Factual Report on Hong Kong Rainfall and Landslides in 2005
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FACTUAL REPORT ON HONG KONG RAINFALL AND LANDSLIDES IN 2005 GEO REPORT No. 223 H.S.W. Kong & A.F.H. Ng GEOTECHNICAL ENGINEERING OFFICE CIVIL ENGINEERING AND DEVELOPMENT DEPARTMENT THE GOVERNMENT OF THE HONG KONG SPECIAL ADMINISTRATIVE REGION FACTUAL REPORT ON HONG KONG RAINFALL AND LANDSLIDES IN 2005 GEO REPORT No. 223 H.S.W. Kong & A.F.H. Ng This report is largely based on GEO Special Project Report No. SPR 2/2006 produced in June 2006 - 2 - © The Government of the Hong Kong Special Administrative Region First published, May 2008 Prepared by: Geotechnical Engineering Office, Civil Engineering and Development Department, Civil Engineering and Development Building, 101 Princess Margaret Road, Homantin, Kowloon, Hong Kong. - 3 - PREFACE In keeping with our policy of releasing information which may be of general interest to the geotechnical profession and the public, we make available selected internal reports in a series of publications termed the GEO Report series. The GEO Reports can be downloaded from the website of the Civil Engineering and Development Department (http://www.cedd.gov.hk) on the Internet. Printed copies are also available for some GEO Reports. For printed copies, a charge is made to cover the cost of printing. The Geotechnical Engineering Office also produces documents specifically for publication. These include guidance documents and results of comprehensive reviews. These publications and the printed GEO Reports may be obtained from the Government’s Information Services Department. Information on how to purchase these documents is given on the second last page of this report. R.K.S. Chan Head, Geotechnical Engineering Office May 2008 - 4 - FOREWORD This report presents a summary of the factual information on rainfall and landslides in Hong Kong throughout 2005. Details of most of the landslides were obtained from the records of incidents reported to the Geotechnical Engineering Office (GEO). Supplementary information was collected from the Agriculture, Fisheries and Conservation Department, Architectural Services Department, Drainage Services Department, Fire Services Department, Highways Department, Housing Department, Lands Department, Water Supplies Department and the GEO’s landslide investigation consultants. The Hong Kong Observatory provided weather and rainfall information. The Standards and Testing Division carried out a review of the available rainfall records and rainfall analysis, and prepared Section 2 of this report. All contributions are gratefully acknowledged. K.K.S. Ho Chief Geotechnical Engineer/LPM Division 1 - 5 - ABSTRACT This report presents a summary of the factual information on rainfall and landslides in Hong Kong throughout 2005. Rainfall information was obtained from the Hong Kong Observatory (HKO) to supplement the information available in the Geotechnical Engineering Office (GEO). Details of the landslides were obtained from the records of incidents reported to the GEO. Supplementary information was provided by the Agriculture, Fisheries and Conservation Department, Architectural Services Department, Drainage Services Department, Fire Services Department, Highways Department, Housing Department, Lands Department, Water Supplies Department and the GEO’s landslide investigation consultants, namely Fugro Scott Wilson Joint Venture and Maunsell Geotechnical Services Limited. Rainfall at the HKO’s Principal Raingauge at Tsim Sha Tsui in 2005 amounted to 3214.5 mm, which was about 45.2% above the mean rainfall of 2214 mm recorded between 1961 and 1990. It was the third wettest year in Hong Kong since records began in 1884, and June and August 2005 was the fourth and the second wettest months respectively. The rainfall in these two months (1865.2 mm) amounted to about 84% of the normal annual rainfall. One Red Rainstorm Warning was issued on 24 June 2005, and 16 Amber Rainstorm Warnings were issued between 8 May 2005 and 20 August 2005. Two Landslip Warnings were issued on 24 June 2005 and 19 August 2005 respectively. A total of 487 incidents that occurred in 2005 were reported to the Government. Of these, 481 were classified as genuine landslides. Of the 481 genuine landslides, 82 were major failures (i.e. with a failure volume of 50 m3 or more, or where a fatality has occurred). One fatality was reported as a result of a landslide incident. Other notable consequences of the landslides included the evacuation of 42 squatter dwellings and 12 buildings. Sixty-two landslides resulted in the temporary closure of sections of roads and another 46 landslides resulted in the temporary closure of sections of pedestrian pavements, footpaths or other forms of minor access. Thirty-nine landslides affected