applicable to sites after disposal. ...... Wonderland Villas, Kwai Chung on 8 May 1997. .... incinerator, indoor games' sport hall, sewage treatment plant, refuse.
GUIDELINES FOR NATURAL TERRAIN HAZARD STUDIES
GEO REPORT No. 138 K.C. Ng, S. Parry, J.P. King, C.A.M. Franks & R. Shaw
GEOTECHNICAL ENGINEERING OFFICE CIVIL ENGINEERING DEPARTMENT THE GOVERNMENT OF THE HONG KONG SPECIAL ADMINISTRATIVE REGION
GUIDELINES FOR NATURAL TERRAIN HAZARD STUDIES
GEO REPORT No. 138 K.C. Ng, S. Parry, J.P. King, C.A.M. Franks & R. Shaw
This report was originally produced in April 2002 as GEO Special Project Report No. SPR 1/2002
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© The Government of the Hong Kong Special Administrative Region First published, December 2003 Prepared by: Geotechnical Engineering Office, Civil Engineering Department, Civil Engineering Building, 101 Princess Margaret Road, Homantin, Kowloon, Hong Kong.
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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 Department (http://www.ced.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 publishes guidance documents as GEO Publications. 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 last page of this report.
R.K.S. Chan Head, Geotechnical Engineering Office December 2003
- 4 FOREWORD Demand for new land for development has not eased in recent years and land nearer to steep natural slopes has to be considered for building and infrastructure projects. There is a need to contain the increase in overall natural terrain landslide risk to the community and to ensure that new developments are not subject to undue risk. The potential hazards posed by natural terrain to developments close to hillsides are studied so that appropriate mitigation measures are incorporated where necessary into development planning. Over the years, studies of natural terrain and other relevant research and development work have led to better understanding of hillside mass wasting processes in Hong Kong. Progressively, a set of procedures has been developed for evaluating natural terrain hazards. In setting up these procedures, overseas practice has been studied and there has been close interaction with the local geotechnical community. This report describes the current requirements for a natural terrain hazard study, and outlines the approaches to studying natural terrain hazards and the design requirements for appropriate mitigation measures. Technical guidelines on the work involved in natural terrain hazard study and mitigation strategy were prepared by consolidating current practices. This document is based on an earlier Geotechnical Engineering Office (GEO) report No. SPR 5/2000 by K.C. Ng, J.P. King, C.A.M. Franks and R. Shaw, issued in August 2000. The revision, based on the experience gained and feedback received over the last year, was carried out by K.C. Ng and S. Parry under the direction of H.N. Wong. In particular, the following areas have been updated or added: revision of the criteria for screening of sites subject to natural terrain hazards, explanation of the differences between Design Event and Quantitative Risk Assessment approaches, the requirement of a detailed aerial photograph interpretation at an early stage of the study, review of notable natural terrain landslide studies, relevance of geomorphology to natural terrain hazard studies, and environmental consideration and maintenance requirements of mitigation measures. Practitioners are encouraged to comment at any time to the GEO on the contents of this document so that improvements can be made to future editions. When needed, the GEO will issue Technical Guidance Notes for promulgation of further amendments to the report.
( R.K.S. Chan ) Head, Geotechnical Engineering Office April 2002
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ABSTRACT Natural terrain covers about 60% of the land area of the Hong Kong Special Administrative Region. Today, most developed areas in Hong Kong extend from the limited areas of level ground onto sloping ground. The need for natural terrain hazard studies is increasing as the pressure on land results in building and infrastructure projects being located nearer to natural hillsides. This report documents current recommended practice and procedures for studying natural terrain hazards. A Natural Terrain Hazard Study (NTHS) normally includes desk study, detailed aerial photograph interpretation, and field mapping. The study should provide sufficient information to determine the hazard models, their respective design events and run-out distances. If the risks posed by an identified hazards needs to be lowered, formulation of a mitigation strategy is required as part of the study. For practical purposes, it may be expedient to proceed with an NTHS in stages, relating to the progression of a development project, typically from land use planning, through engineering feasibility study to the design stage. If it is established at any stage of the study that the natural terrain hazards are inconsequential, there is no need to proceed with further studies. It is much more practical and economical to study and deal with the natural terrain hazards at the time of development, than to address the problem later. If the potential hazards are considered at an early stage of the development project, the hazards can be controlled in a cost-effective manner by such methods as adjusting the facility layout, providing buffer zones, delineating no-build zones, implementing suitable drainage provisions, or constructing check dams and boulder fences. When planning the mitigation strategy, due consideration should also be given to environmental issues and maintenance requirements. The preferred technical approach for dealing with natural terrain hazards is to mitigate the risk through adjustments to the layout of new developments, instead of carrying out stabilization works to large areas of natural terrain, which would be both impractical and environmentally damaging. While studies are still on-going to further our understanding about the complex interplay between the various attributes contributing to natural terrain instability, experienced judgement by competent professionals remains a key aspect in assessing hazards and in formulating appropriate mitigation strategies.
- 6 CONTENTS Page No.
1.
2.
Title Page
1
PREFACE
3
FOREWORD
4
ABSTRACT
5
CONTENTS
6
INTRODUCTION
10
1.1
Purpose
10
1.2
Natural and Quasi-natural Terrain
10
1.3
Natural Terrain Landslides
11
1.4
Natural Terrain Landslide Risk Management
12
1.5
Requirements for the Study of Natural Terrain Hazards
13
1.5.1
Screening of Sites Subject to Natural Terrain Hazards
13
1.5.2
In-principle Objection Criteria
14
1.5.3
Alert Criteria
15
1.6
Objectives of Natural Terrain Hazard Study
16
1.7
Personnel
16
APPROACHES TO A NATURAL TERRAIN HAZARD STUDY
17
2.1
Scope of a Natural Terrain Hazard Study
17
2.2
Site and Study Area
17
2.3
Approaches to the Study
18
2.3.1
Introduction
18
2.3.2
Factor of Safety Approach
18
2.3.3
Quantitative Risk Assessment Approach
18
2.3.4
Design Event Approach
19
2.4
Design Requirements for Natural Terrain Hazards
19
2.5
Hazard Models
20
2.5.1
Introduction
20
2.5.2
Open Hillslope Landslide
21
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2.6
3.
2.5.3
Channelized Debris Flow
21
2.5.4
Deep-seated Slide
21
2.5.5
Rock Fall
22
2.5.6
Boulder Fall
22
Approaches to Study Hazard Models
22
2.6.1
Introduction
22
2.6.2
Open Hillslope Landslide and Channelized Debris Flow
22
2.6.3
Deep-seated Slide
22
2.6.4
Rock Fall
23
2.6.5
Boulder Fall
23
NATURAL TERRAIN HAZARD STUDY
23
3.1
Introduction
23
3.2
Desk Study
24
3.2.1
General
24
3.2.2
Types of Information
24
3.2.3
Topographical Maps
24
3.2.4
Aerial Photographs
25
3.2.5
Geological Maps
25
3.2.6
Terrain Classification Maps
26
3.2.7
Natural Terrain Landslide Inventory
27
3.2.8
Large Landslide Dataset
27
3.2.9
Boulder Field Inventory
27
3.3
3.4
3.2.10 Rainfall Data
28
Aerial Photograph Interpretation
28
3.3.1
Purpose
28
3.3.2
Review of Existing Data
28
3.3.3
Review of the Site Condition
29
3.3.4
Interpretation
29
Collection of Field Data
32
3.4.1
Introduction
32
3.4.2
Site Reconnaissance
32
- 8 Page No. 3.4.3
4.
5
Field Mapping and Site Measurement
33
3.5
Geological and Geomorphological Model
34
3.6
Design Event
35
3.6.1
Overview
35
3.6.2
Determining the Design Event
35
3.6.3
Catchment-specific Landslide Data
37
3.6.4
Regional Landslide Data
37
3.6.5
International Data
38
3.6.6
Special Consideration for Channelized Debris Flows
38
3.7
Run-out Assessment
39
3.8
Presentation of Results
39
3.9
Stages of Natural Terrain Hazard Study
41
3.9.1
Introduction
41
3.9.2
Review
41
3.9.3
Assessment
42
3.9.4
Mitigation Strategy
43
NATURAL TERRAIN HAZARD MITIGATION STRATEGY
43
4.1
Introduction
43
4.2
Typical Mitigation Strategies
43
4.2.1
Review
43
4.2.2
Passive Mitigation Strategies
44
4.2.3
Active Mitigation Strategies
44
4.3
Hong Kong Practice
45
4.4
Environmental Considerations
46
4.4.1
Introduction
46
4.4.2
Bio-engineering
46
4.4.3
Visual Impact of Mitigation Measures
47
MAINTENANCE OF MITIGATION FACILITIES
47
5.1
Introduction
47
5.2
Types of Natural Terrain Hazard Mitigation Measures
48
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6.
5.3
Maintenance Requirements for Mitigation Measures
48
5.4
Other Measures
49
REFERENCES
50
LIST OF TABLES
57
LIST OF FIGURES
67
APPENDIX A: GLOSSARY OF TERMS AND GENERAL CLASSIFICATION OF HILLSIDE FAILURES
72
APPENDIX B: NOTABLE NATURAL TERRAIN LANDSLIDES IN RECENT YEARS
94
APPENDIX C: GEOMORPHOLOGY AND NATURAL TERRAIN HAZARDS
109
APPENDIX D: BIBLIOGRAPHY
130
- 10 1. INTRODUCTION 1.1 Purpose This document outlines current recommended practice and procedures for studying natural terrain hazards that may affect land development in the Hong Kong Special Administrative Region (hereafter referred to as Hong Kong). It is aimed at practitioners who have some familiarity with the subject but who may not be familiar with local conditions, practice or procedures. This report is a guidance document. As such, the methodologies contained within are not mandatory. It is likely that situations will arise for which the report provides inadequate or inappropriate guidance, and the designers must use alternative methods of approach. There will also be improvements in professional practice that will supersede specific recommendations of this report during its lifetime. For proper recognition to be made of advances in our knowledge, it is therefore essential that practitioners continue to provide the Geotechnical Engineering Office (GEO) with suggestions for improvement to the report. 1.2 Natural and Quasi-natural Terrain Natural terrain (see Appendix A) in this document is defined as “terrain that has not been modified substantially by human activity but includes areas where grazing, hill fires and deforestation may have occurred”. Common anthropogenic features occurring on natural terrain in Hong Kong include the following: (a) Hillsides that have previously been used for agriculture. Overgrown, abandoned agricultural terraces (Figures 1a & 1b) with small masonry walls at the front are found in many areas. (b) Walking tracks, commonly along ridgelines, have in some cases developed into deep erosion gullies. Four-wheel drive vehicle tracks with minimal ground cutting have been used by the military to access remote areas in the past and remnants of other military artefacts such as dugouts, foxholes and fortifications (Figures 1c & 1d) also exist. (c) Hillside streams used for water supply and where small dams, pipelines and catchwaters may be present. (d) Traditional village graves of various styles, some with a very similar arcuate shape to a landslide, occurring in groups, or as isolated features, on natural terrain. (e) Previously extensive small scale prospects and mines for various minerals. Overgrown pits, trenches, adits and spoil tips are the present day evidence of these and may be found on natural terrain in many parts of the territory (Figures 1e & 1f). Some indications of the areas where mining
- 11 activities were more common are provided in Fyfe et al. (2000) and Sewell et al. (2000). In Hong Kong, the term “quasi-natural terrain” is used to describe natural terrain which largely retains its original profile and regolith cover, but is partly modified by human activities (see Appendix A). In addition to the above anthropogenic influences, modification of the slopes above and below by cutting, filling and construction may have affected the surrounding ground and surface water conditions. The guidelines given in this document are generally also applicable to studying landslide hazards from quasi-natural terrain. However, in studying quasi-natural terrain, relevant man-made effects need to be taken into account. 1.3 Natural Terrain Landslides Natural terrain covers over 60% of the total land area (about 1,080 km2) of Hong Kong. Nearly 50% of the natural terrain is sloping at 30° or more. As its present day morphology reflects on-going geological and geomorphological processes, much of the natural terrain is only marginally stable over large areas. From a review of high altitude aerial photographs taken between 1945 and 1997 alone, about 27 000 past landslides on natural terrain were identified, of which about 16 000 occurred within the last 50 years. On average, about one natural terrain landslide occurs each year for every 2 km2 of the natural hillsides in Hong Kong. Several landslide-specific natural terrain studies have been carried out in recent years. These well-documented incidents provide relevant information on the hazard model, probable cause and controlling factors of natural and quasi-natural terrain failures in Hong Kong. Consequently, they provide a useful reference for assessing natural terrain hazards. Some notable studied natural terrain landslides are summarised in Appendix B. A broad review of geomorphology with respect to natural terrain landslides is given in Appendix C and further information on the geomorphology of Hong Kong is given in Fyfe et al. (2000). Whilst most natural terrain landslides occur in relatively remote areas, some of them do affect existing developments. Between 1982 and 1999, about 800 natural terrain landslides affecting developed areas were reported to the GEO. Most of these landslides were small-scale failures affecting open spaces, minor roads and footpaths, and other less important facilities. In the past 20 years, failures of “undisturbed” natural slopes have not resulted in any fatalities in planned developments, i.e. excluding squatter areas. However, a boulder fall from natural terrain has resulted in one fatality and a failure of predominantly natural hillside resulted in two fatalities in planned development. Over the same period, 18 fatalities were caused by failures of man-made slopes affecting planned developments. A significant number of fatalities are also known to have occurred in squatter areas as a result of landslides, and current Government policy is to deal with such unplanned developments by clearance of squatters. Although the historical natural terrain landslide data give an indication of the scale of the problem in Hong Kong, they do not fully reflect the inherent landslide risk to the community. Some landslides were “near-miss” incidents that could well have resulted in more serious consequences. This situation will be aggravated as more new developments take place on or close to steep natural hillsides.
- 12 As is the case with other natural disasters, less frequent, larger-scale natural terrain landslides, if they were to take place close to developed areas, could result in severe consequences. These “extreme” events are under-represented by the limited historical records available in Hong Kong. For example, the 20 000 m3 debris flow in 1990 on the eastern hillslope of Tsing Shan, above Tuen Mun, reached the Area 19 platform below and could have had more serious consequences if this area had been developed. This incident highlights the potential risk of natural terrain landslides in Hong Kong. 1.4 Natural Terrain Landslide Risk Management The current practice for natural terrain landslide risk management is aimed at keeping the natural terrain landslide risk to an as low as practically achievable level. It entails the following general principles: (a) For new developments, contain the increase in overall risk through studying the natural terrain hazards and undertaking any necessary mitigation actions where such hazards occur. Avoid new developments on sites that are subject to severe natural terrain hazards. (b) For existing developments, undertake mitigation actions urgently where there exists an immediate and obvious danger. Study natural terrain hazards where there is reason to believe that a dangerous situation could develop, such as persistent landslides affecting an existing development, and undertake mitigation actions when considered necessary. The Geotechnical Manual for Slopes (GCO, 1984) recognizes that natural slopes are frequently close to limiting equilibrium over very large areas, that preventive works can be expensive and difficult, and that it is not advisable to undertake extensive trimming-back of natural slopes in order to achieve what may only be marginal improvement in stability. Therefore, it is considered that disturbance of natural slopes and vegetation and the need for costly preventive or protective measures should be avoided by siting structures away from areas that could be affected by landslide debris. The conventional approach to stabilization works on man-made slopes is generally not suitable for natural terrain. Given the possible large areas of potentially unstable natural terrain involved, such works are likely to be exceedingly expensive, and may not be justified. Also, widespread stabilization works on natural terrain could cause considerable environmental impact, and are difficult to carry out safely. For new developments, the most effective means of risk mitigation is to avoid areas that are exposed to undue natural terrain hazards. Whilst this is the solution commonly adopted in many countries, it is sometimes not practical in Hong Kong where flat land is limited. However, if the potential natural terrain hazards are considered at an early stage of the development project, the hazards can be controlled in a cost-effective manner by such methods as siting important facilities away from the most vulnerable areas, providing buffer zones between the hillsides and the developments, delineating no-build zones for the most
- 13 vulnerable parts of the sites, implementing drainage provisions to divert debris-laden stormwaters from important facilities, specifying minimum clearance of occupied floors from ground level, integrating debris resisting structures as part of the building structures, constructing check dams and boulder fences, etc. It is much more practical, economical and safe to study and deal with the hazards at the time of development, than to attempt to address the problem later. This approach has been successfully adopted in a number of recent developments. Given the current state of knowledge and available technology it is not possible to predict with confidence where and when natural terrain landslides will occur. For existing developments, given the vast number of sites involved, it is neither desirable nor realistic to deal with them all at the same time. However, for existing developments known to be subject to significant hazards, it is prudent and practical to mitigate the hazards. This “react-to-known-hazard” strategy has been adopted in dealing with boulder fall hazards in Hong Kong, and has recently proved practical in applying to mitigate natural slope failure hazards. 1.5 Requirements for the Study of Natural Terrain Hazards 1.5.1 Screening of Sites Subject to Natural Terrain Hazards Not all development sites are affected by natural terrain hazards. Whether a site will be affected depends on how close the site is to the hillside and how susceptible the hillside is to landsliding. To facilitate project planning and early identification of potential constraints, it is good practice to carry out, where necessary, an initial screening as early as possible to examine whether the site may be subject to natural terrain hazards. The screening is not demanding either in terms of resources or time. Sites that are located beyond the influence zone where landslide debris may reach would not be subject to the natural terrain hazards even if landslides occur on the hillside. Such sites may be excluded from further screening and study of natural terrain hazards. For this reason, the GEO has established a set of simple guidelines to assist planners, land administrators, project managers, etc. in identifying whether a site may require screening in respect of natural terrain hazards. This set of “Inclusion” guidelines comprise the following two conditions: (a) the proposed development involves Group 1, 2 or 3 facilities (see Table 1), and (b) there is a ‘hillside’ sloping at more than 15° within 100 m horizontally upslope of the site. For the purpose of applying the “Inclusion” guidelines, the term ‘hillside’ refers to sloping terrain that is undeveloped or largely undeveloped. It excludes built-up areas, such as terraces supported by retaining walls, and a series of paved platforms separated by man-made slopes. Where there are man-made slope features adjoining a sloping terrain that is undeveloped or largely undeveloped, the ‘hillside’ includes both the man-made slope features and the sloping terrain.
- 14 Where a development site satisfies the “Inclusion” guidelines, arrangements should be made for a screening in respect of natural terrain hazards. The GEO has been adopting two sets of technical criteria for screening of sites in respect of natural terrain hazards. These two sets of criteria, viz. the “In-principle Objection Criteria” and the “Alert Criteria” (Figure 2), are described in Sections 1.5.2 and 1.5.3 below. The GEO applies the technical criteria in screening development proposals, to determine whether an in-principle objection should be tendered or whether a natural terrain hazard study (NTHS) is required. The screening process is generally conducted at the land use planning stage when the case is referred to the GEO from the Planning Department, e.g. during Section 16 Application under the Town Planning Ordinance (Cap. 131), or at the land disposal or lease modification stage when Lands Department seeks GEO’s input. The screening should also be carried out as part of the technical feasibility of a proposed public works project (i.e. Preliminary Project Feasibility Study report or Technical Feasibility Statement). The screening will provide information on whether any parts of the site may be affected by natural terrain hazards, which will give an indication of likely scale of the problem, facilitate preliminary design of the site layout, and help to determine where further input on study of natural terrain hazards is required. It should normally come to a conclusion on the feasibility of the proposed development, any need to avoid areas exposed to severe hazards, and any provision to be made in the development for natural terrain hazard study and mitigation. 1.5.2 In-principle Objection Criteria In line with the current practice, the GEO would object in-principle to proposals for the zoning and disposal of a site that fall under either (a) or (b) of the following conditions, denoted as “In-principle Objection Criteria”: (a) The site is faced with severe natural terrain hazards. A site falls within this category if any proposed Facility Groups 1(a), 1(b) and 2(a) (Table 1) is either: (i) located within an angle of reach of 35° from any natural terrain at an elevation of 50 m or more above the proposed site formation level (Figure 2a), or (ii) located on, or immediately below, terrain that is known to be affected by active, large-scale, deep-seated movement (e.g. Area 19, Tuen Mun). (b) The site is proposed for small-scale New Territories Exempted House (NTEH) and is subject to potential natural terrain hazards. However, the required NTHS and mitigation works are disproportionate to the scale of the proposed development and could thereby render the proposed development not economically viable. A site
- 15 falls within this category if it meets all of the following conditions: (i) the site meets GEO’s requirement for study of natural terrain hazards (see Section 1.5.3 below), (ii) the site is not located in a cluster of existing developments of a comparable nature and affected by similar hazards, and (iii) it is unlikely that the requirement for study and mitigation of natural terrain hazards can be lifted by alternative means, such as adjustment of the layout of the proposed development, delineation of no-build zone, simple drainage provisions, confirmation of no need for NTHS and mitigation by a brief review of the natural terrain hazards, etc. Although the criteria are essentially applied to “habitable” facilities, Group 1(b) facilities are included because they are comparable in terms of consequence if affected by natural terrain landslides. Items (b)(i) to (b)(iii) above are to be assessed based on professional judgement on a case-by-case basis, with account taken of the nature of the proposed development, its proximity to the hillside, size and geometry of the site, and available information on the conditions and history of the hillside. The above criteria are not applicable to sites after disposal. 1.5.3 Alert Criteria For a site that may be affected by natural terrain hazards, but does not satisfy the “In-principle Objection Criteria”, an NTHS is required to study the hazards and identify any mitigation measures required. The following criteria, denoted as “Alert Criteria” (Figure 2b), are used by the GEO to help decide whether a site falls under this category: (a) It is a new development site involving provision of Group 1 to 3 facilities (Table 1), or it is a redevelopment that requires modification of the lease conditions and involves either a significant population at risk or a significant increase in population at risk. (b) Where there is natural terrain outside the site, but within the same catchment, that is at an angular elevation of 20° or more from the site and where there is ground sloping at more than 15° within 50 m horizontally upslope of the site, provided that there is a credible debris flowpath to the site. An NTHS may be required for sites that lie beyond the area delineated by the above criteria, as for example for sites where there are historical landslides with long debris run-out extending beyond these limits, and for sites that are either intersected by, or adjacent to, a
- 16 natural drainage course. A “credible debris flowpath” is generally a downhill path followed by surface water. However, flowpaths that debris could follow, but are deemed unlikely to do so would not be regarded as “credible”. For example, a debris flow path down a ridge line, rather than descending into the catchment on either side of the ridge line, would not be “credible”. Another example would be a site that is shielded from debris by a substantial structure such as a large building. In applying the “Alert Criteria” to help decide whether an NTHS is required for a site, professional judgement is exercised with account taken of the nature of the proposed development, its proximity to the hillside, size and geometry of the site, and available information on the conditions and history of the hillside. In addition to recommending that an NTHS be carried out, consideration is also given to alternative means for dealing with natural terrain hazards. These include adjustment of the layout of the proposed development, delineation of no-build zones, simple drainage provisions, etc (see Sections 1.4 & 4.3). 1.6 Objectives of a Natural Terrain Hazard Study The potential risk from landslides on natural hillsides, unless mitigated, will inevitably increase as the population of Hong Kong grows and as building and infrastructure developments continue to spread into areas adjacent to natural hillsides. The relative significance of the natural terrain hazards to any site will vary from non-existent in the middle of a flat plain to very high at a site below steeply sloping ground with a history of landslides. The objectives of an NTHS are to identify any natural terrain hazards that could affect a proposed development and to enable a decision to be reached as to whether the potential risk to the site warrants mitigation. In practice, for sites that are found by screening (Section 1.5) as requiring an NTHS, there is the potential to curtail the NTHS during the study if there is sufficient information to conclude that no significant hazards exist. Alternatively, as more data are obtained about the site, it may become possible to establish that whilst hazards may exist, mitigation measures are unnecessary. 1.7 Personnel Given the nature of the work involved, a multi-skilled team is normally required to undertake an NTHS. The make-up of the team will depend on the complexity and level of the study. In general, the team should include professionals with suitable engineering geological and geotechnical engineering experience in NTHS. Engineering geological expertise is needed for aerial photograph interpretation, engineering geological mapping, interpretation of geological data, and development of geological and geomorphological models. Geotechnical engineering expertise is required for analysis of engineering data, assessment of slope stability and design of mitigation measures. Some tasks, such as determination of design events and development of mitigation strategy would be best carried out jointly by engineering geologists and geotechnical engineers. In some cases, input from risk analysts may also be required in detailed quantitative risk assessment.
- 17 As an NTHS forms part of a geotechnical submission, it should be reviewed by a professionally qualified geotechnical engineer who has suitable local experience in natural terrain hazard studies. 2. APPROACHES TO A NATURAL TERRAIN HAZARD STUDY 2.1 Scope of a Natural Terrain Hazard Study The scope of an NTHS comprises: (a) the determination of the location, type and magnitude of any natural terrain hazards that may affect the development of a site, and (b) where an identified hazard is deemed to affect a site, to determine the design event for such a hazard and outline a mitigation strategy to reduce the risk it poses to the development. The level of study required to determine if any hazards exist and to select suitable mitigation measures if required will vary according to the setting of a site. The resources required to study a site will also vary according to the size of the site and its proposed use. Siting critical facilities away from vulnerable areas, where possible, will often result in a substantial saving with respect to the input required to address the natural terrain hazards. It will also minimize exposure to risk associated with uncertainties or unforeseen conditions. 2.2 Site and Study Area The study of any natural terrain close to a proposed development should be related to a well-defined site boundary, preferably recorded on a topographical map of scale 1:5 000 or larger. In the early stages of a project, there may be a number of possible uses and layouts for a site, which may be subject to changes. At this stage, the study may have to consider the entire area inside the site boundary as potentially habitable. Hence, any hazards would be evaluated in relation to the site boundary. The study should determine if any identified hazards could result in debris being deposited within the site boundary and if so, to establish the debris travel path, magnitude and nature of such materials. The study area for any site is normally the entire upslope catchment of natural terrain above the site boundary. This may include natural terrain separated from the site by existing developments such as roads or buildings, unless these developments are judged to provide a barrier or buffer against debris reaching the site from the natural terrain. It may be possible during the early stage of a study to exclude parts of the catchment on the basis that simple topographical relationships show that the terrain could not affect any part of the site. However, in order to assess the potential natural terrain hazards in its regional context, catchments in the relevant vicinity also need to be considered.
