7 HAZARD RANKINGS
by M G Winter, F Macgregor and L Shackman
The landslides study was commissioned to assess debris flow
hazards on the Scottish road network and address the risks
resulting from these as they affect Transport Scotland’s road
network and the road users. The risk to life and limb was
identified at the outset of this project by Transport
Scotland’s senior management as the primary concern with
socio-economic impacts being secondary (but nonetheless
Risk is classically defined in terms of landslides (Cruden and
Varnes, 1996; Culshaw, 2005) as follows:
where R is the risk.
H is the hazard.
E denotes the elements at risk.
V is the vulnerability of the elements at risk to the
In this work the result of this equation (risk) is described as
Hazard Ranking, RH, as it is recognised that the
work reported does not consider all aspects of risk.
The hazard is as determined in Section 6. The elements at risk,
namely the road and the associated road users, are either present
or not at a given plan location. The elements at risk may thus be
represented by a binary switch that is set to unity in all cases
considered (i.e. where a road and road users are present). The
vulnerability equates to risk to life and limb of road users and
the socio-economic impacts, including diversionary effects, of
temporary closure due to landslides. The binary switch allows a
simplification of Equation 7.1 and for the purposes of this study
may be rewritten as:
where EX represents the
vulnerability of road users to life and limb risks and the
potential socio-economic impacts.
The approach taken herein mirrors that typically followed for
landslide hazard and risk assessment, as described above. It builds
upon the approach outlined by Winter et al. (2005a) in the
precursor to this report and is not a modification of the approach
proposed by Clayton (2001), as has been stated by Anon (2006a).
7.2 EXPOSURE SCORES
The exposure of life and limb may be represented, at a simple
level, by the surrogate of traffic flow. It is accepted that
sightlines and other factors that influence visibility of the road
ahead could also be used to refine the exposure of life and limb
(e.g. McMillan & Matheson, 1997). However, in this study two
issues rendered this additional complication inappropriate: first,
to a large extent traffic flows relate to the type of road
alignment in place and thus to the quality of the sight lines;
second, the extent of the route lengths considered meant that a
simple and straightforward approach was, in this case, more
Similarly, the socio-economic aspects of exposure may be
represented not only by the traffic flow and but also by the
existence, length and quality of any diversion necessary.
As with the different elements that make up the GIS-based hazard
assessment (Section 4), the different elements of exposure must
also be added together in order to achieve an overall score.
Relevant categories were determined and scores then assigned for
both traffic flow and diversionary aspects of exposure for each
site. The scores for these individual factors were then weighted to
reflect their relative importance and then summed to produce the
overall exposure score.
The traffic categories used by Transport Scotland reflect the
traffic flows over the entire network. The lowest flow category
comprises those roads with an Annual Average 2-way 24-hour Daily
Flow (AADF) of less than 10,000 vehicles per day. It was apparent
at the outset of the work to define the traffic flow scores that
this lowest category would cover a large proportion of the
vulnerable sites identified in Section 5. This would mean that the
use of the standard traffic flow categories would not effectively
differentiate between the various sites and a decision was
therefore made to use alternative categories. These new categories
and their associated exposure scores (EXT) were
defined as follows:
AADF _ 2,500 vehicles per day, EXT = 1.0.
2,500 < AADF _ 7,500 vehicles per day, EXT
7,500 < AADF _ 25,000 vehicles per day, EXT
AADF > 25,000 vehicles per day, EXT=
Traffic data was sourced from the Scottish Road Traffic Database
operated by Transport Scotland.
The diversion scores (EXD) were based upon an
informed judgement of the potential consequences of a closure on
the network within a given location section. Where the diversion
was short and effective (e.g. by other trunk and/or
‘A’-roads) then the consequences were defined as
‘Limited’. Where the diversion was long, by difficult
means (e.g. ‘C’, ‘D’ and/or unclassified
road) or does not exist (in practical terms) the consequences were
defined as ‘More significant’.
‘Significant’ represents the middle ground between
these two extremes and the diversion scores were defined as
Limited, EXD = 0.
Significant, EXD = 1.
More significant, EXD = 2.
For any given site, weightings were then applied to the two
exposure scores. The two weighted scores were then added together
to give a total score for exposure. The weightings applied reflect
the paramount importance of reducing the exposure to risks related
to life and limb of the travelling public, and for this reason the
traffic score was weighted more heavily than the largely
disruption-focused diversion score. It should however be noted that
the traffic score does itself include significant elements that
relate to the potential disruption to road users.
The score of the final exposure score is thus given by:
Accordingly, Equation (7.2) may thus be rewritten as
The final exposure scores are detailed in Appendix D.1.
It could be argued that either the hazard or the exposure (and
therefore hazard ranking) should be influenced by the length of the
section in question in a very direct sense by, for example, taking
the Elements at Risk part of Equation 7.1 to be the length of road.
This type of approach is suited to risks such as that of a tanker
over-turning on a straight road – that is, risks that are
entirely uniform along the full length of the section (see also
Section 5.2.2). However, there are a number of factors that count
against this approach, and these are as follows:
1. The relation between length and the probability of event
occurrence is not a constant. The hazards contained within a given
section length are variable in terms of their spacing and magnitude
and the profile of any hazard score along a given length is
therefore also variable. It would therefore not be correct to
proportion the exposure according to the length of the section in
2. Effects of length have already been accounted for
appropriately in the process in which priorities were assigned to
the lengths (see Section 5.2.2). Thus, hazard scores should be
viewed as providing aggregate scores over the length of the
identified hazard rather than average scores at any given point
within the length.
