Transport Scotland began a concerted programme of work following the major landslide events of August 2004, which had led to wide-ranging media and political interest. In general the events observed 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 the major component of Transport Scotland’s response to the August 2004 events and builds upon an earlier report which described the background and objectives behind the work presented. The findings from the work have already been widely presented on both nationally and internationally.

The core of the work addressed by this report is the assessment and ranking of hazards presented by debris flows for the Scottish trunk road network.

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. This approach enabled the rapid analysis of large volumes of data. This desk-based approach to hazard assessment was then supplemented by site-specific inspections 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. The exposure 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.

Processes for the management and mitigation of debris flow hazards have been developed and 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 are based upon the reduction of the exposure of 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 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 rainfall-based analysis to develop, validate and refine a debris flow trigger threshold is under way.

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 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 pro-active 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 the 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.