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1 INTRODUCTION TO LANDSLIDE HAZARDS
by M G Winter, F Macgregor and L Shackman
In August 2004 Scotland experienced rainfall
substantially in excess of the norm. Some areas of Scotland
received in excess of 300% of the 30-year average August
rainfall. In the Perth and Kinross area figures of the
order of between 250% and 300% were typical. While the
percentage rainfall during August reduced to the west,
parts of Stirling and Argyll & Bute still received
between 200% and 250% of the monthly average
1.
The rainfall was both intense and long lasting and a
large number of landslides, in the form of debris flows (
see Section 2), were experienced
in the hills of Scotland. A small number of these
intersected with the trunk road network, notably the A83
between Glen Kinglas and to the north of Cairndow (9
August), the A9 to the north of Dunkeld (11 August), and
the A85 at Glen Ogle (18 August).
While there were no major injuries to those affected,
some 57 people were taken to safety by helicopter after
being trapped between the two debris flows on the A85 in
Glen Ogle (
see cover picture). The A85,
carrying up to 5,600 vehicles per day, was closed for four
days. The A83, which carries around 5,000 vehicles per day
(all vehicles two-way, 24 hour
AADT2), was closed for two days and the A9, carrying 13,500
vehicles per day, was closed for two days prior to
reopening, initially with single lane working under convoy.
The disruption experienced by local and tourist traffic, as
well as to goods vehicles, was substantial.
The need to act has been recognised by the Scottish
Executive and this initial study (Study 1, Part 1) has been
commissioned alongside a second study (Study 2). Study 2 is
designed to identify the potential impacts and consequent
necessary actions in the light of anticipated climate
change and is not considered further in this report,
although it is important to note that action has been taken
to ensure that the two studies are complementary.
As indicated above, this study, termed
Study 1, comprises two parts and it is
Part 1 that is reported here. Part 1 deals
with the following activities:
- Considering the options for undertaking a detailed
review of side slopes adjacent to the trunk road
network and recommending a course of action.
- Outlining possible mitigation measures and
management strategies that might be adopted.
- Undertaking an initial review to identify obvious
areas that have the greatest potential for similar
events in the future.
This work will lead to
Study 1,
Part 2 which will include the development
of a system to allow a detailed review of the network to be
undertaken to identify the locations of greatest hazard and
for those hazards to be ranked and appropriate mitigation
and/or management measures to be selected.
The overall purpose of these studies is thus to ensure
that the Scottish Executive has a system in place for
assessing the hazards posed by debris flows. In addition,
the system will be capable of ranking the hazards in terms
of their potential relative effects on road users. This
will allow the future effects of debris flow events to be
managed and mitigated as appropriate and as budgets permit.
This will ensure that the exposure of road users to the
consequences of future debris flows is minimised whilst
acknowledging that it is not possible to prevent the
occurrence of such events.
A consistent, repeatable and reproducible system is
required. This is especially important as a variety of
consultants will be involved in the data gathering,
analysis and interpretation process. Inevitably each will
have a different, but nonetheless valid, approach when
operating independently. Such a situation would make any
comparison between individual consultant's results and
recommendations impossible for the purpose of, for example,
allocating funds on a priority basis across the network. It
is apparent at the outset that a unified system acceptable
to all of the major players in the industry is
required.
It was thus recognised at an early stage of the
development of the work that the input of a wide range of
experts and stakeholders would be required in order for the
studies to be completed successfully. In particular, the
agreement and input of those most likely to be responsible
for using the system was required.
A Project Workshop was held in order to capture the
knowledge vested with individual experts (
see Appendix). The Project
Workshop was facilitated by Professor Malcolm Horner of the
University of Dundee and comprised presentations given by
acknowledged experts followed by focussed discussion
sessions designed to open out the knowledge base and
determine the way forward with the project. Following the
Project Workshop the Editors assigned tasks to individuals
in terms of the preparation of this report as exemplified
by the authorship of individual sections. The main results
from the Project Workshop are incorporated in the various
sections.
Section 2 gives the background to the Study as a whole.
It describes the different types of landslide, focussing on
debris flows as recently experienced, and illustrates the
recent history of debris flows in Scotland with examples
right up until the present. It also deals with climatic
issues and those issues which relate to third party
ownership of land from which landslides may originate
Section 3 examines sources of relevant information,
including previous literature, the Project Workshop and
available data sets from sources such as the Scottish
Executive and the British Geological Survey.
Section 4 deals with the classification and type of
debris and other types of flows. It explains how rapid
landslides develop from their causes and the underlying
soil failure mechanisms, through the mechanics of their
downslope propagation and, finally, to their run-out at the
base of the slope.
Section 5 examines the relevance of the key factors in
debris flow initiation and propagation that have been
identified from past events, including the events of August
2004. These are considered in terms of factors affecting
the likelihood of debris flow occurrence, including the
effects of run-out, and factors affecting the exposure of
road users to debris flows
Section 6 describes the proposed assessment methodology
in terms of hazard assessment and approach for Study 1,
Part 2 and also details the hazard assessment and exposure
factors that will form the core of the methodology for the
detailed assessment.
Section 7 identifies areas of high hazard that are
considered to have the greatest potential for similar
debris flow events in the future and sets out opportunities
for early actions.
Section 8 describes management and mitigation options.
In terms of management the sequential approach of
Detection, Notification and Action (
DNA) promulgated by the Editors at the
Project Workshop is used. This approach is set out in terms
of a response to both precursor conditions, such as intense
rainfall, and also to the management of future debris flow
events.
Section 9 presents a brief summary of this report and
makes recommendations for the way forward.
In this report reference is made to both debris flows
and landslides. Debris flow is recognised within the
specialist community as a sub-set of the term landslide and
is used in the description of the mechanisms and
characteristics of such events. The term landslide is used
as a common parlance term to describe the broader types of
event that are liable to be encountered.
The work reported herein has been conducted by a Working
Group whose membership was selected in order to enable the
individuals most suited to the various tasks to bring their
knowledge, expertise and experience to bear on the relevant
issues. The work has been funded through a variety of
existing contracts with the close and active involvement
and support of Scottish Executive engineers as key members
of the Working Group. The involvement of
TRL is in providing the facilitating
project manager to lead the experts drawn from Scotland's
geotechnical community as well as making specific and
substantial technical contributions.
TRL's input has been funded through
existing commission arrangements.
TRL has also been responsible for
sub-contracting expertise from the British Geological
Survey, Donaldson Associates,
EDGE Consultants and Arup. The
involvement of Halcrow has been funded through Trunk Road
Division's Geotechnical Certification Commission. The
involvement of
BEAR (represented by Jacobs Babtie) and
Amey (represented by W A Fairhurst & Partners) was
funded through existing arrangements for trunk road
maintenance.
The foregoing refers to the organisations involved in
the project. However, the Working Group, including the
editors of this report, comprised individuals each of whom
was selected on the basis of their knowledge, expertise and
experience and, indeed, their suitability to bring those
characteristics to bear on the issues at hand. Appointments
to the Working Group were for individuals, on the basis of
their knowledge and experience, rather than potentially for
the organisations who employ them. Individual members of
the Working Group did, however, employ the services of
colleagues as appropriate.
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