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9 SUMMARY AND RECOMMENDATIONS FOR DEBRIS FLOWS
IN SCOTLAND
by M G Winter, F Macgregor and L Shackman
9.1 SUMMARY
In August 2004 a series of landslides in the form of
debris flows occurred in Scotland. Some of these affected
the A83, A9 and A85, which form part of the trunk road
network. These incidents were well reported in the
media.
While debris flows occur with some frequency in
Scotland, they only rarely affect the trunk road network or
for that matter the main local road network. However, when
they do impact on the road network the degree of damage
they do, in terms of the infrastructure and the loss of
utility to road users, can have a major detrimental effect
on both economic and social aspects of the use of the
asset. Additionally, there is a high potential for such
events to cause serious injury and even loss of life
although, fortuitously, such consequences have been limited
to date.
The events of August 2004 followed a sustained period of
heavy rainfall and, in addition, intense localised storms
contributed to the triggering of at least some of the
resulting debris flows. Rainfall of up to 300% of the
monthly average fell in certain parts of Scotland during
August 2004.
Within the recent past, debris flow activity in Scotland
has occurred largely in the periods July to August and
November to January, but there is no certainty that such a
pattern will be continued in the future. However, eastern
parts of Scotland do receive their highest levels of
rainfall in August. Additionally, climate change models
indicate that rainfall levels will increase in the winter
but decrease during the summer months but that intense
storm events will increase in number. These factors,
therefore, may change both the frequency and the annual
pattern of debris flow events.
The impacts of such events, when they do happen, are
particularly serious during the summer months due to the
major contribution that tourism makes to Scotland's
economy. Nevertheless, the impacts of debris flow events
during the winter months should not be underestimated.
Following the events of August 2004, this study was
commissioned to take stock of the present situation on the
trunk road network and to determine a sustainable approach
to the management to such occurrences in the future. The
process chosen involves the assessment and ranking of
hazards with a system of management and mitigation also
being proposed. This system is based upon the principles of
Detection, Notification and Action (
DNA) applied both to the response to
landslide events and to precursor rainfall conditions.
9.2 RECOMMENDATIONS
9.2.1 Early Opportunities
A number of areas of perceived high hazard were
identified at the Project Workshop. The lengths of the road
and the slope lengths they involve are substantial.
Accordingly, it is considered unrealistic to undertake
suitably prioritised further evaluations at this stage. The
proposal is for the outputs of the
GIS-based assessment to be used to
corroborate the identification of the localities identified
at the Project Workshop and, in addition, as a validation
tool for the site specific assessment methodology.
In the meantime it is important that maintenance and
construction projects currently in design take the
opportunity to limit any hazards or exposure by
incorporating, where suitable, measures such as higher
capacity or better forms of drainage, or debris traps. Peer
group consultation in the form of the involvement of the
Overseeing Organisation and its Independent Geotechnical
Checker, the corresponding specialists within the Operating
Companies, design organisations or other appropriate
organisations is an essential part of this process.
In the realm of minimising the potential impacts of
debris flows on the network, some retargeting of
maintenance actions could be productive. The checking of
gullies, ditches and catchpits, with a wider view than that
of merely keeping the roadway itself clear of water, could
be undertaken as part of regular inspections. Where
ineffectiveness of the drainage system, or underperformance
under updated drainage criteria, is suspected, this should
be considered in conjunction with the inspection regime for
the roadside side slopes and remedial action addressed via
an appropriate structured asset management plan.
Additionally, critical review of the alignment of culverts
and other conduits close to the road ought to be carried
out as part of inspection and reporting procedures.
Certain monitoring measures are already under
consideration - for example, the installation of a rain
gauge in the A83 Rest and be Thankful area, where debris
flows are generally small but relatively frequent,
potentially yielding more comparable data in a short time
frame. The use of any such data gained, in conjunction with
longer-duration data available from the Meteorological
Office, needs to be managed appropriately to serve a
worthwhile and consistent function. At a later stage,
informed selection of locations for discrete placement of
additional rain-gauging facilities could be productive, and
should be considered in the light of experience of managing
the information from current sources.
