by M G Winter, F Macgregor and L Shackman

The need to act in response to the events of August 2004 was recognised by Scottish Ministers. As an objective, Transport Scotland decided that a system should be put in place for assessing hazards posed by debris flows. It was also recognised that such a system must be capable of ranking the hazards in terms of their potential relative effects on road users. In that way the future effects of debris flow events would be able to be managed and mitigated as appropriate and as budgets permitted, thus ensuring that the exposure of road users to the consequences of future debris flows would be minimised but with the acknowledgement that the prevention of such events is not possible.

As a first step towards that overall objective, the initial landslides study was set in motion by the Minister for Transport to address 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 initial study4 was reported in both a comprehensive Technical Report (Winter et al., 2005a) and in summary form (Winter et al., 2005b), the latter being intended to inform a wider audience of Transport Scotland’s actions since the events of August 2004 and planned for the future. The Technical Report was divided into a number of sections, each of which introduced one or more of the key issues that were to be addressed in order to move the work forward towards implementation. These were as follows:

  • Section 1 introduced landslides in Scotland and the background to the inception of the study (Winter et al., 2005c).
  • Section 2 gave the background to the study as a whole. It described the different types of landslide, focusing on debris flows, and illustrated the recent history of debris flows in Scotland. It also dealt with climatic issues and those issues which relate to third party ownership of land from which landslides may originate (Winter et al., 2005d).
  • Section 3 examined sources of relevant information, including previous literature and available data sets from sources such as the Scottish Executive and the British Geological Survey (McMillan et al., 2005).
  • Section 4 dealt with the classification and type of debris and other types of flows. It explained 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 (Nettleton et al., 2005b).
  • Section 5 examined the relevance of key factors in debris flow initiation and propagation that have been identified from past events, including the events of August 2004. These were 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 (Heald and Parsons, 2005).
  • Section 6 described the proposed assessment methodology in terms of hazard assessment and the approach to the second part of the study, reported herein, and also detailed the hazard assessment and exposure factors forming the core of the methodology for the detailed assessment (Winter et al., 2005e).
  • Section 7 identified areas of high hazard that, based upon collective experience, were considered to have the greatest potential for similar debris flow events in the future and set out opportunities for early actions (Winter et al., 2005f).
  • Section 8 described management and mitigation options, particularly focusing upon the sequential approach to management of Detection, Notification and Action (DNA) that was promulgated by the Editors of the Technical Report at a workshop held at the start of the project. 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 (Sloan et al., 2005).
  • Section 9 presented a summary of the report and made recommendations for the way forward (Winter et al., 2005g).

The findings and recommendations of the Technical Report were used to produce the plan for the second part of the study as reported here. This develops a system to allow a detailed review of the network to be undertaken to identify the locations of greatest hazard, for those hazards to be ranked and for appropriate mitigation and/or management measures then to be selected.

A consistent, repeatable and reproducible system was configured as it was anticipated that a variety of consultants would be involved in the data gathering, analysis and interpretation process. Consultants often have preferred approaches which are different, but nonetheless valid, when operating independently. Should such independent operation occur, this would render any comparison between individual consultant’s results and recommendations unworkable for the purpose of, for example, allocating funds on a priority basis across the network. It was thus apparent at the outset that a system that produced consistent and comparable results was 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. A facilitated Project Workshop was held on 28 September 2004, exactly one month after the events at Glen Ogle, in order to capture the knowledge vested with individual experts who formed a Working Group. Focused discussion sessions at the Project Workshop led to task-assigned activities which eventually formed the chapters of the Technical Report, this being launched along with the Summary Report at a public seminar at The Royal Museum in Edinburgh on 14 June 2005.


As part of the Project Workshop a series of areas of high perceived hazard was identified. The identification of these areas was intended to serve the joint functions of assisting prioritisation of areas for action during this second part of the study whilst providing, in parallel, a shortlist of sites appropriate for validating the debris flow hazard model in its development phase.

The sites identified (in the order in which they were suggested at the Workshop, but not in any order of perceived hazard or hazard ranking) are set out as follows:

  • A83 Ardgarten to Loch Shira (29km).
  • A84 South of Strathyre (8km).
  • A85 Glen Ogle (6km).
  • A87 Glen Shiel (18km, plus a possible further 17km).
  • A82 Fort Augustus to Lochend (29km, plus a possible further 9km).
  • A835 Ullapool to Braemore Junction (16km).
  • A9 Dunkeld to Drumochter (22km).
  • A95 Craigellachie (1km).
  • A86 Spean Bridge (5.5km).
  • A87 (Skye) Gleann Torra-mhichaig to South of Raasay ferry (1.5km).

