Scottish Road Network Landslides Study - University of Glasgow

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EXECUTIVE OVERVIEW INTRODUCTION ‘The surface of the land is made by Nature to decay…. Our fertile plains are formed from the ruins of the mountains’. James Hutton, 1785 In August 2004 a series of landslides in the form of debris flows occurred in Scotland. Some of these affected the A83, A9 and A85, which form part of the trunk road network. These incidents were well reported in the media. While debris flows occur with some frequency in Scotland, they only rarely affect the trunk road network or for that matter the main local road network. However, when they do impact on the road network the degree of damage, in terms of the infrastructure and the loss of utility to road users, can have a major detrimental effect on both economic and social aspects of the use of the asset. Additionally, there is a high potential for such events to cause serious injury and even loss of life although, fortuitously, such consequences have been limited to date. The events of August 2004 followed a sustained period of heavy rainfall and, in addition, intense localised storms contributed to the triggering of at least some of the resulting debris flows. Rainfall of up to 300% of the monthly average fell in certain parts of Scotland during August 2004. Within the recent past, debris flow activity in Scotland has occurred largely in the periods July to August and November to January, but there is no certainty that such a pattern will be continued in the future. However, eastern parts of Scotland do receive their highest levels of rainfall in August. Additionally, climate change models indicate that rainfall levels will increase in the winter but decrease during the summer months but that intense storm events will increase in number. These factors, therefore, may change both the frequency and the annual pattern of debris flow events. The impacts of such events are particularly serious during the summer months due to the major contribution that tourism makes to Scotland’s economy. Nevertheless, the impacts of debris flow events during the winter months should not be underestimated. OBJECTIVES Following the events of August 2004, the need to act was recognised by the Scottish Executive and this study was commissioned to take stock of the present situation on the trunk road network and to determine a sustainable approach to the management of such occurrences in the future. This study, termed Study 1, comprises two parts and it is Part 1 that is reported here. Part 1 deals with the following activities: Considering the options for undertaking a detailed review of side slopes adjacent to the trunk road network and recommending a course of action. 4
5 EXECUTIVE OVERVIEW Outlining possible mitigation measures and management strategies that might be adopted. Undertaking an initial review to identify obvious areas that have the greatest potential for similar events in the future. STRUCTURE A Project Workshop was held in order to capture the knowledge vested with individual experts. The Project Workshop comprised presentations given by acknowledged experts followed by focussed discussion sessions designed to open out the knowledge base and determine the way forward with the project. Following the Project Workshop the Editors assigned tasks to individuals in terms of the preparation of this report as exemplified by the authorship of individual sections. The main results from the Project Workshop are incorporated in the various sections. After a brief introduction in Section 1, Section 2 gives the background to the Study as a whole. It describes the different types of landslide, focussing on debris flows as recently experienced, and illustrates the recent history of debris flows in Scotland with examples right up until the present. It also deals with climatic issues and those issues which relate to third party ownership of land from which landslides may originate Section 3 examines sources of relevant information, including previous literature, the Project Workshop and available data sets from sources such as Scottish Executive and the British Geological Survey. Section 4 deals with the classification and type of debris and other types of flows. It explains how rapid landslides develop from their causes and the underlying soil failure mechanisms, through the mechanics of their downslope propagation and, finally, to their run-out at the base of the slope. Section 5 examines the relevance of the key factors in debris flow initiation and propagation that have been identified from past events, including the events of August 2004. These are considered in terms of factors affecting the likelihood of debris flow occurrence, including the effects of run-out, and factors affecting the exposure of road users to debris flows Section 6 describes the proposed assessment methodology in terms of hazard assessment and approach for Study 1, Part 2 and also details the hazard assessment and exposure factors that will form the core of the methodology for the detailed assessment. Section 7 identifies areas of high hazard that are considered to have the greatest potential for similar debris flow events in the future and sets out opportunities for early actions. Section 8 describes management and mitigation options. In terms of management the sequential approach of Detection, Notification and Action (DNA) promulgated by the Editors at the Project Workshop is used. This approach is set out in terms of a response to both precursor conditions, such as intense rainfall, and also to the management of future debris flow events. Section 9 presents a brief summary of this report and makes recommendations for the way forward.
Page 1 and 2: Scottish Road Network Landslides St
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Page 5 and 6: MINISTERIAL FOREWORD The landslide
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Preparatory Factors 55 TYPES AND ME
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65 TYPES AND MECHANISMS detachment
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67 TYPES AND MECHANISMS Figure 4.21
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69 CONTRIBUTORY FACTORS There is ho
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5.1.2 Geological, Geotechnical and
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73 CONTRIBUTORY FACTORS Other hydro
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77 CONTRIBUTORY FACTORS 5.3 FACTORS
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6 PROPOSED METHODOLOGY FOR DEBRIS F
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85 PROPOSED METHODOLOGY presence of
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A84 South of Strathyre (8km) HIGH H
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HIGH HAZARD AREAS AND EARLY OPPORTU
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8 DEBRIS FLOW MANAGEMENT AND MITIGA
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8.2.3 United Kingdom MANAGEMENT AND
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MANAGEMENT AND MITIGATION OPTIONS D
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MANAGEMENT AND MITIGATION OPTIONS a
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MANAGEMENT AND MITIGATION OPTIONS N
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MANAGEMENT AND MITIGATION OPTIONS F
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9 SUMMARY AND RECOMMENDATIONS FOR D
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SUMMARY AND RECOMMENDATIONS The ini
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SUMMARY AND RECOMMENDATIONS extensi
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115 REFERENCES Chan, R. K. S. 2000.
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117 REFERENCES Moore, R., Lee, E. M
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APPENDIX - PROJECT WORKSHOP AGENDA
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Scottish Road Network Landslides Study - University of Glasgow

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