6 METHODOLOGY FOR SITE-SPECIFIC ASSESSMENTS
by M G Winter, F Macgregor and L Shackman
The site-specific inspection of those parts of the trunk road
network identified as being subject to debris flow hazards is a
necessary element within the process of hazard assessment. These
inspections have three main purposes:
1. To validate the hazards derived from the GIS-based assessment
(Section 4) and their interpretation (Section 5).
2. To provide an interpretation of data that was not available
during the GIS-based assessment.
3. To provide an assessment at a larger scale than the GIS-based
assessment could permit.
There are essentially three stages to the site-specific
assessment process (Section 6.2):
- Desk study.
- Preliminary site inspection.
- Detailed site inspection (where necessary).
The management of the trunk road network in Scotland is
undertaken on the basis of four units run by Operating Companies
employed by Transport Scotland, as follows:
- South-West Unit (currently Amey).
- North-West Unit (currently Scotland TranServ).
- North-East Unit (currently BEAR Scotland).
- South-East Unit (currently BEAR Scotland).
In the first instance it was anticipated that in the main it
would be Priority 1 sites, which exclusively occur in the
North-West Unit, that would be assessed initially during the summer
of 2007. However, the lack of availability of aerial photography in
Scotland meant that only around 44% of the 135km of Priority 1
lengths identified would be able to be examined. Nonetheless,
broadening the assessment out to include the 66% of Priority 2
lengths for which aerial photography was available (including those
lengths in the South-West and North-East Units) raised the total
lengths to be examined to 161km (Tables B.7 to B.11). These 2007
inspections form part of a process which will be ongoing in
The site-specific assessments carried out formed the end point
of a staged hazard assessment process for a given route that began
with the GIS-based assessment. Interpretation of this data using
Ordnance Survey 1:50,000 mapping and low resolution aerial
photography meant that many of the factors that would inform an
assessment of this nature were already implicit.
The site-specific assessments were therefore to supplement and
validate the initial process by utilising high resolution aerial
photography and initial site inspections from road level. Where
appropriate more detailed inspection of any given site was
subsequently conducted and involved excursions from road level to
the adjacent hillsides.
The work centred on various locations across the Scottish trunk
road network, predominantly in the north-west sector (see Tables
B.7 and B.8). The site evaluation element of the work involved
inspecting adjacent hillsides up to, potentially, two to three
kilometres from the trunk road itself; such inspections and the
maximum distance of the inspection from the road were dependent
upon the needs of the site.
6.2 THE INSPECTION PROCESS
The primary evaluation of hazards was achieved through the
GIS-based assessment. It was then supplemented by site-specific
studies involving the use of aerial photography and site visits.
The intention of this latter exercise was thus to validate and make
relatively small adjustments to the scores derived from the
Factors such as lithology and water condition were not able to
be readily assessed, in most cases, from the type of imagery
available (see Section 6.2.1). These factors did, however, form a
key part of the GIS-based assessment, which has been a major
contributor to deciding on the areas to be subject to site-specific
inspections. Notwithstanding this however, the site inspection
process did provide an essential opportunity to validate
assessments made in regard to at least some of these factors on the
The site inspection process was reported primarily through the
completion of a standardised Microsoft Excel® spreadsheet at
each of the three main stages of the process (Table 6.1). In
addition, a short report (circa. One or two pages) was
prepared for each site with photographs to illustrate specific
features and decisions made on scoring.
As noted above the site inspection process involved three main
stages as described below.
1. Desk study: These activities were intended to be
carried out prior to embarking upon on-site activities.
Key desk study activities included the assembly and printing out
of relevant information including OS Map imagery (1:50,000 and/or
1:25,000), high resolution aerial photography, and the GIS-based
This information then enabled those carrying out the inspections
to familiarise themselves with the detail of the OS mapping, to
reconcile the positioning of the OS detail with the GIS-based
assessment imagery and to examine the GIS-based assessment imagery
with respect to detail features on the OS mapping.
Attention was then focused on the high resolution aerial
photography in order to obtain an overview of the area and then to
enable a virtual inspection of particular features. The printed
imagery was marked-up and notes made to enable the site-specific
spreadsheet (Table 6.1) to be preliminarily completed.
2. Preliminary Site Inspection: This was intended to
allow a provisional, but necessarily limited, view of the site
setting. This was achieved by a drive-through of the length of road
in question, with the inspector as a passenger, stopping as
necessary to observe and note features from road level. Photographs
were taken to illustrate features and decisions made.
Table 6.1 – Reporting spreadsheet for site-specific
From the notes made further entries to the site-specific
spreadsheets were made (Table 6.1) as appropriate and an evaluation
was made as to whether further excursion up the hillside was
required. This essentially involved answering the questions:
"Will the information obtained make a substantive improvement to
"If so, will a more detailed site assessment (as in (3) below)
provide adequate answers for the purposes of the spreadsheet?"