- 18 2.3 Approaches to the Study 2.3.1 Introduction Three different approaches, namely, Factor of Safety, Quantitative Risk Assessment and Design Event, may be used, either individually or in combination, to evaluate natural terrain hazards. 2.3.2 Factor of Safety Approach The Factor of Safety approach is set out in the Geotechnical Manual for Slopes (GCO, 1984). This approach requires the study of the stability of the natural terrain and design of any necessary slope stabilization measures to meet the requirements given in the Manual. As stipulated in Section 5.2.3 of the Manual, natural terrain does not need to meet the factors of safety given in Table 5.1 of the Manual if it is “undisturbed” and “a careful examination is made to determine that there is no evidence of instability or severe surface erosion”. The term “evidence of instability” used in the Manual refers to “evidence relevant to future instability”. By implication, if one chooses to adopt the Factor of Safety approach and if the natural terrain does not satisfy the two criteria given in Section 5.2.3 of the Manual, the natural terrain has to meet the factors of safety stipulated in Table 5.1 of the Manual. In practice, few natural hillsides satisfy both criteria. The Factor of Safety approach can be used by designers who opt for prevention of natural terrain failures. In the past, the Factor of Safety approach was adopted in the study of natural terrain below development areas to check that the sites would not be adversely affected by failure of the natural hillside. It has also been used in assessing the stability of hillsides above development sites against large failures or deep-seated landslides. 2.3.3 Quantitative Risk Assessment Approach The Quantitative Risk Assessment (QRA) approach entails detailed assessment of the probability and consequence of natural terrain hazards to a development site and determines any necessary mitigation measures. A comprehensive review of the QRA concepts as applied to slope engineering is given in Ho et al. (2000). Lo (2001) gave an interim review of pilot application of QRA to landslide problems in Hong Kong. This approach provides a formalized methodology to gauge the relative importance of different parameters and the uncertainties associated with them, and so to determine the acceptability of a calculated risk level by comparing it to established guidelines. Furthermore, it provides a common, definitive basis for applying the “As Low as Reasonably Practicable (ALARP)” principle, and for evaluating the cost-benefit of alternative risk mitigation strategies to optimise design. The risk guidelines currently adopted by the GEO are given in ERM (1998), and are for interim use as a basis for the evaluation of QRA results. These risk guidelines may be
- 19 further refined as necessary. QRA can be applied by designers who opt for mitigation of landslide risk instead of prevention of natural terrain failures. The study calls for expert input, and the assessment may be fairly involved and costly. Therefore, it would not be appropriate to apply QRA to routine problems that can be adequately handled by conventional techniques. Relevant QRA studies carried out in recent years include Atkins Haswell (1995), Atkins (1998, 1999a), Halcrow (2000) and Fugro Maunsell Scott Wilson (2001). 2.3.4 Design Event Approach The Design Event approach is based on a qualitative risk framework, with account taken of both the susceptibility (or likelihood of failure) of the natural terrain and failure consequence in a semi-quantitative manner. Under this approach, the relevant design failure events are assessed for the different natural terrain hazards and any necessary mitigation measures are evaluated. The design requirements for the Design Event approach are given in Section 2.4 below. Like QRA, the Design Event approach can be used by designers who opt for mitigation of landslide risk instead of prevention of natural terrain failures. The Design Event is relatively easy to apply as it does not demand formal and rigorous quantification of risk. Many practitioners in Hong Kong favour the use of the Design Event approach where the required mitigation measures are not disproportionate to the scale of the development. However, as this approach does not explicitly consider the risk tolerability, it cannot evaluate mitigation actions based on the ALARP principle, or justify not taking actions on the grounds of a broadly acceptable risk level. 2.4 Design Requirements for Natural Terrain Hazards For the Design Event approach, the need for any further study and the design requirements for mitigation measures are evaluated, normally in an early stage of the study, based on Table 2. This involves consideration of the consequence of failure and susceptibility of the hillside to failure. (a) Consequence of failure is categorized into five classes based on the types of facilities (Table 1) affected and their proximity to the hillside (Table 3). (b) Susceptibility of the hillside to failure is categorized into four classes based on historical landslide data and other information (Table 4). According to Table 2, further study is not required if the susceptibility of the hillside to failure and the consequence of failure are insignificant. This is consistent with the provisions in the Geotechnical Manual for Slopes (GCO, 1984), where there is no evidence of instability and negligible consequence slopes. Otherwise, further study should be carried out to establish the need for any mitigation measures to deal with the relevant design failure
- 20 events. Depending on the susceptibility of the hillside to failure and the consequence at the site, the required design event may be a “conservative event” or a “worst credible event”. The return periods given below are notional, and are intended to reflect the design principles. They should not be taken as accurate values derived from detailed statistical analysis. (a) A “Conservative event” is a reasonably safe but not overly cautious estimate of the hazard that may affect the site, with a notional return period in the order of 100 years, and is generally based on the largest historical landslide over the past 50 to 100 years (e.g. identified from aerial photographs) in the catchment and its vicinity as appropriate. (b) A “Worst credible event” is a very conservative estimate such that the occurrence of a more severe event is sufficiently unlikely. Its notional return period is in the order of 1 000 years, which indicates an order of magnitude difference compared with that of a “conservative event”. This is intended to exclude historical events that occurred in the geological past. It generally corresponds to the largest credible landslide based on interpretation of historical landslide data, geomorphological evidence in the catchment and its vicinity as appropriate, and any other relevant evidence from similar terrain in Hong Kong. The design requirements for the Design Event approach involve consideration of the susceptibility of the hillside to landsliding. Hence, in applying the Design Event approach, conservative assumptions may have to be made in the early stage of an NTHS on the susceptibility of the hillside, to ensure that the design requirements would not be underestimated due to lack of information on the susceptibility of the hillside. As further work is carried out in the NTHS, the assumptions made on the susceptibility of the hillside should be reviewed in order to confirm the design requirements. For calibration purposes, the technical framework proposed for determining the design requirements for the Design Event approach have been applied to some previous cases of NTHS and to some selected sites of proposed development (Table 5). The framework gives reasonable results. 2.5 Hazard Models 2.5.1 Introduction Natural terrain hazards can be grouped into hazard models on the basis of the transport mechanism, the nature of the displaced material and the topographical location. Five hazard models that are generally applicable to most sites in Hong Kong are presented below. These may not cover all possible situations and it may be useful to define sub-models or other models as appropriate to the prevailing conditions. Some notable natural terrain landslides occurring in recent years are summarized in Appendix B. Typical landslide types that have been identified in Hong Kong are discussed in Section C.6 of Appendix C.
- 21 The five hazard models described in the following sections should be considered in the study of each site. For certain sites it may be possible to discount some or all of them at an early stage. As a study progresses, the hazard models may need to be refined in the light of additional information. 2.5.2 Open Hillslope Landslide This type of hazard results from a landslide that remains wholly on the open hillside and is not channelized along a stream course. It may include landslides classified as debris slides, debris avalanche or debris flow (Figure C2). An open hillslope landslide commonly involves a rapid to extremely rapid, shallow detachment of partially or fully saturated debris on steep slopes. The initial displaced mass disintegrates and loses its original structure during displacement but is not channelized in a stream bed. There is minimal additional water admixed into the displaced material during transport. Debris transport can include the mechanisms of flow, sliding, rolling and bouncing. These landslides may occur on any part of a hillside but as the landslide debris moves downhill it will converge into hollows (especially linear depressions) in its path. Thus, the hazard may be much greater where the site boundary intersects such hollows. This model may be sub-divided on the basis of particular initiation points such as coastal slopes, river slopes or previous landslide scars. 2.5.3 Channelized Debris Flow This type of hazard results from saturated landslide debris being transported by slurry flow or hyper-concentrated stream flow and can only occur along a stream (Figure C3). Channelized debris flows generally have much greater mobility than open hillslope landslides and normally develop when debris from one or more open hillslope landslides enters a stream channel, and becomes mixed with stream water. The relative volume of debris and the cross-sectional area of the channel (the channelization ratio) control the degree to which the debris is confined by the channel. Collapse of the channel sides and erosion and entrainment of material from the stream bed and banks may add additional debris. Channelized debris flows may occur repeatedly in the same channel and such locations can sometimes be recognized by the presence of debris fans from previous events. 2.5.4 Deep-seated Slide This type of hazard involves displacement of an intact mass by sliding along a basal rupture surface. Landslides in this model are generally slower moving and with a deeper surface of rupture than open hillslope debris slides. The model does not include soil creep, which is not generally considered to be a hazard; however, evidence of soil creep may provide an indication of potential hazards. Deep-seated, intact sliding is relatively rare in Hong Kong and usually occurs only as landslides with very short travel distances, where the displaced material has not moved from the source area (slump) or where tension cracks and bulging are the sole surface expression of movement, e.g. Siu Sai Wan (Ho & Evans, 1993) and Island School (Irfan, 1989). In Hong Kong, slow, small sliding slope movements are particularly associated with sites where natural terrain has been disturbed by excavation, in some cases into degraded ancient landslide scars.
- 22 2.5.5 Rock Fall This type of hazard results from one, or several, typically angular rock fragments being transported initially by free fall but may include sliding, bouncing and rolling. Rock fragments are generally joint-bounded and displaced from a rock face by sliding, wedge failure or toppling. Initiation may often be related to transient high water pressure along joints and where smaller “key” blocks have been displaced. Locations where this hazard is present are relatively easy to identify as it is limited to rock faces. 2.5.6 Boulder Fall This type of hazard results from one, or several, typically rounded rock fragments being transported by rolling, bouncing and sliding. Boulders originate either as corestones in a weathered profile or are derived from colluvium. They become exhumed during hillslope erosion, although in some instances they may have originated higher up the slope from rock outcrops and been displaced to their present position as a rock fall. They are usually dislodged from the natural hillside as a result of soil erosion or undercutting due to landsliding. Where vegetation growth is minimal, locations of boulders on steep slopes can be easily identified from interpretation of aerial photographs. 2.6 Approaches to Study Hazard Models 2.6.1 Introduction The three approaches outlined in Section 2.3 above should not be considered as independent of one another. A site may be exposed to several types of natural terrain hazard and of varying scales. Some approaches may be more applicable to certain hazards than others. Therefore, the designer should choose the most suitable approach, or combination of approaches, for a site. 2.6.2 Open Hillslope Landslide and Channelized Debris Flow As large areas of natural terrain may be susceptible to open hillslope landslides which may develop into channelized debris flows, stabilization works on natural terrain to prevent these hazards are usually impractical. Therefore, the QRA and Design Event approaches are usually more appropriate for addressing these two types of hazard. 2.6.3 Deep-seated Slide If the extent of the hillside that is susceptible to this type of hazard can be defined, the Factor of Safety approach may be applied to determine the need for any slope stabilization works. However, stabilization works on the hillside may be costly, particularly if a large volume of potentially unstable ground is involved.
- 23 2.6.4 Rock Fall The Factor of Safety approach is generally applicable to dealing with rock fall hazard because potentially unstable blocks can commonly be identified from field mapping. Alternatively, in cases where the extent of the unstable blocks is difficult to ascertain for in-situ stabilization works, or if it is more cost-effective to provide mitigation measures, the Design Event approach may be more suitable. However, implementation of some mitigation measures, such as rock ditches and slope drapery, is of prescriptive or empirical in nature. 2.6.5 Boulder Fall There is an established methodology for studying boulder fall hazard and engineering solutions have been available and adopted for many years (e.g. Au & Chan, 1991). Therefore, this document does not specifically address boulder fall hazard. Depending on the hillside conditions, all three approaches may be used to tackle this type of hazard. 3. NATURAL TERRAIN HAZARD STUDY 3.1 Introduction The purpose of an NTHS is to identify natural terrain hazards that may affect a particular site and to propose a mitigation strategy if required. The study of natural terrain hazards involves consideration of the site in the context of its regional geological and geomorphological settings, any man-made influences that may have modified this setting and the history of landsliding in the area. This information together with an engineering geological investigation allows a geological model of the catchment to be developed. An NTHS generally comprises a desk study of available information about the study area, an examination of all available sets of aerial photographs, collection of field data, assessment of the natural terrain hazards, and determination of the hazard events to be mitigated against. Ground investigation may also be required. Account may have to be taken of catchments in the relevant vicinity or other similar locations to identify all possible hazards. The results of the study should allow a decision to be reached as to whether the potential risk can be considered negligible, where the risk can be contained by mitigation works, or whether the risk is so severe that it would render the proposed development not feasible. Where applicable, the study should include determination of the location, potential magnitude (i.e. the design events) and run-out characteristics of specific hazard models that are likely to affect the site being studied. The economic viability of the proposed development may form part of the study. The following sub-sections of Section 3 outline the key components and procedures of good practice recommended for an NTHS. The aspects on mitigation strategy of an NTHS are given in Section 4.
- 24 3.2 Desk Study 3.2.1 General The desk study should comprise a thorough review of geological and geotechnical information pertinent to the study area so as to identify those features that may be hazardous to the site. 3.2.2 Types of Information Desk study information relevant to site investigation in Hong Kong is described in Geoguide 2 “Guide to Site Investigation” (GEO, 1987). Much of the information described in Sections 4, 5 and 6 and Appendices A, B and C in the Geoguide is relevant to the desk study. Appendix B of the Geoguide has been superseded by GEO Technical Guidance Note (TGN) No. 5. Information concerning the history of developed areas is only likely to be relevant to the site itself. A useful source of information covering natural catchments in Hong Kong is the Stormwater Drainage Manual (Drainage Services Department, 1994). Published maps that provide information on natural terrain for the whole of Hong Kong include topographical maps, geological maps, and the Geotechnical Area Study Programme (GASP) series maps based on terrain classification. Another map series based on high altitude aerial photograph interpretation is the GEO Natural Terrain Landslide Inventory. An inventory of boulder fields mapped from the 1963 low altitude aerial photographs is also available. Most of these sources of data are available for consultation by the public in the Geotechnical Information Unit (GIU) of the Civil Engineering Library (CEL) maintained by the Civil Engineering Department (CED). A considerable amount of slope safety-related information is contained in a computerized Slope Information System (SIS). The SIS displays the locations of registered man-made slopes, past landslide incidents and ground investigation records along with base maps and lot boundaries. These are linked to slope inspection records, maintenance information and photographs. Public access to the SIS is available through the GEO Hong Kong Slope Safety Website, http://hkss.ced.gov.hk. By early 2002, the website will also maintain an up-to-date list of geotechnical publications and technical papers on geology and geotechnical engineering in Hong Kong. If the site or other parts of the study area have previously been investigated, or developed, then useful data may be available. This information may include records of past landslide incidents, ground investigations, building records and details of man-made slopes. Besides the building records which are held at the Buildings Department, a great deal of this information is held in the GIU of the CEL and can be searched for by location using the Geotechnical Information Library System (GILS) database. A bibliography of geological and geotechnical publications in Hong Kong is given in Brand (1996). 3.2.3 Topographical Maps Topographical maps of Hong Kong are available from the Map Publications Centre of the Lands Department Survey and Mapping Office at the North Point Government Offices.
- 25 Full coverage of Hong Kong is available at 1:20 000, 1:5 000 and 1:1 000 scales and each is appropriate for different aspects of natural terrain hazard studies. The 1:20 000-scale maps have been used as base plans for various thematic maps that cover the whole of Hong Kong, including geology and GASP terrain classification. In general, this scale is too small to show details of a study area but can be used to place the study area in a regional context. The 1:5 000-scale maps are useful for delineating the study area, and for large catchments they can be useful base maps to present data and information. The 1:1 000-scale maps are a suitable base for aerial photograph interpretation and field mapping. Slope angles derived at this scale are similar to those derived from the 1:5 000-scale maps, but the greater detail provides better resolution of smaller features. 3.2.4 Aerial Photographs The earliest available aerial photographs of parts of Hong Kong were taken in 1924 with the next set of photographs taken in 1945. Other photographs of variable quality and coverage are available up to the first complete survey of Hong Kong undertaken in 1963. This is a set of high quality low altitude aerial photographs taken at a time when there was generally low vegetation cover in Hong Kong, which allows clear observations of subtle ground features and consequently provide a useful “baseline” for comparison with subsequent observations. Since 1973, reasonable quality photographs covering most of Hong Kong have been taken annually, although the low altitude coverage of many of the more remote natural terrain areas of Hong Kong has been sporadic. In addition to vertical aerial photographs, a collection of oblique aerial photographs is held in Planning Division of GEO and should also be consulted. Some areas in Hong Kong have been photographed using infra-red aerial photography, which may in some cases be useful for identifying areas of seepage. Potential applications of remote sensing techniques for identifying areas of seepage in Hong Kong and for geotechnical applications are discussed in Scott Wilson (1999a) and Mason (2001). Aerial photographs are available for sale from the Lands Department’s Map Sales Unit at the North Point Government Offices. For public works projects, much of the Government collection can be borrowed from the GEO Aerial Photograph Library, housed in the Planning Division of the GEO. The lending policy of the library and general background information on aerial photography in Hong Kong are contained in GEO Circular No. 12. 3.2.5 Geological Maps Geological maps cover the whole of Hong Kong at 1:20 000 scale and are available from Government Publications Centres. These maps are regional surveys, and should not be used to determine site-specific lithology. However, they are useful in forming an initial geological model as the basis for detailed site mapping and ground investigation. These 1:20 000-scale maps are supported by a series of six memoirs describing the geology of Hong Kong. The 1:10 000-scale field sheets that record the original field mapping observations can be viewed at the Hong Kong Geological Survey, which is part of the Planning Division of the GEO. The geology of Hong Kong is comprehensively described in two memoirs, one of
- 26 which focuses on the Quaternary geology, including aspects of weathering and geomorphology (Fyfe et al., 2000), and the other provides details on the solid geology (Sewell et al., 2000). A limited number of 1:5 000-scale maps, supported by sheet reports, have also been published. They are mainly located in the development areas of North Lantau, Ma On Shan and Yuen Long (Appendix D). These sheet reports can be obtained from the Planning Division of the GEO. Some site-specific geological data are available in the Hong Kong Geological Survey Archive of Maps and Reports and can be viewed in the Planning Division of the GEO. Some of the geologically orientated data held by Planning Division of the GEO have been digitized and are stored in a geographical information system. These include the 1:20 000-scale geological maps, some drillhole locations, and the locations of geophysical surveys. 3.2.6 Terrain Classification Maps A series of eleven 1:20 000-scale terrain classification maps (TCM) covering the whole of Hong Kong were produced by the GEO in the 1980s. Each map covers an irregular area based on catchment boundaries. Derivative maps and accompanying reports have been published for each area as the Geotechnical Area Studies Programme (GASP) series. The series contains 12 volumes (Appendix D), of which Volume XII is a summary report for the whole of Hong Kong. They are available for consultation in the GIU. The terrain classification system recorded three attributes, slope angle, landform, and instability and erosion. This information was derived from interpretation of pre-1980 aerial photographs. The slope angle information was derived from topographical maps and visual estimates and should now be replaced with slope angles derived from more updated large-scale maps. The landform data are still generally relevant although the erosion and instability characteristics may have changed due to erosion control measures, particularly revegetation. The terrain classification maps were used to create thematic maps that include a Physical Constraints Map and the Geotechnical Land Use Map (GLUM). Appendix A to each GASP report explains the system of terrain evaluation used. It should be noted that these maps were prepared for general planning and resource evaluation purposes. Whilst the information they contain provides a good basis for engineering feasibility studies they should not be used to interpret parcels of land smaller than 3 ha in size. Nine catchments with extensive colluvial deposits were also identified for GAS mapping at 1:2 500 scale. These maps contain details of old landslide scars. In 1990, a 1:5 000-scale terrain classification mapping exercise was carried out for eleven 1:5 000-scale map sheets in North Lantau (Appendix D). These are available for consultation in the CEL and have been incorporated into three engineering geology reports on the North Lantau Area (Franks, 1991, 1992; Woods, 1992).
- 27 3.2.7 Natural Terrain Landslide Inventory The Natural Terrain Landslide Inventory (NTLI) was compiled from the interpretation of high altitude aerial photographs taken in various years between 1945 and 1994. Emery (1996) and King (1999) document the methodology and procedures of the study. The NTLI 1:5 000-scale maps are available on the SIS (http://hkss.ced.gov.hk). Factors such as coverage, scale and resolution of the high altitude aerial photographs impose certain limitations on the data set. Consequently, some landslides may not have been identified and some landslides shown may be other features mis-identified as landslides. Furthermore, identification of landslides on the aerial photographs may have been hindered by poor image resolution, cloud cover, ground shadows or vegetation cover. These maps are useful for reviewing the landslide history of an area of natural terrain on a regional basis, but should not be used as the only basis for individual site assessments. The NTLI has been updated for 1995 to 1997 as reported in Scott Wilson (1999b). An update of the NTLI for 1998 to 2000 is currently being underway and will be available in late 2002. 3.2.8 Large Landslide Dataset The Large Landslide Dataset, covering the whole of Hong Kong, was prepared under the Large Landslide Study. In this study, a geomorphological interpretation was conducted to identify features thought to be landslides with source areas greater than 20 m wide. The main source of the large landslide data was from the interpretation of the 1963/64 low altitude aerial photographs (Scott Wilson, 1999b, 1999c). The study also examined the large landslides recorded in the NTLI, GASP maps and geological maps. The 1:5 000-scale maps containing the features are available for viewing in the Planning Division of the GEO. The features that were identified range from fully vegetated, degraded features of valley-side dimensions to single recent landslide scars. It should be noted that some features might be the result of multiple smaller landslides. Furthermore, old landslides might have occurred under environmental conditions different from the present. The classifications of sliding, flowing or toppling/falling need to be confirmed by field observations. 3.2.9 Boulder Field Inventory A series of fifteen 1:20 000-scale boulder field inventory maps covering the whole of Hong Kong was completed in 1998. These maps were derived mainly from interpretation of the low altitude aerial photographs taken in 1963/64. A multiple attribute mapping technique was adopted to present the results - an area of land considered to have a relatively uniform pattern of boulder deposits was delineated and enclosed within a polygon and drawn onto the 1:20 000-sacle topographical maps (Emery, 1998). The polygons were defined to ensure that the units were essentially homogeneous for the range of attributes described. Four attributes were mapped - percentage area covered by boulder, boulder type, boulder size and boulder shape. These maps, available for reference in CEL, provide information for identification of boulder fall hazard and on the number, size, shape and type of boulders present. For site specific assessment, these maps should be supplemented by detailed interpretation of low altitude aerial photographs and by field mapping.
- 28 3.2.10 Rainfall Data The GEO operates an extensive network of automatic rain gauges providing real-time rainfall data to support the Landslip Warning System. This network, which dates back to 1984, has recently been upgraded to improve coverage in the new development areas. The network now comprises 86 rain gauges located throughout Hong Kong. The data capture, control and processing system, which has also recently been upgraded, receives data from the GEO raingauges and from an additional 24 automatic rain gauges operated by the Hong Kong Observatory and 5 gauges operated by the Drainage Services Department. Since 1984, the GEO has compiled annual reports to document information on rainfall and landslides occurring in Hong Kong. Detailed reports on significant landslides, prepared by the GEO, also analyse the relevant rainfall data. 3.3 Aerial Photograph Interpretation 3.3.1 Purpose Aerial Photograph Interpretation (API) should be used as the first step in the development of geomorphological and geological model as well as to examine the history of development and past instability in the study area. Being one of the most important parts of an NTHS, interpretation of aerial photographs should be undertaken by an experienced professional. The objective of the API is to determine, in as much detail as possible, the terrain attributes of the study area and its surroundings that may influence hazards, as well as details of past events. These include the characteristics, magnitude and observed consequences of past natural terrain hazards, the extent of any boulder fields, drainage characteristics, slope gradient, slope aspect and morphology, characteristics of vegetation cover, location and types of rock outcrops and superficial deposits, etc. Table 6 gives further guidance on terrain attributes that can be mapped from aerial photographs. The API results help identify areas for field data collection and provides the basis for determining landslide susceptibility. 3.3.2 Review of Existing Data Many of the datasets available for an NTHS were derived from the interpretation of aerial photographs (e.g. Terrain Classification Maps, Natural Terrain Landslide Inventory, Large Landslide Dataset and Boulder Field Inventory). As described in their respective sections (Section 3.2), these data were prepared under various constraints; some due to data quality whereas others due to method of mapping. Consequently, it is recommended that these datasets should be compared with the results of the API in order to evaluate the relevance of the desk study information for the site of interest. For example, the NTLI (Section 3.2.7) suffers from certain limitations inherited in the coverage and quality of the high altitude aerial photographs. Therefore, the NTLI features should be confirmed and supplemented by detailed interpretation of low altitude aerial photographs as part of the NTHS. Account should also be taken of the coverage and quality of the datasets, corresponding rainfall history at the site and any environmental changes (e.g. hill fires, man-made effects) that have occurred in the area.
- 29 Using the NTLI data (Section 3.2.7), Evans and King (1998) integrated geology and slope angle to derive the regional susceptibility of debris avalanches in Hong Kong. However, in applying this regional susceptibility, due consideration should be given to the methodology and assumptions adopted in deriving the correlation. In general, this regional correlation should not be regarded as sufficiently reliable for use in site specific assessment. 3.3.3 Review of the Site Condition The most recent aerial photographs provide information on the current condition of the study area. This is a useful check on the current status of the base map to be used for the API and may provide data to update it. Observations from these photographs can also be useful for planning the site reconnaissance by identifying suitable locations for an overview of the site and possible access points. The best access is normally along footpaths and streams and the current vegetation cover in these locations is often the most important factor that affects access. Vegetation cover can change rapidly in Hong Kong due to hill fires or clearance by man and regrowth can leave a cleared area difficult to access. 3.3.4 Interpretation An important part of the API is mapping topographical, geomorphological and geological features to assist in the development of a geological/geomorphological model. The 1:1 000-scale topographical maps generally represent sufficient details of the topography to be used as the base maps for recording information. The observations need to be transferred onto a suitable base map, preferably at a scale similar to that of the photographs. The use of ortho-photographs is extremely beneficial in that it allows the accurate location of features and enables scaled measurements to be made. A number of studies have used ortho-rectified photographs for measurement of ground features and as base plans for field mapping. The mapping should be carried out using a well-defined legend that includes all the aspects of engineering geology and geomorphology covered by the mapping. A generally accepted system is described in Anon. (1972). Important aspects to consider include the scale, flying height, year of photography, time of year and quality of the photographs. Both high and low altitude photographs should be included in any interpretation. Interpretation of good quality oblique aerial photographs and site photographs can also provide useful information. The interpretation should make use of as complete a history as possible, preferably using both dry season and wet season coverage as this may allow more precise age-bracketing of specific landslides, and/or determination of the effect of changed conditions such as slope excavations or hill fires. It is important to review aerial photographs from as many consecutive wet seasons for a catchment as possible to build up a history on the likelihood of landslide initiation. A glossary of commonly used terms for landslide description and related subjects is given at Appendix A. Relevant features that can be recorded from API are discussed below.
