by M G Winter, F Macgregor and L Shackman

The GIS-based assessment (Section 4) covers, to all intents and purposes, the entirety of Scotland and can thus be applied to both trunk and local road networks in any part of the country. This section and those that follow it detail the work that has been undertaken on the trunk road network to obtain a greater understanding of the hazards that exist and their relative rankings (see Section 3.2). The results of the assessment have also been distributed to local authorities for use in assessing their networks.

The interpretation of the GIS-based assessment imagery and relating this to the road network as it exists on the ground is, in many ways, the key to the study. While the GIS-based assessment deals with the potential for triggering debris flow, the interpretation detailed in this section of the report assesses the potential for such flows to reach the network. This then allows the prioritisation of sites (Section 6), the development of the levels of exposure of road-users, the consequent hazard rankings (Section 7) and the assignment of management strategies that result (Sections 8 and 9).

It should be stressed that this interpretation phase was configured to be an entirely desk-based exercise underpinned by comparing digital mapping (and low resolution aerial photography where available) with the GIS-based results, and augmenting this with extensive individual knowledge of the routes and adjacent landscape on the network.


Imagery of various types was utilised in the process of relating the GIS-based information to potential hazards affecting the network. Initially the layer derived from the GIS-based assessment was used (see Figure 5.1). Other layers available within the GIS included land elevation contours, the local and trunk road network, elevation data and coastline were used where appropriate. In addition, flat areas above roads, which were identified as part of the GIS-based hazard assessment process, were available as a layer as were the results of bespoke assessments of the superficial geology which were made as part of the project for areas where such information was not otherwise available.

Digital, two-dimensional Ordnance Survey mapping at 1:50,000 scale (Figure 5.2) was used to relate the GIS imagery to salient features of the network. The purpose of this was to aid perception of the nature of the terrain in areas where potential hazards had been identified. This was supplemented by relatively low resolution, two-dimensional aerial photography (Figure 5.3), where this was available at the time the interpretation was undertaken.

Ordnance Survey and low resolution aerial photography were examined in two-dimensions and, where clearer information about the topography of the landscape was required, in three-dimensions also (Figures 5.2 and 5.3). All viewing of the imagery was undertaken digitally, via a computer monitor, and latterly a 24-inch wide-screen device was used to allow the two main sets of imagery to be viewed side-by-side.


The Scottish trunk road network comprises some 3,200 kilometres of route length. This network is of a widely varying nature, ranging from heavily trafficked motorways to, in certain localities, single-track roads providing essential transport and communications links for remote communities. The entire network was inspected in detail using the available imagery in order to achieve a valid interpretation of the GIS-based assessment. An essential factor within the process was the need to ensure consistency in the interpretative outputs, and this was built into the methodology.

Figure 5.1 – GIS-based imagery for A85 Glen Ogle. The length of road shown is approximately 5km.

Figure 5.1

Figure 5.2 – Digital Ordnance Survey mapping at 1:50,000 for A85 Glen Ogle in: (left) two-dimensions; (right) three-dimensions. (Note that the image itself is not to scale.) (© Crown Copyright. All rights reserved Scottish Government 100020540, 2008.)

Figure 5.2

A two-phase approach was taken to the interpretation of the GIS-based assessment. The first phase primarily entailed selecting lengths of route for further study. At the same time account was taken of lengths of road that could become vulnerable to hazards if major works, such as realignment, were to be undertaken and, also, of data anomalies (see Section 4.5.1). The second phase of the interpretation then prioritised the route lengths selected in the first phase for further study in the form of site-specific assessments.

Figure 5.3 – Relatively low resolution aerial photography imagery for A85 Glen Ogle in: (left) two-dimensions; (right) three-dimensions. Note that the white areas represent those for which the aerial photography was not available.

Figure 5.3

The assessments entailed a close visual examination of the available imagery. In particular, given the spatially-distributed potential for debris flow trigger conditions defined by the GIS-based hazard assessment, an informed judgement was made of the existence of plausible flow paths that could allow debris flow to reach the road. Clearly the process by which such triggers propagate flows downslope are complex (e.g. Hungr et al., 2005; van Asch, 2006), proceeding through a sequence of erosion and deposition to the final runout zone, and potentially reaching a distal piece of infrastructure, such as, in this case, a road. However, using highly detailed approaches such as mathematical and numerical models, while appropriate to a small number of sites, would involve disproportionate resources in order to allow their implementation across a significant portion of the Scottish trunk road network. The assessment and interpretation presented here is thus regional, rather than local, and semi-quantitative/qualitative, rather than quantitative. It is, however, above all, appropriate to the problem under consideration, the area to be covered and the resources available.