catchwaters, two of which had significant knock-on effects on existing developments, resulting in serious consequences. - 6 - CONTENTS Page No. Title Page 1 PREFACE 3 FOREWORD 4 ABSTRACT 5 CONTENTS 6 1. INTRODUCTION 9 2. RAINFALL 9 2.1 The Raingauge System 9 2.2 Rainfall Records 9 2.3 Rainstorms in 2005 11 2.4 Warnings Issued by the Hong Kong Observatory 12 3. LANDSLIDES 12 3.1 Landslide Occurrence in 2005 12 3.2 Consequence of Landslides 13 3.2.1 General 13 3.2.2 Buildings 13 3.2.3 Roads and Transport Facilities 13 3.2.4 Squatter Dwellings 14 3.2.5 Catchwaters and Reservoirs 14 3.2.6 Construction Sites 14 3.2.7 Other Facilities 14 3.3 Types of Slope Failures 14 3.3.1 General 14 3.3.2 Fill Slopes 14 3.3.3 Cut Slopes 15 3.3.4 Retaining Walls 15 3.3.5 Natural Hillsides 15 - 7 - Page No. 3.3.6 Registered Disturbed Terrain Features 15 3.4 Landslide Volume Distribution 15 4. NOTABLE LANDSLIDES 15 4.1 General 15 4.2 The 20 August 2005 Landslide Incident on a Hillside at Fu Yung 16 Shan Tsuen, Tsuen Wan (Incident No. 2005/8/0362) 4.3 The 20 August 2005 Landslide Incident on the Natural Hillsides at 16 Bowen Road near No. 120 Kennedy Road (Incident No. 2005/8/0304) 4.4 The 20 August 2005 Debris Flood Incident at Lo Wai, Tsuen Wan 17 (Incidents Nos. 2005/8/0338, 2005/8/0348, 2005/8/0402, 2005/8/0417 and 2005/09/0528) 4.5 The 20 August 2005 Landslide Incident on the Natural Hillside behind 17 Nurses’ Quarters Block A, Queen Mary Hospital, Pok Fu Lam (Incident No. 2005/8/0464) 4.6 The 22 August 2005 Landslide Incident on the Natural Hillside 18 above Slope No. 7SW-D/C2, Lion Rock Tunnel Road (Incident No. 2005/8/0428) 4.7 The 19 August 2005 Landslide Incident on Slope No. 7SW-D/C167 18 at Shatin Heights (Incident No. 2005/8/0320) 4.8 The 21 August 2005 Landslide Incident on Slope Nos. 7SE-B/C195 18 and 7SE-B/C131 at Ma On Shan Upper Village (Incident No. 2005/8/0384) 4.9 The 20 August 2005 Landslide Incident on Slope No. 11SW-A/C415 19 at No. 404 Victoria Road (Incident No. 2005/8/0368) 4.10 The 20 August 2005 Landslide Incident on Slope No. 11SW-C/FR126 19 below Canossian Retreat House Honeyville, Victoria Road (Incident No. 2005/8/0308) 4.11 The 20 August 2005 Landslide Incident on Slope No. 11NE-A/C153 19 at Ping Ting Road, Ngau Chi Wan (Incident No. 2005/8/0299) 4.12 The 21 August 2005 Landslide Incident on the Natural Hillside 20 above Fei Ngo Shan, Choi Hung (Incident No. 2005/8/0381) 4.13 The 20 August 2005 Landslide Incident on Slope No. 13NE-B/C166 20 above South Lantau Road, approximate 500 m East of Tong Fuk, Lantau Island (Incident No. 2005/8/0331) 4.14 The 20 August 2005 Landslide Incident at Route Twisk near 21 Kong Pan Tin Tsuen, Tsuen Wan (Incidents Nos. 2005/8/0351A, 2005/8/0351B and 2005/8/0480) - 8 - Page No. 4.15 The 20 August 2005 Landslide Incident on Slope No. 11SW-D/C543 21 above Middle Gap Road, Wanchai Gap (Incident No. 2005/8/0326) 4.16 The 11 August 2005 Landslide Incident on the Natural Hillside above 21 Slope No. 11NW-A/C49 at Lai King Hill Road, Kwai Chung (Incident No. 2005/8/0279) 4.17 The 22 August 2005 Channelised Debris Flow Incident on the 22 Natural Hillside at Kwun Yam Shan (Incident No. 2005/8/0422) 5. CONCLUSIONS 22 6. REFERENCES 23 LIST OF TABLES 24 LIST OF FIGURES 33 LIST OF PLATES 55 APPENDIX A: RAINFALL OF SELECTED RAINSTORMS 73 RECORDED AT GEO RAINGAUGES APPENDIX B: LIST OF LANDSLIDE INCIDENTS REPORTED 78 TO THE GOVERNMENT - 9 - 1. INTRODUCTION This report presents a summary of the factual information on rainfall and landslides that occurred in Hong Kong throughout 2005. Rainfall information was obtained from the Hong Kong Observatory (HKO) to supplement the information available in the Geotechnical Engineering Office (GEO). Details of the landslide were obtained from the records of incidents reported to the GEO. Supplementary information was provided by the Agriculture, Fisheries and Conservation Department (AFCD), Architectural Services Department (Arch SD), Drainage Services Department (DSD), Fire Services Department (FSD), Highways Department (HyD), Housing Department (HD), Lands Department (Lands D), Water Supplies Department (WSD) and the GEO’s landslide investigation consultants, namely Fugro Scott Wilson Joint Venture and Maunsell Geotechnical Services Limited under Agreements Nos. CE 29/2003 (GE) and CE 15/2004 (GE) respectively. In this report, a landslide is defined as the detachment or excessive displacement of soil or rock mass, and includes failure of fill slope, cut slope, retaining wall, natural hillside, disturbed terrain, together with rockfall and boulder fall. A 'major' landslide is defined as a failure in which the estimated/recorded volume of the detached or displaced mass is >50 m3 or where a fatality has occurred. A 'very minor' landslide is defined as a failure that is small in scale (i.e.