- 30 (a) Catchment Characteristics - Catchment boundaries can be delineated on the basis of the 1:1 000 scale maps but details should be confirmed by API. Hollows, channels and streams should also be marked. (b) Superficial Geology - The ground surface should, as far as possible, be divided into areas of rock outcrop and regolith. It may be possible to interpret structure or lithology from the rock outcrop and open joints and loose blocks may be observed. Regolith may be divided into saprolite and transported soils. Colluvium can often be recognized by distinct morphological features such as fans, lobes and valley infill. The colluvium may be further sub-divided depending on topographical position, morphology and material properties (see Section C.4 of Appendix C). Areas of boulders should be mapped and if possible interpreted as saprolite with corestones or as colluvial boulders. Colluvial boulders may form boulder trails or fans on the surface of distinct colluvial landforms. (c) Bedrock Geology - With the assistance of geological maps, it may be possible to interpret some details of the bedrock geology. Dykes, faults and lithological contacts can often be seen as linear features in the topography. Photo-lineaments should also be recorded. (d) Landslide Scars - In Hong Kong, landslides occur on natural terrain at an average frequency of about one in every 2 km2 per year (Evans & King, 1998). It is likely that landslide scars are present in most study areas. As the past instability of terrain can provide information for the assessment of future events, it is critical to record the location and dimensions of all landslide scars and to estimate their date of occurrence. The terminology used to describe landslide scars and the parameters recorded should be clear and unambiguous. A useful reference for landslide description is given by Cruden and Varnes (1996). A set of terms that are adopted for the description of landslides in Hong Kong is given in Appendix A. A landslide scar is defined as the land surface affected by a landslide and includes the source, the displaced material and any trail. Landslide scars can be divided into recent, which can be observed as a light toned area bare of vegetation on aerial photographs, and relict, which are covered with vegetation and their exact date of occurrence cannot be established as they occurred prior to the earliest available aerial photographs. Care should be taken in the identification of recent landslide scars, particularly on high
- 31 altitude aerial photographs, because some features, such as gully erosion and grave sites, can easily be confused with landslides. Parameters such as source dimensions and run-out extent can be recorded from API. The identification of relict landslide scars is much more interpretative. Although it may be possible to estimate source dimensions, it is seldom possible to record run-out information with confidence. Vegetated concave features with colluvial deposits downslope are common in the Hong Kong landscape and may be interpreted as relict landslide sources. However, such landscape features could result from a large single past event, a number of smaller landslide events, protracted surface erosion, or any combinations of these processes. As such landscape features are often the largest that may be interpreted as past landslides, this interpretation can be crucial to an understanding of the largest events that may have affected the site. Consideration should also be given to the likely age of these landslides and their relevance to the current climatic and site conditions. Great care should therefore be taken with their interpretation, and it should always be backed up by thorough field observations wherever possible. (e) Landslide Deposits - Locations of landslide deposits such as colluvial lobes and fans, boulder fields or trails, and scree slopes should be recorded. (f) Slope Features - Breaks in slope, localized slope inversions, ridges, knolls, hollows and other small slope features that may not be reflected on the contour map should also be mapped. In particular, attention should be paid to locally steep slopes that could be the source of landslides, and for hollows and ridges that could channelize flowing landslide debris or surface runoff. Areas of hummocky ground, reverse scarps, tension cracks, and impeded drainage should also be recorded. These may offer clues to areas where sliding movements have occurred. (g) Channel/Stream Features - Where possible, particular attention should be paid to stream beds that could act as channels for channelized debris flows. The roughness of the stream bed, as represented by the presence or absence of vegetation, bedrock or boulders, should be recorded. Local steep reaches and waterfalls should be noted. The steepness and character of the channel banks and their potential to confine and channelize debris should be established if possible. A note should be made of any material in the stream bed and on the banks that could be
- 32 eroded and entrained in a debris flow. (h) Anthropogenic Features - A number of anthropogenic features may occur on natural terrain in Hong Kong and they are easy to miss from interpretation of aerial photographs. They may include walking tracks, abandoned agricultural terraces, traditional village graves, military fortifications and overgrown trenches and adits left from previous small scale mining (Figure 1). (i) Erosion - Unvegetated areas where sheet or gully/rill erosion occurs are present in some parts of Hong Kong, mainly on granite bedrock. Eroded areas, bare of vegetation, were more common in the past and because much revegetation has occurred in recent years, the relevance of historical information should not be discounted. Where erosion has resulted in steep scarps or undercutting of boulders, these may form landslide sources and should be mapped as such, even if the surface has become revegetated. (j) Vegetation - Vegetation patterns should be mapped and may provide information on ground or surface water flow, and past erosion and instability. High lush vegetation cover may indicate areas of shallow groundwater table or groundwater discharge; linear strips of low vegetation oriented down slope may indicate overgrown landslide trails. As hill fires are common in Hong Kong, care should always be taken to relate the current vegetation pattern to past conditions. 3.4 Collection of Field Data 3.4.1 Introduction Field data collection is an integral part of any NTHS and can be broadly differentiated into site reconnaissance and detailed field mapping. The field work, anticipated to be predominantly mapping and site measurements, should be of sufficient detail to validate the findings of the API and to enable a well reasoned assessment of any natural terrain hazards in the catchment. Ground investigation, including vegetation clearance for access and excavation of trial pits, may be required to supplement the field mapping. 3.4.2 Site Reconnaissance For site reconnaissance, it is essential to find a location where an overview of the site, and preferably the whole study area, can be gained. This is generally from a hill or tall building close by. Photographs, in some cases oblique stereo pairs, can be taken to illustrate site conditions for field mapping in the next stage and for further analysis in the office.
- 33 Following the overview, the whole upslope site boundary of the site should be traversed, as this is the location where any landslide debris would impinge upon it. Any materials along the boundary, such as colluvium or boulder deposits, that could be debris from past landslides should be noted. For known recent landslides, particularly those that reached the site boundary, an initial inspection to determine the slope conditions, slope processes and material involved should also be carried out at this stage. Careful observation of the local topography along the site boundary and consideration of any proposed site formation levels should also be made. This can be one of the most important aspects in identifying any possible hazards to a site. If the development is located higher than the adjacent ground or potential debris transport paths such as streams and channels, then the hazard from any postulated event is likely to be relatively low. The final proposed site formation level needs to be considered when making these judgements. 3.4.3 Field Mapping and Site Measurement Although the API will have determined the range of features associated with any natural terrain hazards in the study area, these features (as indicated in Table 6) should be confirmed and quantified further during the field mapping. Additional features as shown in Table 7 may be recorded. The mapping may be required to extend outside the catchment of concern in order that the geological and geomorphological setting is fully understood. The contour interval of 1:1 000-scale topographical maps is 2 m and the topography shown usually bears a closer resemblance to the ground conditions than that on smaller-scale maps; therefore, this scale is the most useful for field work where reliable positioning is essential. However, locating specific features on a broad, vegetated catchment can be difficult and it may be necessary to place surveyed markers across the study area or to use a Global Positioning System. Where access is restricted by dense vegetation, emphasis should be placed on examining features relevant to the potential magnitude of natural terrain hazards. Specific features to target for detailed field examination will come from the detailed API of the study area and its environs. The location of all traverses undertaken during the field mapping should be marked on the map. The field work should enable relevant information (Table 7) to be collected on all accessible landslide scars, drainage lines and hollows, regolith (e.g. rock outcrops, boulders and boulder fields, soil exposures; see Section C.4 of Appendix C), seepages and stream flows, open slope and drainage line morphologies, and vegetation types and densities. Traverses should be made both across the open hillside and along important drainage lines, to examine the nature and extent of superficial materials, and to determine areas that may be susceptible to failure during heavy rainfall. Traverses along drainage lines should examine their characteristics, such as channel width and height, slope gradient, and channel bed materials. The level of detail should be sufficient to assess the likely travel paths, velocities, depths and transport distances of any failure debris that enters the drainage line. Information about past landslides should be collected, including measurements of the recent and relict source areas, such as their depth and length, and width of the main scarps for volume estimates. Where possible for recent landslides, materials displaced from the source areas and their degree of disaggregation should be described. In addition, characteristics of
- 34 debris trails should be determined, including their length, width and processes of transport and deposition, and related erosion. Other significant observations that should be made, if possible, to determine the cause and mechanisms of recent landslides and their implications for future landslide hazards. These may include: (a) super-elevation of debris trails, based on vegetation flattening, erosion scars, marks on trees, and/or characteristics of materials deposited in levees (including thickness and volume of materials) for estimating velocity of debris flows (Johnson & Rodine, 1984), (b) Evidence of scour within stream beds or scars on channel sides to determine whether entrainment of additional debris from within the debris trail has occurred, and (c) the location and character of tension cracks above the main scarp to establish the possibility of future landslide hazards. An examination of rock mass characteristics should be made at all accessible bedrock outcrops. Where pertinent, rock type, material weathering grade, measurements of joint attitude and spacing, geological features that could affect stability (e.g. bedding, eutaxitic fabric, clay layers, metamorphic texture), and seepage locations and flow rates should be recorded. The character of the weathering profile in stream sections, landslide scarps or rock outcrops should also be recorded, as should descriptions of colluvial materials if present. Where boulder falls are considered to be a hazard to the development, mapping of boulder fields should be at sufficient detail to allow an assessment of the locations and sizes of all boulders that are considered to pose a significant risk to the site under consideration. Potentially unstable boulders should be identified on site, particularly if in-situ stabilization is to be adopted for hazard mitigation. The likely travel path of such boulders in the event of failure should, where possible, also be assessed in the field. The field work should also attempt to distinguish between rock outcrops and individual corestones/boulders. In order to estimate the likelihood of boulder fall initiation and subsequent movement, information on boulder type, boulder shape, boulder volume, boulder location (travel distance to development site/boundary), local slope angle, embedding material, slope angle, and vegetation density (Maunsell & ERM, 1999) are useful. 3.5 Geological and Geomorphological Model Based on the information collected and interpreted, a geological and geomorphological model of current and past processes in the study area should be developed. This will form a basis for assessing the likely sources, nature and volume of any instability that could affect the site. It is beneficial to look beyond the catchment boundary so as to observe processes and events in adjacent or similar catchments that may be relevant for determining possible hazards to the site.
- 35 The geological and geomorphological model should be specifically designed for each assessment and will combine the API with the engineering geological mapping. Historical landslide data often provide useful information for development and calibration of the model. Fookes (1997) notes that “The strength of the geological model is in providing an understanding of the geological processes which made the site. This enables predictions to be made or situations anticipated for which explorations need to be sought in the geological materials, geological structure and the ancient and active geological processes in the area”. Mass movements have displaced much of the weathered mantle on hillslopes, forming extensive colluvial deposits on the shallower slopes, and large debris sheets and lobes on the steeper slopes and in the steep tributary valleys. Recent scars can be identified either in the field or on aerial photographs, a record that covers most of the territory from 1945 to the present. On the other end of the spectrum, large relict scars, which have a degraded morphology, probably extend the historical record to thousands of years. In the context of an NTHS, the geological/geomorphological model is useful for assessing the types of landslide hazard that may occur, the likelihood of their occurrence, and the possible size of failures. 3.6 Design Event 3.6.1 Overview A design event refers to the magnitude of the hazard model that is selected to be mitigated against as part of a development. As such, it requires information about the proposed layout of the site. If the Design Event approach is adopted, the relevant design requirements (i.e. worst credible event or conservative event) set out in Section 2.4 should be followed. If the QRA approach is adopted, the design event is normally determined by reference to risk guidelines (ERM, 1998). In both cases, the determination of the design event requires professional judgement. The geological/geomorphological model and design requirements (i.e. conservative or worst credible event) for individual identified hazards obtained from the technical framework (Section 2.4), and subsequently refined with field information from the NTHS (Section 3.4), should be used to determine the design event for each hazard model in each sub-catchment. The design event needs to be defined in terms of the volume, velocity (or impact load) and nature of the landslide debris (or boulder) at the site boundary, or any other upslope location where construction of mitigation measures might be proposed. 3.6.2 Determining the Design Event The design event can be derived in several ways, largely dependent on the nature of the natural terrain hazards. For the more frequent, shallow failures it may suffice to determine the design event based on historical data whereas for the infrequent, large events it requires the development of an appropriate geological and geomorphological model as well as the exercise of professional judgement. Some typical means of assessing the design events are given as follows:
- 36 (a) Open hillslope landslide - Most natural terrain landslides in Hong Kong are relatively shallow open hillslope failures. They are well represented by historical data, such as from landslide records, aerial photograph interpretation and field mapping. As a result, historical data usually provide a good basis for assessing the frequency, mechanism and scale of such landslides. Given the relatively large amount of data available, development of site-specific landslide susceptibility models may be practical in some cases (e.g. Parry, 2001). Furthermore, the historical landslide data may also be used to construct the catchment-specific frequency-magnitude relationships in selecting the source volume for a design event. Section 3.6.3 summarizes this approach that has been adopted for a number of cases in Hong Kong. (b) Deep-seated slide or large scale (area extent) shallow failure - Such landslides are less frequent, and may not be well represented by historical landslides at the site. Hence, correlation by reference to historical landslide data could be of limited use. Consideration of the geological and geomorphological model, as well as any geomorphological evidence of relic events, is essential to assessing the likelihood of occurrence of such failures. Historical landslides in the vicinity of the site, and in other places with a similar geological/geomorphological setting, could also give insight to evaluating whether factors contributing to development of such failures are present at the site. For example, well documented large failures include the Tsing Shan debris flow involving colluvium infilled gullies on steep terrain (King, 1996a) and the quasi-natural terrain Shum Wan Road landslide in thick weathering profile related to fault and weak clay seams (GEO, 1996). The designer should be aware of the geological environments in which these events have occurred, and make a comparison with that of the study area. Given the relatively sparse data for this type of infrequent, large magnitude events, a catchment-specific cumulative frequency-landslide volume relationship is likely to be poorly defined. An attempt to circumvent this limitation using landslide data from similar geological settings to extrapolate a catchment-specific frequency-volume relationship (Section 3.6.4; see also Atkins, 1998; Lo, 2000). was described in Tse et al. (1999). However, it is essential that the extrapolation is compatible with the geological and geomorphological setting of the catchment concerned.
- 37 (c) Channelized debris flow - This type of failure has several distinctive characteristics that deserve special consideration (Section 3.6.6). Assessment of the magnitude of the design event involves consideration of the possibility of multiple landslide sources, entrainment along travel path, and development of dam break scenario. Like deep-seated slides, determination of the design event for a channelized debris flow requires the establishment of an appropriate geological and geomorphological model, and the exercise of professional judgement. For example, API and field mapping can identify superficial deposits along drainage lines and hence provide useful indications of probable entrainment material and its likely volume. 3.6.3 Catchment-specific Landslide Data This approach makes the assumption that past events in the study area and its relevant vicinity give an indication of the potential for future events. The approach has been used in the assessment for the Foothills Bypass, Tuen Mun by Scott Wilson Kirkpatrick (1996), Tsing Yi North Coastal Road by Lam (1999), Sham Tseng San Tsuen by Maunsell (2000) and Shek Pik School and Hostel by Parry (2001). The first case was note-worthy for the presence in the study area of the largest recent debris flow recorded in Hong Kong (King, 1996a). It was taken to be a reasonable representation of the more extreme events that could occur in the study area. Records of the date and source dimensions of past landslides in the study area can be used to construct local frequency-magnitude relationships that are useful in selecting an appropriate source volume for a design event. Such catchment-specific data should also be considered in relation to the rainfall history of the study area. A basic problem of using historical landslide records to help assess future hazards is that catchment-specific data may not be entirely representative of all events that could occur at that location. The smaller the catchment area and the more limited the available geological information is, the greater the uncertainty becomes. Catchments with a past landslide history are likely to reflect better the future occurrence of landsliding although even these may not include evidence of more extreme events. Therefore, it may be useful to consider data from wider regional studies. However, care needs to be taken to ensure that only data with a similar geological/ geomorphological setting to that being assessed is used. 3.6.4 Regional Landslide Data Regional landslide data can be used to check that a design event estimated from a catchment-specific geological model is reasonable in the context of a wider population of past events from similar events and similar environments. Regional data may also be the only logical basis for determining a design event where catchment-specific landslide records and data are insufficient or not available. It may be possible to extend relationships established from catchment-specific data by incorporating regional data. Where the two sets of data
- 38 give comparable results, this would lend confidence to the determined design event. Sources of regional data on landslides in Hong Kong include the NTLI, which is a database of some 27 000 landslides identified on high altitude aerial photographs (Emery, 1996; King, 1999) and the Large Landslide Dataset which contains nearly 2 000 features identified on low altitude aerial photographs (Scott Wilson, 1999b, 1999c). Compilations of field data from landslide area studies are reported in Franks (1998) and Wong et al. (1998). An overview of some landslides in Hong Kong and elsewhere, and their use in selecting a design event, is given in Atkins (1999b). A number of individual landslide case studies have also been carried out for natural terrain landslides in Hong Kong (see Appendix B). Again, care should be taken to ensure that the datasets used are from similar geological/ geomorphological settings. 3.6.5 International Data Review of data and methods used in other countries can also be a useful aid when determining a design event. For example, methods of quantitative analysis of debris flows used in Canada are reported in Hungr et al. (1984) and VanDine (1985). These are further developed in work carried out at the Cheekye Fan in British Columbia, described in Hungr and Rawlings (1995) and Sobkowicz et al. (1995). The relationship between angle of reach and landslide volume is presented for worldwide data by Corominas (1996). Some methods for predicting design event parameters in Hong Kong and elsewhere are given in the Proceedings of the Seminar on Planning, Design and Implementation of Debris Flow and Rockfall Hazards Mitigation Measures (AGS & HKIE, 1998). 3.6.6 Special Consideration for Channelized Debris Flows If the study has determined that the catchment contains drainage lines that have the characteristics to channelize debris flows, then the possibility of multiple landslide sources, the potential entrainment of eroded debris along the travel path, and the probable development of a dam-break scenario need to be considered. These phenomena can result in a significant increase in the total volume of the landslide debris and/or a significant increase in the debris mobility. For drainage lines that are considered susceptible to channelize debris flows, the debris yield rate (volume of eroded material per linear metre) of each drainage line should be determined. This will typically be carried out by reference to the thickness, characteristics and distribution of the materials present within the sections of the drainage line susceptible to erosion and entrainment of debris. Debris yield rate is affected by the characteristics of the stream course. Even for the same stream course, debris yield rate can be affected by the volume and mechanism of the source failure. In Hong Kong, debris yield rates for North Lantau were estimated to be 2.3 m3/m to 3.7 m /m, for landslide source volumes of about 450 m3 to 1 500 m3 and where the average stream bed gradients were in the range of 20° to 30° (Franks, 1998). Wong et al. (1998) estimated that the total debris volume for some landslides at South Lantau could be three 3
- 39 times the source volume. King (1996a) reported a debris yield rate of up to 24 m3/m for the Tsing Shan debris flow in 1990, with a source volume of about 3 000 m3 and mean stream bed gradients of 22°. For those sections of a drainage line that have exposed bedrock in the stream bed, debris yield rates are likely to be much lower. For example, the debris yield rate of the Sham Tseng San Tsuen debris flow was estimated to be negligible (Fugro Maunsell Scott Wilson, 2000a). Halcrow (2001) documented the formation of a debris dam along a drainage line, some 30 m below the source area of a landslide above Leung King Estate, Tuen Mun. The dam, represents the initial debris lobe resulting from the failure, had a travel angle of about 27°. Subsequently breaching of the dam, presumably due to damming of water behind the debris lobe, resulted in the formation of a wet channelized debris flow with a travel angle of about 17°. This case demonstrates that debris mobility can be significantly increased in a dam break scenario. 3.7 Run-out Assessment Where the probable sources for future landsliding have been identified, the potential run-out of landslide debris from them should be estimated in respect of the required design event, i.e. a worst credible event or a conservative event. Determination of the debris travel distance is an important component in assessing if the hazards may affect the site. A reliable run-out assessment requires information on the likely failure volume, characteristics of the travel path and the momentum or energy of the moving debris which is related to the velocity and density of the debris. It should be assessed either with respect to the site boundary, or other suitable reference point critical to the consideration or mitigation of the hazards (e.g. check dam or boulder fence). Catchment-specific data on the extent of debris trails of past landslides can be used if available. However, it is often difficult to map the extent of past landslide debris from aerial photographs and such debris may be difficult to recognize on the ground after several years. Colluvial deposits provide some guidance as to the possible reach of past landslide debris but the possibility that they were deposited under a different climatic and geomorphological regime than today must be considered. Mobility of landslide debris, in terms of the distal point of debris deposition, was discussed by Lau and Woods (1997). Lo (2000) presented a comprehensive review of various approaches for debris run-out assessment and suggestions for assessing debris mobility and debris impact loads in the design of landslide debris-resisting barriers. 3.8 Presentation of Results The NTHS should establish the following: (a) Geological/geomorphological model for the area with particular reference to existing and potential landslide locations.
- 40 (b) Hazard models and their respective design requirements based on the detailed study. (c) The location, magnitude, and run-out distance of each specific hazard model that may affect the site or other relevant facilities (e.g. check dam). (d) Whether measures are required to mitigate against the specific hazards that may affect the site. In the report, the study area should be described in the context of its regional setting with specific regard to the current and/or proposed development of the site and its topography, geomorphology, geology, drainage and rainfall characteristics. The limitations of the study (e.g. access problems, limited aerial photograph coverage) should be stated. The report should include the scope and methodology adopted and assumptions used to establish items (a) to (d) above. For cases where mitigation measures are required, sufficient detail for formulation of mitigation strategy (Section 4) should be given. Adequate photographs, tables, figures and drawings, including cross-sections, should be used. The following illustrations are suggested as part of the report: (a) Cross-sections along each potential debris travel path from identified natural terrain hazard source locations to the site boundary. (b) An Engineering Geology Map of the study area at an appropriate scale (e.g. 1:1 000) showing: (i) the solid geology and regolith, (ii) geomorphological setting, (iii) the locations of exposures of soil and rock, annotated where necessary with key information about the nature of the materials (e.g. stereonet analyses), (iv) sites of past instability (e.g. landslide scars, boulder fall locations), annotated with key information about the nature of the instability, (v) catchment boundary and drainage courses with comments regarding any areas of concentrated surface water flow or seepage, and (vi) the location of field traverses. (c) A Natural Terrain Hazard Map of the study area at an appropriate scale (e.g. 1:1 000) showing:
- 41 (i) potential landslide sources, for open hillslope landslide, deep-seated slide or channelized debris flow, annotated for easy reference in the report, (ii) potential boulder fall and/or rock fall locations, (iii) open hillslope landslide paths from identified potential landslide sources with the likely distance that debris will travel, (iv) channelized debris flow paths with their likely maximum extent, and (v) boulder/rock fall trajectories with the estimated distance that boulders and rock blocks will travel. Where mitigation measures are required, the report should clearly present the rationale for the mitigation strategy formulated to reduce the risk to the development. The general types and layout of the measures should be shown on a Mitigation Measures Strategy Map at an appropriate scale (e.g. 1:1 000 or 1:5 000). If in-situ stabilization measures at the source area are recommended, their locations should be marked on the map. This is particularly applicable to boulder and rock falls hazards where the exact location of the hazards should be indicated on a map of 1:1 000 scale or better. The report should recommend a programme, including the scope and frequency for maintenance inspections, and should list the parties responsible for carrying out the inspections and maintenance works. 3.9 Stages of the Natural Terrain Hazard Study 3.9.1 Introduction For large scale projects, such as site formation and highway projects (GEO, 2000a), it may be beneficial to divide the study into sequential stages (review, assessment and mitigation strategy), in increasing details, that meet the requirements for different phases of the projects from land use planning through engineering feasibility study to design stage. However, for projects that are relatively straightforward, a staged approach is unlikely to be necessary. Should project proponents opt to undertake an NTHS in stages, consideration may be given to splitting the study into stages (Figure 3) as described below. 3.9.2 Review This first level of study is a desk and reconnaissance level study to identify in broad terms, any natural terrain hazards that might affect the proposed development. The review is to decide whether further assessment will be necessary, and also to identify whether the hazards are so significant that the site is deemed uneconomic to develop. If the review identifies significant hazards, the study should progress to the next stage. This process is generally carried out at the earliest possible stage of a project, such as Building Plan stage for private developments, or as part of the feasibility study or preliminary
- 42 design for public works projects. Consideration should also be given at this stage of the broad costs that may be involved for future financial estimates. A natural terrain hazard review should include the following aspects: (a) desk study of available information (Section 3.2), (b) detailed interpretation of all available aerial photographs (Section 3.3), and (c) site reconnaissance (Section 3.4.2). At the end of the review stage, the study should conclude the following: (a) Whether further study is required or whether the proposed development is considered not feasible. (b) If further study is required, the recommended design approach and the design requirements for each type of hazard models for each sub-catchment above the proposed site should be provided. If the Design Event approach is to be followed, the design requirements (i.e. conservative or worst credible event) should be assessed in accordance with the procedures given in Section 2.4. The indication of the design requirements provides information about the probable scale of the natural terrain hazards posed to the proposed development and relative to the site boundary (Section 2.2). This would give the designer the opportunity to consider reducing the hazards through adjusting the internal layout of the critical facilities at an early stage of the development. The results of the review should be presented in a report that summarises all the geological and geotechnical information about the study area and, in particular, the results of the aerial photograph interpretation. Sufficient text should be given to explain the reasoning and the methodology adopted for the review and to justify the conclusions made. The report should be supported by relevant photographs, an aerial photograph interpretation map using 1:1 000 or 1:5 000-scale topographical base maps, and representative cross-sections. 3.9.3 Assessment The assessment requires a detailed field study of the natural terrain, sufficient to determine the location, type and magnitude of any hazards. Quantifying the design event from the catchment above the site can be carried out when the detailed design and layout of the development have been decided. The assessment will establish whether the risk necessitates mitigation measures. This process is generally carried out at the design stage for both private and public works projects.
- 43 A natural terrain hazard assessment should include the following aspects: (a) all items covered in a natural terrain hazard review (Section 3.9.2), and (b) detailed field mapping (Section 3.4.3). At the end of the assessment stage, the study report should cover all the items (excepting the mitigation strategy) given in Section 3.8 above. 3.9.4 Mitigation Strategy The final stage, which is to formulate a suitable mitigation strategy (Section 4), may be needed once the assessment stage is complete. The NTHS may not proceed through to this stage because as more data are obtained about the site, it may become possible to establish that mitigation measures are unnecessary. This process is generally carried out at the design stage for both private and public works projects. At the end of the mitigation strategy stage, the study report should provide the mitigation strategy formulated for reducing the risk to the development, with all the supporting rationales. 4. NATURAL TERRAIN HAZARD MITIGATION STRATEGY 4.1 Introduction Upon determination of the design event, the risk to the development can be assessed and an appropriate mitigation strategy formulated if the risk has to be reduced. Mitigation measures can either be “passive”, for example by re-locating a development site beyond the area considered at risk and so avoiding the hazard, or “active”, involving the construction of measures to reduce the effect of the anticipated hazard and therefore the risk to the development. Active mitigation measures can be further subdivided into preventive and protective. The former may include in-situ stabilization (at source areas) and the latter commonly involves defensive mitigation works in the travel path or within the site to retard, store or deflect failure debris. 4.2 Typical Mitigation Strategies 4.2.1 Review Franks and Woods (1997), Greg Wong & Associates (1999) and Lo (2000) have reviewed typical mitigation measures and their applicability to Hong Kong conditions.
- 44 4.2.2 Passive Mitigation Strategies Passive mitigation strategies involve precluding development in areas that may be affected by severe natural terrain hazards. This may be accomplished by land use planning, statutory regulations and development moratoria. The Mid-Levels construction moratorium was introduced in 1973 to safeguard the stability of slopes in the district. In some cases, mitigation can be achieved by closure or relocation of facilities at risk, provision of suitable buffer areas on the upslope side of the development site or adjacent to drainage lines, or erection of warning signs. The option of adopting passive mitigation measures should be considered at an early stage of the project so as to ensure that any necessary arrangements can be accommodated. 4.2.3 Active Mitigation Strategies Active mitigation strategies commonly include stabilization in the source area, and energy dissipation, deflection and containment measures along the travel path, or in the deposition area. The following active mitigation strategies that may be applicable for the main hazard models in Hong Kong are extracted primarily from Franks and Woods (1997): (a) Open Hillslope Landslide (i) Source - afforestation, surface drainage, surface protection, shallow subsurface drainage, slope regrading and soil nailing. (ii) Deposition - energy dissipators, deflectors and containment structures to induce deposition of debris away from the elements at risk. (b) Deep-seated Slide (i) Source - afforestation, surface drainage, surface protection, subsurface drainage, buttresses, slope regrading and soil nailing. (ii) Deposition - not relevant for this hazard model. (c) Channelized Debris Flows (i) Source - as for open hillslope landslides, and also including channel works, such as revetment works and spur dykes, to prevent production of debris due to erosion of the stream bed and banks, and slope stabilization measures. (ii) Deposition - dykes, check dams, deflection walls, sacrificial baffles and containment basins built along the path and in the depositional area to dissipate energy and
- 45 allow containment of debris. (d) Boulder and Rock Falls (i) Source - excavations, shotcrete, rock bolts, dowels, anchors, buttresses and wire catch nets to stabilize rock fragments. (ii) Deposition - catch fences, deflection walls and catch ditches along the path and in the deposition area to mitigate the effects of boulder and rock falls. 4.3 Hong Kong Practice Lo (2000) provides a review of the types of debris- and boulder-resisting barriers deployed in Hong Kong. These include rock/boulder fences, gabions, reinforced concrete retaining walls, earthfill berms and check dams. Boulder fences, gabions and concrete walls have been used to arrest loose boulders; reinforced concrete retaining walls have been employed as debris-resisting barriers. Some of these examples are shown in Figure 4. Besides the typical debris-resisting structures, some alternative measures that have been successfully implemented are given below for reference: (a) Adjusting facility layout - This can be an effective action for a large site. It is generally possible to avoid or reduce the exposure to natural terrain hazards by siting the critical facilities (e.g. residential blocks) away from the hillside. (b) Making use of natural barrier - During site formation planning stage, the possibility of making use of natural features (e.g. existing ridge) as a barrier for landslide debris should be considered. (c) Delineating non-building area - In some cases, it may be possible to delineate the most hazardous part of the site as no-build zone. (d) Drainage provision - For sites intersected by natural drainage courses that may act as a flow path for channelized debris flow, it may be possible to deal with the hazard by having adequate drainage provisions such as debris basins and boulder straining structures to intercept, retain and/or convey the debris away form the site. (e) Elevated platform - For a narrow site, there may be little room to accommodate debris barriers. A possible option is to have the superstructure built on an elevated platform so as to allow debris to pass underneath the building.