The interpretative process was thus focused upon the morphology of the ground between areas of potential hazard and the road itself. Slope angles, the presence of stream channels that might aid the passage of debris and any potential barriers to flow were, amongst other factors, considered in all their forms. Consequently the interpretation may be summarised as a semi-quantitative/qualitative determination of potential debris flow tracks and run-out zones to determine whether they intersect with the trunk road asset. Figure 5.4 illustrates how the GIS-based assessment and its subsequent interpretation allowed sites of ‘high hazard’ were progressively identified n a systematic manner.

As the time of writing this report the editors are not aware of any instance of a systematic interpretation of the GIS-based assessment having been carried out for all or part of the local road network.

At an early stage in the development process it was decided that the interpretation of the GIS-based assessment would be carried out in its entirety by the project management team. This decision was taken in order to ensure consistency of output which would follow naturally from familiarity with the various facets of the development of the project as a whole.

Figure 5.4 – Diagram illustrating the process by which the GIS-based assessment and its subsequent interpretation ‘home-in’ upon sites of high hazard.

Figure 5.4

One particular aspect to be taken account of was that of the team becoming more familiar with the interpretative process itself as the work progressed, and the need to establish the effects, if any, of this familiarity on the outputs. This was addressed by revisiting, as a conclusion to the interpretative process, a selection of the route sections which had been examined early in the exercise to verify that the outputs remained consistent.

5.2.1 Initial Interpretation and Outcomes

The specific methodology for undertaking the initial interpretation was to deal with the network on a route-by-route basis, working consistently from one end of the route to the other, and identifying sites of interest as they occurred by means of the route name followed by a numerical identifier (e.g. the thirty-seventh section identified on the A9 route would be coded as A9-37). Sites showing potential indicators of hazard on the GIS-based assessment imagery were be categorised under three descriptors, namely:

  • Other (None): Although indicators were present on the GIS-based imagery, assessment had established that no potential hazard was present and these sites were considered benign in terms of the asset under consideration. As examples, there are localities where the GIS-based algorithms interpret flat, wet, low-lying ground as a hazard (see Section 4.5.1), or, in other situations, where an identified hazard would be directed to a location away from, or more remote from the road in question. For route lengths so categorised no further action is required.
  • Opportunistic: This descriptor was used for localities where the hazards and risks were assessed to be less than would justify a main detailed study as part of this project, but would be required to be considered and assessed if and when any major works were planned – reconstruction or realignment for example. For route lengths so categorised no further action is required until specific plans to alter, upgrade or otherwise affect the route are planned.
  • Main Study: These sites would be the lengths of route where significant potential hazards had been identified. These would be the sites recommended to be taken forward for full detailed assessment as part of this study. Care was to be taken that such Main Study sections were subdivided into separate sections such that all the individual sections exhibited hazards of consistent character. For route lengths so categorised further interpretation is required (see Section 5.2.2) with a view to site-specific inspections being carried out as appropriate and on a programmed basis.

For the full 3,200km of trunk road route length, imagery from the two main sources (i.e. Ordnance Survey and the GIS-based assessment) was inspected, along with the low resolution aerial photography where available. Three-dimensional views were used to assist in the process where appropriate.

The inspection progressed along the route in question until an area of interest was identified. In all cases, a swathe of several kilometres to each side of the road in question was inspected on the GIS-based imagery, in order to identify any more remote hazards, in addition to ones closer to the road.

For each area of potential hazard identified from the GIS-based assessment, the potential for any debris flow to reach the road infrastructure was determined as described previously. Any section of road that was identified as being potentially vulnerable to a hazard was then marked-up by category on a ‘master’ 1:50,000 Ordnance Survey plan.

In cases where flat low-lying land was indicated as a potential ‘hazard’, as a function of the way in which the GIS-based assessment was undertaken (see above and Section 4.5.1), a simple extrapolation from the ‘hazard zone’ to the road was made in order to rapidly define the route length and preclude it from further consideration, consequently marking it as ‘Other (None)’.

The three categories, as previously described, were colour coded as follows:

  • Other (None) – Grey.
  • Opportunistic – Purple.
  • Main Study – Red.

Figure 5.5 illustrates an area of the GIS-based assessment imagery for the A9 at Glen Garry, while Figure 5.6 illustrates the two-dimensional Ordnance Survey mapping with identified route lengths corresponding to the above categories.

Table 5.1 summarises the results for this initial interpretation, the results of which are presented in full in Appendix B.1 (Table B.1). In total 1,684km of the trunk road network was categorised as either Other (None), Opportunistic or Main Study. Table 5.1 shows how these categorisations relate to the total length of the network.

Figure 5.5 – GIS-based imagery for A9 Glen Garry. Not to scale.

Figure 5.5

Figure 5.6 – Ordnance Survey two-dimensional imagery for A9 Glen Garry showing lengths categorised as Opportunistic (A9-36), Main Study (A9-37) and Other (None) (A9-38) as viewed from left to right. Digital Ordnance Survey imagery at 1:50,000 is shown but the image itself is not to scale. (© Crown Copyright. All rights reserved Scottish Government 100020540, 2008.)

