited

Abstract

This report presents the findings of a detailed study of the debris flood (Incident No. 2005/08/0351a) that occurred at about 6:00 p.m. at Route Twisk, Tsuen Wan on 20 August 2005. The debris flood was probably triggered by a major landslide (Incident No. 2005/08/0480) on slope No. 6SE-B/CR249, involving a failure volume of about 50 m³. The slope is situated immediately above the section of the Shing Mun Catchwater northeast of Route Twisk near Kwong Pan Tin San Tsuen, Tsuen Wan. Another slope No. 6SE-D/FR98 located along the debris trail and on the downhill side of Route Twisk was also severely eroded (Incident No. 2005/08/0351b). A section of Route Twisk was temporarily closed.

The key objectives of the study were to document the facts about the debris flood and the associated landslides, present relevant background information and establish the probable causes of the debris flood and associated landslides. The scope of the study comprised desk study, site reconnaissance, detailed field mapping and theoretical hydraulic assessment of the catchwater channel. Recommendations for follow-up actions are presented separately.

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