- 46 (f) Integrated structure - In some cases, it may be possible to incorporate debris-resisting wall into the design of the permanent structure. This is particularly appropriate for development sited very close to hillside. (g) Potentially Hazardous Installation (PHI) - For erection of PHIs (e.g. petrol filling station), a QRA is normally required. Natural terrain landslide hazard risk, being one of the risks that the facility would be exposed to, should be taken into account as part of the QRA. This eliminates duplicating effort in undertaking a separate natural terrain hazard study. 4.4 Environmental Considerations 4.4.1 Introduction The majority of natural terrain landslides occur in areas where humankind has had little or no influence. As a result both mitigation and remedial works as applied to natural terrain should consider landscape aesthetics in that the measures should blend with their surroundings as far as practicable. Although both Works Bureau Technical Circular (WBTC) No. 25/93 and 17/2000 specifically apply to the design and maintenance of man-made slopes, their principle of minimizing visual impact is also relevant to natural terrain. Emergency landslide repairs are however exempt from both circulars. For any mitigation works carried out within the Country Parks or in environmentally sensitive areas, or for any works classified as designated projects under the Environmental Impact Assessment (EIA) Ordinance, the potential environmental impacts of these works should be assessed. If found necessary, appropriate monitoring and/or measures should be implemented to ensure that the impacts to the environment are kept to an acceptable level. 4.4.2 Bio-engineering Bio-engineering has been defined in Hong Kong (GEO, 2000b) as “the use of living vegetation, either alone or in conjunction with non-living plant material and civil engineering structures, to stabilize slopes and/or reduce erosion; and the use of any form of vegetation, whether a single plant or collection of plants, as an engineering material (i.e. one that has quantifiable characteristics and behaviour)”. In Hong Kong, only simple bio-engineering techniques in the form of vegetation cover have been used. However, more sophisticated techniques have been used in other countries and information on these techniques is contained in GEO (2000b). Natural terrain landslides occur as part of the on-gong, natural processes of erosion and landform evolution. Therefore, aesthetic reasons alone may not be sufficient to qualify a natural terrain landslide scar for revegetation. However, where landslide reactivation is likely, or where the landslide debris may be remobilized, or deterioration of the conditions of the hillsides needs to be minimized, revegetation may form an integral, cost-effective part of any mitigation proposals. Bio-engineering may be adopted in reparation of hillsides affected by landslides or hill fires so as to control surface erosion and soil loss, and to prevent further
- 47 deterioration of the hillsides. There is also scope for application of bio-engineering as mitigation measures to reduce the consequence of natural terrain landslides by partial retention of coarse debris or to reduce entrainment by stabilizing loose deposits Most natural terrain landslides in Hong Kong are relatively shallow and commonly occur along geological/hydrogeological boundaries such as the colluvium/saprolite interface. The topsoil and other suitable growth media are commonly removed and natural revegetation can be slow. Furthermore, natural terrain landslides can take many forms and hence they result in a wide range of soil damage. Therefore, each location needs to be carefully analysed to determine the objective of the bio-engineering application and the most appropriate techniques for that site. Experience gained from application of vegetation cover to cut slopes in Hong Kong may be relevant to natural terrain. Characteristics of various plant species commonly used in Hong Kong as surface protection are summarized in the Geotechnical Manual for Slopes (GCO, 1984). The General Specification for Civil Engineering Works (Hong Kong Government, 1992) provides specifications and guidance on landscaping works and establishment. Wong et al. (1999) noted that it is essential to provide both erosion protection mats and a biodegradable protection fabric to reduce erosion and protect the seeds while the vegetation establishes. Requirements for plant growth, typical plant mixes and prescriptive and potential landscape treatments for soil cut slopes are given in GEO (2000b). The recent use of landscaping and bio-engineering of cut slopes is summarized in Martin (2001). As natural hillside is generally remote, consideration should be given to future access requirements for maintenance of the mitigation works. If planting of vegetation is to be used, special consideration should be given to use of watering and maintenance free plant species. While there is little experience in Hong Kong on revegetating landslide scars on natural terrain, there is considerable experience in other places (e.g. Ministry of Forests, 1997). It may be possible to adapt some of their practice with suitable modification to cater for the Hong Kong climate and the natural terrain conditions. 4.4.3 Visual Impact of Mitigation Measures In addition to bio-engineering approach, the visual impacts of any hard mitigation measures, such as deflection barriers, should be considered. Where possible visual intrusion should be reduced, for example, through plantation of vegetation screens to hide the facilities or blending of the measures with the natural environment. 5. MAINTENANCE OF MITIGATION FACILITIES 5.1 Introduction Maintenance requirements for the mitigation measures should be proposed by the designers. Besides the maintenance schedule required for the works, preliminary estimates of the related capital and recurrent costs should be prepared where necessary. The proposed maintenance requirements follow the principle that the maintenance agent’s work is confined
- 48 to maintaining the physical integrity and continued functionality of the Measures. The maintenance agent is neither required to maintain the natural hillside, nor to review the adequacy of the Measures provided. However, special cases that warrant more stringent maintenance requirements would be treated separately such as by imposing “Green-hatchedblack” geotechnical clause. 5.2 Types of Natural Terrain Hazard Mitigation Measures Natural Terrain Hazard Mitigation Measures are classified into two categories for registration in the Slope Catalogue. They are: (a) Stabilization Measures constructed on the natural hillside to prevent failure, e.g. boulder buttresses, soil nails, raking drains and retaining walls. (b) Defense Measures to contain landslide debris or boulder fall from the hillside above, e.g. check-dams, earth bunds and boulder fences. 5.3 Maintenance Requirements for Mitigation Measures Natural hillsides do not require maintenance. Natural Terrain Hazard Mitigation Measures do not normally result in substantial modification to the geometry and conditions of the natural hillside. Where this is the case, the purpose of maintenance is confined to ensuring the physical integrity and continued satisfactory function of the Measures. Where stabilization measures have been adopted and the hillside has not been substantially modified by the Measures, the hillside concerned does not require maintenance. Likewise, defense Measures are designed to contain landslide debris or boulder falls, and the hillside where landslides or boulder falls may occur does not require maintenance. The general principles given in Geoguide 5 (GEO, 1998), such as the recommended good practice in maintenance management and personnel requirements, and attention to access and safety precautions, are applicable to maintenance of Natural Terrain Hazard Mitigation Measures. However, since the Measures are of different nature from man-made slope features, their technical requirements for maintenance differ and these are set out below. Routine Maintenance Inspections on stabilization measures and defence measures should be carried out annually by maintenance staff at the rank of Assistant Clerk of Works, Technical Officer or Work Supervisor, to identify any maintenance works required. The inspection should cover the Measures, and the area containing the Measures and the adjoining ground. The inspection should be completed well before the onset of the wet season. This would allow sufficient time to carry out the necessary routine maintenance works, which may typically include: (a) clearance of debris from drainage channels, catch trenches and pits, containment basins and straining structures,
- 49 (b) repair or replacement of damaged sections, (c) unblocking of weepholes and outlet drain pipes, (d) removal of any vegetation causing severe cracking, (e) repair or reinstate the ground adjoining the Measures if affected by severe erosion, and (f) other routine maintenance works to upkeep the integrity and continued function of the Measures. In cases where unusual conditions or problems are observed, e.g. check dam filled with a large amount of landslide debris or significant movement observed at boulders supported by buttresses, advice should be sought from a professional geotechnical engineer. In line with the recommendations of GEO Report No. 56 (Wong et al., 1999) raking drains used as prescriptive measures should not be considered as “Special Measures” as defined by Geoguide 5. Regular inspections and routine maintenance of the raking drains should however be carried out. Unless specified otherwise by the designer or by special provisions (e.g. delineation of “Green-hatched-black” area), Engineer Inspections for Maintenance are not required for stabilization measures and defence measures in view of the following: (a) Unlike in the case of maintenance of man-made slope features, the natural hillside itself does not require maintenance. Hence, the guidelines set out in Geoguide 5 on Engineer Inspections for man-made slope features are not applicable to natural hillside, and are outside the scope of work required of the maintenance agent. (b) The maintenance works required for stabilization measures and defence measures are relatively simple, and do not normally require input from a professional geotechnical engineer. If stabilization measures were adopted and the hillside was substantially modified by the Measures (e.g. major regrading), then the hillside has been turned into a man-made slope feature. In such case, it should be registered as a man-made slope feature and maintained in accordance with the guidelines given in Geoguide 5. 5.4 Other Measures In some circumstances, dealing with natural terrain landslide hazards may involve the use of other mitigation measures that do not require registration in the Slope Catalogue. Examples include:
- 50 (a) provision of a buffer zone (e.g. an open space) between the hillside and developed facilities, and (b) incorporation of debris basins, sand traps, etc. as part of the drainage facilities. Unless specified otherwise by the designer, there are no maintenance requirements for such measures from the geotechnical point of view, apart from regular clearance of debris. Re-vegetation may be carried out in a prescriptive manner to repair hillsides that are affected by landslides, hill fires, etc. The vegetation species to be adopted in such circumstances should be maintenance free. Where special bio-engineering measures are adopted for mitigation of natural terrain hazards, the maintenance requirements should be specified by the designer. 6. REFERENCES AGS & HKIE (1998). Proceedings of the One Day Seminar on Planning, Design and Implementation of Debris Flow and Rockfall Hazards Mitigation Measures. The Association of Geotechnical Specialists (Hong Kong) Limited and The Hong Kong Institution of Engineers (Geotechnical Division), 27 October 1998, Hong Kong. Anon. (1972). The Preparation of Maps and Plans in terms of Engineering Geology: Report by the Geological Society Engineering Group Working Party. Quarterly Journal of Engineering Geology, Vol. 5, pp 295-382. Atkins China Ltd. (1998). Geotechnical Assessment Fanling Area 49A Phases 1 & 3 Natural Slope, Vol. 1. Report to Hong Kong Housing Authority, Hong Kong, 88 p. Atkins China Ltd. (1999a). Geotechnical Assessment for Natural Slope Adjoining Tsing Yan Temporary Housing Area, Tsing Yi. Report to Housing Department, Hong Kong. Atkins China Ltd. (1999b). Scoping Study for a Global Quantitative Risk Assessment of Natural Terrain Landslides in Hong Kong. Report to Geotechnical Engineering Office, Hong Kong, 51 p. Atkins Haswell (1995). Quantitative Landslide Risk Assessment for the Squatter Villages in Lei Yue Mun. Report to Geotechnical Engineering Office, Hong Kong, 37 p. plus 4 drgs. Au, S.W.C. & Chan, C.F. (1991). Boulder treatment in Hong Kong. Selected Topics in Geotechnical Engineering - Lumb Volume, (Edited: Li, K.S.), pp 39-171. Brand, E.W. (1996). Bibliography on the Geology and Geotechnical Engineering of Hong Kong to March 1996. GEO Report No. 50, Geotechnical Engineering Office, Hong Kong, 111 p.
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Discussion
King, J.P. (1999). Natural Terrain Landslide Study - The Natural Terrain Landslide Inventory. GEO Report No. 74, Geotechnical Engineering Office, Hong Kong, 127 p. Lam, A.Y.T. (1999). Tsing Yi North Coastal Road - Natural Terrain Hazards and Mitigation Measures. Advisory Report ADR 1/99, Geotechnical Engineering Office, Hong Kong, 6 p. plus 2 Appendices. Lau, K.C. and Woods, N.W. (1997). Review of Methods for Predicting the Travel Distance of Debris from Landslides on Natural Terrain. Technical Note TN 7/97, Geotechnical Engineering Office, Hong Kong, 48 p. Lo, D.O.K. (2000). Review of Natural Terrain Landslide Debris-resisting Barrier Design. GEO Report No. 104, Geotechnical Engineering Office, Hong Kong, 91 p. Lo, D.O.K. (2001). Interim Review of Pilot Application of Quantitative Risk Assessment to Landslide Problems in Hong Kong. Technical Note TN 4/2001, Geotechnical Engineering Office, Hong Kong, 69 p. Martin, R.P. (2001). Panelist Report - Landscaping and bio-engineering of slopes in Hong Kong. Geotechnical Engineering Meeting Society’s Needs, Proceedings of the Fourteenth Southeast Asian Geotechnical Conference, (Edited: Ho, K.K.S. & Li, K.S.), 10-14 December 2001, Hong Kong, pp 661-670. Maunsell Geotechnical Services Ltd. (2000). Detailed Design of Check Dam at Sham Tseng San Tsuen, Debris Flow Barrier (Check Dam) Design, Vol. I & II, Report to Geotechnical Engineering Office, Hong Kong. Maunsell Geotechnical Services Ltd. & ERM-Hong Kong Ltd. (1999). Territory Wide Quantitative Risk Assessment of Boulder Fall Hazards, Stage 1. Report to Geotechnical Engineering Office, Hong Kong, 116 p. plus Appendices A-H.
- 55 Ng, K.C., King, J.P., Franks, C.A.M. and Shaw, R. (2000). Natural Terrain Hazard Study: Interim Guidelines. Special Project Report SPR 5/2000, Geotechnical Engineering Office, Hong Kong, 132 p. Parry, S. (2001). Natural Terrain Hazard Study Shek Pik. Advisory Report ADR 2/2001, Geotechnical Engineering Office, Hong Kong, 60 p. plus 3 drgs. Resources Inventory Committee (1996). Terrain Stability Mapping in British Columbia - A review and suggested methods for landslide hazard and risk mapping (Final Draft). Government of British Columbia, British Columbia, Canada, 79 p. Scott Wilson (Hong Kong) Ltd. (1999a). Specialist API Services for the Natural Terrain Landslide Study - Potential Application of Remote Sensing Techniques for Identifying Areas of Seepage in Hong Kong. Report to Geotechnical Engineering Office, Hong Kong, 10 p. Scott Wilson (Hong Kong) Ltd. (1999b). Specialist API Services for the Natural Terrain Landslide Study - Task B Factual Report. Report to Geotechnical Engineering Office, Hong Kong, 9 p. plus 4 Appendices. Scott Wilson (Hong Kong) Ltd. (1999c). Specialist API Services for the Natural Terrain Landslide Study - Interpretive Report. Report to Geotechnical Engineering Office, Hong Kong, 32 p. plus 6 Appendices. Scott Wilson Kirkpatrick (1996). Foothills Bypass, Tuen Mun Road/Wong Chu Road Interchange and Other Road Improvement Road, Geotechnical Appraisal Report. Report to Territory Development Department, 23 p. plus figures. Sewell, R.J., Campbell, S.D.G., Fletcher, C.J.N., Lai, K.W. & Kirk, P.A. (2000). The Pre-Quaternary Geology of Hong Kong. Geotechnical Engineering Office, Hong Kong, 181 p. plus 4 maps. Sobkowicz, J., Hungr, O. & Morgan, G. (1995). Probabilistic mapping of a debris flow hazard area: Cheekye Fan, British Columbia. In: Canadian Geotechnical Conference, September 25-27, 1995, Vancouver, B.C., pp 519-530. Tse, C.M., Chu, T., Lee, K., Wu, R., Hungr, O. & Li, F.H. (1999). A risk-based approach to landslide hazard mitigation measure design. In: Proceedings of the Hong Kong Institution of Engineers Geotechnical Division Annual Seminar on Geotechnical Risk Management. Hong Kong Institution of Engineers and Association of Geotechnical Specialists, Hong Kong, pp 32-42. VanDine, D.F. (1985). Debris flows and debris torrents in the southern Canadian Cordillera. Canadian Geotechnical Journal, Vol. 22, pp 44-68. Wong, C.K.L. (1998). The New Priority Classification Systems for Slopes and Retaining Walls. GEO Report No. 68, Geotechnical Engineering Office, Hong Kong, 117 p.
- 56 Wong, H.N., Lam, K.C. & Ho K.K.S. (1998). Diagnostic Report on the November 1993 Natural Terrain Landslides on Lantau Island. GEO Report No. 69, Geotechnical Engineering Office, Hong Kong, 98 p. plus 1 drg. Wong, H.N., Pang, L.S., Wong, A.C.W., Pun, W.K. & Yu, Y.F. (1999). Application of Prescriptive Measures to Slopes and Retaining Walls. GEO Report No. 56, Geotechnical Engineering Office, Hong Kong, 73 p. Woods, N.W. (1992). Engineering Geology of North Lantau: Tung Chung, Vol I & II. Special Project Report SPR 1/93, Geotechnical Engineering Office, Hong Kong, 94 p. plus 8 drgs.
- 57 LIST OF TABLES Table No.
Page No.
1
Grouping of Facilities
58
2
Design Requirements for the Design Event Approach
59
3
Consequence Classes
60
4
Susceptibility Classes
61
5
Application of the Proposed Framework to Previous NTHS Cases and Proposed Development Sites
62
6
Natural Terrain Features that Should be Determined as Part of the API, Supported by 1:1 000 Scale Topographical Maps
65
7
Natural Terrain Features that Should be Recorded during Field Mapping
66
- 58 Table 1 - Grouping of Facilities (adapted from Wong, 1998) Group No.
Facilities (a) Buildings - any residential building, commercial office, store and shop, hotel, factory, school, power station, ambulance depot, market, hospital/polyclinic/ clinic, welfare centre
1
2
(b) Others - bus shelter, railway platform and other sheltered public waiting area - cottage, licensed and squatter area - dangerous goods storage site (e.g. petrol station) - road with very heavy vehicular or pedestrian traffic density (a) Buildings - built-up area (e.g. indoor car park, building within barracks, abattoir, incinerator, indoor games’ sport hall, sewage treatment plant, refuse transfer station, church, temple, monastery, civic centre, manned substation) (b) Others - road with heavy vehicular or pedestrian traffic density - major infrastructure facility (e.g. railway, tramway, flyover, subway, tunnel portal, service reservoir)
3
- densely-used open space and public waiting area (e.g. densely-used playground, open car park, densely-used sitting out area, horticultural garden) - quarry - road with moderate vehicular or pedestrian traffic density
4
- lightly-used open-air recreation area (e.g. district open space, lightly-used playground, cemetery, columbarium) - non-dangerous goods storage site - road with low vehicular or pedestrian traffic density
5
- remote area (e.g. country park, undeveloped green belt, abandoned quarry) - road with very low vehicular or pedestrian traffic density
Note:
For roads, the Facility Group should be based on Figure A13 (Wong, 1998) taking into account the actual Annual Average Daily Traffic and the number of road lanes. For footpaths alongside roads, it may be assumed that footpaths are within the same group as the adjoining roads, except for Expressways (EX), Urban Trunk Roads (UT) and Rural Trunk Roads (RT). Footpaths alongside EX, UT and RT roads may be taken, by default, as a Group 5 facility, unless dictated otherwise by site-specific conditions.
- 59 Table 2 - Design Requirements for the Design Event Approach Susceptibility Class (see Table 4)
Consequence Class (see Table 3) I
II
III
IV
A
WCE
WCE
WCE
CE
B
WCE
WCE
CE
CE
C
WCE
CE
CE
N
D
N
N
N
N
Notes:
(1) The recommended minimum design requirements are given in this Table. The designer may adopt a more conservative design or provision of other precautionary/warning measures if considered necessary. (2) This Table will be applied to each sub-catchment and normally the predominant type of hazard will control the design requirements. (3) WCE = Adopt a “worst credible event” as the design event. CE = Adopt a “conservative event” as the design event N = Further study is not required
- 60 Table 3 - Consequence Classes Proximity
Facility Group 1&2
3
Very Close (e.g. if angular elevation from the site is ≥ 30°)
I
II
Moderately Close (e.g. if angular elevation from the site is ≥ 25°)
II
III
Far (e.g. if angular elevation from the site is < 25°)
III
IV
Notes:
(1) Facility groups are given in Table 1. Facilities of Groups 4 and 5 normally do not require natural terrain hazard study and hence they are not included in Table 3. (2) For channelized debris flow, if the worst credible event affecting the site is judged to have a volume exceeding 2000 m3, the angular elevation given in the above examples should be reduced by 5°. (3) The examples given above are for general guidance only. Other factors, such as credible debris path, topographical conditions and site-specific historical data, should also be taken into account in assessing the “proximity” of the natural terrain to the site. (4) This Table is applicable only to sites that require natural terrain hazard study.
- 61 Table 4 - Susceptibility Classes Susceptibility Class
A
B
C
D
Note:
Description The natural terrain is extremely susceptible to the type of failure under consideration, with a notional annual probability of occurrence in the order of 1/10 or higher. For example: there are signs of instability, continued movement, or records of repeated recent failures (say over the past 50 to 100 years as observed from aerial photographs) in the catchment and its relevant vicinity. The natural terrain is highly susceptible to the type of failure under consideration, with a notional annual probability of occurrence within the order of 1/10 to 1/100. For example: there are records of occasional recent failures in the catchment and its relevant vicinity. The natural terrain is moderately susceptible to the type of failure under consideration, with a notional annual probability of occurrence within the order of 1/100 to 1/1000. For example: there are few records of recent failures, but there are indications of relict failures, or geomorphological evidence of potential problems in the catchment and its relevant vicinity, or any other evidence from similar terrain in Hong Kong. The natural terrain is of low susceptibility to the type of failure under consideration, with a notional annual probability of occurrence less than 1/1000. For example: there are no records of recent and relict failures, little geomorphological and other evidence of potential problems in the catchment and its relevant vicinity, and little other evidence from similar terrain in Hong Kong.
In assessing the susceptibility of the hillside to failure, consideration should be taken of the potential effects of changes in environmental factors, e.g. any changes to the overall setting of the terrain such as hill fires and construction upslope, and the relevance of the available historical landslide records. Most of the information on environmental changes can be obtained from a review of past aerial photographs.
Table 5 - Application of the Proposed Framework to Previous NTHS Cases and Proposed Development Sites (sheet 1 of 3)
Site
Area 49A, Fanling
Area 19, Tuen Mun Golf Centre
Design Requirement (see Table 2)
Consequence Class (see Table 3)
Facility Group
Angular Elevation
Proximity
Class
Susceptibility Class
Required Design Event
Actual design Event
Remarks
Actual design event based on extrapolation using regional data to a 1-in-300 years failure at source together with mobilisation of material along trail and with no deposition before reaching the debris barrier. Factor of Safety Approach adopted in checking against deep-seated failures.
1
Channelized debris flow
1 (residential building)
26°
VC (WCE >2000 m3)
I
B
WCE
WCE
1 (catchment of 1990 failure)
Channelized debris flow
2 (road)
29°
VC (WCE > 2000 m3)
I
A (given the 1990 large scale debris flow)
WCE
WCE
2 (catchment of 2000 failure North)
Channelized debris flow
2 (road)
30°
VC (WCE > 2000 m3)
I
A/B
WCE
WCE
3 (catchment of 2000 failure South)
Channelized debris flow
2 (road)
30°
VC (WCE > 2000 m3)
I
A/B
WCE
WCE
1 (catchment of 1990 failure)
Channelized debris flow
4 (lightly- used open recreation area)
28°
VC (WCE > 2000 m3)
III
A (given the 1990 large scale debris flow)
WCE
WCE
2 (catchment of 2000 failure North)
Channelized debris flow
4 (lightly- used open recreation area)
28°
VC (WCE > 2000 m3)
III
A/B
WCE/ CE
WCE
Actual design event based on the 1990 Tsing Shan debris flow, which was assessed as an event in the order of 1 in 1000 years.
Actual design event based on the 1990 Tsing Shan debris flow, which was assessed as an event in the order of 1-in-1000 years. Due to the presence of a buffer zone between the site and the hillside, the mitigation measures provided are not very substantial.
- 62 -
Tsing Shan Foothill Bypass
Catchment No.
Predominant Type of Hazard
Table 5 - Application of the Proposed Framework to Previous NTHS Cases and Proposed Development Sites (sheet 2 of 3)
Site
Catchment No.
1
Design Requirement (see Table 2)
Consequence Class (see Table 3)
Predominant Type of Hazard
Facility Group
Channelized debris flow
2 (road)
Angular Elevation
Proximity
18°
F (WCE > 2000 m3)
Class
III
Susceptibility Class
C
Required Design Event
CE
Actual design Event
Remarks
WCE
Actual design event based on x10 the largest historical failure. Since the facility is quite far away from the hillside, the mitigation measures provided are not very substantial.
CE
Although the debris barriers are designed for a CE, some stabilisation measures are provided to the hillside to reduce the chance of open slope failure. In this respect, the actual design event may also be taken as equivalent to a WCE.
N. Tsing Yi Coastal Road 2
2 (road)
Open slope failure
1 (factory)
Open slope failure
1 (factory)
28°
MC
II
B
WCE
27°
MC
II
C
CE
CE
By provision of buffer zone, angular elevation from edge of site set back to 23°, which was considered adequate given the design event is not large in volume.
30°
VC
I
C
WCE
WCE
By provision of buffer zone, angular elevation from edge of site set back to 8°. This is conservative since the hillside in question is small in size.
CE
The debris barrier is designed for a CE which corresponds to more than twice the volume of the 1999 failure. Additional buffer zone is provided downstream of the barrier to cater for possible over-spill from a larger event. The mitigation works were urgent protective measures for an existing squatter village in response to a landslide.
1
VC (if WCE > 2000 m3)
Sham Tseng San Tsuen squatter village
1
Proposed development site, Black’s Link
1
Channelized debris flow
Channelized debris flow
1 (squatters)
1 (residential building)
I
B
WCE
28°
26°
MC (if WCE ≤ 2000 m3)
II
B
WCE
VC (WCE > 2000 m3)
I
C (based on preliminary assessment)
WCE
Proposed development The design requirements are reasonable. (NTHS yet to be carried out)
- 63 -
To Kau Wan, N. Lantau
Open slope failure
Table 5 - Application of the Proposed Framework to Previous NTHS Cases and Proposed Development Sites (sheet 3 of 3)
Site
Proposed bus shelter, Lion Rock Tunnel Road
Proposed development site, South Bay Road
1
1
Open slope failure
1 (bus shelter)
Channelized debris flow
4 (non-dangerous goods storage site)
29°
32°
Proximity
MC
VC
1
1 (residential building)
29°
MC
1
Channelized debris flow
1 (residential building)
24°
MC
Open slope failure
1 (residential building)
33°
2
Notes:
Facility Group
Angular Elevation
Open slope failure
Shatin Heights
Design Requirement (see Table 2)
Consequence Class (see Table 3)
VC
Class
Susceptibility Class
II
C/D (based on preliminary assessment)
III
C/D (based on preliminary assessment)
Required Design Event
CE/N
CE/N
II
C/D (based on preliminary assessment)
CE/N
II
B
WCE
I
A/B
WCE
Actual design Event
Remarks
Proposed development (NTHS yet to be carried out)
The less stringent design requirements are reasonable. For this site, it may be possible to show that the landslide debris will not reach the site and hence no mitigation measures are required, if the CE is not of large volume.
N
The less stringent design requirements for this case which is of less serious consequence are reasonable. The critical facilities will be located farthest away from the hillside, and subsequently the NTHS requirement has been waived.
Mitigation measures will not be required if the hillside is assessed as of low susceptibility. Alternatively, if the proposed buildings are Proposed development placed on the part of the site that is further (NTHS yet to away from the hillside and if the CE is not of be carried out) large volume, it may be demonstrated that the debris will not reach the buildings and no mitigation measures are required.
QRA approach QRA approach
An existing development affected by several hillside failures in 1997.
(1) VC = Very Close; MC = Moderately Close; F = Far. See Table 3 for details. (2) WCE = Worst credible event; CE = Conservative event; N = Further study is not required. See Table 2 for details. (3) See Table 3 for details of Consequence Classes, Table 1 for facility types, and Table 4 for Susceptibility Classes.
- 64 -
Proposed Short Term Tenancy site, Mt. Parker Road
Catchment No.