If the answer to both questions (above) was "yes" then
proceeding to further detailed site inspection was deemed to be
required, otherwise not. In actuality the decision to proceed with
further inspections was made in all cases. A number of factors may
have, however, informed such decisions. It was appreciated that the
inspectors were not, and nor could they have been, party to the
complete assessment process. The information available to them, and
their experience of the complete process, may thus have proved to
be insufficient to allow them to decide, with confidence, not to
undertake further inspections.
3. Detailed site inspection: This process essentially
completed the hazard assessment process by relating the information
considered thus far (which was either image/data-based or a
physical view from a remote location) to the ground itself. In
practice the detailed site inspection comprised a walkover from
road level and excursions up slope (or down where necessary) as
required, but typically every 0.5km to 1.0km. Further entries were
then made to the site-specific spreadsheet (Table 6.1) and the
scores obtained at this stage taken as final scores. Photographs
were taken to illustrate both the features encountered on site and
the decisions made, as appropriate.
6.2.1 Aerial Photography
The orthographic digital aerial photographs were supplied as
25cm resolution JPEG images with both JGW and TAB files for the
purposes of geo-referencing by Getmapping.
A typical example of such an image for Glen Ogle (south-west
corner NN 570 260, or 2570 7260 using the eight-digit referencing
system applied to the filenames of the images supplied) is
illustrated in Figure 6.1. The original image was 33.87cm square at
a resolution of 300ppi (300 pixels per inch or 118.11 pixels per
centimetre) and has been scaled for use in this report. Both the
north and south debris flows at Glen Ogle are annotated on the
image, but note that the source areas appear in the images of
adjacent areas to the north and east (see Figure 6.2).
The aerial photograph in Figure 6.1 and other adjacent images
have been digitally ‘stitched’ together to give more
extensive coverage in Glen Ogle. Key features are marked on the
image as described in the figure heading. Of particular interest,
in addition to the different elements of the debris flows are the
subsequent carriageway repairs and the rockfalls/rock slides that
cross the old railway line on the west side of the glen. The
railway line was closed in 1965 when the line, already scheduled
for closure, was blocked by one or more major landslides. It is
believed that these may have been one or both of the rock-based
landslides illustrated in Figure 6.2.
Figure 6.1 – Aerial photograph showing the northerly
(N) and southerly (S) debris flows that occurred in Glen Ogle in
August 2004 with key features marked. The photograph represents a
1km by 1km square; north is to the top.
1. North debris flow: (a) potential source areas, (b) debris
track, (c) runout/debris fan and (d) subsequent carriageway
2. South debris flow: (a) potential source areas, (b) debris
track, (c) runout/debris fan and (d) subsequent carriageway
3. Historic rock falls.
4. Other debris flows assumed to have occurred in August 2004.
6.3 SITE-SPECIFIC ASSESSMENT RESULTS
The data collection spreadsheet (Table 6.1) contains four main
categories, in each of which between three and five sub-categories
are scored. The four main categories are as follows:
- Vegetation and land-use.
It is important to note that the scores which derive from the
site-specific assessment are additive to the scores established
from the interpretation of the GIS-based interpretation. This
reflects the importance of information new to the hazard assessment
process, as any information available to the GIS-based assessment
and/or its interpretation is prevented from influencing scoring at
the site-specific assessment stage.
Figure 6.2 – Aerial photograph showing a large part of
Glen Ogle. The image was made by stitching 12 adjacent 1km by 1km
photographs, including Figure 6.1, in a 4km (vertical) by 3km
(horizontal) grid; north is to the top and the marked features are
as for Figure 6.1.
In addition, limits were placed upon the amount by which the
scores could go up or down. This was intended to ensure that the
maximum score was restricted in any instance to a nominal 100 also
that lower ranked sites could not automatically jump to an
inappropriately higher category (and vice versa).
The initial hazard scores based upon the interpretation of the
GIS-based assessment, the allowable adjustments and the associated
possible range of final score for each of Priority 1 to 4 are given
in Table 6.2.
Table 6.2 – Adjustments to hazard scores from
50 to 100
30 to 100
10 to 100
0 to 85
Summary results from the site-specific assessments are given in
Table 6.3 and example score sheets and associated reports are given
in Appendix C, along with a more detailed breakdown of the
Table 6.3 shows that the average increase in the hazard score as
a result of the site-specific inspections was around 13. In order
to ensure that those Priority 1 and 2 sites that were not inspected
due to a lack of aerial photography were not effectively downgraded
when it came to determining what actions should be taken, an
across-the-board increase of 10 was applied to the scores for the
un-inspected Priority 1 and 2 sites; the final hazard scores are
presented in Appendix D.
Table 6.3 – Summary hazard scores.