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The physical vulnerability of roads to debris flow
La vulnérabilité physique des routes aux coulées de boue
Winter M.G., Smith J.T.
Transport Research Laboratory (TRL), Edinburgh, United Kingdom
Fotopoulou S., Pitilakis K.
Aristotle University of Thessaloniki,Thessaloniki, Greece
Mavrouli O., Corominas J.
Technical University of Catalona, Barcelona, Spain
Agyroudis S.
Aristotle University of Thessaloniki,Thessaloniki, Greece
ABSTRACT: The physical vulnerability of roads to debris flow is expressed through fragility functions that relate flow volume to
damage probabilities. Fragility relations are essential components of quantitative risk assessments (QRA) and allow for the estimation
of risk within a consequence-based framework. This paper describes fragility curves produced in order to provide the conditional
probability for a road to be in, or to exceed, a certain damage state for a given debris flow volume. Preliminary assessments were
undertaken by means of a detailed questionnaire. Fragility curves have been defined for three damage states for high speed (reported
herein) and for low speed roads (to be reported later) in order to cover the typical characteristics of roads vulnerable to debris flow.
The probability of any given damage state being met or exceeded by a debris flow of a given volume (10m
3
to 100,000m
3
) was
derived from the mean of the responses received. The development of the fragility curves is described and data issues discussed.
RÉSUMÉ : La vulnérabilité physique des routes aux coulées de boue s'exprime à travers des fonctions de fragilité qui mettent en
rapport le débit volumique et les probabilités de dommages. Les relations de fragilité sont des composantes essentielles des
évaluations quantitatives de risques (QRA) et permettent d’estimer le risque au sein d'un cadre basé sur les conséquences. Cet article
décrit les courbes de fragilité produites afin d’indiquer la probabilité conditionnelle qu’une route se trouve, ou dépasse, un certain état
d’endommagement pour un débit volumique donné de boue. Des évaluations préliminaires ont été réalisées au moyen d'un
questionnaire détaillé. Des courbes de fragilité ont été définies pour les trois états d’endommagement pour les routes à grande vitesse
(présentées dans ce document) et pour les routes à petite vitesse (qui feront l’objet d’un rapport ultérieur) afin de couvrir les
caractéristiques typiques des routes vulnérables au coulées de boue. La probabilité d’atteinte ou de dépassement d’un état
d’endommagement donné en présence d’une coulée de boue d'un volume donné (10m
3
à 100 000m
3
) a été dérivée de la moyenne des
réponses reçues. L'élaboration des courbes de fragilité est décrite et les problématiques liées aux données sont abordées.
KEYWORDS: Landslides, debris flows, hazard, risk, probability, fragility, QRA.
1 INTRODUCTION
Fragility curves are a graphical means of describing the physical
vulnerability of elements at risk to a given hazard. They give the
conditional probability of a particular element at risk to be in, or
to exceed, a certain damage state as a result of a hazard of a
particular type or intensity (Mavrouli & Corominas 2010).
Fragility relationships are essential components of quantitative
risk assessments (QRA) as they allow for the estimation of risk
within a consequence-based framework.
For the purposes of this work the element at risk is a road
and the hazard is debris flow. Damage probabilities have been
assigned for specific debris flow volumes; these should not be
confused witht he probability of event occurence. Fragility
curves have been produced which indicate the probability of a
debris flow of a given volume exceeding each of three damage
states. To the best of the Authors’ knowledge this is the first
time that fragility curves have been developed for the effects of
debris flow on roads. Fragility relationships are widely adopted
in seismic ‘expected loss’ and risk assessments, being a
valuable tool to explicitly assess the vulnerability of structures
to earthquake hazard (Pitilakis et al. 2006).
While several possible approaches were available, including
analytical and empirical ones, for the development of fragility
curves, it was decided that expert engineering judgement should
be used due to a lack of a comprehensive empirical dataset as
well as the complex nature of the problem.
This paper describes the questionnaire sent to experts
globally to collect data for the fragility curve development. It
also describes the analysis and interpretation of the data
collected, and its validation using real world examples.
2 METHODOLOGY
2.1 Road characterisation
Many different classifications of roads could be considered,
covering numerous key factors such as construction type,
stiffness, and traffic speed. However, in order to reduce the
questionnaire to a reasonable size some simplification was
needed. Primarily it was decided that, for the purposes of this
exercise, all roads could be considered to be relatively stiff and
brittle (the low strain stiffness of even an unbound pavement is
such that it is likely to behave in a stiff, brittle manner). In order
to further simplify the analysis roads have been divided into low
and high speed roads, characterized as follows:
• High speed: speed limit between 80 and 110km/h and one
or more running lane in each direction, most likely in
conjunction with a hardstrip or hard shoulder.
• Local (or low speed) roads: speed limit typically <50km/h
on a single-carriageway (one lane for each traffic direction) or
single-track. This category is intended to encompass both paved
(bituminous, unreinforced or reinforced concrete) and unpaved
constructions.
The gap between the speed limits of the two classes of road,
reflects the transition between local roads and high speed roads,
which is by no means geographically consistent. This reflects
reality – in some countries and regions certain road geometries
are more closely aligned with the definition of local roads and