Predominant Type of Hazard
- 65 Table 6 - Natural Terrain Features that Should be Determined as Part of the API, Supported by 1:1 000 Scale Topographical Maps (modified from Resources Inventory Committee, 1996) Terrain Feature Slope Morphology Gradient Uniformity of slope Lateral curvature Position Elevation Length Aspect Identified Processes Landslides Erosion Other processes Age Areal extent Depth Incipient Processes Instability signs Abundance Superficial Material Origin, genesis Geomorphological features Geomorphological processes Solid Geology Outcrops Structural features Drainage Lines Order and status Channel bed material Channel processes Sideslope/Headslope material Sideslope/Headslope processes Groundwater Soil drainage Seepage regime Vegetation Type Age Deforestation/fire history Human Activity Type Quantity
Examples (not exhaustive) Typical, average or range over a specified slope length Straight, stepped, benched, concave, convex, etc. Broad, narrow, re-entrant, ridge Near crest, mid-slope, near toe, etc. Typical, range Slope length with similar features Quadrant with respect to north Rock fall, open hillslope landslide, deep-seated slide, channelized debris flow, etc. Sheet, rill, drainage line, etc. Flooding Recent, relict, or date if known Width and length Typical Tension cracks, disrupted road, tilted trees, etc. Number of indications per area or length Fluvial, colluvial, alluvial, boulder field, etc. Debris fan, apron, cone, landslide scar, rock exposure, etc. Gullying, erosion, failing, etc. Location and extent Bedding, faults, folds, dykes, other discontinuities First, second, third, etc; permanent vs ephemeral or dry Inorganic (boulders, bare bed rock etc) vs vegetation debris Flood, debris flood, debris avalanche, etc. As for superficial material above As for identified and incipient processes above Rapidly, well, moderate, etc. Recharge area, discharge area, undrained area, etc. Trees, shrub, grass, bare 30 years, etc. Deforestation plus years since, hill fire plus years since Fill at top of slope, cut, agricultural terraces, graves, footpaths, road across slope, etc. Road length/unit area, etc.
- 66 Table 7 - Natural Terrain Features that Should be Recorded during Field Mapping (modified from Resources Inventory Committee, 1996) Terrain Feature
Examples (not exhaustive)
Identified Processes Landslides, Erosion, other Processes Areal extent
Width & length (of source area and debris trail, debris run out, deposition, erosion)
Depth
Typical (of source, erosion, etc.)
Superficial Material Type
Saprolite, Colluvium, alluvium, scree
Boulder Shape
Rounded, sub-rounded, angular, tabular, etc.
Properties of soil
Strength, consistency, density, etc.
Thickness
Typical, average or range
Genesis of Contrasting Layers
Saprolite overlain by colluvium, etc.
Solid Geology Rock Type
Granite, feldsparphyric rhyolite, eutaxitic tuff, etc.
Decomposition Grade
Fresh, slightly decomposed, moderately decomposed
Block size and shape
Spacing of discontinuities, cubes, etc.
Structural attitudes
Strike, dip, dip direction
Properties of rock mass
Strength, fracture roughness, RQD, PW 90/100, etc.
Description of Contrasting Layers Layer description
Loose, dense etc, together with material type
Thickness
Typical, average or range
Substrate description
Residual soil, saprolite, bedrock, etc.
Nature of contact
Weak soil horizon, distinct, etc.
Drainage Lines Channel gradient
Typical, average or range (in degrees, over typical length)
Uniformity of gradient
Uniform, stepped, etc.
Channel width
Typical, average or range
Sideslope/Headslope height
Typical, average or range
Sideslope/Headslope gradient
Typical, average or range
- 67 LIST OF FIGURES Figure No.
Page No.
1
Examples of Anthropogenic Features on Natural Terrain
68
2
Application of In-principle Objection Criteria and Alert Criteria
69
3
Overall Process of Natural Terrain Hazard Study
70
4
Examples of Debris- and Boulder-resisting Barriers Used in Hong Kong
71
- 68 -
(a) Abandoned Agricultural Terraces, Route Twisk (1963)
(b) Abandoned Agricultural Terraces, Route Twisk (1999)
(c) Military Redoubt, Ying Pun, New Territories (1963)
(d) Foxholes from a Military Exercise, Route Twisk (1963)
(e) Mineral Exploration Trenches, South Lantau (1963)
(f) Mining Induced Failures, Ma On Shan (1977)
Figure 1 - Examples of Anthropogenic Features on Natural Terrain
- 69 -
Figure 2 - Application of In-principle Objection Criteria and Alert Criteria
- 70 -
Figure 3 - Overall Process of Natural Terrain Hazard Study
(b) Landslide Debris/Boulder Barrier (Fanling)
(c) Boulder Fence (Mid-Levels)
(d) Gabion Barrier (Shau Kei Wan)
Figure 4 - Examples of Debris- and Boulder-resisting Barriers Used in Hong Kong
- 71 -
(a) Check Dam (Sham Tseng San Tsuen)
- 72 -
APPENDIX A GLOSSARY OF TERMS AND GENERAL CLASSIFICATION OF HILLSIDE FAILURES
- 73 CONTENTS Page No. Title Page
72
CONTENTS
73
A.1 GLOSSARY OF TERMS FOR NATURAL TERRAIN LANDSLIDES
74
A.1.1 Introduction
74
A.1.2 Glossary of Terms
74
A.2 GENERAL CLASSIFICATION OF HILLSIDE FAILURES
86
A.2.1 Introduction
86
A.2.2 Classification Framework
86
A.3 REFERENCES LIST OF FIGURES
87 91
- 74 A.1 GLOSSARY OF TERMS FOR NATURAL TERRAIN LANDSIDES A.1.1 Introduction This glossary of terms has been compiled to help promote a consistent use of terms in the study of natural terrain hazards in Hong Kong. Much of it is extracted from Cruden and Varnes (1996) which is a widely accepted reference on landslide types and processes. A recent review on the landslide terminology is given by Hungr et al. (2001). For many Hong Kong natural terrain landslides, it is difficult to establish the mechanism of initial displacement and transport and use of the wider definitions proposed below may be more practical. The terminology is primarily prepared for study of natural terrain landslides in Hong Kong. As such, it is biased towards terms used for describing elongate, relatively shallow natural terrain landslides which are dominated by flow and alluvial processes. The glossary includes some additional and more detailed terms than those of Cruden and Varnes (1996), and some new definitions. Additional definitions of landslide types are based on Hutchinson (1988) and for the description of sediment-water flows, on Pierson and Costa (1987). Users of the terminology may define and use their own terms for any specific project although consistency is recommended wherever possible. Figure A1 illustrates the main features of a typical natural terrain landslide scar. A.1.2 Glossary of Terms Angle of Reach The arc-tangent of the height of a landslide (H) divided by its length (L) where; H = elevation difference between the crown and the toe and L = the horizontal distance from the crown to the toe. Note that this is similar to the apparent angle of friction (Sassa, 1987), equivalent coefficient of friction (Hsu, 1975) and average coefficient of friction (Scheidegger, 1973). However, these terms have in some cases been used when H and L are defined on the basis of the centre of gravity of the displaced material. The term reach angle is proposed in conformity with its use by Corominas (1996) as this reference may be used to help estimate run out distance. The same angle is termed Travel Angle by Cruden and Varnes (1996). Blocky Landslide Debris Landslide debris comprising loose blocks of intact displaced material, commonly rock and/or soil. Block A discrete boulder-sized fragment of rock significantly detached from its outcrop source and characterized by an angular shape of tabular or blocky form. It may lie on or adjacent to the source or have travelled downslope. Boulder A rock fragment greater than 200 mm diameter that is not part of a rock mass.
- 75 Boulder Fall The displacement of a boulder where the movement may include free fall through the air, sliding, leaping and rolling. Note that in Hong Kong boulder falls commonly result from undercutting by erosion, or landsliding of exhumed corestones or colluvial boulders on steep natural hillsides. Boulder Field An area covered by a veneer of boulders. Catchment An area of a hillside with a relatively uniform geomorphology from which one or several well-defined hazard models could affect the site. Channel A linear depression of the ground surface in which water may flow. Channelize To convey in a channel. A landslide is considered to be channelized when the moving landslide debris converges into a channel that confines its lateral extent. Note that this commonly results in a longer travel distance than for an open slope landslide of the same volume. Channelized Debris Flow A debris flow that has become channelized in a drainage line with significant amounts surface water mixed into the debris. Note that while debris flows may occasionally develop on hillslopes and become confined in hollows, these are difficult to distinguish from debris avalanches on the basis of mobility or by API, or even by field observations soon after the failure. Therefore, for the purposes of hazard recognition and data collection, the term is best used for debris flows that are coincident with a streambed. Channelization Ratio The width to depth ratio of the cross section area in a channel occupied by a pulse of landslide debris. Channelization may be defined in relation to this ratio. Hungr et al. (1984) suggested 5; the Tsing Shan Debris Flow started to deposit at about 7 (King, 1996). Cliff Any steep to sub-vertical or over-hanging face of rock. Colluvium A general term applied to generally structureless and commonly heterogeneous mass of soil
- 76 and/or rock material and sometimes organic matter, deposited on, and at the base of, natural slopes predominantly by mass-wasting processes (after Bates & Jackson, 1987). It is usually landslide debris. Colluvial Fan A colluvial deposit shaped like a fan or cone, usually with the apex at a point where a drainage line ends. Such a feature is usually the result of deposition from a number of debris flows over a period of time. Colluvial Lobe A colluvial deposit with convex sides that forms a positive landscape feature. Often located along the bottom of a valley. Such a feature is usually interpreted as a landslide debris deposit from a single landslide. Confined Scar A landslide scar in which the surface of rupture comprises a distinct main scarp and floor and may include a rising downslope part. The source material cannot be displaced until it is transformed into a kinematically feasible geometry by internal displacements or shears (Hutchinson, 1988). A confined scar has no trail. Crown The practically undisplaced material still in place and adjacent to the highest parts of the main scarp (IAEG, 1990). Debris Predominantly coarse soil. In Hong Kong such material is usually the weathered profile and/or colluvium and may be referred to as regolith. Landslide displaced material that has disintegrated is commonly called “debris”. To retain conformity with international practice the term landslide debris is used for this in the terminology (see Landslide Debris). Debris Avalanche A landslide comprising a rapid to extremely rapid, shallow movement of partially or fully saturated mainly granular debris on steep slopes with only minimal water mixed into the displaced material after failure. Movement is by sliding and/or liquefaction and/or rolling and/or bouncing and/or saturated or inertial granular flow (Pierson and Costa, 1987; Varnes, 1978). Debris Flood The transport of significant load of sediment by the dominant mechanisms of stream flow or hyperconcentrated stream flow. The load can range in grain size from clay to boulders. A debris flood may originate from outwash of landslide debris or gully erosion.
- 77 Debris Flow A landslide in which the landslide debris moves by the dominant mechanism of slurry flow. Note that the presence of remoulded landslide debris is a good indication that slurry flow has occurred. In Hong Kong, debris flows usually develop from debris slides or debris avalanches. If the moisture content of the displaced material is high enough then slurry flow may develop during movement without additional water. More commonly a debris flow develops when landslide debris mixes with stream water. Debris flows may form open hillslope failures on relatively planar slopes or in hollows but commonly become confined in drainage lines forming channelized debris flows. Debris Slide A landslide in which an intact displaced mass moves by the dominant mechanism of sliding. Note that when these occur on steep hillsides they typically form an elongate scar. Debris Torrent A term used in Canada by D.F. VanDine (1985) to mean the same as “channelized debris flow” and defined by him as “a mass movement that involves water-charged, predominantly coarse grained inorganic and organic material flowing rapidly down a steep confined preexisting channel”. Torrent is used by Aulitzky (1989) to describe steep mountain streams. Debris Run-Out The travel distance of landslide debris beyond a specified point. The “landslide run-out” may be the horizontal length of the scar. Run-out may also be considered beyond a point with a limiting angle where deposition starts (deposition point) or the “slope toe”. Deposition Point A notional point on a landslide trail beyond which most landslide debris is assumed to be deposited. Design Event The particular size specification for a hazard model that is selected to be mitigated against as part of a development. Displaced Material The material moved from its original position on the slope by a landslide (IAEG, 1990). Note that an accumulation of displaced material forms a deposit. Drainage Line A channel with a stream bed in which water flows for at least part of the year.
- 78 Drainage Line Order A first order drainage line has no confluences (i.e. no other contributing drainage lines), two first order drainage lines make a second order, and two second orders drainage lines make a third order (a second and first order confluence remains a second order) (after Strahler, A.N., 1952). Elongate Scar A landslide scar in which some or all of the displaced material has mobilized from the surface of rupture forming landslide debris. Note that an elongate scar has a trail and the source is often a spoon shaped or tabular depression. Elongate Channelized Scar An elongate scar in which the landslide debris has been channelized in a drainage line or depression and the trail has a low channelization ratio. Entrainment Factor The volume of additional material that has been entrained by a landslide expressed as a proportion of the source volume. Entrained Material The displaced material from any location other than the source. Floor Any exposed shallow part of the surface of rupture. Flow A spatially continuous movement in which surfaces of shear are short-lived, closely-spaced, and usually not preserved. The distribution of velocities in the displacing mass resembles that in a viscous liquid (Cruden and Varnes, 1996). Frequency The number of events that occur in a specific period of time. Geological Model A representation of the geology of a particular location. The form of the model can vary widely and may include written descriptions, two-dimensional sections or plans, block diagrams, or be slanted towards some particular aspect such as groundwater or geomorphological processes, rock structure and so on (Fookes, 1997).
- 79 Gully Erosion The incision of ground along flow lines by surface water. Hazard A physical situation with a potential for human injury, damage to property, damage to the environment or some combination of these (ERM, 1998). Hazard Model A type or types of landslide that will result in a well-defined risk to a development site. It is described in terms of volume, nature and velocity of material passing the site boundary at a specified location. Hazard Intensity A relative measure of the potential for harm from a hazard. potential for injury and loss than low intensity.
High intensity has a greater
Head The upper parts of a landslide along the contact between the displaced material and the main scarp (IAEG, 1990). Note that an elongate landslide may not have a definable head. Hollow A depression, commonly linear, on hillside. Hyperconcentrated Stream Flow The flow of a mixture of water and sediment that possesses a small but measurable yield strength but that still appears to flow like a liquid. Particles settling out of a hyperconcentrated stream flow suspension settle independently, giving deposits sorted by grain size (Pierson and Costa 1987). Inertial Granular Flow A flow in which the full weight of the flowing granular mass is borne by grain-to-grain contact or collisions in which grain inertial effects dominate but frictional effects are still significant (Pierson and Costa, 1987). Intact Displaced Mass The full mass of displaced material (soil, colluvium or rock) from a landslide when it largely retains its original morphology.
- 80 Intact Displaced Material A displaced block of soil, colluvium or rock that retains its original structure. Note that intact displaced material may occur as clasts, blocks or slabs within landslide debris. Intermediate Deposit The landslide debris accumulation at some point along the trail between the source and the toe. Landslide A general, all-encompassing term used to describe an event comprising the relatively rapid downslope movement of a discrete mass of soil and/or rock. Note that the term makes no inference to process, i.e. true sliding may not be involved. It is proposed that extremely slow slope movements such as soil creep and gravitational sagging are not included under the general term landslide. Landslide Cause An environmental factor that gradually brings a slope to failure such as the chemical or physical weathering of slope materials (Weiczorek, 1996). Landslide Debris The displaced material from a specific landslide that has disintegrated and lost its original morphology. Landslide debris may include clasts of intact displaced material. Note that landslide debris is very young colluvium and it is proposed that the term is used to distinguish the landslide debris of a specific landslide from pre-existing in-situ colluvium in the regolith which may have the same composition. The term landslide debris may be shortened to “debris” for convenience where this will not be ambiguous or confused with regolith debris. Landslide Erosion Feature A relatively large depression defined by scarps or a sharp break in slope. This is assumed to be landslide related. Smaller recent landslides may occur along the perimeter scarp but there is no debris deposit that can be unambiguously interpreted as the result of a single failure. This feature could be the source of a single very large landslide, or the result of a number of smaller landslides over time, or an erosion scarp (possibly landslide generated) cutting back into an older landscape. Landslide Trigger An external stimulus that causes a near-immediate response in the form of a landslide by rapidly increasing the stresses or by reducing the strength of slope materials. Typical triggers are intense rainfall, collapse of a soil erosion pipe, earthquake shaking, storm waves or rapid stream erosion (Weiczorek 1996).
- 81 Lateral Deposit A landslide debris deposit comprising ridges aligned along the trail at or near its lateral margins. Liquefaction The generation of high positive excess pore water pressures during shearing and hence a substantial reduction of the effective stress and the shearing resistance. Note that in Hong Kong liquefaction is known to have occurred in the rain-induced failure of loose fill slopes. Liquefaction Slide A slide in which liquefaction occurs within a zone at the sliding surface and the landslide debris is not saturated (Casagrande, 1971; Hutchinson, 1988). Main Scarp The exposed steep part of the surface of rupture (IAEG, 1990). Mass Transport The carrying of soil and rock material in a moving medium such as water, air, or ice (after Bates & Jackson, 1987). Mass Movements A general term for the dislodgement and downslope transport of soil and rock material under the direct application of gravitational body stresses. In contrast to other erosion processes, the debris removed by mass movement processes is not carried within, on, or under another medium. The mass properties of the material being transported depend on the interaction of the soil and rock particles and on the moisture content. Mass movements include slow displacements, such as creep, and rapid processes such as rock falls, rock slides, and debris flows (Syn. mass wasting) (after Bates & Jackson, 1987). Mobilization The movement of displaced material from the source. Usually expressed as the proportion of the material that is displaced. Mud Flow A landslide with the same characteristics as a debris flow in which the displaced material is predominantly fine-grained. Natural Terrain Terrain that has not been modified substantially by human activity such as site formation works, agricultural terracing, cemetery platforms or squatter habitation. Note that in most of
- 82 Hong Kong natural terrain has been influenced by deforestation and fire and locally may have been influenced by prehistoric agriculture. Natural terrain and natural hillside are synonyms Natural Terrain Landslide A landslide in which the source is located entirely within natural terrain (see Section A.2.2 below). Note that a landslide involving both natural terrain and terrain that has been modified substantially by human activity is not considered to be a natural terrain landslide, e.g. failure immediately above and extending into an excavation or immediately below fill. Open Hillslope Lanslide A landslide with its scar entirely on the hillside and is not channelized along a stream course. It is initiated on a hillside slope and transported by the mechanism of debris avalanche, debris slide or debris flow. Outcrop An exposed area underlain by a specific material or rock unit. Outwash The redistribution of landslide debris by stream flow and hyper-concentrated stream flow. Parent Landslide A discrete initial landslide, the landslide debris from which increases in volume through erosion and entrainment of the downslope substrate to become a debris flow. Quasi-natural Terrain Predominantly natural terrain which is partly modified by human activities but largely retains its original profile and regolith cover. Minor surface features such as agricultural terracing, grave sites or squatter platforms may be present. Modification of the slopes above and below by cutting, filling and construction may have affected the surrounding ground and surface water conditions. Quasi-natural Terrain Landslide A failure in predominantly natural terrain triggered by, or possibly triggered by, a relatively small human activity. Landslides for which there is an element of doubt with respect to the importance of human activity are placed in this category. Section A.2.2 below provides further information on classification of such landslides. Recent Landslide A landslide that occurred within the aerial photograph coverage. In the NTLI recent landslides are defined as those with a light tone on aerial photographs and are generally bare of vegetation.
- 83 Regolith A general term for the layer or mantle of fragmental and unconsolidated rock material (engineering soil), whether residual or transported and of highly varied character, that nearly everywhere forms the surface of the land and overlies or covers the bedrock. It includes rock debris of all kinds (after Bates & Jackson, 1987). Relict Scar A scar on which vegetation has re-established but which still has a well-defined main scarp. In the NTLI, relict is defined in relation to landslides first observed on the earliest available photographs and with vegetation class C or D (Grass or Trees). Remoulded Landslide Debris Landslide debris that has largely lost its original structure and comprises a remoulded matrix that supports particles of gravel size or larger. Note that remoulded landslide debris may occur as steep-sided deposits such as lobate fronts, levees, and accumulations on the uphill side of, or capping, obstructions in the trail. Risk The likelihood of a specified undesired event occurring within a specified period or in specified circumstances. It may be viewed in terms of either a frequency (the number of specified events occurring in unit time) or a probability (the likelihood of a specified event following a prior event) depending upon the circumstances (ERM, 1998). It commonly refers to as the product of frequency (or probability) and consequence. Rock Fall The displacement of a piece of rock from a rock face chiefly by free fall through the air but may include sliding, leaping and rolling. Note that this is a simple landslide if the initial displacement does not involve another mechanism such as sliding or toppling. Rotational Scar A scar in which the surface of rupture is curved and concave upwards and imparts a degree of backward rotation or tilt to the surface of the displaced ground. Note this was termed a slump by Varnes (1978) and a slip by Hutchinson (1988). Run-out The process of transport of displaced material beyond the source. it will be length less the horizontal extent of the source.
If expressed as a distance
Scar The land surface affected by a landslide. This includes the source, the displaced material and any trail.
- 84 Sheet Erosion The uniform removal of soil or decomposed rock by the surface flow of water or a mixture of water and sediment. Side/Head wall The area of a catchment or sub-catchment that could contribute debris to a stream channel. Site A proposed development area demarcated by a continuous line on a topographical base map of 1:5 000 or larger scale. The whole site should be considered habitable unless buffer or no build zones are incorporated into the lease. Site Catchment The area from which surface runoff would intersect the site boundary or flow into a drainage line adjacent to the site, effectively the water catchment to a site. Slide A landslide in which the movement of debris is sliding. Note that this may be further qualified by a prefix describing the morphology of the scar and specifying the material involved, e.g. slides may result in translational, rotational, compound or confined scars from failures of colluvium, saprolite or rock. Sliding Downslope movement occurring dominantly on surfaces of rupture or on relatively thin zones of intense shear strain (Cruden and Varnes, 1996). Slump A scar with an intact displaced mass which partially overlies the surface of rupture Slurry Flow The movement of a saturated sediment/water mixture having sufficient yield strength to exhibit plastic flow behaviour in the field. When movement ceases, fine and coarse particles settle together with no inter-particle movement (Pierson and Costa 1987). Sorted Landslide Debris Landslide debris in which grain size sorting and/or layering is present and that has little or no fines.
- 85 Source The space between the surface of rupture and the original ground level. Stream Bed The bottom of a relatively narrow but clearly defined channel, where surface water flows or may flow over bedrock and/or loose granular deposits. Stream Flow The flow of water with insufficient sediment concentration to affect the flow behaviour (Pierson and Costa 1987). Study Area The area from which natural terrain landslide hazards could affect the site. this should include both the site and its catchment.
In most cases
Substrate In-situ material over which landslide debris is transported. Surface of Entrainment A surface on undisturbed ground that was originally below ground level and from which material has been picked up and moved away during the landslide. This is separate, and downslope from the source. Surface of Rupture A surface on undisturbed ground that was originally below ground level and from which a discrete mass of displaced material has moved away (IAEG, 1990). Terminal Deposit A landslide debris deposit at the toe of the trail. It is commonly lobate or fan shaped. Toe The lower margin of the displaced material of a landslide. It is the most distant part of the scar from the main scarp IAEG (1990). Note that for a landslide with a trail this will be the distal part of the trail. Topple Movement of a detached rock mass by overturning about a pivot point below the centre of gravity of the unit (Varnes, 1978). Note that topples are often multiple.
- 86 Trail The part of a scar downslope from the source. This may include displaced material, ground over which displaced material has passed and any surface of entrainment. Translational Scar A scar in which the surface of rupture is relatively planar or gently undulating in downslope section and often broadly channel-shaped in cross section (Cruden and Varnes 1996). Uniform Deposit A landslide debris deposit comprises a sheet of landslide debris of uniform thickness within the trail. Weathered Mantle A regionally widespread and usually deep zone of weathered rock materials, formed in situ over a geologically long interval by relatively uniform chemical weathering (Syn. weathered profile, weathering crust) (after Bates & Jackson, 1987). A.2 GENERAL CLASSIFICATION OF HILLSIDE FAILURES A.2.1 Introduction There is a need for use of consistent terminology in denoting different types of hillside failures in Hong Kong. This will facilitate systematic landslide classification and statistical analysis. In this note, “hillside failure” refers to a landslide that occurs mainly on a hillside instead of man-made features. “Man-made feature” refers to cut slope, fill slope or retaining wall. “Hillside” includes disturbed terrain. For the purpose of general classification, the terminology should be practical and consistent with the nature and quality of the information that would normally be available for most of the reported hillside failures. A.2.2 Classification Framework For general classification, hillside failures are classified according to the types of terrain involved in the source of the landslide (Figure A2): (a) Natural hillside failure - occurs on natural hillside which has not been modified by man-made activities (e.g. cutting, filling, cultivation, mining, etc.) (b) Disturbed-hillside failure - occurs on hillside which has locally been modified by man-made activities but there is no indication that the landslide involves registerable man-made features. Where information is available, such landslide
- 87 should be further classified as follows: (i) Disturbed-hillside failure (Type A) - which is not significantly controlled by non-registerable man-made features (ii) Disturbed-hillside failure (Type B) - which is significantly controlled by non-registerable man-made features (c) Hillside failure involving registerable man-made features occurs predominantly on hillside but partly involves registerable man-made features. Selected cases of hillside landslides are given in Figure A2 as examples of classification based on the proposed framework. The above classification may also be applied to individual rock or boulder falls from hillside. The mode, mechanism and cause of failure are not accounted for in the general classification, as reliable information on these is not normally available. Where such information is available, say from detailed landslide studies, the landslides can be further described or classified, on top of the general classification. It should be noted that a “natural hillside failure” (Item (a) above) may be triggered by factors that involve man-made influence (e.g. concentration of surface water overflowed from a catchwater). Conversely, a “disturbed-hillside failure” (Item (b) above) and a “hillside failure involving registerable man-made features” (Item (c) above), which are to some extent affected by man-made activities, may be triggered by purely natural causes (e.g. heavy rain). Strictly speaking, only “natural hillside failures” that are caused by natural factors should be considered as “genuine” natural terrain landslides. The other types of hillside landslides, i.e. “disturbed-hillside failures”, “hillside failures involving registerable man-made features”, and “natural hillside failures” involving man-made triggers, were often denoted collectively as “quasi-natural terrain landslides” (see Section A.1.2 above) in the past. The proposed classification provides a practical framework to differentiate these landslides. A.3 REFERENCES Glossary of Terms for Natural Terrain Landslides Aulitzky, H. (1989). The debris flows of Austria. Vol. 40, pp 5 - 13.
Bulletin of Engineering Geology,
Bates, R.L. & Jackson, J.A. (1987). Glossary of Geology. Geological Institute, Alexandria, Virginia, 788 p.
Third Edition, American
- 88 Casagrande, A. (1971). On liquefaction phenomena: report of a lecture. Geotechnique, Vol. 21, pp 197-202. Corominas, J. (1986). The angle of reach as a mobility index for small and large landslides. Canadian Geotechnical Journal, Vol. 33, pp 260 -271. Cruden, D.M. and Varnes, D..J. (1996). Landslide Types and Processes. In: Landslides Investigation and Mitigation (Edited: Turner, A.K. and Schuster, R. L.), Special Report 247 of the Transportation Research Board, National Research Council, National Academy Press, Washington, D.C, pp 36-75. Fookes, F.G. (1997). Geology for engineers: the geological model, prediction and performance. Quarterly Journal of Engineering Geology, Vol. 30, pp 293-424. ERM-Hong Kong Ltd. (1998). Feasibility Study for QRA of Boulder Fall Hazard in Hong Kong. GEO Report No. 80, Geotechnical Engineering Office, Hong Kong, 61 p. Hsu, K. (1975). Catastrophic debris streams (surtzstrons) generated by rockfalls. Bulletin of Geological Society of America, Vol. 86, pp 129-140. Hungr, O., Morgan, G.C. & Kellerhals, R. (1984). Quantitative analysis of debris torrent hazards for design of remedial measures. Canadian Geotechnical Journal, Vol. 21, pp. 663-677. Hungr, O., Evans, S.C., Bovis, M.J. and Hutchison, J.N. 2001. A review of the classification of landslides of the flow type. Environmental & Engineering Geoscience. Vol. VII, pp 221-238. Hutchinson, J. N. (1988). General Report: Morphological and geotechnical parameters of landslides in relation to geology and hydrogeology. In: B, Proceedings of the Fifth International Symposium on Landslides (Edited: Bonnard, C.), Lausanne, Switzerland, Vol. 1, pp 3-35. IAEG (1990). Suggested nomenclature for landslides. Bulletin of the International Association of Engineering Geology, Vol. 41, pp 13-16. Pierson, T.C. & Costa, J.E. (1987). A rheological classification of sub-aerial sediment-water flows. In: Debris Flows/Avalanches: Process, Recognition and Mitigation (Edited: Costa, J.E. & Wieczorek, G.F.), Geological Society of America, Boulder, Colorado, pp 1-12. Sassa, K. (1987). The Jizkukiyama Landslide and the interpretation of its long scraping motion. In: Proceedings of the 5th International Conference and Field Workshop on Landslides (Edited: Sloane, D.J. & Bell, D.H.), Christchurch, New Zealand, pp 215-223. Scheidegger, A. (1973). On the prediction of the reach and velocity of catastrophic landslides, Rock Mechanics, Vol. 5, pp 231-236.
