1 INTRODUCTION

Transport Scotland instigated the Scottish Road Network Landslides Study in August 2004, when extensive and excessive rainfall caused a number of serious landslides which adversely affected the operation of the strategic road network.

During that month, some areas of central and western Scotland experienced levels of rainfall more than three times the average for the time of year including intense storms. As a result a large number of landslides – more specifically debris flows, a particular type of landslide – was experienced in the hills of Scotland. Among those which reached the trunk road network were those on the A85 in Glen Ogle, the A83 at Glen Kinglas and Cairndow, and the A9 north of Dunkeld.

Debris flows occur with some frequency in the hills of Scotland and although these only rarely affect the main road network, when they do they can have a major effect on communities, the economy and public safety.

Fortunately, during the events of August 2004 there were no injuries, but other factors were significant. Road closures and long diversions had the effect of restricting access to already relatively remote communities and were a substantial inconvenience to road users. This included tourist traffic, which is generally at its peak in the summer months when landslides can often occur.

In light of the above, and acknowledging that climate change may further increase the prevalence of landslides, Transport Scotland recognised the need to act.

The first part of the Scottish Road Network Landslides Study,^{1 2} considered the cause and effect of landslides and proposed the development of a system for assessing and ranking the hazards posed by debris flows. (A second study, the Scottish Road Network Climate Change Study^{,3 4}, examined the distinct, but related, issue of climate change and its broader impacts on the road network) This system for assessing and ranking debris flow hazards has since been developed and applied across the entire coverage area of Scotland’s trunk road network and the results are reported in this report on the second part of the study⁵.

The objective of the study is to allow budget and resources to be focused on areas where debris flow hazards and impacts are most severe and where they can be effectively managed and mitigated to reduce the exposure to road users, while at the same time acknowledging that debris flows themselves cannot be prevented.

Section 2 of the full report⁵ summarises the types of landslide that exist, the events that have been experienced in Scotland in recent years and the time of year during which these are most likely to occur.

Section 3 provides a brief overview of the actions which were undertaken in the immediate aftermath of the August 2004 landslide events.

Section 4 describes the methodology which was applied to devise the system of assessing debris flow hazards across Scotland. The use of this system essentially identifies the areas most susceptible to debris flow triggering.

Section 5 reports how this information was interpreted with the aid of other data and imagery to establish plausible flow paths from the susceptible zones identified in Section 4 and determine whether these debris flows might impact on the road network.

Using these findings the study progresses from desk-based work to actual site inspections to augment and verify the findings and modify the hazard scores. The methodology and results from these surveys are reported in Section 6.

Section 7 concludes the assessment of hazards posed by debris flows by considering the likely impacts of any flows on the road network, road users, communities and the economy. The results are presented as a listing of higher hazard sites in Scotland.

Section 8 outlines management and mitigation strategies which could be applied to or on the network. It draws on international experience and focuses on two key areas – reducing the exposure of road users through education, the use of signs and, where appropriate, temporary road closures; and reducing the hazard through engineering works.

Approaches which could be applied in the longer term are considered in Section 9. This looks in particular at methods for forecasting landslides from rainfall data and how such a system might be developed for Scotland.

Conclusions and recommendations for action and further investigation and study are outlined in Section 10.

Supporting information and detail is presented in a series of eight appendices.