An important action which could be introduced on an
early basis is bringing
NADICS into the management loop with
regard to route advice when weather conditions conspire to
create situations where sections of the network might be
considered 'at-risk'.
9.2.1 Study 1, Part 2
The initial stage of Study 1, Part 2 will be 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 will
be to test the methodology before applying it more widely
to the trunk road network.
Figure 9.1 presents a flowchart of the work to be
undertaken.
The initial stage of this work is itself 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.
- The identification and development of appropriate
management processes for each category of hazard
ranking.
Figure 9.1 - Management and mitigation options
within Study 1, Part 2.
The
GIS-based assessment will be
used as a first stage in the hazard assessment process.
This will enable site specific assessments to be targeted
in order to obtain better value from such relatively
resource-intensive activities. It will also allow the
elimination of large areas of the network having minimal
hazard.
It is also particularly important to note that the
site-specific assessment will not be a
'driveby' survey; it will require a highly specialised
detailed site examination which will need to be carried out
using an overall consistent approach. Prior to undertaking
any site surveys it is important that the system is
established for consistently describing and identifying
hazards and the associated exposure. Some of the factors
that will need to be incorporated in such a system, such as
slope angle and the broad nature of the geology, will be
incorporated into the
GIS assessment. Other, more detailed,
factors such as the effects of forestation will need to be
incorporated into the site-based survey. Once a hazard
assessment has been completed it may be combined with an
assessment of the exposure of the road user to that hazard
to give a hazard ranking. This will allow, in-turn, an
appropriate management option to be selected from the range
of options to be developed.
There are a number of potential options which could be
applied to the management of debris flows. 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
landslide event affecting such areas it is seen as
unlikely, largely unforeseeable and/or the exposure is less
serious than at other locations where resources may 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 and any residual
exposure cannot readily be quantified and is unlikely to
justify the commitment of additional resources which may 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 and/or actions of the road-using public on the
network at times either when events occur or precursor
rainfall has indicated a high likelihood of landslides
occurring.
In the case of short-term to medium-term reaction to
such occurrences, then the Detection-Notification-Action (
DNA) approach can be implemented by
pre-planned actions such as issuing an advisory warning or
closing the road. There may be a case for reacting to
extremely heavy rainfall events in a similar fashion,
especially with warnings. A caveat to this is the need to
consider carefully at what levels the triggers should be
set, in so far as the relationship between rainfall and
landslides in Scotland is by no means fully understood.
Considering the longer-term approach, precursor
triggering conditions (
i.e. rainfall) may enable many of the actions
described above to be taken prior to the occurrence of
major events. Either an extensively enhanced network of
rain gauges installed across Scotland or access to data
derived from radar and of sufficient resolution would be
required. Such work initially be concentrated on known
storm tracks, if these are available from the
Meteorological Office, and vulnerable slopes. Clearly, if
this approach is taken then a close consultation with both
the Geotechnical Engineering Office in Hong Kong, which has
extensive experience of operating such a system albeit in
different climatological and geological conditions, and the
UK Meteorological Office would be highly
desirable.
It is fully expected that it will take some considerable
time and effort to ensure that sufficient data has been
obtained and analysed so as to be able to introduce a
warning system. Even then it must be expected that atypical
events, which are not the subject of warnings, may occur.
Also a number of false alarms may inevitably be expected. A
programme of public and media education and
awareness-raising is also likely to be desirable to
minimise any potential adverse reaction to such
scenarios.
'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 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 the Scottish
Executive'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.
Clearly, and as illustrated in Figure 9.1, M
onitoring and Feedback is fundamental to
the success of the system and key to deriving best value
from the arrangements proposed. The system developed is an
active one and lessons learned from future landslide
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.
There exists a need to ensure that actions identified by
the existing Rock Slope Hazard Index system (as developed
in the early 1990s) are carried out on a priority budget
basis. These will include both maintenance works and
re-inspection activities. While the rock slope system and
the proposed landslide 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.
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