3. The purpose of the work was also to rank hazards rather than
to perform an all-encompassing risk assessment.
4. The hazards and hazard rankings are intended to apply to each
likely point of potential incident on a route length and therefore
it would be inappropriate to undertake a length-based approach.
5. A length-based approach does not lend itself readily to
spatial assessments (such as those relating to debris flows) as
opposed to purely linear hazards (such as that of a fuel tanker
7.3 HAZARD RANKINGS
The overall hazard ranking scores were thus able to be computed
by taking the results presented in Section 6 and applying them,
along with the exposure scores, to Equation 7.4. The detailed final
hazard ranking scores obtained are then able to be set-out and are
presented in tabular form in Appendix D.2. A truncated form of the
table, detailing sites with final hazard ranking scores of 100 or
greater, is presented as Table 7.1 and the geographical
distribution of the sites is illustrated in Figure 7.1.
In Section 3, Figure 3.1 identified site rankings in terms of
Low, Medium, High and Very High Hazard Ranking. An explanation of
these categorisations was given in the text.
The distinction that was made between Low and Medium Hazard
Ranking sites (Winter et al., 2005a) has been superseded by
the development of Traffic Scotland’s Emergency Standard
Diversion Routes (ESDR) (see also Section 8.3.1). This means that
the actions planned for Medium Hazard Ranking sites are in the
process of being implemented for all sites, including Low Hazard
Ranking sites; the ‘do nothing’ scenario thus becoming
Table 7.1 – Sites with a hazard ranking score of 100 or
In terms of the High and Very High Hazard Ranking sites the
primary intention is to concentrate on exposure reduction and a
mixture of exposure and hazard reduction respectively. The
distinction between these two approaches is more fully described in
Section 8, but, in summary, the key issue is one of cost, with
hazard reduction generally being significantly more expensive than
Figure 7.1 – Sites with a hazard ranking score of 100
or greater. (© Crown Copyright. All rights reserved
Scottish Government 100020540, 2008.)
The cost of hazard reduction measures, beyond routine drainage
maintenance and improvement measures, is such that embarking upon
such work needs to be considered very carefully. Indeed, the costs
need to be set within the context of the overall maintenance and
construction budgets operated by Transport Scotland. Additionally,
any intended work should be reviewed in terms of existing programme
plans for significant upgrading and/or realignment of existing
routes (see also Section 8.2).
Table 7.1 details those sites determined to be of High and Very
High Hazard Ranking.
7.4 RE-INSPECTION PROGRAMME
In Section 3, Figure 3.1 indicates a Monitoring and Feedback
activity in the flowchart. An essential component of that activity
is having in place an effective programme for re-inspection of
slopes identified as being hazardous or potentially so. As an
overall consideration, the GIS-based assessment should be
re-visited in (say) 10 years to take account of:
1. New and improved data sets.
2. New and improved technologies for handling such date
This work would also require reinterpretation of the GIS-based
assessment (either manually as reported here or automatically if
technology, including processing power, allows).
In terms of re-inspection of the sites themselves, those with a
hazard score of (say) 70 (i.e. Priority 2 sites with the 10 uplift
added for site-specific inspection) and above, should also be
subject to a reassessment exercise. This would go some way towards
taking account of temporal changes to the volume and nature of
material available for triggering debris flows.
The combination of revisiting the GIS-based assessment,
interpretation and site-specific re-inspection after the interval
suggested, should also ensure that the appreciation of debris flow
hazard to the network remains soundly-based in future years.
7.5 ROCK SLOPES
Clearly debris flows are not the only hazards that may affect
roads in Scotland and amongst the others are those presented by
falls of geological material from rock slopes and cliffs alongside
the network. Between 1994 and 1999 Transport Scotland (in a
previous guise) initiated and operated a structured programme of
rock slope risk assessment and management on the trunk road network
(McMillan, 1995; McMillan & Matheson, 1997). The process
involved the computation of a Hazard Index that then determined the
actions required in terms of further inspections and more detailed
surveys to determine Hazard Ratings.
The Hazard Index categories that were developed are as
- Urgent detailed inspection (hazard rating survey).
- Detailed inspection (hazard rating survey).
- Review in five years.
- No action.
In recent years, a review of progress with both inspections and
recommended remedial works was undertaken (Blair & McMillan,
2004). This review identified the sites at which urgent detailed
inspections had been required to be undertaken as part of the
Hazard Rating process. Whilst most of those had been, indeed,
undertaken prior to the review, some outstanding inspections were
identified, as follows:
- A82 Tyndrum to Fort William (one outstanding Hazard Rating
- A87 Invergarry to Cluanie (two outstanding Hazard Rating
- A887 Invermoriston to Moriston Bridge (one outstanding Hazard
- A86 Newtonmore to Spean Bridge (two outstanding Hazard Rating
- A82 Fort William to Fort Augustus (one outstanding Hazard
The 2004 review is, of course, unable to detail how many of the
sites flagged for detailed inspection have now been inspected.
Transport Scotland is currently assessing the future actions
required to address those Hazard Rating surveys and re-inspections
that remain to be carried out.