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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
835
DOI: 10.4408/IJEGE.2011-03.B-091
DEVELOPMENT OF PRELIMINARY ASSESSMENT TOOLS TO EVALUATE
DEBRIS FLOW HAZARD
f
RanCesCo
BREGOLI
(*)
, a
llen
BATEMAN
(*)
, v
iCente
MEDINA
(*)
& f
abio
CIERVO
(**)
Marcel HÜRLIMANN
(***)
& G
uillaume
CHEVALIER
(*,***)
(*)
University of Catalonia - Department of Hydraulic, Marine and Environmental Engineering -
Technical Sediment Transport Research Group (GITS) - Spain
(**)
Univesitá degli Studi di Salerno - Consorzio Inter-Univesitario per la Previsione e la Prevenzione dei Grandi Rischi - Italy
(***)
Technical University of Catalonia - Department of Geotechnical Engineering and Geosciences - Spain
assessments at regional scale generally imply a Geo-
graphic Information System (GIS), in combination
with statistical analysis (e.g., m
aRk
et alii, 1995)
simple physical based and dynamic approaches
(e.g., i
veRson
et alii., 1998; G
uzzetti
et alii, 1999).
Detailed studies require numerical models and com-
prehensive field work to determine the hazard in the
debris flow deposition areas.
Focusing on hazard assessment at regional scale
the identification of prone areas is substantially differ-
ent from the mapping that is usually performed by the
basin authority in order to give guidance for the plans
of urban development and, in general, for the manage-
ment of the territories. Actually, in case of early warn-
ing, a comparatively coarser identification of the areas
at risk can be sufficient. In this perspective the hazard,
as a component of risk, may be estimated in a simpli-
fied way and it will be the topic of the present study.
The hazard is defined as a combination of event
intensity and its probability of occurrence. Therefore,
in order to construct a hazard map it is necessary to
estimate, for each elementary portion of the area ex-
amined, the intensity of possible events and the cor-
responding event probability.
The intensity of a debris flow is defined as its
ability to cause damage and is generally estimated
through the impact energy of the flow against an ob-
stacle, which depends on the characteristics of flow
depth and velocity (R
iCkenmann
, 2005).
This document proposes a preliminary methodol-
ABSTRACT
With the objective of providing guidance for an
early detection of phenomena potentially giving raise
to Debris Flow, one of the main topics is the prelimi-
nary identification of areas at risk. In case of early
warning a coarser identification of areas at risk should
be sufficient. In this perspective, the hazard of phe-
nomena, as component of risk, can be estimated in a
simplified way. In the framework of the IMPRINTS
European Research Project (FP7), a toolbox for fast
assessment of debris flow hazard has been developed.
The aim of this toolbox is to implement different exist-
ing models inside a common package useful for a fast
evaluation of potential hazard. The identification of
hazard is performed by different levels of accuracy, de-
pending on the availability of input data. As an exam-
ple, the result could be achieved by a rough handling
of topographical data but could be improved in quality
by adding geological and hydrological data. Both the
initiation and propagation of the debris flow are mod-
elled. For this study, the methodology has been applied
in a catchment located in the North East of Spain.
K
ey
words
: debris flow, hazard assessment, run out, shallow
landslide
INTRODUCTION
Concerning the hazard, usually two types of as-
sessments can be distinguished: studies at regional
scale and studies at local scale. Debris flow hazard
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order to perform the simulations only in those areas
and save computational time. Past studies have inves-
tigated the occurrence of debris flows and reported
the area of the catchments affected by various de-
bris flows events (s
toCk
& d
ietRiCH
, 2003; m
aRCHi
& d’a
Gostino
, 2004). From those studies seem to
emerge a threshold value for the area of catchments
known to have experienced debris flows. Scheidl
(2009) proposed a threshold value of 25 km
2
. In his
geomorphologic study of Spanish debris flows, C
Hev
-
alieR
et alii (2010) report that many of the Strahler’s
second-order catchments, where debris flows oc-
curred, has a similar maximum value.
An important pre-process is the application of
the typical “fillsinks” algorithm to the DEM in or-
der to fill the natural depressions that could influence
seriously the execution of the subsequent steps. Be-
cause of that lakes and dams are removed from the
DEM, making those areas as “no data”. It means that
in case of the presence of dams the method is not
valid. A “fillsinks” algorithm is included in the meth-
odology implemented..
INITIATION MODELS
The initiation of debris flow is possible by means
of various mechanisms (C
oussot
et alii, 1996; H
unGR
et alii, 2001) but the mobilization from rainfall-trig-
gered landslides (i
veRson
et alii, 1997) seems to be
the most common process.
Starting from that, it has been decided to include
in that framework a model that describes this common
behaviour of debris flow initiation.
These movements are triggered during intense
rainfall when high pore pressure is produced inside a
loose sediment layer, thus reducing the factor of safe-
ty. This behaviour is described by the typical Coulomb
failure approach in the infinite slope stability model
(i
veRson
et alii, 1997)
The water pore pressure may be estimated, in a
simplified way, by assuming that steady-state condi-
tions are reached after a rainfall having constant inten-
sity and indefinite duration (d
ietRiCH
et alii, 1995). If
the assumption of complete saturated material (water
table coincident with the free surface) is also made, a
very simple relation between rainfall and soil trans-
missivity may be derived (d
ietRiCH
et alii, 1995) for
each cell of the DEM:
ogy to evaluate the hazard due to debris flow triggered by
shallow landslides and his propagation through the paths
Different methodologies are selected in order to
define a “multilevel approach” to the problem, de-
pending on the data input availability and on the detail
required by the analysis.