Collectively, the above correspond to a total length of 162km.

A number of shorter-term actions were also instigated following the events of August 2004. A significant programme of clearing vegetation and rocks from, and adjacent to, ditches, gullies, catchpits and culverts was undertaken and some new ditches were added at the crest of slopes to limit water ingress.

On a related theme the national drainage standards have since been updated and enhanced and these are used for the design of all construction and maintenance operations. The new standards upgrade the design storms used in determining the various required capacities (e.g. from a 1 in 2 year return period to 1 in 5 years).

  • With regard to the specific areas of high hazard identified in the report the following works have been progressed: A83 between Ardgarten and Loch Shira (29km).  Culvert realignment and renewal works were completed in 2005 including upgrading of Ardgarten Culvert to provide increased capacity.  A further phase of boulder stabilisation and repair/improvement to cascades has also been undertaken at Rest and be Thankful. Installation of Rain gauges is being progressed in conjunction with SEPA and the Met Office. 
  • A85 in Glen Ogle (6km).  A Scheme is currently at design stage to improve the road alignment and reduce the potential impact of landslides. 
  • A87 in Glen Shiel (18km, plus a further 17km either end of Glen Shiel). Numerous small rock falls which have been blocking culverts and ditches have been cleared in addition to routine maintenance activities.
  • A82 between Fort Augustus and Lochend (29km, plus a further 9km to the south). No additional work done other than routine maintenance as the presence of a rock face along the length of the trunk road makes improvements difficult at reasonable cost.
  • A835 between Ullapool and Braemore Junction (16km). Ditching and vegetation clearance has been undertaken in addition to general routine maintenance.
  • A9 between Dunkeld and Drumochter (22km).  Drainage improvements imminent at the site of the Dunkeld landslip.  The local authority are also working at this location to minimise the hazard.  In addition extensive re-ditching works have been undertaken along this length of the A9 during 2005.
  • A95 in the Craigellachie area (1km). Top of slope ditching has been undertaken but further works are required to address short term problems. It is likely that improvements will require carriageway reconstruction.
  • A86 around Spean Bridge (5.5km).  Extensive ditch clearance works and improvements to drainage and cross road culverts have been undertaken during 2005.

A87 (Skye) between Gleann Torra-mhichaig and South of the Raasay ferry (5.5km).  Ditching to the top of the cutting slope has been undertaken to arrest minor rock slips.


The initial stage of the work may be divided into four elements and can be summarised as follows:

  • Development of a debris flow hazard and exposure assessment system to provide a hazard ranking of ‘at-risk’ areas of the road network.
  • Undertaking a computer-based GIS assessment as a first stage in the hazard assessment process.
  • Undertaking site-specific hazard and exposure assessments of areas identified by the GIS as being of higher hazard.
  • Identification and development of appropriate management processes for each category of hazard ranking.

Figure 3.1 presents a flowchart of the work undertaken. The initial stage of the process was to develop the methodology for the assessment of hazard and exposure to provide a hazard ranking, together with the selection of an appropriate management approach. The second stage was to test the methodology and apply it more widely to the trunk road network.

Figure 3.1 – Outline flowchart of the current study.

Figure 3.1

A GIS-based assessment was used as a first stage in the hazard assessment process. This enabled site-specific assessments to be targeted in order to obtain better value from such relatively resource-intensive activities. It also allowed the elimination of large areas of the network having minimal hazard (see Sections 4 and 5).

It is particularly important to note that the site-specific assessment, described in Sections 6 and 7, was not a simple ‘drive-by’ survey; it comprised a highly specialised detailed site examination using an overall consistent approach. Prior to undertaking any site surveys the system for consistently describing and identifying hazards and the associated exposure was established. Some of the factors that needed to be incorporated into such a system, such as slope angle and the broad nature of the geology, were already incorporated into the GIS assessment. Other, more detailed, factors such as the effects of forestation were incorporated into the site-based survey. The site-specific assessments were predicated upon the principle that the hazard assessment derived from the interpretation of the GIS-based assessment should be changed only on the basis of information that had not been previously taken into account. Once a hazard assessment was been completed it was combined with an assessment of the exposure of the road user to that hazard to give a hazard ranking. This in turn allowed an appropriate management option to be selected from the range of options developed.