- 89 Strahler, A.N. (1952). Hypsomertic (area altitude) analysis of erosional topography. Bulletin of Geological Society of America, Vol. 63, pp 1117-1142. VanDine, D.F. (1985). Debris flows and debris torrents in the southern Canadian Cordillera. Canadian Geotechnical Journal, Vol. 22, pp 44-68. Varnes D.J. (1978). Slope Movement Types and Processes. In: Landslides Investigation and Mitigation (Edited: Turner, A.K. and Schuster, R. L.), Special Report 247 of the Transportation Research Board, National Research Council, National Academy Press, Washington, D.C, pp 11-33. Wieczorek, G.F. (1996). Landslide Triggering Mechanisms. In: Landslides Investigation and Mitigation (Edited: Turner, A.K. and Schuster, R. L.), Special Report 247 of the Transportation Research Board, National Research Council, National Academy Press, Washington, D.C, pp 76-90. Classification of Hillside Failures Fugro Maunsell Scott Wilson Joint Venture (2000). Report on the Debris Flow at Sham Tseng San Tsuen of 23 August 1999 - Findings of the Investigation. Geotechnical Engineering Office, Hong Kong, 92 p. Fugro Scott Wilson Joint Venture (1999). Detailed Study of the Natural Terrain Landslide at No. 30 Pak Sha Wan on 9 June 1998. Landslide Study Report LSR 1/99, Geotechnical Engineering Office, Hong Kong, 47 p. Fugro Scott Wilson Joint Venture (1999). Detailed Study of the Landslide at The Outward Bound School on 9 June 1998. Landslide Study Report LSR 7/99, Geotechnical Engineering Office, Hong Kong, 88 p. GEO (1996). Report on the Shum Wan Road Landslide of 13 August 1995 - Findings of the Landslide Investigation (Vol. 2). Geotechnical Engineering Office, Hong Kong, 51 p. Halcrow Asia Partnership (1998). Detailed Study of the Landslides at Tao Fung Shan Christian Cemetery on 2 July 1997. Landslide Study Report LSR 2/98, Geotechnical Engineering Office, Hong Kong, 46 p. Halcrow Asia Partnership (1998). Detailed Study of the Landslide Near Lido Beach, Castle Peak Road on 2 July 1997. Landslide Study Report LSR 8/98, Geotechnical Engineering Office, Hong Kong, 38 p. Halcrow Asia Partnership (1998). Detailed Study of the Landslides below Sha Tin Heights Road on 4 June 1997. Landslide Study Report LSR 10/98, Geotechnical Engineering Office, Hong Kong, 47 p.
- 90 Halcrow Asia Partnership (1998). Detailed Study of the Landslide Behind Block 3 and 4 Wonderland Villas, Kwai Chung on 8 May 1997. Landslide Study Report LSR 11/98, Geotechnical Engineering Office, Hong Kong, 30 p. Halcrow Asia Partnership (1998). Detailed Study of the Landslide Opposite Shing On Temporary Housing Area, Ma On Shan Road on 2/3 July 1997. Landslide Study Report LSR 19/98, Geotechnical Engineering Office, Hong Kong, 35 p. Halcrow Asia Partnership (1998). Detailed Study of the Landslides at Ka Tin Court, Shatin on 2 July 1997. Landslide Study Report LSR 24/98, Geotechnical Engineering Office, Hong Kong, 29 p. Halcrow Asia Partnership (1998). Report on the Landslides at Hut No. 26 Kau Wah Keng Upper Village of 4 June 1997 - Findings of the Landslide Investigation. Geotechnical Engineering Office, Hong Kong, 47 p. King, J.P. (1996a). The Tsing Shan Debris Flow. Special Project Report SPR 6/96, Geotechnical Engineering Office, Hong Kong, 3 volumes, 427 p, 129 p & 166 p. Wong, H.N., Chen, Y.M. and Lam, K.C. 1996. Factual Report on the November 1993 Natural Terrain Landslides in Three Study Areas on Lantau Island. Special Project Report SPR 10/96, Geotechnical Engineering Office, Hong Kong, 21 p. plus 3 Appendices.
- 91 LIST OF FIGURES Figure No.
Page No.
A1
Schematic Diagram of a Typical Natural Terrain Landslide Scar
92
A2
General Classification of Hillside Failures
93
- 92 -
Figure A1 - Schematic Diagram of a Typical Natural Terrain Landslide Scar
Disturbed-hillside failure
Classification
Natural hillside failure
Type A
Natural hillside not modified by man-made activities
Disturbed hillside locally modified by man-made activities
Disturbed hillside locally modified by man-made activities Non-registerable cut and fill slopes
Debris
Type B
Registerable man-made feature
Natural or disturbed hillside
Non-registerable cut and fill slopes Debris
Debris
Debris Registerable man-made feature
Hillside failure involving registerable man-made features
Registerable man-made feature
Registerable man-made feature
Example
- 93 -
1990 Tsing Shan debris flow (SPR 6/96) 1997 Wonderland Villas (LSR 11/98) 1998 Pak Sha Wan (LSR 1/99) 1998 Outward Bound School (LSR 7/99) 1999 Sham Tseng San Tsuen (Forensic)
1993 Lantau Trail (SPR 10/96) 1997 Lido Beach (LSR 8/98) 1997 Ka Tin Court (LSR 24/98)
1997 To Fung Shan Cemetery (LSR 2/98) 1997 Shatin Heights Road (LSR 10/98)
Figure A2 - General Classification of Hillside Failures
1994 Shum Wan Road (Forensic) 1997 Ma On Shan (LSR 19/98) 1997 Kau Wah Keng (Forensic)
- 94 -
APPENDIX B NOTABLE NATURAL TERRAIN LANDSLIDES IN RECENT YEARS
- 95 CONTENTS Page No. Title Page
94
CONTENTS
95
B.1 LIU POK, LO WU
96
B.2 LIDO BEACH, CASTLE PEAK ROAD
96
B.3 WONDERLAND VILLAS, KWAI CHUNG
96
B.4 OPPOSITE SHING ON TEMPORARY HOUSING AREA, MA ON SHAN ROAD
97
B.5 PAK SHA WAN, SAI KUNG
97
B.6 OUTWARD BOUND SCHOOL, SAI KUNG
98
B.7 QUEEN’S HILL, BURMA LINES CAMP, FANLING
98
B.8 LUK KENG WONG UK, LUK KENG
98
B.9 7C BOWEN ROAD
99
B.10 NO. 37 MOUNT DAVIS ROAD
99
B.11 KAP LUNG, SHEK KONG
100
B.12 SHAM TSENG SAN TSUEN
100
B.13 NO. 92 TAK SHEK WU KIU TAU, PAT HEUNG
101
B.14 VTC POKFULAM SKILLS CENTRE, POKFULAM
101
B.15 THE 2000 TSING SHAN DEBRIS FLOW
102
B.16 LEUNG KING ESTATE, TUEN MUN
102
LIST OF FIGURES
104
- 96 The information below presents the key features of recent notable natural terrain landslides that have been studied and documented in a systematic manner. In addition to the study reports, the main findings of some of these landslides are available on the GEO Hong Kong Slope Safety Website, http://hkss.ced.gov.hk under “Findings of Landslide Investigation”. B.1 LIU POK, LO WU The landslide occurred in September 1993 (Figure B1a). It involved approximately 300 m3 of debris, which was channelized along an ephemeral streamline and partially destroyed an abandoned school building. The travel angle of the landslide debris was about 24º. The failed material comprised colluvium, which overlies slightly to moderately decomposed metamorphosed sandstone. In parts the rupture surface comprises sandstone bedding planes dipping obliquely out of the slope at 28º. Reference: King, J.P. (1997). Damage to Liu Pok School by a Natural Terrain Landslide. Discussion Note DN 2/97, Geotechnical Engineering Office, Hong Kong, 66 p. B.2 LIDO BEACH, CASTLE PEAK ROAD The 2 July 1997 landslide occurred on a 15 m high (Figure B1b), partly modified natural hillside and affected an adjacent fill slope that was formed in the 1950s. The landslide involved two phases of detachment, with a source volume of approximately 750 m3, predominately comprising completed decomposed granite with some fill and residual soil. The debris travelled some 70 m and completely blocked the Castle peak Road. Eight people were trapped and injured by the debris associated with the second phase of failure. The travel angle of the debris was about 20º, which reflects that the failure was mobile when compared to other rain-induced landslides of similar volume in Hong Kong. The landslide was probably triggered by an increase in groundwater pressure in the slope caused by the effects of direct infiltration of rainwater and surface water ponding within an abandoned property directly upslope of the landslide. Reference: Halcrow Asia Partnership (1998). Detailed Study of the Landslide Near Lido Beach, Castle Peak Road on 2 July 1997. Landslide Study Report LSR 8/98, Geotechnical Engineering Office, Hong Kong, 38 p. B.3 WONDERLAND VILLAS, KWAI CHUNG The 8 May 1997 landslide occurred on the natural terrain behind Blocks 3 and 4 of the development (Figure B1c). The landslide debris travelled about 200 m down the slope and partially blocked one lane of Wah King Hill Road. The landslide occurred at the head of a poorly defined drainage line and the travel angle of the landslide was about 34º. The landslide material predominately comprised completely decomposed granite with the surface of rupture formed at the interface between
- 97 PW 0/30 and PW 50/90. From aerial photographs, numerous relict scars are evident in the drainage lines to the north of the failure. Reference: Halcrow Asia Partnership (1998). Detailed Study of the Landslide Behind Block 3 and 4 Wonderland Villas, Kwai Chung on 8 May 1997. Landslide Study Report LSR 11/98, Geotechnical Engineering Office, Hong Kong, 30 p. B.4 OPPOSITE SHING ON TEMPORARY HOUSING AREA, MA ON SHAN ROAD The 2/3 July 1997 landslide, which had a source volume of about 3 000 m3, occurred in a drainage line on a hillside that varies from 38º-40º (Figure B1d). The failure was probably initiated by a small-scale failure of a cut slope at the toe of the natural terrain that resulted in retrogressive failure. It affected an abandoned cut slope and two fill platforms. The surface of rupture was within completely decomposed granite, and inclined at 32º-38º. Part of the rupture surface was formed by a clay-coated discontinuity. The travel angle was about 28º. Numerous relict landslides were identified from aerial photographs. The failure was associated with the very severe rainstorm on 2/3 July 1997. This resulted in the highest maximum daily rainfall since the installation of the nearest rain gauge in 1983. Based on record from the Hong Kong Observatory, the storm had an estimated return period of about 1 000 years. Reference: Halcrow Asia Partnership (1998). Detailed Study of the Landslide Opposite Shing On Temporary Housing Area, Ma On Shan Road on 2/3 July 1997. Landslide Study Report LSR 19/98, Geotechnical Engineering Office, Hong Kong, 35 p. B.5 PAK SHA WAN, SAI KUNG The 9 June 1998 landslide with a source volume of 300 m3 occurred in colluvium within a drainage line (Figure B1e). The natural hillside slopes at about 30º and has no evidence of past instability from the available aerial photographs. More than one phase of landsliding occurred over a period of about two hours. The landslide deposited debris on the platform of two village houses at the toe of the slope. The debris travelled downslope in the form of a channelized debris flow and was relatively mobile, with a travel angle of about 25º. There is no evidence of past instability from the available aerial photographs. The failure was probably caused by the development of transient elevated water pressures in the surface mantle of colluvium following a severe rainstorm (estimated return period of about 100 years). Concentrated surface and subsurface water flows, leading to water ingress into the landslide site, probably contributed to the failure. Reference: Fugro Scott Wilson Joint Venture (1999). Detailed Study of the Natural Terrain Landslide at No. 30 Pak Sha Wan on 9 June 1998. Landslide Study Report LSR 1/99, Geotechnical Engineering Office, Hong Kong, 47 p.
- 98 B.6 OUTWARD BOUND SCHOOL, SAI KUNG The 9 June 1998 landslide occurred on natural hillside (Figure B1f). The landslide source volume was 900 m3 and the debris was deposited within 20 m of a house, which was temporarily evacuated. The failed material comprised completely decomposed volcanic tuff and residual soil. The failure occurred in an area with a distinct break in slope, probably the location of an old landslide scarp. The failure, with a travel angle of 27º, probably involved a “retreating” hillside mechanism, i.e. successive discrete failures retrogressing uphill over a period of time. The landslide was triggered by a severe rainstorm (estimated return period of about 70 years). The presence of tension cracks as well as open joints suggests that the hillside was progressively deteriorating with time. The presence of complex local geological and hydrogeological regimes (partly due to faults traversing the hillside) was a contributory factor to the landslide. Reference: Fugro Scott Wilson Joint Venture (1999). Detailed Study of the Landslide at The Outward Bound School on 9 June 1998. Landslide Study Report LSR 7/99, Geotechnical Engineering Office, Hong Kong, 88 p. B.7 QUEEN’S HILL, BURMA LINES CAMP, FANLING This incident involved extensive distress on natural hillside (Figure B1g), with a displaced mass of over 1 500 m3. The identification of distress on the hillside resulted in the evacuation of a number of dwellings at the toe of the hillside. The distress predominantly involved slope movement, tension cracks and localised detachment, affecting both the colluvium and, in parts, the underlying completely decomposed volcanic tuff. Aerial photographs suggest that some of the distress dates back to 1924. The hillside instability was not identified during the detailed study of the cut slope at the toe of the natural hillside. There is evidence, over parts of the hillside, that progressive ground movement and deterioration of the ground condition with time develop into more mobile failures. Reference: Fugro Scott Wilson Joint Venture (1999). Detailed Study of Slope Distress at Queen’s Hill, Burma Lines Camp, Fanling. Landslide Study Report LSR 10/99, Geotechnical Engineering Office, Hong Kong, 102 p. B.8 LUK KENG WONG UK, LUK KENG The 24 May 1998 landslide (Figure B1h), which had a source volume of 150 m3, occurred on a natural hillside with a history of failures. It affected the open areas of some dwellings within the village at the toe of the hillside. The failed material mainly comprised highly to completely decomposed siltstone.
- 99 The landslide debris travelled along a stream course as a channelized debris flow, and the travel angle was about 23º. The landslide occurred under moderately heavy rainfall (estimated return period of 8 years). Reference: Fugro Scott Wilson Joint Venture (1999). Preliminary Study of the Landslide at Luk Keng Wong Uk on 24 May 1998. Landslide Study Report LSR 14/99, Geotechnical Engineering Office, Hong Kong, 33 p. B.9 7C BOWEN ROAD The 11 June 1998 landslide affected a portion of natural hillside (Figure B1i) with no evidence of past instability from the available aerial photographs. The landslide debris was deposited at the rear of a building at Bowen Road and some parts of the building suffered minor damage. As the run-out of the landslide debris was constrained by a building, the travel angle of the landslide debris cannot be determined reliably. The failed material comprised colluvium. The rainfall preceding the landslide (estimated return period of 8 years), together with leakage from an exposed water main along the access road above the crest of the hillside, probably contributed to the landslide. Reference: Fugro Scott Wilson Joint Venture (1999). Detailed Study of the Landslide behind 7C Bowen Road on 11 June 1998. Landslide Study Report LSR 19/99, Geotechnical Engineering Office, Hong Kong, 61 p. B.10 NO. 37 MOUNT DAVIS ROAD The 24 August 1999 failure was a “near-miss” incident in that the debris narrowly missed an apartment block below Mount Davis Road (Figure B1j). The incident involved the fracture of a 150 mm diameter fresh water pipe and undermining of a 1.4 m diameter fresh water main located at the crown of the landslide scar. The latter is an important trunk main of the Mount Davis Service Reservoir, supplying potable water to the Hong Kong South area. The rainfall that triggered the landslide was not severe (estimated maximum return period of about 4 years). The surface runoff from a significant catchment above the landslide site was channelled along a road due to a blocked catchpit, over-spilling the road onto the hillside near a bend. Inadequate provision and maintenance of the surface water drainage provisions above the access road and the lack of surface drainage or an upstand on the downhill side of the access road were probably key contributory factors in causing the failure. Reference: Fugro Maunsell Scott Wilson Joint Venture (2000). Initial Review of the Landslide above No. 37 Mount Davis Road on 24 August 1999. Geotechnical Engineering Office, Hong Kong, 52 p.
- 100 B.11 KAP LUNG, SHEK KONG This August 1999 landslide occurred on a natural hillside (Figure B1k) beneath an electricity transmission pylon situated in a remote area. Extensive vegetation clearance was carried out in the vicinity of the pylon during its construction in 1995. The vegetation remained sparse at the time of failure. The landslide was in the form of a channelized debris flow. The relatively low travel angle of the landslide debris (15º) indicates that the channelized debris flow was mobile, possibly involving a large volume of surface water running down the stream course. The time of failure is not known. The hillside was previously assessed and checked to the required standards, as part of the foundation design for the pylon. However, the stability assessment did not consider a sharp convex break of slope and an area with a steeper ground profile, which probably corresponds to previous instability. Reference: Fugro Maunsell Scott Wilson Joint Venture (2000). Detailed Study of the Landslide above Kap Lung, Shek Kong, during the Severe Rainstorms between 22 and 24 August 1999. Landslide Study Report LSR 3/2000, Geotechnical Engineering Office, Hong Kong, 52 p. B.12 SHAM TSENG SAN TSUEN On 23 August 1999 four landslides occurred on a natural hillside above Sham Tseng San Tsuen (Figure B1l). The landslide debris demolished a house and its detached kitchen structure, and severely damaged several other dwellings within the squatter village. It resulted in one fatality and thirteen injuries. The largest landslide, which was the primary source of the debris flow, had a source volume of 600 m3. The failure occurred at the location of a very old landslide. Much of the colluvium at the landslide location probably originated from a lower landslide scarp, which has progressed up the hillside over a long period of time as a result of weathering and intermittent small- to large-scale movements of the colluvium. The landslide developed into a channelized debris flow down a stream course, with a travel angle of about 24º. The debris flow occurred during a severe rainstorm, with an estimated maximum return period of about 49 years. The subject hillside suffered numerous failures (>10 notable failures) during severe rainstorms in 1982. The August 1999 rainstorm resulted in seven notable landslides, six of which were at different locations compared to the failures in 1982. The landslides were probably caused by the build-up of elevated water pressures within the surface colluvium. The thin mantle of bouldery colluvium overlying relatively less permeable colluvium is favourable to the build-up of perched water tables during heavy rain. The presence of relict failures suggests that the rock mass has been disturbed and weakened. This in turn may have adversely affected the local hydrogeology by promoting direct infiltration of rainwater through the open or partially infilled joints. A hill fire about 4 to 5 years before the failure had also removed much of the vegetation cover. There is evidence of progressive deterioration of the hillside with discontinuities opening up with time, probably associated with past intermittent slope movements.
- 101 Reference: Fugro Maunsell Scott Wilson Joint Venture (2000a). Report on the Debris Flow at Sham Tseng San Tsuen of 23 August 1999 - Findings of the Investigation. Geotechnical Engineering Office, Hong Kong, 92 p. B.13 NO. 92 TAK SHEK WU KIU TAU, PAT HEUNG The 24 August 1999 landslide, with a 260 m3 source volume, occurred on a natural hillside above No. 92 Tak Shek Wu Kiu Tau near Pat Heung (Figure B1m). The landslide debris became channelized and was deposited on a platform immediately adjacent to a residential building near the toe of the hillside. The travel angle of the landslide debris was about 20º, which indicates that the debris was relatively mobile. The channelized debris flow was probably triggered by rainfall, with an estimated return period of about 25 years. The landslide occurred towards the top of an ephemeral drainage line, with its main scarp developed along a pre-existing tension crack (which was noted in the 1996 and 1998 aerial photographs). The tension crack was located below two relict landslide scars. Other relict landslide scars were identified within a number of depressions on the hillsides in the area. The presence of rafts of relatively intact material within the source area, together with the exposure of planar surfaces on the surface of rupture, suggest that the initial instability involved a shallow sliding failure (up to 1 m deep). In addition, the location of the landslide towards the top of the drainage line suggests that the failure may be part of the geomorphological development of the hillside. There is evidence of progressive deterioration of the hillsides. Reference: Fugro Maunsell Scott Wilson Joint Venture (2000). Review of the 24 August 1999 Landslide on the Hillside above No. 92 Tak Shek Wu Kiu Tau near Pat Heung. Geotechnical Engineering Office, Hong Kong. B.14 VTC POKFULAM SKILLS CENTRE, POKFULAM On 23 August 1999 two landslides (Figure B1n) occurred within the same local catchment of the hillside, which has a history of instability. One landslide (100 m3) involved unauthorised end-tipped fill whilst another (200 m3) involved natural terrain with the source material comprising colluvium and weathered volcanic tuff. As a result of the landslide, Victoria Road was temporarily closed to traffic. Debris from both landslides became channelized in an ephemeral stream course and reached Victoria Road, with a travel angle of about 25º. The natural terrain failure occurred in an area showing evidence of progressive deterioration of the ground condition, in the form of localised tension cracking and ground movement. The rainstorm that triggered the landslides was not particularly severe (estimated maximum return period of less than two years). The landslides were probably caused by the build-up of transient elevated groundwater pressure in the soil mass following the rainstorm. Probable contributory factors to the failures were the blockage of surface drainage provisions
- 102 above one of the landslides, changes in slope gradient caused by pervious local instability, and possible deterioration of ground conditions. Reference: Fugro Maunsell Scott Wilson Joint Venture (2000). Detailed Study of the Landslides below VTC Pokfulam Skills Centre, Pokfulam Road on 23 August 1999. Landslide Study Report LSR 7/2000, Geotechnical Engineering Office, Hong Kong, 99 p. B.15 THE 2000 TSING SHAN DEBRIS FLOW On 14 April 2000 a landslide originated at about 360 mPD on the rocky upper slopes of Tsing Shan and extended down to a spur at about 180 mPD where it divided to form channelized debris flows in the two drainage lines on either side of the spur (Figure B1o). The source volume was about 150 m3. A significant amount of materials (colluvium and weathered rock) was eroded along the travel path and the estimated total volume of the natural terrain landslide about 1 600 m3. The event is interpreted to have started in the rocky gully as a debris avalanche that continued down over the open colluvial slope. At the spur some debris flowed down the southern drainage line while a bouldery lobe accumulated on the spur at a power transmission pylon before flowing down the northern drainage line. Channelized debris flows occurred in both drainage lines and the travel angles for the northern and southern branches are 27º and 24º respectively. The trigger for the initial landslide was probably elevated pore water pressure in the bouldery colluvium and weathered profile of the source area. However, it is worth noting that the estimated maximum return period for the rainfall was less than two years. Besides the steep slope gradient (about 40º), the environmental factors that pre-disposed the generation of the debris flow were the accumulation of loose bouldery deposits on the steep slope above well-incised colluvium-lined drainage lines that serve to channelize the debris and contribute additional material. Reference: King, J.P. (2001). The 2000 Tsing Shan Debris Flow. Landslide Study Report LSR 3/2001, Geotechnical Engineering Office, 54 p. plus 1 drg. B.16 LEUNG KING ESTATE, TUEN MUN On 14 April 2000 during heavy rain, a series of landslides occurred on the hillside above Leung King Estate, Tuen Mun. One of the natural terrain landslides involved about 600 m3 of material (source volume 450 m3 and entrained volume 150 m3) which developed into a channelized debris flow (Figure B1p) which came to rest close to the toe of the hillside. The subsequent outwash blocked the drainage at the crest of a cut slope above Leung King Estate, causing inundation of mostly silty sand into the northern portion of the Estate. The source area of the landslide is a natural depression, located at the head of a minor natural drainage line below a granite cliff. The surrounding natural hillside slopes at 30º to 40º. The failed material comprised colluvium and weathered profile including slightly
- 103 decomposed rock. The surface rupture generally followed a series of undulating sheeting joints orientated sub-parallel with the ground surface. A notable feature about the landslide is the probable formation and subsequent breaching of a debris dam within the natural drainage line. Landslide debris that formed the dam had a travel angle of about 27º. The breaching of the debris dam resulted in a travel angle of 17º for the debris flow, which is indicative of a mobile failure involving fluidized debris. A number of recent and relict landslides have been identified from aerial photographs in the vicinity, most of them of small size. There is evidence of disturbance within the rock mass exposed at the landslide, including open sediment infilled joints, offset joints, offset quartz veins and fractured rock. Reference: Halcrow China Ltd. (2001). Detailed Study of Selected Landslides above Leung King Estate of 14 April 2000. Landslide Study Report LSR 9/2001, Geotechnical Engineering Office, Hong Kong, 140 p.
- 104 LIST OF FIGURES Figure No.
Page No.
