In August 2004 Scotland experienced rainfall substantially in excess of the norm. The rainfall was both intense and long lasting and as a result a large number of landslides, in the form of debris flows, were experienced in the hills of Scotland. A small number of these intersected the trunk (strategic) 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).

The most dramatic events occurred at Glen Ogle, where 57 people had to be airlifted to safety when they became trapped between two major debris flows (see cover picture). It was, perhaps, fortuitous that there were no major injuries to those involved. However, the real impacts of the August events were economic and social, in particular the severance of access to and from relatively remote communities.

The need to acknowledge such natural processes and act accordingly was recognised by Transport Scotland and an initial landslides study was commissioned alongside a second study on climate change. The landslides study comprises two parts. The initial study collated and presented the background information and developed the plan for the second part. The second part of the landslides study presents the proposed means of debris flow management on the trunk road network and is documented in this report.

The overall purpose of the landslides study is to ensure that Transport Scotland has systematically assessed and ranked the hazards posed by debris flows and has in place a management and mitigation strategy for the Scottish trunk road network. The purpose of the ranking system is to allow the future effects of debris flow events to be appropriately managed and mitigated as budgets permit, thus ensuring that the exposure of road users to the consequences of future debris flows is minimised.

It is important to recognise that it is not possible to prevent landslide events from occurring and some may occur in such close proximity as to affect the operation of the trunk road network.

The work undertaken and set out in this report is therefore targeted at developing the evidence base for allocating resources to reduce the exposure of road users to landslide hazards and/or to reduce the physical hazard. Notwithstanding this, the latter actions involve higher cost solutions and are likely to be applied only in rare cases.


The landslide events of August 2004 had a substantial effect on the operation of Scottish trunk road network and led to wide-ranging media and political interest. The nature of these events broadly conformed to the relatively fast-moving, shallow debris flow-type of landslide with which this report primarily deals. There have since been other debris flows of a similar nature including, for example, those that affected the A9 in 2006 and the A83 in 2007, as well as a wide range of similar occurrences that affected the local road network. In general the events detailed in this report confirm that landslides typically occur in Scotland in two seasons, namely:

  • Summer: July and August.
  • Winter: November to January (with events sometimes occurring in October).

The work reported here forms a component of Transport Scotland’s response to the August 2004 events and builds upon the earlier report (Winter et al., 2005) which described the background and objectives behind the work presented in this report. The findings from the work have already been widely presented on both nationally and internationally.

Consideration of the socio-economic aspects of landslide risk illustrates the diverse approaches taken by different societies and cultures. These considerations support the principle that the landscape itself has both a social and an environmental value and that a drive towards risk mitigation and/or reduction is only one part of the wider picture.

Notwithstanding this, the core of the work addressed by this report is the assessment and ranking of hazards presented by debris flows.

The hazard assessment process involves the GIS-based spatial determination of zones of susceptibility which are then related to the trunk road network by means of plausible flow paths to determine specific hazard locations. The approach taken, using a GIS-based assessment, enabled large volumes of data to be analysed relatively quickly and was able to rapidly deliver a scientifically-sound platform for the assessment. This desk-based approach to hazard assessment was then supplemented by site-specific inspections, including site walkovers, to give a hazard score for each site of interest.

The subsequent hazard ranking process involved the development of exposure scores predicated primarily upon the risk to life and limb, but also taking some account of the socio-economic impact of debris flow events.

Finally, these scores were combined with the hazard scores to give site-specific scores for hazard ranking from which a listing of high hazard ranking sites in Scotland was produced.

An approach to the management and mitigation of debris flow hazards has also been developed. Two approaches are described:

  • Exposure reduction, which involves for example education, warning, signing and road closure.
  • Hazard reduction, which includes engineering measures that protect the road, reduce the opportunity for debris flow to occur, or involve realignment of the road.

Most of the recommendations (see Section 10.2) are based upon the reduction of the exposure of the road users to debris flow hazards as a reaction to events and utilise lower cost and less environmentally intrusive approaches rather than the typically high cost, environmentally intrusive approach of specific hazard reduction. Exposure reduction is predicated upon the simple and easily-remembered, three-part management tool, Detection-Notification-Action (DNA).

Weather and climate are clearly key influences upon the triggering of debris flows in Scotland and climate change models generally indicate that such events may become more frequent and/or more intense in the future. In the longer term the ability to forecast of debris flow from rainfall data is clearly desirable in order to allow, at least, the Detection and Notification aspects of the DNA process to be carried out in advance of events.

In support of this a variety of international approaches to the back analysis and forecast of landslide events resulting from rainfall have been researched and described. Back analysis of the rainfall associated with a selection of Scottish debris flow events has enabled a tentative debris flow trigger threshold, in terms of rainfall intensity-duration, to be proposed. This threshold, however, needs to be further validated against observations in the future and it is estimated that at least five years of data will be required prior to implementing such a system. Work is currently in progress to develop the dataset and validate the threshold. During the development period a system will also need to be put in place to allow ‘real-time’ capture and analysis of data to enable forecasting.

The work presented in this report gives Transport Scotland the means to apply appropriate management measures to the sites of highest risk on the trunk road network. Specific recommendations to achieve this and to further develop and improve the management process are given in the following section.

The main recommendations relate to:

  • A series of management actions predicated towards exposure reduction.
  • Opportunities for physical hazard reduction on new works and rehabilitation schemes.
  • The vital role of the development of rainfall-monitoring systems and interpretative techniques to enable proactive warning of debris flows to be brought into play in future years.
  • The value of studying the ongoing effects of climate change on the prevalence of debris flows, of carrying out an evaluation of the economic effects of debris flow events, and working with Forestry Commission in order to ensure that best practices are adopted in terms of forestry harvesting and hill slope stability.
  • The need for a continuing site inspection programme to validate all four priorities of sites on the network, and the role of re-assessment and re-inspection at some time in the future.
  • Consideration of actions relating to rock slope surveys.
  • The need for separate assessment of scree-slope sections in Glen Coe and on Skye.