There are a number of outline options which could be applied to the management of debris flows depending upon the level of hazard ranking pertaining at any given site. These are addressed in the following paragraphs.

The ‘Do-Nothing’ approach is intended to be applied to sites of low hazard ranking for which substantial expenditure is inappropriate. For such sites, whilst it is not possible to eliminate the chance of a debris flow event affecting such areas, it is seen as unlikely, largely unforeseeable and/or the exposure is less serious than at other locations where resources might be better expended.

The ‘Do-Minimum’ option, with the potential to mitigate the impacts of debris flows to some extent, involves simply ensuring that forward plans are in place to ensure that diversion routes are available and may be exploited in an expedient and well-organised manner. Diversion route maps and contingency plans are currently held for many areas of the trunk road network.

Whilst it is not possible to eliminate the chance of a debris flow event affecting such areas any occurrence is seen as unlikely and largely unforeseeable. Any residual exposure cannot readily be quantified and is unlikely to justify the commitment of additional resources which might be better expended at other locations.

‘Do-Something 1’ is the first management option where site-specific action is contemplated. Such action is essentially exposure reduction by managing the access to the network and/or actions of road-users at times when events occur or precursor rainfall has indicated a high likelihood of debris flows occurring.

‘Do-Something 2’ involves more major works in order to achieve hazard reduction (as opposed to exposure reduction in the ‘Do-Something 1’ case). The approaches involved here entail physical measures such as the protection of the road, reduction of the opportunity for a debris flow to occur or realignment of the road away from the area of high hazard. Such options need to be considered in the context of the policy governing Transport Scotland’s overall trunk road maintenance and construction programme. In general, these are likely to be of high cost, necessitating their restriction to the very few areas of highest hazard ranking.

For all trunk road routes, irrespective of the particular type of risk or incident that might force a closure (landslide, flood, road traffic accident, etc), diversion routes are in place and these have been agreed in advance with the relevant local authority and with the police. In all of the cases described above, in the event of a landslide incident closing the road such diversionary routes will be used.

The approach to and specific methods of exposure and hazard reduction are specifically addressed in Section 8.

Clearly, and as illustrated in Figure 3.1, Monitoring and Feedback is fundamental to the success of the system and key to deriving best value from the arrangements proposed. The system developed operates actively and lessons learned from future debris flow events, whether they occur in areas of high or very high hazard ranking or not, will produce valuable data which needs to be taken into account in adjusting the parameters that form the cornerstone of the assessment methodology.

In parallel with this there exists a need to ensure that actions identified by the existing Rock Slope Hazard Index system (as developed in the early 1990s: McMillan & Matheson 1997) are carried out on a priority budget basis. Such actions will include both maintenance works and re-inspection activities. While the rock slope system and the proposed debris flow system have very different structures, great efforts have been made to ensure that the critical exposure evaluation and the output categories are capable of being mutually compatible.


Dissemination activities include a wide range of presentations to a wide range of audiences, both specialist and otherwise. A range of publications has been published in technical journals (Winter et al., 2006a) and in international conference proceedings aimed at disseminating the work undertaken in the UK (Winter et al., 2006b; 2007a), the USA (Winter et al., 2007b; 2007c) and Hong Kong (Winter et al., In Press). This is a continuing process and further technical papers are in preparation including ones intended to be delivered at international conferences in Asia and Europe.

More than 40 separate dissemination activities have been undertaken to date; Table 3.1 lists the key activities.

Table 3.1 – Key dissemination activities.


Audience, Location and Date

Project Workshop

Project participants, North Queensferry, September 2004

Cover picture for the Quarterly Journal of Engineering Geology and Hydrogeology

Profession, all four journal issues in 2005

International Conference on Land Risk Management (attendance and awareness raising), part-funded by Royal Academy of Engineering

Profession, Vancouver (Canada), June 2005

Report to Royal Academy of Engineering on visit to Canada (see above)

Profession, 2005

Study Technical Report

Profession, June 2005

Study Summary Report

Public and politicians, June 2005

Launch Seminar for Technical and Summary Reports

Profession, Edinburgh, June 2005

Paper in Proceedings of International Conference on Landslides and Avalanches: ICFL 2005

Profession, Norway, 2005

Interview on BBC Radio Scotland (Good Morning Scotland) on anniversary of Glen Ogle events