B1a-d
Notable Natural Terrain Landslides in Recent Years
105
B1e-h
Notable Natural Terrain Landslides in Recent Years
106
B1i-l
Notable Natural Terrain Landslides in Recent Years
107
B1m-p
Notable Natural Terrain Landslides in Recent Years
108
- 105 -
(a) Liu Pok, Lo Wu
(b) Lido Beach, Castle Peak Road
(c) Wonderland Villas, Kwai Chung
(d) Opposite Shing On Temporary Housing Area, Ma On Shan Road
Figure B1a-d - Notable Natural Terrain Landslides in Recent Years
- 106 -
(e) Pak Sha Wan, Sai Kung
(f) Outward Bound School, Sai Kung
(g) Queen’s Hill, Burma Lines Camp, Fanling
(h) Luk Keng Wong Uk, Luk Keng
Figure B1e-h - Notable Natural Terrain Landslides in Recent Years
- 107 -
(i) 7C Bowen Road
(j) No. 37 Mount Davis Road
(k) Kap Lung, Shek Kong
(l) Sham Tseng San Tsuen
Figure B1i-l - Notable Natural Terrain Landslides in Recent Years
- 108 -
(m) No. 92 Tak Shek Wu Kiu Tau, Pat Heung
(n) VTC Pokfulam Skills Centre, Pokfulam
(o) The 2000 Tsing Shan Debris Flow
(p) Leung King Estate, Tuen Mun
Figure B1m-p - Notable Natural Terrain Landslides in Recent Years
- 109 -
APPENDIX C GEOMORPHOLOGY AND NATURAL TERRAIN HAZARDS
- 110 CONTENTS Page No. Title Page
109
CONTENTS
110
C.1 INTRODUCTION
111
C.2 ROCK TYPE AND STRUCTURE
112
C.2.1 Introduction
112
C.2.2 Volcanic Rocks
112
C.2.3 Granitic Rocks and Rhyolitic Dykes
112
C.2.4 Sedimentary Rocks
113
C.2.5 Structures
113
C.3 WEATHERING
114
C.4 REGOLITH
115
C.5 HYDROGEOLOGY
115
C.5.1 Groundwater Conditions
115
C.5.2 Piping
116
C.6 MASS MOVEMENTS
117
C.6.1 Introduction
117
C.6.2 Interpretation from Aerial Photograph
117
C.6.3 Landslide Types
118
C.6.3.1 General
118
C.6.3.2 Rock and Boulder Falls
118
C.6.3.3 Slumps to Debris Flows
118
C.6.3.4 Large Relict Landslides
121
C.7 REFERENCES LIST OF FIGURES
121 126
- 111 C.1 INTRODUCTION The morphology of the present day landscape of Hong Kong is generally considered to have developed mainly during the Quaternary Period (1.4 million years ago to the present), but it records processes that probably began during the Tertiary (Ruxton & Berry, 1957), perhaps as long as 60 million years ago. The last glaciation commenced 25 000 years ago, reaching a maximum 17 000 to 18 000 years ago, when sea level was at around -120 mPD or -130 mPD. At that time, the coastline was about 120 km south of Hong Kong (Fyfe et al., 2000). When global deglaciation began about 11 000 to 12 000 years ago, the sea level rose and a new coastline was established. A postglacial sea level high, possibly 1 to 3 m above its present level, was attained about 7 000 years ago. Substantial erosion occurred at the new coastline(s). This involved, and continues to involve, coastal landslides, and reactivation of relict coastal landslides, as a major component of modification of the landscape. Mass movements have dominated the erosion of mountain slopes, particularly during wetter periods, which are postulated at about 12 500 to 11 000, 10 500 to 8 000 and 7 000 to 5 500 years ago. Tregear (1958) compiled the first regional description of the geomorphology of Hong Kong. Later, Berry and Ruxton (1960) carried out a detailed study of the evolution of the Victoria Harbour area. Berry (1961) was the first to recognize erosion surfaces in Hong Kong, identifying planation surfaces at around 130 mPD and 230 mPD, with concordant summit levels at about 480 mPD. Other regional geomorphological summaries have been compiled by Hansen (1984) and So (1986). The agricultural soils of Hong Kong were mapped and described by Grant (1962, 1986), who also discussed soil development and erosion (Grant, 1968) in the context of the geomorphology of the area. The landscape of Hong Kong has a distinctive drowned appearance with an intricate coastline. This is particularly noticeable in the east where the hillslopes plunge directly into the sea, without the development of coastal plains or extensive wave cut platforms that would indicate prolonged coastal erosion. The limited erosion that has occurred has only succeeded in over-steepening the hillslopes to form steep fringing cliffs. Similarly, most of the drainage lines are short and appear to have been truncated. Extensive deposition in the drowned coastal extremities of the valleys has formed distinctive alluvial flats around the coastline. As is typical of subtropical upland areas, current geomorphological processes are dominated by mass movements and large-scale, but intermittent, erosional events. The effects of these periodic processes are superimposed on locally deeply weathered profiles. Weathering has produced a smooth, rounded topography with a thick regolith cover on the ridge tops and side slopes, incised by the valleys of a slowly evolving drainage system. Slope failures and fluvial erosion add an irregular and angular overprint of failure scars, free faces, and rocky ridgelines. Mass movements are currently the most important geomorphological processes in Hong Kong. They have displaced much of the weathered mantle on hillslopes, forming extensive colluvial deposits on the shallower slopes, and large debris sheets and lobes on the steeper slopes and in the steep tributary valleys. Fluvial processes of rilling, gullying and stream flow have generally played a lesser role, primarily serving to progressively erode the matrix from weathered profiles and to rework mass movement debris.
- 112 C.2 ROCK TYPE AND STRUCTURE C.2.1 Introduction Three main rock types make up the geology of Hong Kong. These are, in order of importance, volcanic rocks, granitic rocks and sedimentary rocks. Volcanic and granitic rocks together crop out over about 70% of the area. Faults control the major topographical grain, largely determining the orientations of the major valleys and ridges. In particular, northwestand east-northeast- to east-trending faults have been preferentially weathered and eroded, forming distinctive linear depressions. Late Pleistocene and Holocene superficial deposits form a discontinuous cover, with colluvium on hillsides and alluvium on the valley floors. The geology of Hong Kong is described in Sewell et al. (2000) and Fyfe et al. (2000). C.2.2 Volcanic Rocks Volcanic rocks are exposed over about 48% of the land area of Hong Kong and generally form the higher, more angular and rugged uplands. These rocks occur over much of the New Territories, especially in eastern Hong Kong and most of the country parks, Lantau Island, and southern Hong Kong Island. Volcanic terrain, which commonly includes interbedded sedimentary units, is typically angular, characterized by moderate to steep slopes, gently concave sideslopes and narrow convex ridges. Bedrock exposures are a feature of ridgelines, with free faces occurring on spur ends and steeper slopes. Large boulders are common, littering slopes below these exposures. Commonly, a thin veneer of bouldery colluvium mantles the slopes. Volcanic rocks underlie the highest peaks and ridges of Hong Kong. Tai Mo Shan, Sunset Peak, Lantau Peak, Fei Ngo Shan to Tate’s Cairn, Ma On Shan, High Junk Peak, and Sharp Peak all form prominent, angular facets of the landscape that stand in sharp contrast to the more rounded forms of the granitic hills. Similarly, volcanic rocks tend to form the steeper, cliffed coastlines, particularly in the east. Massive, coarse ash tuff weathers in an analogous way to the granitic rocks, with moderately thick weathered profiles, corestones, and uplands with generally smoothed outlines. Fine ash tuff and eutaxite have locally pronounced bedding structures or bedding parallel foliation. They weather less deeply, rarely develop corestones, and produce shallow, clayey soils on very angular uplands. The eutaxitic rocks display a pronounced layered structure that tends to form escarpments and distinctive topographical features. Evidence of mass movements, both recent and relict, is common in the shallow, clayey weathering residues that are characteristic of the volcanic rocks. Erosion gullies are rarely developed, in contrast to the deeply weathered granitic rocks. Colluvial deposits, derived from the shallow weathered profiles, are thinner than over the more deeply weathered granitic rocks. C.2.3 Granitic Rocks and Rhyolitic Dykes Granitic rocks are exposed over about 28% of Hong Kong, with most of the main urban areas of Hong Kong built on these rocks. Granite topography is distinguished by low rounded hills, with a more subdued topography than that developed on the volcanic rocks. Weathered mantles and soils are generally thicker than on the volcanic lithologies and there is
- 113 generally less evidence of mass movements, although gully erosion is more common. The distinctive scenery of the granitic rocks characterizes many areas of Hong Kong, in particular the western New Territories, the central New Territories from Tsuen Wan to Sha Tin, the Kowloon Peninsula and Lion Rock ridge, the northern and central parts of Hong Kong Island from Happy Valley to Shau Kei Wan, and the southern parts of Lamma and Lantau islands. Granite landscapes typically exhibit a subdued relief, comprising rounded summits with long convex upper slopes and a relief amplitude of generally less than 70 m. Deep weathering is a distinctive feature, with most areas underlain by a thick, granular weathered mantle. Deep gullies are a distinctive feature in some areas of granitic terrain, with many unvegetated ridge crests and side slopes being extensively gullied. Tors are common features in granite landscapes. They occur as prominent exposures of jointed, relatively unweathered granite in a variety of topographical situations, as hill summit, ridge crest, spur end and valley side features. Granite boulders, which are typically rounded, litter the surrounding slopes. The boulders originate as exhumed corestones, either by erosion of the finer weathered matrix on slopes, or by toppling from cliff faces and tors. Boulder fields are common on summit plateaus, the sides of ridges, and on open slopes, but characteristically choke valley lines as distinctive boulder trains. Mass movements are less common in areas of weathered granite than on the more clayey, less freely drained volcanic rocks. However, colluvial deposits are generally thicker over the granites, reflecting derivation from deeper weathered profiles. Large-scale sheet jointing is common, paralleling the smooth hillslopes. Differential weathering of the feldsparphyric dykes in the northeast of Lantau Island has produced a distinctive lineated topography with prominent ribs, ridges and ledges that are oriented in an east-northeast direction. These ridges comprise coarse, joint-bounded blocks that topple and collapse to feed debris streams and sheets on the flanking slopes. C.2.4 Sedimentary Rocks Sedimentary rocks are exposed over only about 5% of the Hong Kong onshore area. They occur mostly in the northern and northeastern New Territories, where they form the intricate and varied scenery of the outlying islands. Pronounced bedding structures impart a clear grain to the topography, forming continuous ridges that support a trellised drainage pattern. Vertically dipping rocks, such as at Bluff Head, form a distinctive ridgeline along the northern shore of the Tolo Channel. Sedimentary rocks, in general, have a higher incidence of landslide scars than the other lithologies (Evans & King, 1998). C.2.5 Structures Geological structures occur on a wide range of scales. They can be distinguished at the regional, local and outcrop level, and include faults, joints, and bedding planes. Weathering and erosion have exploited these structural discontinuities to produce a distinctive grain or fabric to the topography and to individual bedrock exposures. The regional grain is
- 114 reflected in the form of the hills and the outline of the coast. Differential weathering of exposed rock outcrops commonly emphasizes the rock structure or fabric, producing a range of geometrical patterns. The dominant structural trend in Hong Kong is aligned northeast, varying to east-northeast. A subordinate, orthogonal northwest trend is also present and becomes dominant in east and southeast Hong Kong. Alignments of the main valleys, ridges and uplands show a broadly rectilinear pattern that reflects the effects of weathering and erosion along these main structural trends, as does the coastal estuaries, channels, bays and headlands. C.3 WEATHERING There is a close correlation between the topography of the weathering front and the main structural trends in Hong Kong (e.g. Shaw, 1997; Owen & Shaw, 2000). Geological structures determine local hydrogeology, and consequently the intensity of weathering and eluviation. In most slopes, the base of the weathered mantle, or weathering front (i.e. “rockhead” in engineering use), is subparallel to the topography. Generally, the weathering front tends to rise nearer the ground surface below hills, which usually occur in less well-jointed rocks, although it is not unusual to find a thick weathering profile preserved below ridge crests. In many areas the weathering front is stepped, a phenomenon that depends on the geometry of the joint pattern and its infilling by impermeable clay minerals. The steps (varying from millimetres to 20 m or more) commonly coincide with subvertical or steeply inclined faults, or major joints. The weathering front commonly descends abruptly below fault-controlled valleys as a result of preferential weathering along the discontinuity. The linear, discontinuity-controlled zones of deep weathering are easily eroded to create the main valleys. However, most of the minor stream valleys have rock floored channels. Geomorphologically, the weathering profile can be subdivided into several broad zones. The term saprolite is usually applied to the lower, sedentary part of the profile, that has not undergone marked volume changes and preserves original structures, such as joints and quartz veins, features directly inherited from the parent rock. Above the saprolite zone, towards the ground surface, there is usually a mobile zone of slightly transported material. This shows evidence of disturbance by gravitational creep, settling, and bioturbation, notably by termites and plant roots. The mobile zone is commonly subject to extensive leaching of soluble components and eluviation of the fines. Loss of rock texture, the disturbance of structures such as relict joints, the development of stone lines, and the appearance of outcrop curvature, are features that distinguish this portion of the profile. However, in most weathered profiles in Hong Kong, the mobile zone is usually only a few metres thick. Within a given rock type, the depth of weathering is primarily a function of the depth to which oxygenated surface water penetrates and circulates through the rock mass. As discontinuities are the only permeable pathways in most igneous rocks, regional and local patterns, spacing and openness of discontinuities in the rock mass largely determine the groundwater regime, and thus the character of the weathered profile, which ultimately influences the subsequent topography. The thickest weathered profiles in Hong Kong are generally developed in uniform granitoid rocks, ranging from a few metres to a several tens of metres. For example,
- 115 weathering depths over the Kowloon peninsula vary from about 10 metres over the relatively high areas, to greater than 40 metres in the low lying districts (Shaw, 1997). In contrast, fine-grained tuff, rhyolite, and sedimentary rocks tend to generate relatively shallow weathering profiles of no more than a few metres thick, and with few corestones. Zones of particularly deep weathering are usually associated with faults, irrespective of the lithology. Studies have shown that, at its deepest point, weathering of volcanic rocks in the fault-controlled Tuen Mun valley extends to a depth of -86 mPD with a weathering profile over 80 metres thick (Owen & Shaw, 2000). Similarly, fault controlled weathering at Tung Chung New Town has resulted in a weathering profile more than 170 m thick (Fyfe et al., 2000). Weathering also affects colluvium and alluvium. Because the degree of weathering increases with the age of the deposit, features such as the thickness of the weathering rind around colluvial clasts can be used to determine the relative age of these deposits (Lai & Taylor, 1984). C.4 REGOLITH Colluvium blankets many slopes although, commonly, it is too thin to have been mapped previously. The Hong Kong Geological Survey regional geological maps at 1:20 000-scale show only superficial deposits greater than 2 m thick. Colluvium was generally classified as debris flow deposits, although talus was recognized in specific localities. Mapped deposits of colluvium are thickest on the footslopes. They form extensive deposits over the base of the Lion Rock ridge, the Mid-levels area, the slopes of Fei Ngo Shan, and other large hill masses in the New Territories, such as below Castle Peak, Tai Mo Shan and on Lantau Island. With a detailed interpretation of aerial photographs followed by field verification, it is possible to develop a process-based colluvium classification based on its morphology and topographical location. For example, relatively recent colluvium can be subdivided into rockfall debris below rock outcrops, valley colluvium along drainage lines, and recent landslide debris (Figure C1); older colluvium can be subdivided into residual colluvium which has lost its original form due to subsequent weathering and erosion, and relict landslide deposits which retains its morphological characteristics such as fan-like or lobate form (Figure C1). It is also possible to identify corestone bearing saprolite, non-corestone bearing saprolite, or rock at or near ground surface (Figure C1). This type of regolith mapping is proving useful in studying landslide initiation factors in the on-going area study of natural terrain hazard study for the Tsing Shan foothills. C.5 HYDROGEOLOGY C.5.1 Groundwater Conditions In Hong Kong groundwater conditions are largely a function of the seasonal rainfall patterns, the geology and the local topography. Discontinuities, such as faults and joints, act as important conduits for the movement and storage of groundwater. In general, the igneous rocks have a low porosity that may be increased by tectonic fracturing and the opening of joint sets by weathering and related stress relief. Consequently, groundwater movement and
- 116 storage is essentially a function of the spacing and aperture of the discontinuities. Commonly, the rise in the groundwater table during a major rainstorm is of the order of several metres and, in extreme cases, may be up to 20 m or more (e.g. Sun & Campbell 1998; Nandy & Jiao, 2001). Groundwater table response may be rapid, and occur broadly synchronously with the rainstorm, or may be delayed for periods of up to several days, reflecting the complexity of the hydrogeological regimes and the distance travelled from the point of infiltration. Transient elevation of pore pressure, during and following heavy rainfall, is the main cause of slope failure in Hong Kong. The four main hydrogeological factors, consistently identified as contributing to landsliding, are: (i) the direct rainwater infiltration forming a wetting front; (ii) the development of perched water tables, often in association with relatively impermeable, persistent clay seams (e.g. GEO, 1996a); (iii) throughflow along high permeability layers or soil pipes subparallel to the ground surface creating seepage pressures and, if the conduits are blocked, a rapid and possibly very high local increase in pore water pressure; and (iv) a general rise in the groundwater table at a site due to artesian seepage from bedrock at the toe of large hill masses. The latter is commonly a delayed response that may occur several days after heavy rainfall. Individually or in combination, these mechanisms can cause a rapid and extreme elevation of the groundwater table (e.g. Sun & Campbell, 1998; Nandy & Jiao, 2001). Natural vegetation distribution is a function of the slope hydrology, which is a function of geomorphology, and specifically the slope aspect, slope angle, exposure, and elevation, which influence microclimate and soil development at a particular site. Man’s influence, particularly the role of frequent hill fires during the dry season (October to April), continues to modify these natural patterns. C.5.2 Piping Soil piping, or natural tunnel erosion, is widespread in Hong Kong, and has been reviewed by Nash and Dale (1984), and Hansen and Nash (1985). A comprehensive model for soil piping in Hong Kong has been developed by Nash and Dale (1984). Soil pipes are essentially preferential flow paths for groundwater. They evolve progressively due to eluviation leading to interconnection of voids and further erosion and expansion of the pipe. Most reported examples in Hong Kong are less than 100 mm diameter, and occur within 1 m of the surface. They commonly occur within superficial deposits, most notably colluvium and residual soils. However, Whiteside (1996) has demonstrated the presence of a network of pipes at the base of a more than 10 m thick regolith. Initiation of soil pipes at the surface can be attributed to various features that facilitate water ingress. These include shallow surface depressions, desiccation cracking, in particular of clay and organic rich soils, hollows and tubes formed by decayed tree roots, animal burrows, and even insect nests. In addition, factors such as excavation, tension cracks associated with landsliding, and the removal of vegetation cover can also promote soil pipe development. Piping commonly occurs at textural or permeability boundaries in the regolith, including colluvium-in situ soil boundaries and boundaries within the weathering profile.
- 117 Shallow soil pipes are most common in the coarser-grained granitic and volcanic rocks, generally in corestone-bearing profiles, and other highly permeable soils, including gravel and cobble rich colluvium. Erosion within shallow pipes can lead to collapse of their roofs, and to the initiation of gully erosion. Shallow pipes may coalesce and feed into deeper seated, more substantial pipes in completely to highly decomposed in situ material. These deeper pipes are influenced in their development by relict joints, especially within coarser-grained rocks (Nash & Dale, 1984). The deeper-seated pipes (e.g. exposed in landslide main scarps) may be up to 1 m in diameter. Where open-ended and free draining, the soil pipes may provide efficient drainage within the slope. However, soil pipes may be closed, have become blocked, or their capacity exceeded, during a period of extreme rainfall. In such circumstances, they have been implicated at many locations in Hong Kong as having had an adverse influence on slope stability because they lead rapidly to very high pore pressures (e.g. Nash & Dale, 1984; Koor & Campbell, 1998; GEO, 1998; Whiteside, 1996). Infilling of pipes by sediment may be partial (Nash & Dale, 1984) or complete (GEO, 1998). C.6 MASS MOVEMENTS C.6.1 Introduction In Hong Kong, deep weathering profiles, steep topography and high rainfall have resulted in mass movements being an important landscape forming process. Landslide scars, which range in size from small depressions to valley-side amphitheatres, are a common feature of natural hillsides (Evans et al., 1997). Together, they probably represent a landslide history spanning at least the Holocene Period (the last 10 000 years). Recent scars are largely unvegetated and can be easily and unambiguously identified either in the field or on aerial photographs, a record that covers most of the territory from 1945 or 1963 to the present. Relict scars retain a sharp morphology, but are completely overgrown with vegetation. Most of the larger scars, which have a degraded morphology, are assumed to be very old and are referred to as ancient landslides. C.6.2 Interpretation from Aerial Photograph Aerial photograph studies have shown that revegetation of landslide scars in Hong Kong is relatively rapid. In general, landslide trails are completely vegetated within 18 years of the event. The source areas vegetate rather more slowly. They are typically 70% revegetated after 20 to 30 years, and are commonly about 90% revegetated after 35 years (Evans et al., 1997). Therefore, field verification of landslides that are more than 20 years old can prove difficult. Similarly, the identification of landslide scars that significantly pre-date available aerial photographs must rely on recognizing the degraded landforms. Interpretation of high level aerial photographs has determined that an average of about 320 landslides occurred each year between 1945 and 1994 (Evans & King, 1998). Landslides are commonly initiated on the middle and upper sections of natural slopes and occur as clusters on slopes of 30° to 40° gradient. They are usually concentrated in the footprint of intense rainstorms, with densities of more than 10 per km2 occurring if the 24 hour rainfall exceeds about 20% of the mean annual rainfall (Evans, 1997). Most of the landslides triggered under the present climatic regime are shallow debris avalanches that have typical volumes of between 50 m3 and 2000 m3. Much less commonly, larger natural terrain
- 118 landslides may occur (King 1996a) or large failures of natural terrain can be triggered by human activity (GEO, 1996b; Cooper, 1992). C.6.3 Landslide Types C.6.3.1 General Historically, landslides have been classified according to a wide range of geological, geomorphological and engineering criteria, depending upon the intended purpose of the scheme (e.g. Sharpe, 1938; Varnes, 1958, 1978; Hutchinson, 1988; Cruden & Varnes, 1996). The most commonly used international landslide classification is that of Cruden and Varnes (1996). Definitions of landslide types are given in the Glossary of Terms at Section D1 of Appendix D. A general classification scheme for hillside failures in Hong Kong is given in Section D2 of Appendix D. Most natural terrain landslides in Hong Kong involve displacement of the weathered profile and colluvium. The displaced material is termed debris. Generally it is difficult to determine the dominant mechanism of movement, largely because many landslides are rapid with most of the displaced material transported beyond the source area. For this reason the more general term avalanche is used to describe the movement of these types of landslide. Several other types of landslide are recognized in Hong Kong, namely rock falls, boulder falls, earth slumps, debris slides, debris flows and channelized debris flows. Some well-documented landslides that occurred in recent years are given in Appendix A. C.6.3.2 Rock and Boulder Falls Rock and boulder falls are restricted to specific materials and facets of the landscape. Rock falls originate from rock faces as falls, slides or topples of joint-bounded, and thus generally angular, rock fragments. These blocks move downslope by combinations of rolling, bouncing and sliding. Boulder falls occur when generally rounded corestone or colluvial boulders, located at or near the ground surface, are dislodged from the regolith, either by fluvial undercutting or by landslides. On straight slopes, once displaced, the rounded boulders could be expected to be more prone to rolling than would the angular rockfall debris. C.6.3.3 Slumps to Debris Flows Slumps, debris slides, debris avalanches, and channelized debris flows are points on a continuum that reflects the increasing run-out of generally granular materials on slopes. This continuum presents the end points for a classification scheme, and also a series of stages through which an individual landslide may progress as a channelized debris flow develops. Consequently, individual landslides could be classified anywhere between the four definitive types. The source areas of most of these landslides have similar slope angles, materials, and hydrogeological characteristics. The displaced material is normally debris, and commonly soil pipes are exposed in the main scarp and source floor. Slides are failures in which the dominant mechanism of movement is sliding.
In this
- 119 case the displaced material will remain an intact mass (Figure C2d). Inevitably there will be some attrition of the displaced intact mass around its unsupported edges and at the base where sliding occurs, but this should not involve a significant proportion of the whole mass. Sliding along a basal plane in the source area can be confirmed by fieldwork if slickensides are present. However, these have only been found at a small proportion of natural terrain landslides in Hong Kong, mainly where the failure occurs along relict joints or a planar bedrock/colluvium interface. Even then, only a few of these landslides were classified as debris slides, because in most cases the displaced material broke up resulting in debris avalanches. A slump is a form of slide in which the intact displaced mass partially overlies the surface of rupture. Slumps range considerably in size. Smaller examples of debris slumps are the relatively common shallow displacements of hillslope debris defined by tension cracks or small landslide scarps, such as the Burma Lines failure (Fugro Scott Wilson, 1999a). At the other end of the scale are the larger, deeper seated, translational or rotational, inactive ancient failures that have been recognized at several localities, for example at Area 19 in Tuen Mun (Scott Wilson, 1999). In addition, a number of failures of natural terrain triggered by excavations fall into this class (Koor & Campbell, 1998; Lau & Franks, 1998). A debris slide is the next stage of mobilisation where the displaced, but still intact, mass moves beyond the plane of rupture by sliding. True debris slides are seldom encountered due to the propensity of a coarse granular soil mass to break up when sliding over an irregular surface, although isolated rafts of intact displaced material are commonly found associated with remoulded landslide debris. These features probably represent a transitional stage between a debris slide and debris avalanche. The Shum Wan Road landslide of 1995 (GEO, 1996b) was a very large (26 000 m3) rock/debris slide. Observations during fieldwork revealed that the displaced mass at the slope toe was essentially intact, although this was not discernible from aerial photographs. Debris avalanches (Figure C2) are features in which most of the displaced material breaks up and becomes remoulded, but no additional surface water is incorporated into the debris. The initial displacement of material at the source may be by sliding at the base of the displaced mass, although there are few examples with clear field evidence for this. In most cases the surface of rupture is rough, comprising in-situ weathered rock with embedded corestones or colluvium containing large clasts. Thin layers of remoulded debris and rafts of intact soil may locally overlie the surface of rupture. Together this evidence suggests that other processes may be involved such as the localized flow of the displaced material due to elevated pore water pressure. This could be caused by perched water tables from infiltration and soil pipes, seepage pressures, and the collapse of soil structure in undrained conditions. As movement continues, and the intact material breaks up into landslide debris, components of undrained loading, rolling, bouncing and inertial granular or slurry flow (Pierson & Costa, 1987) may also be important. Where the landslide debris is mobile, but the slope below the source is planar or convex, the debris usually spreads out over the ground with lobes and fingers extending downslope as an open slope debris avalanche (Figure C2a). Depending upon the local topography, the debris may be funneled into channels. This results in a trail that is narrower than the source (Figures C2a & 2b) and the avalanche is described as channelized. Although the overall movement of the landslide debris has a flow-like morphology, a number of mechanisms may have been involved in addition to flow. If the material is saturated, then slurry flow may become the dominant mechanism and this part of
- 120 the event could be classified as a debris flow. Debris avalanches are the most common type of natural terrain landslide in Hong Kong. Open slope types are generally small with limited run-out, therefore they have not been singled out for specific case studies. A possible example is the 1998 landslide at the Outward Bound School (Fugro Scott Wilson, 1999b), which was an intermediate stage between a debris slide and an open slope debris avalanche. The more mobile the debris, the longer will be the run-out of a debris avalanche, thus the more likely it is to become channelized. The upper section of the 1990 Tsing Shan Debris Flow (King, 1996a) was a very large channelized debris avalanche, which developed into a debris flow lower down the slope. Debris flows develop when the landslide debris is mixed with sufficient water for slurry flow to occur. If the moisture content of the displaced material is sufficiently high, then slurry flow may develop during movement without the further addition of water. However, more commonly the debris enters a stream where it mixes with the streamflow and saturated debris is channelled along the drainage line (Figure C3). This mechanism occurred at Liu Pok in 1995 (King, 1997). As dilution increases the debris slurry will become progressively more mobile until slurry flow develops into hyperconcentrated stream flow. With further dilution the mechanism grades into stream flow, which is no longer a mass movement process but an alluvial process. At this stage the phenomenon is termed a debris flood (e.g. King, 1996b). Field work is required to confirm that deposition from slurry flow has occurred. Diagnostic features include a fines-rich matrix or loose sand and gravel that indicate redistribution of the landslide debris, or the presence of debris fans. It is proposed that the category of channelized debris flow is restricted to landslides in which stream water is mixed with the debris and that follow an established stream bed. These channels will commonly have a history of previous channelized debris flows. Some of the longest channelized debris flows in Hong Kong have resulted from the coalescing of multiple debris avalanches to form a single channelized debris flow (Figure C3). A notable example, about 1.2 km long, occurred south of Shek Kong in 1982. Channelized debris flows commonly increase in volume due to erosion and entrainment of loose deposits from the stream bed and banks. Typical entrainment rates are up to 3.5 m3 per linear metre of trail (Franks, 1998) or up to 3 times the source volume (Wong et al., 1998). The 1990 Tsing Shan debris flow (King, 1996a) was an exceptional event where an initial landslide of about 3 000 m3 eroded and entrained a further 18 000 m3 of material containing large boulders and deposited it as a fan at the foot of the slope. This exceptional entrainment, of some 900%, occurred because a large source was located at the crest of a small cliff that accelerated the saturated remoulded landslide debris into a steep-sided, colluvium-lined stream channel. In Hong Kong, many landslide sources are located near the pronounced breaks of slope that are common on valley sides or at the head of hollows. These breaks may be either an erosion front on the slope or the main scarp of an ancient landslide. Sources are found both directly above the break in slope, where a thinning soil profile and permeability contrasts can cause seepage pressures to build up, and on the break in slope itself, where the locally steeper slope forms an erosion scarp. Other common locations of the larger landslides are accumulations of loose rock and soil on the midslope below steeper rocky outcrops that form the upper slopes.
