Report No. : GEO Report No. 281

Report Title : Tsing Shan Debris Flow and Debris Flood, by J.P. King (2013), 265 p. plus 11 drgs.

Author : J.P. King

Abstract

Section 1: This Section describes the study of the Tsing Shan Debris Flow (TSDF) that occurred on the steep natural slopes to the west of Tuen Mun in September 1990, which involved approximately 19,000 m³ of soil and rock and the debris trail extended some 1,035 m to Area 19 Tuen Mun. The work included desk study, aerial photograph interpretation, field mapping, a ground investigation and the study of a debris flood in an adjacent drainage line. The debris flow occurred during an unexceptional rainstorm of increasing intensity in which 136 mm fell in 5 hours. It was generated by a parent landslide on the steep upper slopes of Tsing Shan. This occurred on a spur line sloping at 28° where a thick deposit of loose granite colluvium was actively accumulating over completely decomposed granite from which seepage was occurring. The debris flowed over an exposed sheeting joint into a steep drainage line where it eroded and entrained bouldery colluvium and increased in volume by 200%. Significant deposition started at the mouth of the drainage line but debris continued a further 500 m over an excavated area, cut slope and platform, eroding insitu material and depositing a total of 19,000 m³ of bouldery debris. Flooding late in the event was responsible for about 3,000 m³ of gully erosion and redistribution of the earlier flow deposits.

Section 2: This Section describes the Tsing Shan debris flood which occurred in a drainage line close to the TSDF but it was almost two orders of magnitude smaller and did not extend beyond the steep natural slopes. Consequently a brief investigation of the debris flooed was carried out as part of the Tsing Shan Debris Flow Study. The investigation comprised a desk study, several days field survey, aerial photograph interpretation, a ground investigation comprising three water replacement insitu density tests and associated sampling for laboratory index and mobility moisture content tests. The Tsing Shan debris flood was a linear erosive flow of water and sediment that cut a 350 m long, 4 - 10 m wide scar through the vegetation on the upper slopes of Tsing Shan on 13th/14th June 1992. It originated from the toe of a rubbish filled hollow at 485 mPD, about 100 m below the summit of Tsing Shan. It extended down to a concrete footpath on the lower side slopes at 200 mPD. The event had an estimated volume of less than 250 m³ and comprised boulders, cobbles and sand with only minor traces of silt and clay and no evidence of a slurry matrix.

Section 3: This Section describes the study of the 2000 Tsing Shan Debris Flow that occurred on 14 April 2000, which started during a rainstorm as a landslide at about 360 mPD on the rocky upper slopes of the eastern side of the Tsing Shan Range. The landslide debris moved down to a spur at 180 mPD where it bifurcated to flow down two drainage lines and deposited bouldery debris at the mouth of both drainage lines. Floodwater and some debris entered the Foothills Bypass construction site, blocking drainage and eroding unprotected soils. Sand, silt and gravel were washed off the site and deposited on the Light Rail Transit tracks, the Tuen Mun Golf Centre driving range and Lung Mun Road. The estimated volume of natural slope landslide debris involved in the event is about 1,600 m³. Some 800 m³ of debris was deposited along the scar, of which 500 m³ was on a spur in the vicinity of a China Light and Power pylon. About 800 m³ of the debris was transported off the steep natural terrain, 300 m³ from the northern drainage line and 500 m³ from the southern. In addition 3,000 m³ of material was estimated to have been eroded and washed from the Foothills Bypass site by floodwater that were primarily due to overflow from drainage channels blocked by the landslide debris.

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