Public, 18 August 2005

Article in TRL News

Profession, October 2005

Book review of Technical Report in Engineering Geology journal

Profession, 2005

Article in Surveyor magazine

Profession, November 2005

Presentation to Seminar on Landslides and Sediment Control – Implications of Climate Change for the Design and Management of Forestry

Forestry Commission, Dunkeld, December 2005

Article in Surveyor magazine

Profession, February 2006

Paper in Quarterly Journal of Engineering Geology and Hydrogeology

Profession, 2006

Presentation to Scottish Universities Geotechnical Network – Landslides Masterclass

Profession, Dundee, April 2006

Presentation to Central Scotland Regional Group of the Geological Society

Profession, May 2006

Presentation to Ground Engineering Magazine Seminar on Slope Engineering

Profession, London, July 2006

Article in Surveyor magazine

Profession, July 2006

Presentation to Climate Impact Forecasting for Slopes (CLIFFS) network seminar

Profession, Kingston-upon-Thames, July 2006

Paper in Proceedings of Engineering Geology for Tomorrow’s Cities: Proceedings, 10th International Association of Engineering Geology Congress

Profession, Nottingham, September 2006

Presentations to RoadEXPO 2006

Profession, Edinburgh, October 2006

Consultation on Scottish Executive’s State of Scottish Soils

Government Organisations, 2006

Article posted on CLIFFS website

Profession, 2007

Leaflet drafted for Transport Scotland on Scottish Roads and Landslides

Public, 2007

Presentation to HR Wallingford

Profession, Oxfordshire, March 2007

Presentation to TRL (Landslides Masterclass)

Profession, Berkshire, May 2007

Presentation at IAT National Conference

Profession, Telford, May 2007

Paper in Proceedings of International Conference on Landslides and Climate Change: Challenges and Solutions

Profession, Isle of Wight, May 2007

Presentation at Climate Change and the Roads Seminar

Profession, Nottingham, June 2007

Two papers in Proceedings of First North American Landslides Conference: Landslides and Society – Integrated Science, Engineering, Management and Mitigation

Profession, Vail (Colorado, USA), June 2007

Presentation to SCOTS Training Module II – Design and Construction

Profession, Hamilton, September 2007

Presentation to Northern Ireland Geotechnical Group Earthworks Seminar

Profession, Belfast, September 2007

Paper in Proceedings of the International Forum on Landslide Disaster Management

Profession, Hong Kong, December 2007

Presentation to Hong Kong Regional Group of the Geological Society

Profession, Hong Kong, December 2007

Presentation to public CLIFFS network seminar

Profession, Loughborough, February 2007

Presentation to Royal Meteorological Society

Profession, Norwich, March 2007

Presentation to EGU Session on The role of plants on slope stability and the impacts of climate change and land-use change on landslides (co-convened by lead editor)

Profession, Vienna (Austria), April 2008

Paper accepted for the Proceedings of 10th International Conference the Application of Advanced Technologies in Transportation

Profession, Athens (Greece), May 2008

Papers submitted to the First World Landslide Forum

Profession, Tokyo (Japan), November 2008

Other relevant publications that expressly relate to or refer to the work include (Nettleton et al., 2005a; Winter et al., 2006c; 2007d; 2007e).


The affirmation that we live in a ‘risk-averse society’ is becoming a common viewpoint and implies that the willingness to accept, or to tolerate, risk is low. In many spheres of life such a statement may well be accurate, but it remains relatively meaningless unless it is viewed in a broader context. Such a context includes the willingness (and/or ability) of society (as an individual, a corporation, an organisation, or as a sector of government) to pay for risk reduction measures and the willingness to alter the environment in order to accommodate such measures.

The United States of America is often cited as a definitive example of a risk averse society. However, the evidence does not always support this assertion. Interstate 70, the main east-west route through Colorado, traverses the toe of the DeBeque Canyon landslide (Figure 3.2). During the last reactivation of the landslide in April 1998, the road heaved 4.3m and shifted 3m laterally towards the nearby river (White et al., 2007). The landslide continues to move forewarning of possibly future rockslides from above and heaving of the road associated with rotational failure. The Colorado Department of Transportation (CODoT) have undertaken a series of remediation measures as described by White et al. (2007) and commissioned a long term monitoring system. The overall approach seems to be that the movements described above are at an acceptable level and can be managed on an emergency works basis as and when they happen.

Figure 3.2 – DeBeque Canyon landslide showing Interstate 70 passing over the toe.

