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GEO Report No. 134
Report No. : GEO Report No. 134

Report Title : Luminescence Dating of Colluvium and Landslide Deposits in Hong Kong & Toe Slope Angle of Natural Terrain Landslides (2003), 114 p.

Author : J.P. King & A.S.W. Choi, E.K.S. Fung, A.M.H. Law & J.P. King

Abstract

This GEO Report consists of two Technical Note in two separate research and development projects carried out by the Planning Division in 2001.

They are presented in two separate sections in this Report.

Section 1 : Luminescence Dating of Colluvium and Landslide Deposits in Hong Kong

Some colluvial deposits in Hong Kong are believed to have been derived from landslides that were considerably larger than those described in the recent historical record. An understanding of their age and genesis is necessary to assess whether similar landslides could occur in the current environmental regime. This information is important if the risk from such events, and their return periods, need to be considered in engineering design.

Conventionally, recent superficial deposits such as colluvium are dated using radiocarbon (14C) methods, which require the presence of organic matter. However, materials suitable for 14C dating are rare in Hong Kong colluvium and this technique has not been able to provide the necessary dating information for colluvial deposits. The GEO has been investigating other dating methods. Average Response Luminescence Dating is a technique that has been developed in the last few years for establishing the length of time since a sediment was last exposed to sunlight. A more refined statistical technique of Dose Distribution Optically Stimulated Luminescence is currently being developed. The GEO has collaborated with the leading researchers in this field (Dr A. Murray of the Nordic Laboratory for Luminescence Dating and Dr S.H. Li from the University of Hong Kong) by providing samples of colluvium and control samples of recent landslide debris for testing.

This report summarises the luminescence-dating results obtained from colluvium samples in Hong Kong. These include both the Average Response and Dose Distribution methods and control samples of modern landslide debris. The Dose Distribution dates suggest that a large colluvium lobe (1 million m3) on North Lantau has been deposited within the last 10,000 years, and may be as young as 2000 years. The results are subject to a certain degree of uncertainty due to the pioneering nature of this technique. To further evaluate the potential of the Dose Distribution method, additional trials should be carried out, and their results compared to dates obtained from other techniques.

Section 2 : Toe Slope Angle of Natural Terrain Landslides

Natural terrain landslides typically originate on steep slopes and the debris moves down hill before coming to rest on flatter slopes. The ground slope angle at the distal end of a landslide debris trail (toe slope angle) is one of the parameters that may give an indication about the mobility of landslide debris. Data on the toe slope angle was collected for 569 of the large recent landslides (>15 m wide) in the Natural Terrain Landslide Inventory, using 1:5000 and 1:1000-scale maps and a digital terrain model based on 1:20,000-scale input data. The 1:5000-scale data show that debris from 67 of the landslides reached ground with a slope angle less than 15oC . None of these landslides resulted in debris reaching more than 50 m beyond the 15oC break-in-slope.

The slope angles recorded from the different scale maps were compared. The correlation is poor between the 1:20,000 data and those from the 1:5000 and 1:1000-scale maps. The 1:5000 and 1:1000 data generally show a reasonable match although local steep slopes and flat areas shown by the 1:1000-scale contours are not given in the 1:5000-scale maps.

The findings from this study have been used in developing the "Alert Criteria" currently adopted by the GEO for screening new development sites that may be affected by natural terrain landslides.


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