- 121 In addition to the landslides that originate on the upper sections of hillslopes, larger natural terrain landslides are also common near the base of slopes, particularly on over-steepened slopes at the coast and along incised river valleys. Cliffs can occur at both locations, in which case the landslides may include falls and slides of rock in addition to sliding failures. Although these landslides may have relatively large volumes, they do not generally show long run-outs as the shoreline or the flat river valley floor confines them. These types of landslide include some large scars that contain multiple sources where several adjacent smaller landslides have occurred over a number of years. This may have developed through nested instability within a much older relict landslide scar, or from backsapping and growth of the main scarp from an originally smaller source. C.6.3.4 Large Relict Landslides The degraded scars of ancient, more deep-seated landslides in the Hong Kong landscape can be recognized on aerial photographs. However, the source areas are usually indistinct due to modification by more recent small landslides on the scarps. Also the displaced material commonly forms fans and aprons of colluvium which make it difficult to distinguish the deposits of individual events. A number of such locations have been identified (Scott Wilson, 1999), but until these have been investigated in the field, the materials, mechanism and activity of these scars cannot be established. Fieldwork has been carried out to investigate an 850 000 m3 colluvial lobe at Sham Wat on North Lantau (King, 1998). Results indicate that the lobe is a composite feature comprising several large rock and soil failures. Photoluminescence dates of a large colluvium lobe (about 1 million m3) on North Lantau suggest that the events may have occurred within the last 10 000 years (King, 2001). Several very large, relict earth slides of completely decomposed andesite have also been identified in the pre-development landscape at Area 19 Tuen Mun, which is in the foothills of Tsing Shan (Scott Wilson Kirpatrick, 1986). Further detailed investigations of the ancient landslide scars in the Hong Kong landscape may enable an appropriate classification for these events to be established, and also clarify their significance with respect to present day slope development processes. C.7 REFERENCES Berry, L. (1961). Erosion surfaces and merged beaches in Hong Kong. Bulletin of the Geological Society of America, Vol. 72, pp 1383-1394. Berry, L. & Ruxton, B.P. (1960). The evolution of Hong Kong Harbour basin. fur Geomorphologie, Vol. 4, No. 2, pp 97-115.
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- 122 Evans, N.C. (1997). Preliminary Assessment of the Influence of Rainfall on Natural Terrain Landslide Initiation. Discussion Note DN 1/97, Geotechnical Engineering Office, Hong Kong, 28 p. Evans, N.C., Huang, S.W. & King, J.P. (1997). The Natural Terrain Landslide Study: Phases I and II. Special Project Report SPR 5/97, Geotechnical Engineering Office, Hong Kong, 119 p. plus 2 drgs. Evans, N.C. & King, J.P. (1998). The Natural Terrain Landslide Study: Debris Avalanche Susceptibility. Technical Note TN 1/98, Geotechnical Engineering Office, Hong Kong, 96 p. Franks, C.A.M. (1998). Study of Rainfall Induced Landslides on Natural Slopes in the vicinity of Tung Chung New Town, Lantau Island. GEO Report No. 57, Geotechnical Engineering Office, Hong Kong, 102 p. plus 3 maps. Fugro Scott Wilson Joint Venture (1999a). Detailed Study of Slope Distress at Queen’s Hill, Burma Lines Camp, Fanling. Landslide Study Report LSR 10/99, Geotechnical Engineering Office, Hong Kong, 102 p. Fugro Scott Wilson Joint Venture (1999b). Detailed Study of the Landslide at the Outward Bound School on 9 June 1998. Landslide Study Report LSR 7/99, Geotechnical Engineering Office, Hong Kong, 88 p. Fyfe, J.A., Shaw, R., Campbell, S.D.G., Lai, K.W. & Kirk, P.A. (2000). The Quaternary Geology of Hong Kong. Geotechnical Engineering Office, Hong Kong, 209 p. plus 4 maps. GEO (1996a). Report on the Fei Tsui Road Landslide of 13 August 1995 - Volume 2. Findings of the Landslide Investigation. Geotechnical Engineering Office, Hong Kong, 68 p. GEO (1996b). Report on the Shum Wan Road Landslide of 13 August 1995 - Volume 2. Findings of the Landslide Investigation. Geotechnical Engineering Office, Hong Kong, 51 p. GEO (1998). Report on the Ching Cheung Road Landslide of 3 August 1997. No. 78, Geotechnical Engineering Office, Hong Kong, 142 p. Grant, C.J. (1962). The Soils and Agriculture of Hong Kong. Printer, Hong Kong, 164 p.
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Hong Kong Government
Grant, C.J. (1968). Gullying and erosion characteristics of Hong Kong soils. Proceedings of the Symposium on Recent Advances in Tropical Ecology, Varanasi, India, pp 94-107. Grant, C.J. (1986). Soil. In: T.N. Chiu & C.L. So (editors) A Geography of Hong Kong, (second edition), Oxford University Press, Hong Kong, pp 110-117.
- 123 Hansen, A. (1984). Engineering geomorphology: The application of an evolutionary model of Hong Kong’s terrain. Zeitschrift fur Geomorphologie, suppl. Vol. 51, pp 39-50. Hansen, A. & Nash, J.M. (1985). A brief review of soil erosion causes, effects & remedial measures. Proceedings of the conference on geological aspects of site investigation. Bulletin of the Geological Society of Hong Kong Bulletin No. 2, pp 139-150. Hutchinson, J.N. (1988). General report: Morphological and geotechnical parameters of landslides in relation to geology and hydrogeology. Proceedings of the Fifth International Symposium on Landslides, Lausanne, Balkema, Rotterdam, pp 3-35. King, J.P. (1996a). The Tsing Shan Debris Flow. Special Project Report SPR 6/96, Geotechnical Engineering Office, Hong Kong, 3 volumes, 427p, 129 p & 166 p. King, J.P. (1996b). The Tsing Shan Debris Flood of June 1992. Special Project Report SPR 7/96, Geotechnical Engineering Office, Hong Kong, 83 p. plus 2 drgs. King, J.P. (1997). Natural Terrain Landslide Study: Damage to Liu Pok School by a Natural Terrain Landslide. Discussion Note 2/97, Geotechnical Engineering Office, Hong Kong, 66 p. King, J.P. (1998). Natural Terrain Landslide Study: The Sham Wat Debris Lobe; Interim Report. Technical Note TN 6/98, Geotechnical Engineering Office, Hong Kong, 37 p. plus 1 drg. King, J.P. (2001). Luminescence dating of colluvium and landslide deposits in Hong Kong. Technical Note TN 1/2001, Geotechnical Engineering Office, Hong Kong, 65 p. Koor, N.P. & Campbell, S.D.G. (1998). Geological Characterization of the Lai Ping Road Landslide. Geological Report GR 3/98, Geotechnical Engineering Office, Hong Kong, 178 p. plus 5 maps. Lai, K.W. & Taylor, B.W. (1984). The classification of colluvium in Hong Kong. In: W.W.S. Yim (editor) Geology of Surficial Deposits in Hong Kong, Geological Society of Hong Kong Bulletin No. 1, pp 75-86. Lau, K.C.. & Franks, C.A.M. (1998). A Study of the Landslide at CLP Substation Site, Sham Shui Kok, Lantau Island. Special Project SPR 5/98, Geotechnical Engineering Office, Hong Kong, 53 p. Nandy, S. & Jiao, J.J. (2001). Ground-water flow analysis in the slope above Shum Wan Road, Hong Kong. Environmental & Engineering Geoscience, Vol. VII, pp 239-250. Nash, J.M. & Dale, M.J. (1984). Geology and hydrogeology of natural tunnel erosion in superficial deposits in Hong Kong. In: W.W.-S Yim (editor) Geology of Surficial Deposits in Hong Kong. Geological Society of Hong Kong Bulletin No. 1, pp 61-72.
- 124 Owen, R.B. & Shaw, R. (2000). Weathering profile development over volcanic rocks in the Tuen Mun valley, Hong Kong: S. Reels & A. Page (editors) The Urban Geology of Hong Kong. Geological Society of Hong Kong Bulletin No. 6. Pierson, T.C. & Costa, J.E. (1987). A rheological classification of sub-aerial sediment-water flows. In: J.E. Costa & G.F. Wieczorek (editors), Debris Flows/Avalanches: Process, Recognition and Mitigation, Geological Society of America, Boulder, Colorado, pp 1-12. Ruxton, B.P. & Berry, L. (1957). The weathering of granite and associated erosional features in Hong Kong. Bulletin of the Geological Society of America, Vol. 68, pp 1263-1292. Scott Wilson (1999). Specialist API services for the Natural Terrain Landslide Study: Interpretative Report Task B. Report to the Geotechnical Engineering Office/Planning Division by Scott Wilson (Hong Kong) Ltd., 32 p. plus 6 Appendices. Scott Wilson Kirkpatrick (1986). Tuen Mun New Town Area 19 Slope Monitoring: April 1983 - March 1986. Summary Report, Report to Territory Development Department, Hong Kong, 87 p. Sewell, R.J., Campbell, S.D.G., Fletcher, C.J.N., Lai, K.W. & Kirk, P.A. (2000). The Pre-Quaternary Geology of Hong Kong. Geotechnical Engineering Office, Hong Kong, 181 p. plus 4 maps. Sharpe, C.F.S. (1938_. Landslides and Related Phenomena; A study of Mass-movements of Soil and Rock. Columbia University Press, New York, 137 p. Shaw, R. (1997). Variations in sub-tropical weathering profiles over the Kowloon Granite, Hong Kong. Journal of the Geological Society, London, Vol. 154, Part 6, pp 1077-1085. So, C.L. (1986). Geology and Geomorphology. In: C.J. Peng, F.S. Sit and C.L. So (editors) Hong Kong and Macau, Commercial Press, Hong Kong (In Chinese), pp 25-45. Sun, H. W. & Campbell, S.D.G. (1998). The Lai Ping Road Landslide of 2 July 1997. Landslide Study Report LSR 27/98, Geotechnical Engineering Office, Hong Kong, 138 p. Tregear, T.R. (1958). A Survey of Land Use in Hong Kong and the New Territories. Hong Kong University Press, 76 p. Varnes, D.J. (1958). Landslide types and processes. In: E.B. Eckel (editor), Landslides and Engineering Practice. Highway Research Board, Special Report 29, Washington, DC, pp 20 47. Varnes, D.J. (1978). Slope movement types and processes. In: R.L. Schuster & R.J. Krizek (editors), Landslides: Analysis and Control. Transport Research Board, Special Report 176, National Academy of Sciences, Washington, DC, pp 11-33.
- 125 Whiteside, P.G.D. (1996). Technical Note: Drainage characteristics of a cut soil slope with horizontal drains. Quarterly Journal of Engineering Geology, Vol. 30, pp 137-142. Wong, H.N., Lam, K.C. & Ho, K.K.S. (1998). Diagnostic Report on the November1993 Natural Terrain Landslides on Lantau Island. GEO Report No. 69, Geotechnical Engineering Office, Hong Kong, 98 p.
- 126 LIST OF FIGURES Figure No.
Page No.
C1
Examples of Regolith Mapping Units
127
C2
Examples of Debris Slides and Debris Avalanches
128
C3
Examples of Channelized Debris Flows
129
- 127 -
Rock fall debris: forms below rock slopes or escarpments as a result of individual rock falls or rock avalanches
Valley colluvium: narrow linear or ribbon like features topographically confined within drainage lines
Residual colluvium: relatively old colluvial deposit not topographically defined
Relict landslide: exhibits distinct morphological characteristics, generally in fan-like or lobate form
Saprolite: typically occurs on hilltops, spur/ridgelines as well as slopes
Bedrock at or within 1m of the ground surface
Figure C1 - Examples of Regolith Mapping Units
- 128 -
(a) Open Hillslope Debris Avalanche, North Tsing Shan Range
(b) Channelized Debris Avalanche, South Tsing Shan Range
(c) Channelised Debris Avalanche, Mid-Tsing Shan Range
(d) Debris Slide, South Lantau
Figure C2 - Examples of Debris Slides and Debris Avalanches
- 129 -
(a) North of Leung King Estate, Tuen Mun
(b) Tsing Shan (2000) Channelized Debris Flow
(c) North of Leung King Estate, Tuen Mun
(d) Pat Heung, Shek Kong
Figure C3 - Examples of Channelized Debris Flows
- 130 -
APPENDIX D BIBLIOGRAPHY
- 131 CONTENTS Page No. Title Page
130
CONTENTS
131
D.1 GEOLOGICAL MEMOIRS
132
D.2 1:20 000-SCALE GEOLOGICAL MAPS
132
D.3 1:5 000-SCALE SHEET REPORTS
134
D.4 GASP REPORTS
134
D.5 GEOGUIDES
135
D.6 REPORTS PREPARED BY GEO OR BY GEO CONSULTANTS
135
D.7 GENERAL PUBLICATIONS ON HONG KONG
136
D.8 OTHER RELEVANT DOCUMENTS
137
- 132 The documents listed below are additional reference materials on local geology and geotechnical assessment, particularly GEO publications. It is not intended to be an exhaustive list of reading materials on the subjects. D.1 GEOLOGICAL MEMOIRS Addison, R. (1986). Geology of Sha Tin: 1:20 000 Sheet 7. Survey Memoir No. 1, Geotechnical Control Office, 85p
Hong Kong Geological
Fyfe, J.A., Shaw, R., Campbell, S.D.G., Lai, K.W. and Kirk, P.A. (2000). The Quaternary Geology of Hong Kong. Geotechnical Engineering Office, Hong Kong, 208 p. plus 6 maps. Langford, R.L., James, J.W.C., Shaw, R., Campbell, S.D.G. Kirk, P.A. & Sewell, R.J. (1995). Geology of Lantau District. 1:20 000 Sheets 9, 10, 13 and 14. Hong Kong Geological Survey Memoir No. 6, Geotechnical Engineering Office, 173 p. Langford, R.L., Lai, K.W., Arthurton, R.S. & Shaw, R. (1989). Geology of the Western New Territories: 1:20 000 Sheets 2, 5 and 6. Hong Kong Geological Survey Memoir No. 3, Geotechnical Control Office, 140 p. Lai, K.W. Campbell, S.D.G., Shaw, R. (1996). Geology of the North Eastern New Territories. 1:20 000 Sheets 3 and 4. Hong Kong Geological Survey Memoir No. 5, Geotechnical Engineering Office, 144 p. Sewell, R.J., Campbell, S.D.G., Fletcher, C.J.N., Lai, K.W. & Kirk, P.A. (2000). The Pre-Quaternary Geology of Hong Kong. Geotechnical Engineering Office, Hong Kong, 181 p. plus 4 maps. Strange, P.J. & Shaw, R. (1986). Geology of Hong Kong Island and Kowloon: 1:20 000 Sheets 11 and 15. Hong Kong Geological Survey Memoir No. 2, Geotechnical Control Office, 134 p. Strange, P.J., Shaw, R. & Addison, R. (1990). Geology of Sai Kung and Clearwater Bay. 1:20 000 Sheets 8, 12 and 16. Hong Kong Geological Survey Memoir No. 4, Geotechnical Control Office, 111 p. D.2 1:20 000-SCALE GEOLOGICAL MAPS Addison, R. & Purser, R.J. (1986). Sha Tin. Hong Kong Geological Survey Sheet 7. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong. Addison, R. & Shaw, R. (1990). Waglan Island. Hong Kong Geological Survey Sheet 16, Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong.
- 133 Arthurton, R.S., Lai, K.W. & Shaw, R. (1988). Tsing Shan (Castle Peak). Hong Kong Geological Survey Sheet 5. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong. Lai, K.W. & Shaw, R. (1989). San Tin. Hong Kong Geological Survey Sheet 2. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong. Lai, K.W. & Shaw, R. (1991). Sheung Shui. Hong Kong Geological Survey Sheet 3. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong. Lai, K.W. & Shaw, R. (1992). Kat O Chau (Crooked Island). Hong Kong Geological Survey Sheet 4. Solid and Superficial Geology. 1;20 000 Series HGM20. Geotechnical Engineering Office, Hong Kong. Langford, R.L., Addison, R., Strange, P.J. & James, J.W.C. (1991). Silver Mine Bay. Hong Kong Geological Survey Sheet 10. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Engineering Office, Hong Kong. Langford, R.L. & James, J.W.C. (1995). Shek Pik. Hong Kong Geological Survey Sheet 13. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Engineering Office, Hong Kong. Langford, R.L., Kirk, P.A. & James, J.W.C. (1994). Tung Chung. Hong Kong Geological Survey Sheet 9. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Engineering Office, Hong Kong. Langford, R.L., Lai, K.W. & Shaw, R. (1988). Yuen Long. Hong Kong Geological Survey Sheet 6. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong. Langford, R.L., Shaw, R. & Fyfe. J.A. (1995). Cheung Chau. Hong Kong Geological Survey Sheet 14. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Engineering Office, Hong Kong. Strange, P.J. & Shaw, R. (1986). Hong Kong Island and Kowloon. Hong Kong Geological Survey Sheet 11, Solid and Superficial Geology. 1;20 000 Series HGM20. Geotechnical Control Office, Hong Kong. Strange, P.J. & Shaw, R. (1987). Hong Kong South and Lamma Island. Hong Kong Geological Survey Sheet 15. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong. Strange, P.J. & Shaw, R. (1989). Clear Water Bay, Hong Kong Geological Survey Sheet 12. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong.
- 134 Strange, P.J., Addison, R. & Shaw, R. (1989). Sai Kung Peninsula. Hong Kong Geological Survey Sheet 8. Solid and Superficial Geology. 1:20 000 Series HGM20. Geotechnical Control Office, Hong Kong. D.3 1: 5 000-SCALE SHEET REPORTS Sheet Report 1: Frost, D.V. (1992). Geology of Yuen Long. Hong Kong Geological Survey Sheet Report No.1, Geotechnical Engineering Office, Hong Kong, 69 p. Sheet Report 2: Langford, R.L. (1994). Geology of Chek Lap Kok. Hong Kong Geological Survey Sheet Report No.2, Geotechnical Engineering Office, Hong Kong, 61 p. Sheet Report 3: Sewell, R.J. & Fyfe, J.A. (1995). Geology of Tsing Yi. Hong Kong Geological Survey Sheet Report No. 3, Geotechnical Engineering Office, Hong Kong, 43 p. Sheet Report 4: Sewell, R.J. & James, J.W.C. (1995). Geology of North Lantau Island and Ma Wan. Hong Kong Geological Survey Sheet Report No. 4, Geotechnical Engineering Office, Hong Kong, 46 p. Sheet Report 5: Sewell, R.J. (1996). Geology of Ma On Shan. Hong Kong Geological Survey Sheet Report No.5, Geotechnical Engineering Office, Hong Kong, 45 p. D.4 GASP REPORTS Geotechnical Control Office (1987). Geotechnical Area Studies Programme - Hong Kong and Kowloon. GASP Report No. I, Geotechnical Control Office, Hong Kong, 170 p. plus 4 maps. Geotechnical Control Office (1987). Geotechnical Area Studies Programme - Central New Territories. GASP Report No. II, Geotechnical Control Office, Hong Kong, 165 p. plus 4 maps. Geotechnical Control Office (1987). Geotechnical Area Studies Programme - West New Territories. GASP Report No. III, Geotechnical Control Office, Hong Kong, 155 p. plus 4 maps. Geotechnical Control Office (1988). Geotechnical Area Studies Programme - North West New Territories. GASP Report No. IV, Geotechnical Control Office, Hong Kong, 120 p. plus 3 maps. Geotechnical Control Office (1988). Geotechnical Area Studies Programme - North New Territories. GASP Report No. V, Geotechnical Control Office, Hong Kong, 135 p. plus 4 maps.
- 135 Geotechnical Control Office (1988). Geotechnical Area Studies Programme - North Lantau. GASP Report No. VI, Geotechnical Control Office, Hong Kong, 124 p. plus 4 maps. Geotechnical Control Office (1988). Geotechnical Area Studies Programme - Clear Water Bay. GASP Report No. VII, Geotechnical Control Office, Hong Kong, 144p. plus 4 maps. Geotechnical Control Office (1988). Geotechnical Area Studies Programme - North East New Territories. GASP Report No. VIII, Geotechnical Control Office, Hong Kong,144 p. plus 4 maps. Geotechnical Control Office (1988). Geotechnical Area Studies Programme - East New Territories. GASP Report No. IX, Geotechnical Control Office, Hong Kong, 141 p. plus 4 maps. Geotechnical Control Office (1988). Geotechnical Area Studies Programme - Islands. GASP Report No. X, Geotechnical Control Office, Hong Kong, 142 p. plus 4 maps. Geotechnical Control Office (1988). Geotechnical Area Studies Programme - South Lantau. GASP Report No. XI, Geotechnical Control Office, Hong Kong, 148 p. plus 4 maps. Geotechnical Control Office (1989). Geotechnical Area Studies Programme - Territory of Hong Kong, by K.A. Styles & A. Hansen. GASP Report No. XII, Geotechnical Control Office, Hong Kong, 346 p. plus 14 maps & 1 chart. D.5 GEOGUIDES Geotechnical Control Office (1984). Geotechnical Manual for Slopes (Second edition). Geotechnical Control Office, Hong Kong, 295 p. Geotechnical Control Office (1987). Guide to Site Investigation (Geoguide 2). Geotechnical Control Office, Hong Kong, 353 p. Geotechnical Control Office (1988). Guide to Rock and Soil Descriptions (Geoguide 3). Geotechnical Control Office, Hong Kong, 189 p. Geotechnical Engineering Office (1998). Guide to Slope Maintenance (Geoguide 5) (Second edition), Geotechnical Engineering Office, Hong Kong, 91 p. Geotechnical Engineering Office (1998). Layman’s Guide to Slope Maintenance (Second edition) (Bilingual), Geotechnical Engineering Office, Hong Kong, 54 p. D.6 REPORTS PREPARED BY GEO OR BY GEO CONSULTANTS Cooper, A.J. (1992). Reassessment of the Po Shan Road Landslide of 18th June 1972, Special Project Report 16/92, Geotechnical Engineering Office, Hong Kong, 75 p.
- 136 ERM-Hong Kong, Ltd (1998). Feasibility Study for QRA of Boulder Fall Hazard in Hong Kong, GEO Report No. 80, Geotechnical Engineering Office, Hong Kong, 61 p. Halcrow Asia Partnership Ltd. (1998-2000). Investigation of some Selected Landslide Incidents in 1997 (Volume 1 to 6), GEO Report No. 79, Geotechnical Engineering Office, Hong Kong, 142 p. Hungr, O. (1997). Mobility of Landslides in Hong Kong, Pilot Analysis using a Numerical Model. Report to Geotechnical Engineering Office, Hong Kong. Irfan, T.Y., Tang, K.Y. (1993). Effect of the Coarse Fractions on the Shear Strength of Colluvium, GEO Report No. 23, Geotechnical Engineering Office, Hong Kong, 223 p. Koor, N.P. & Campbell, S.D.G. (1998). Geological Characterization of the Lai Ping Road Landslide. Geological Report 3/98, Geotechnical Engineering Office, Hong Kong, 152 p. Malone, A.W. (1997). Areal Extent of Intense Rainfall in Hong Kong 1979 to 1995. GEO Report No. 62, Geotechnical Engineering Office, Hong Kong, 85 p. Premchitt, J. (1991). Hong Kong Rainfall and Landslides in 1985. GEO Report No. 2, Geotechnical Engineering Office, Hong Kong, 108 p. plus 1 drg. Premchitt, J., Lam, T.S.K., Shen, J.M. & Lam, H.F. (1992). Rainstorm Runoff on Slopes. GEO Report No. 12, Geotechnical Engineering Office, Hong Kong, 211 p. Tang, M.C. (1993). Report on the Rainstorm of May 1982. GEO Report No. 25, Geotechnical Engineering Office, Hong Kong, 137 p. plus 1 drg. Wong, C.K.L. (1997). Hong Kong Rainfall and Landslides in 1995. GEO Report No. 59, Geotechnical Engineering Office, Hong Kong, 125 p. plus 1 drg. Wong, H.N., Chen, Y.M. & Lam, K.C. (1997). Factual Report on the November 1993 Natural Terrain Landslides in Three Study Areas on Lantau Island. GEO Report No. 61, Geotechnical Engineering Office, Hong Kong, 42 p. D.7 GENERAL PUBLICATIONS ON HONG KONG Bennett, J.D. (1984a). Review of Superficial Deposits and Weathering in Hong Kong. Geotechnical Control Office, Hong Kong, 51 p. (GCO Publication No. 4/84). Burnett, A.D. & Lai, K.W. (1984). A review of the photo-geological lineaments and fault system of Hong Kong. Geological Society of Hong Kong, Bulletin No. 2, I. (Edited: McFeat-Smith (Editor), pp 113 - 131.
- 137 Hansen, A. Franks, C.A.M., Kirk, P.A., Brimicombe, A.J. & Fung, T. (1995). Application of GIS to hazard assessment, with particular reference to landslides in Hong Kong. In: Geographical Information systems in Assessing Natural Hazards, (Edited: Carrara, A. & Guzzetti, F.), pp 273-298. Lai, K.W. (1982). Discussion on the colluvium of Hong Kong (8th October 1980). Hong Kong Baptist College Academic Journal, Vol. 9, pp 139-158. Lai, K.W. & Taylor, B.W. (1984). The classification of colluvium in Hong Kong. Geological Society of Hong Kong, Bulletin No. 1, (Edited: W.W.S. Yim), pp 75-85. Langford, R.L. & Hadley, D. (1990a). New debris flow on the flanks of Tsing Shan, Hong Kong. Geological Society of Hong Kong Newsletter, Vol. 8, part 3, pp 2-12. (Discussion, Vol. 8, part 4, pp 40-45). Langford, RL. & Hadley, D. (1990b). New debris flow on the flanks of Tsing Shan, Hong Kong. (Photographic feature). Geotechnical Engineering, Vol. 21, pp 105-107. Shaw, R. (1993a). Granite weathering and erosion in Hong Kong. Improving Degraded Lands: Promising Experiences from South China. Bishop Museum Bulletin in Botany No. 3, Bishop Museum Press, Honolulu (Edited: W.E. Parham, P.J. Durana & A.L. Hess) pp 189-196. D.8 OTHER RELEVANT DOCUMENTS Costa, J.E. & Wieczorek, G.F. (1987). Debris Flows/Avalanches: Process, Recognition and Mitigation. The Geological Society of America, 239 p. Dearman, W.R. (1991). 387 p.
Engineering Geological Mapping.
Butterworth/Heinemann,
Fookes, P.G. (1997). Geology for Engineer: the geological Model, prediction and performance. Quaternary Journal of Engineering Geology. Vol. 30, pp 293 - 424. Fookes, P.G. (Editor) (1997). Tropical Residual Soils. Geological Society Engineering Group Working Party Revised Report. The Geological Society, London, 184 p. Fookes P.G., Frederick, J.B. & Hutchison, J.N. (2000). Total geological history: a model approach to the anticipation, observation and understanding of the site conditions (Invited Paper). Proceedings of the International Conference on Geotechnical and Geological Engineering GeoEng2000, Melbourne, Vol. 1, pp 370-460. Hansen, A. (1984). Landslide hazard analysis. In: Slope Instability. Ltd, (Edited: Brunsden, D & Prior, D.B.), pp 523 - 602.
John Wiley & Sons
Lawrence, C.J., Byard, R.J. & Beaven, P.J. (1993). Terrain Evaluation Manual. Transport Research Laboratory State-of-the-Art, Review 7, HMSO Publications, 285 p.
- 138 Ollier, C.D. & Pain, C. (1996).
Regolith, Soils and Landforms.
John Wiley & Sons, 316 p.
Selby, M.J. (1993). Hillslope Materials and Processes. Oxford University Press, 451 p. Thomas, M.F. (1994). Geomorphology in the Tropics: A Study of Weathering and Denudation in Low Latitudes. John Wiley & Sons, 460 p. Turner, A.K. & Schuster, R.L. (Editors) (1996). Landslides: Investigation and Mitigation. Transportation Research Board Special Report No. 247, National Research Council, National Academy Press, Washington, D.C., 673 p.
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