Figure 3.2

The example of DeBeque Canyon, cited above, implies a high level of willingness to accept risk and an associated low level of willingness to pay, possibly driven by an unwillingness to affect the environment. There also may be higher levels of risk elsewhere which may take priority. Provided that the willingness to accept risk, to pay and to affect the environment can be consistently described at a conceptual level then the approaches in different parts of the world and in different situations may be straightforwardly and graphically compared to gain a deeper understanding of the drivers for the approach to risk mitigation.

This has been achieved by means of the ternary ‘Willingness (ternary) Diagram’ (Winter et al., In Press) (Figure 3.3). The Willingness Diagram inter-relates three parameters, thus constraining any one of the three in terms of the levels assigned to the other two, the implicit assumption being that there is a fixed amount of ‘willingness’ to share between the following parameters:

1. Willingness to accept (or tolerate) risk.
2. Willingness (and/or ability) to pay.
3. Willingness to alter the environment in the pursuit of lower risk.

The example of DeBeque Canyon, cited above, implies a high level of willingness to accept risk and an associated low level of willingness to pay, possibly driven by an unwillingness to affect the environment and, potentially, higher levels of risk elsewhere which may take priority.

The situation in Hong Kong, where life has been valued at a high, but nevertheless realistic, level and the willingness to accept risk is relatively low, provides and interesting counterpoint. In the 1980s the willingness to affect the environment was also at a relatively high level with hard engineering solutions often dominating the scene (e.g. Figure 3.4). In the latter part of the 1990s and beyond there was an apparent shift in the approach in Hong Kong and the willingness to affect the environment was much reduced leading to softer vegetative solutions where appropriate. This change in approach may have been associated with an increase in the willingness to accept risk as some of the design solutions used may be less robust. There may also have been an associated increase in the willingness to pay, if only in terms of an increase in the long-term maintenance expenditure required for such soft solutions.

Figure 3.3 – The Willingness Diagram showing the different approaches to landslide risk in respect of the Scottish main road network, the Undercliff at Ventnor, I-70 DeBeque Canyon and in Hong Kong.

Figure 3.3

Figure 3.4 – A shotcrete slope in Kowloon, Hong Kong SAR.

Figure 3.4

In the Isle of Wight, the willingness to accept risk is also low and the willingness to pay is high, despite the fact that the risks are generally to property rather than to life and limb. At the same time the willingness to affect the environment is low and these factors drive the use of the generally discrete and ‘invisible’ solutions that are implemented.

In respect of Scotland’s roads the both the willingness to affect the environment and the willingness to pay are relatively low, and management solutions are thus favoured over intrusive engineering solutions. With this comes an acceptance that a certain level of risk must be accepted and that these risks are generally significantly less than those posed in other situations – by road traffic accidents, for example.

In terms of the Scottish environment some of the key drivers for the willingness (or indeed unwillingness) to accept risk are social, economic and environmental and often include components of all three. Roads in Scotland provide vital communication links to residents of remote communities from both the social and economic viewpoint and the effects of the severance of the communities from services and markets for goods is highly undesirable.

An example of the adverse impacts that severance may have on communities may be drawn from Jamaica. In this case (Figure 3.5) a landslide has occurred on the B1 route in the Blue Mountains in Jamaica effectively severing the local coffee production industry from the most direct route to markets accessed from the island’s north coast.

Figure 3.5 – Landslide on the B1 road at Section in Portland Parish, Jamaica.

Figure 3.5

The landscape has both a social and an environmental value, but what is often forgotten is that, for Scotland, its economic value is substantial as it attracts much business in the form of tourism, especially important to many of the remote communities potentially affected by landslides. The height of the tourist season does also coincide with the summer landslides season of July and August and thus, in parallel with the need to maintain access, detrimental effects on tourism from negative publicity are unwelcome to all involved parties, including both politicians and the public. At the same time adverse visual impacts on the landscape by large defence/remediation structures (e.g. debris basins, overshoots, shelters, etc.) are seen as undesirable and, as a result, the underlying philosophy of any remediation must be to preserve the natural landscape as much as is possible insofar as this is what tourists come to enjoy.

The avoidance of adverse impacts on other valuable natural resources is also a key issue. Examples of such adverse affects might include measeres that result in the alteration of the hydrogeological regime of protected peat bogs and activities which may add silt to protected/valuable salmon fishing/spawning rivers.