Figure 5.6

5.2.2 Secondary Interpretation and Outcomes

With some 607km of the network falling into the Main Study category as an outcome of the initial imagery-based assessment, it was considered that further prioritisation of the Main Study sites would be necessary for reasons of allocating available resources for the detailed inspection phase.

Table 5.1 – Initial outcomes from the interpretation of the GIS-based imagery.

Table 5.1

Accordingly the Main Study sites on the network were critically reassessed for severity of potential hazard and ranked in priority categories ranging from 1 (most severe) to 4 (lesser severity). At that point, relatively arbitrary hazard values (out of a nominal 100 ‘maximum’) of 80, 60, 40 and 20 were then assigned on an interim basis to Priority 1, 2, 3 and 4 sites respectively. These interim values would then be confirmed, or modified appropriately, as part of the detailed site inspection phase described later in this report. A score of 80 was also assigned at this juncture to the two sites selected for Separate Assessment (see below).

The hazards identified through the process described here are clearly variable within sections and between sections. This raises the issue of how to determine the aggregate hazard within any given section length. In reality this variability means that it would be inappropriate to use length as a multiplicand in the exposure scoring (see Section 7.2) and as a result the effects of section length have been incorporated into the process used to assign the hazard priorities. Notwithstanding this some relatively short sections, of high potential hazard and/or high traffic are likely to remain in the group of higher hazard ranking evolved in Section 7.2.

In addition, two significant sections of the network, the A82 through Glen Coe and parts of the A87 on Skye (and amounting to a total of 46km), were identified for separate evaluation as a result of their particular characteristics. Both of these localities are considered to be of a different character to the bulk of the sites. They are predominantly affected by scree (talus) slopes rather than more heavily weathered and finer-grained materials. In addition, these areas, particularly the A82 in Glen Coe, have been subject to intense commercial and academic study over many years. Thus, any resources used to assess the risk in these areas would be best allocated to a literature review, at least initially. It was thus considered that best value would not be derived from detailed site-specific assessments that are recommended for other ‘Main Study’ sites in Section 6.

This resulted in the Main Study network length of 607km being subdivided into the various categories as shown in Table 5.2, the results of which are presented in full in Appendix B (Tables B.2 to B.6).

Figure 5.7 illustrates an area of the GIS-based assessment imagery for the A82 at Loch Ness, while Figure 5.8 illustrates the two-dimensional Ordnance Survey mapping with identified route lengths corresponding to the categories in Table 5.2.


The secondary categorisation exercise identified some 607km of the trunk road network subdivided into four fairly equal quarters of approximately 110km to 160km each (plus a total of slightly less than 50km for the two separate assessments). This provided the basis for a staged annual process of carrying out the detailed site inspections, within the anticipated limits of annual budget for this work.

Table 5.2 – Secondary outcomes from the interpretation of the GIS-based imagery.

Table 5.2

Figure 5.7 – GIS-based imagery for A82 Loch Ness. Not to scale.

Figure 5.7

However, the availability of the essential, higher resolution, recent aerial photography for many of the sites in question, placed constraints on the sequencing. Transport Scotland had contracted to obtain the imagery necessary for this exercise. For the surveys planned for Summer 2007, however, imagery was not available for a number of Priority 1 and 2 sites, not least on the Arrochar to Inverary section of the A83. The absent areas were due for flying at some time in 2007, so the imagery needed to complete the remaining detailed site inspections (in all Priorities) would be to hand for 2008. As a result, a decision was taken to use the available high resolution aerial photography imagery to undertake detailed site inspections for Priority 1 and 2 sites in 2007. Details of coverage available in early 2007 are given in Appendix B.3 (Tables B.7 to B.11).

Figure 5.8 – Ordnance Survey two-dimensional imagery for A82 Loch Ness. Showing lengths categorised at Priority 3 (A82-03), Priority 1 (A82-04) and Priority 2 (A82-05) from top to bottom. Digital Ordnance Survey imagery at 1:50,000 is shown but the image itself is not to scale. (© Crown Copyright. All rights reserved Scottish Government 100020540, 2008.)

Figure 5.8

While not all of the Priority 1 and 2 routes were located in the North West Unit, by far the majority were. For this reason the decision was taken that the site inspection work for 2007, and potentially for subsequent years, would be primarily undertaken by the North-West Unit Operating Company (Scotland TranServ).

As a result of the above considerations, it was decided that the detailed site inspections would be carried out for all of the Priority 1 and 2 sites that had aerial photography available in 2007. Scotland TranServ, as Operating Company, for the North-West Unit, would resource a suitable team to do the work. In addition the TranServ team would also undertake the four non-NW Category 1 and 2 sites for which aerial photography was available, namely on the A77 at Glen App (South-West Unit) and on the A95 (North-East Unit). Scotland TranServ liaised with Amey, the South-West Unit Operating Company, for purposes of the A77 site and with BEAR Scotland for those on the A95.