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
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
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.
CONCLUSIONS AND RECOMMENDATIONS
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
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
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,
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
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
- Opportunities for physical hazard reduction on new works and
- 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
- 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.