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by M G Winter, F Macgregor and L Shackman

In August 2004 Scotland experienced rainfall substantially in excess of the norm. Some areas of Scotland received in excess of 300% of the 30-year average August rainfall. In the Perth and Kinross area figures of the order of between 250% and 300% were typical. While the percentage rainfall during August reduced to the west, parts of Stirling and Argyll & Bute still received between 200% and 250% of the monthly average 1.

The rainfall was both intense and long lasting and a large number of landslides, in the form of debris flows ( see Section 2), were experienced in the hills of Scotland. A small number of these intersected with the trunk road network, notably the A83 between Glen Kinglas and to the north of Cairndow (9 August), the A9 to the north of Dunkeld (11 August), and the A85 at Glen Ogle (18 August).

While there were no major injuries to those affected, some 57 people were taken to safety by helicopter after being trapped between the two debris flows on the A85 in Glen Ogle ( see cover picture). The A85, carrying up to 5,600 vehicles per day, was closed for four days. The A83, which carries around 5,000 vehicles per day (all vehicles two-way, 24 hour AADT2), was closed for two days and the A9, carrying 13,500 vehicles per day, was closed for two days prior to reopening, initially with single lane working under convoy. The disruption experienced by local and tourist traffic, as well as to goods vehicles, was substantial.

The need to act has been recognised by the Scottish Executive and this initial study (Study 1, Part 1) has been commissioned alongside a second study (Study 2). Study 2 is designed to identify the potential impacts and consequent necessary actions in the light of anticipated climate change and is not considered further in this report, although it is important to note that action has been taken to ensure that the two studies are complementary.

As indicated above, 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.
  • 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.

This work will lead to Study 1, Part 2 which will include the development of a system to allow a detailed review of the network to be undertaken to identify the locations of greatest hazard and for those hazards to be ranked and appropriate mitigation and/or management measures to be selected.

The overall purpose of these studies is thus to ensure that the Scottish Executive has a system in place for assessing the hazards posed by debris flows. In addition, the system will be capable of ranking the hazards in terms of their potential relative effects on road users. This will allow the future effects of debris flow events to be managed and mitigated as appropriate and as budgets permit. This will ensure that the exposure of road users to the consequences of future debris flows is minimised whilst acknowledging that it is not possible to prevent the occurrence of such events.

A consistent, repeatable and reproducible system is required. This is especially important as a variety of consultants will be involved in the data gathering, analysis and interpretation process. Inevitably each will have a different, but nonetheless valid, approach when operating independently. Such a situation would make any comparison between individual consultant's results and recommendations impossible for the purpose of, for example, allocating funds on a priority basis across the network. It is apparent at the outset that a unified system acceptable to all of the major players in the industry is required.

It was thus recognised at an early stage of the development of the work that the input of a wide range of experts and stakeholders would be required in order for the studies to be completed successfully. In particular, the agreement and input of those most likely to be responsible for using the system was required.

A Project Workshop was held in order to capture the knowledge vested with individual experts ( see Appendix). The Project Workshop was facilitated by Professor Malcolm Horner of the University of Dundee and 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.

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 the 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.

In this report reference is made to both debris flows and landslides. Debris flow is recognised within the specialist community as a sub-set of the term landslide and is used in the description of the mechanisms and characteristics of such events. The term landslide is used as a common parlance term to describe the broader types of event that are liable to be encountered.

The work reported herein has been conducted by a Working Group whose membership was selected in order to enable the individuals most suited to the various tasks to bring their knowledge, expertise and experience to bear on the relevant issues. The work has been funded through a variety of existing contracts with the close and active involvement and support of Scottish Executive engineers as key members of the Working Group. The involvement of TRL is in providing the facilitating project manager to lead the experts drawn from Scotland's geotechnical community as well as making specific and substantial technical contributions. TRL's input has been funded through existing commission arrangements. TRL has also been responsible for sub-contracting expertise from the British Geological Survey, Donaldson Associates, EDGE Consultants and Arup. The involvement of Halcrow has been funded through Trunk Road Division's Geotechnical Certification Commission. The involvement of BEAR (represented by Jacobs Babtie) and Amey (represented by W A Fairhurst & Partners) was funded through existing arrangements for trunk road maintenance.

The foregoing refers to the organisations involved in the project. However, the Working Group, including the editors of this report, comprised individuals each of whom was selected on the basis of their knowledge, expertise and experience and, indeed, their suitability to bring those characteristics to bear on the issues at hand. Appointments to the Working Group were for individuals, on the basis of their knowledge and experience, rather than potentially for the organisations who employ them. Individual members of the Working Group did, however, employ the services of colleagues as appropriate.