Further copies of this document are available, on request, in audio and large print formats and in community languages, please contact: 0131 244 3258 SCOTTISH ROAD NETWORK LANDSLIDES STUDY: IMPLEMENTATION [email protected] or phone 0141 272 7100 An Agency of © Crown copyright 2008 This document is also available on the Transport Scotland website: www.transportscotland.gov.uk RR Donnelley B57494 10/08 Further copies are available from Blackwell’s Bookshop 53 South Bridge Edinburgh EH1 1YS Telephone orders and enquiries 0131 622 8283 or 0131 622 8258 Fax orders 0131 557 8149 Email orders [email protected] www.transportscotland.gov.uk SCOTTISH ROAD NETWORK LANDSLIDES STUDY: IMPLEMENTATION Editors M G Winter (Transport Research Laboratory), F Macgregor (Consultant to Transport Scotland) and L Shackman (Transport Scotland) Transport Scotland, Edinburgh 2008 Cover Photograph (© Perthshire Picture Agency, PPA: www.ppapix.co.uk): The A85 in Glen Ogle blocked by two debris flows on 18 August 2004. RAF and Royal Navy helicopters are pictured airlifting some of the 57 occupants from the 20 trapped vehicles to safety. The views expressed in this report are those of the Editors and Authors and do not necessarily represent those of Transport Scotland or Scottish Ministers. © Crown Copyright 2008. Except as otherwise stated. Limited extracts from the text may be reproduced provided the source is acknowledged. For more extensive reproduction, please write to the Chief Road Engineer, Transport Scotland, Buchanan House, 58 Port Dundas Road, Glasgow G4 0HF, United Kingdom. Due to OS licence conditions, you/your agent may only use the maps contained in this report for official business dealings with the Scottish Government. If you wish to use the maps for other uses, you must first obtain a separate licence from OS. Aerial Photography imagery is Licensed to Transport Scotland for PGA, through Next PerspectivesTM. Permitted use: Transport Scotland core business only. ISBN: 978-1-906006-38-9 The Scottish Government St Andrew’s House Edinburgh EH1 3DG Produced for Transport Scotland by RR Donnelley B57494 10/08 Published by Transport Scotland, October 2008 TABLE OF CONTENTS MINISTERIAL FOREWORD 1 EDITORS AND REPORT CONTRIBUTORS 2 EXECUTIVE OVERVIEW 5 1 INTRODUCTION 8 2 LANDSLIDE EVENTS 11 2.1 Landslides and Debris Flow 11 2.2 Events of August 2004 13 2.3 Other Events 23 2.4 Seasonality 34 3 RESPONSE TO THE 2004 EVENTS 36 3.1 Immediate Actions 37 3.2 Development of Future Management Options 39 3.3 Dissemination 41 3.4 Risk Issues in Context 43 4 GIS-BASED ASSESSMENT 47 4.1 Introduction 47 4.2 Background 47 4.3 Debris Flow Potential Assessment 49 4.4 GIS Methodology 60 4.5 Results 61 4.6 Conclusions 64 5 INTERPRETATION OF THE GIS-BASED ASSESSMENT 65 5.1 Description of Data 65 5.2 Methodology and Outcomes 66 5.3 Selection of Routes for Site-Specific Assessments 71 6 SITE-SPECIFIC ASSESSMENT METHODOLOGY 74 6.1 Introduction 74 6.2 The Inspection Process 75 6.3 Site-Specific Assessment results 78 7 SITE-SPECIFIC ASSESSMENT RESULTS 82 7.1 Introduction 82 7.2 Exposure Scores 82 7.3 Hazard Rankings 84 7.4 Re-Inspection Programme 86 7.5 Rock Slopes 87 TABLE OF CONTENTS 8 MANAGEMENT OF HIGH HAZARD RANKING AREAS 88 8.1 Techniques for Exposure Reduction 88 8.2 Techniques for Hazard Reduction 97 8.3 Management Actions 109 9 LANDSLIDES, CLIMATE CHANGE, RAINFALL AND FORECASTING 114 9.1 Introduction 114 9.2 Climatic Influences on Landslides 115 9.3 Generalised Forecasting Methods 118 9.4 Case Studies 119 9.5 A Trigger Threshold for Scotland 120 9.6 Conclusions 128 10 CONCLUSIONS AND RECOMMENDATIONS 129 10.1 Summary and Conclusions 129 10.2 Recommendations 130 ACKNOWLEDGEMENTS 133 REFERENCES 135 TABLE OF CONTENTS APPENDIX A SUPPLEMENTARY INFORMATION FOR THE GIS ASSESSMENT 146 APPENDIX B GIS-BASED ASSESSMENT INTERPRETATION RESULTS 153 B.1 Initial Interpretation 153 B.2 Secondary Interpretation 162 APPENDIX C RESULTS FROM SITE-SPECIFIC ASSESSMENTS 170 C.1 Sample Site Inspection Reports and Score Sheets 170 C.2 Hazard Scores 198 APPENDIX D HAZARD, EXPOSURE AND HAZARD RANKINGS RESULTS 199 D.1 Exposure Scores 199 D.2 Final Hazard Scores and Hazard Rankings 201 APPENDIX E A BRIEF INTERNATIONAL REVIEW OF THE SIGNING OF LANDSLIDE AND OTHER HAZARDS 203 E.1 United Kingdom 203 E.2 New Zealand 204 E.3 Greece 205 E.4 Canada 205 E.5 Hong Kong SAR 207 E.6 Croatia 209 E.7 United States of America 209 APPENDIX F DRAFT TRANSPORT SCOTLAND LEAFLET: SCOTTISH ROADS AND LANDSLIDES 214 F.1 Precautions for Safer Journeys 214 F.2 Context 214 F.3 Transport Scotland Actions 216 APPENDIX G LANDSLIDES AND RAINFALL: CASE STUDIES 218 G.1 Australia 218 G.2 Hong Kong SAR 219 G.3 Italy 226 G.4 Jamaica 234 G.5 Nepal 237 G.6 Norway 237 G.7 Singapore 241 G.8 Slovenia 241 G.9 Switzerland 242 G.10 United Kingdom 244 G.11 United States of America 245 G.12 Other Regions and Countries 249 Annex G.1 Method Adopted in Preliminary Assessment by Evans (1997) 251 APPENDIX H DEVELOPMENT OF A RAINFALL TRIGGER THRESHOLD FOR DEBRIS FLOW 255 H.1 Methodology and Sample Results 255 H.2 Discussion of Results 260 H.3 Conclusions 265 Annex H.1 System for Future Back Analysis 267 MINISTERIAL FOREWORD The implications of landslides on the operation of the road network and thus on the economy of Scotland were brought into sharp focus in August 2004, when significant rainfall led to serious events on roads in the north and west of Scotland. Improving communications, enhancing the country’s transport infrastructure and supporting a stable economy are vital elements of the work of the Scottish Government and Transport Scotland. For these reasons the importance of advancing our understanding of landslides in Scotland was immediately recognised. The Scottish Road Network Landslides Study, a programme of detailed research, began immediately after the events of 2004 and continues today. The study sets a benchmark in terms of the assessment of such large areas at relatively large scale. The results documented here provide us with a comprehensive picture of the future risk of landslides in Scotland and the evidence that we require to properly plan for and manage that risk, reducing as far as possible the impact on our roads and road users. This study has been delivered primarily by experts from Scotland’s geotechnical community. They have drawn on their own international experience, and that of others, and experience from other disciplines, as appropriate. They have used technology in innovative ways to achieve the objectives of the study. The body of work produced places Scotland amongst other leading nations involved in the study of landslides and landslide management. I would like to thank all of those involved. A number of the recommendations made in this study have already been taken on board and activities are underway in key locations to manage the exposure of road users to landslide hazards. I believe that continued investment in this study, its recommendations and the associated study of the broader implications of climate change on the road network will ensure that Scotland is well placed to deal effectively with landslide events in the future. Stewart Stevenson Minister for Transport, Infrastructure and Climate Change 1 EDITORS AND REPORT CONTRIBUTORS Peter Dempsey: Peter is a hydrologist, specialising in hydro-meteorology, working within the Met Office’s Scientific Consultancy Group. He has experience working with the UK Met Office, the Environment Agency and consulting engineers delivering research and consultancy projects. Current interests include the application of weather radar and Met Office precipitation forecasts for flood forecasting. He is a Fellow of the Royal Meteorological Society and a Member of Chartered Institution of Water & Environmental Management. James Dent: James is a Chartered Meteorologist and Fellow of the Chartered Institution of Water & Environmental Management. Specialising in hydro-meteorology, he is mostly involved with both ends of hydrological extremes – floods and droughts and has worked extensively overseas and in the UK. Recently he has been involved with studies on rain gauge distribution and methods for early identification of extreme rainfall events. His experience of heavy rainfalls includes many years in Papua New Guinea and Bangladesh, in the latter being World Meteorological Organisation’s Chief Technical Advisor to the National Flood Warning Centre. David Entwisle: David is a chartered Geologist and Senior Engineering Geologist at the British Geological Survey. David has 25 years’ experience in engineering geology, in particular description and classification and material behaviour. He is currently leading the research into 3D engineering geological models and geotechnical data presentation for desk study and planning purposes, which includes the use of GIS. David is the Vice-Chair of the Engineering Group of the Geological Society, a member of the Association of Geotechnical and Geoenvironmental Specialists laboratories committee and the BSI technical committee B/526/3, Site Investigation and Ground Testing. Alan Forster: Alan is a Chartered Geologist and prior to retirement was Principal Engineering Geologist at the British Geological Survey, managing the Geological Hazards sub- programme. Alan has over 30 years’ experience in a wide range of subject areas in engineering geology, including a particular emphasis on slope instability in the past 10 years. Alan is former Secretary to the Engineering Group of the Geological Society and former Scientific Editor of the Quarterly Journal of Engineering Geology and Hydrogeology. Dr Andrew Gibson: Andy Gibson is an engineering geomorphologist and Head of Science (Land Use and Development) at the British Geological Survey. Andy has worked extensively on geohazard assessments over the past 10 years, more recently focusing on developing BGS techniques for mapping and recording landslides and assessing landslide risks.