The methodologies selected are applied on the upper
part of the basin of Llobregat River in the North East and
some preliminary results are presented. The validation of
the models is still a work in progress..
DEBRIS FLOW HAZARD ASSESSMENT
METHODOLOGIES
As the hazard assessment at regional scale is con-
cerning a wide area it is necessary to find a general and
common methodology able to describe the phenomenon
in a wide range of cases. A flow chart of methodology is
described in Fig. 1.
As said before, the hazard is given by the combi-
nation of intensity and probability of occurrence. For
the debris flows triggered by rainfall of particular in-
tensity and duration, the probability of occurrence of
an event may be related to the return period of the trig-
gering rainfall. The connection between the rainfall
and its effects can be reconstructed by the simulation
of the different processes, which take place during the
spatial and temporal evolution of the flow from its
mobilization to its stop. In this sense we can distin-
guish two distinct phases: the first aims to estimate the
volume potentially mobilized by a given precipitation,
with an assigned return period, (initiation models), the
second has the objective to estimate the area of inva-
sion and the resulting intensity (propagation models)
It is of crucial importance to extract the areas in
which potential debris flow triggering is expected in
Fig. 1 - debris flow hazard assessment methodology’s flow
chart
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837
derived by the Intensity- Duration-Frequency curves
(IDF). An example of those curves is given in Figure 2.
PROPAGATION MODELS
A stochastic model and a 2D model of propagation
are implemented.
The stochastic model consists in a flow routing
algorithm incorporated into a random walk to generate
trajectories of debris flow. G
amma
(1999) & H
üRli
-
mann
et alii (2008) combined a D8 flow routing algo-
rithm (o’C
allaGHan
& m
aRk
, 1984) with Montecarlo
and random walk theory. The method was successful-
ly applied in the European Alps and the Spanish Pyr-
enees catchments. The model used here is a modifica-
tion of the previous one with the incorporation of local
flow velocity computation and a stopping mechanism
Starting from initiation points evaluated with
methods previously described, that procedure permits
to obtain a flow path of propagation for each point,
and subsequently n
iter
flow trajectories were calculat-
ed. Finally the probability P
xy
, was computed for each
cell of the DEM using the following equation.
where n
afect
is the number of debris-flow trajectories
that invaded a cell. The output of this method is a map
containing information on the probability of each cell
of the DEM to be affected by a future debris flow. The
result depends strongly on the DEM resolution and on
the number of iterations, which is recommended to be
set to 10
4
(H
üRlimann
et alii, 2008)
. Computation of flow velocity is achieved apply-
ing the Voellmy Fluid Flow Rheology for Granular
Debris Flow (1955):
where q is the rainfall intensity, T is the soil transmissiv-
ity, )α is the slope, a/b is the cumulated area per width of
flow, p
w
is the density of water, z is the thickness of soil,
c’ is the soil cohesion,φ is the soil internal friction angle
and p
s
is the saturated bulk density of the soil.
In this extremely simple approach the output of
the model will assess only the most prone areas of
ruptures and it is not possible to compute the total un-
stable debris volume. For these reasons this approach
is here called “qualitative-steady state”.
Removing the assumption of complete saturated
layer, the ratio between the water table depth h and the
thickness of the soil layer z, may be derived by equa-
tion [2] (d
ietRiCH
et alii, 1995).
The safety factor F
s
may be then computed as fol-
lows:
where γ
s
is the specific weight of saturated soil, 1w is
the specific weight of water.
Such equation is valid in the hypothesis of con-
stant intensity and indefinite duration rainfall. Conse-
quently the corresponding return period is not defined.
To overcome this difficulty, the duration of the rainfall
event is fixed equal to the time necessary for the soil
to reach to steady state condition. A simple relation to
evaluate such interval time is proposed in equation [4]
(P
aPa
et alii, 2010):
where n is the basin cells number, τ
s
is the time to satu-
ration, τ
s
is the water content at saturation.
In that case prone areas and volume will be as-
sessed and the model proposed is named “quantita-
tive-steady state”
.
Once the duration time of rainfall (τ
s
) is assessed,
the rainfall intensity for the different return periods is
(1)
(2)
(3)
(4)
Fig 2 - IDF curves in the Upper Llobregat Basin in Cata-
lonia, Spain
(5)
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
and turbulent ones (Manning, Chezy). The model is
based on the shallow flow hypothesis and is depth in-
tegrated. A bi-dimensional approach is used for mo-
mentum conservation..
The main characteristics of the model are:
• monophasic flow is considered
• constant density flow is considered
• no pore pressure effect is considered
• Terrain curvatures are neglected
• Steep slopes are considered
• Multiple rheologies are implemented
Apart from rheological parameters (from back-
analysis), the necessary input data are two raster data
sets including a DEM and a raster defining the initial
extension and volume of the debris flow. The accuracy
of calibration of the rheological parameters and the
computational time requirement represent the major
drawback of this technique, but the outputs can be di-
rectly used to generate intensity maps, since velocity
and flow depth are simulated within the entire study
area. The computational cost also increases consider-
ably with the number of initiation points..
MULTILEVEL APPROACH
Three levels of models system characterize the
multilevel approach: S_mall, M_edium, L_arge (see
Tab. 1). Obviously the quality of output depends on the
system applied: detailed input data allow for the use of
more complex models and give better results. The three
systems proposed (S_mall, M_edium and L_arge) are
composed by two models, an initiation model and a
propagation model.
where v is velocity of the mixture, s is the flow path
line, μ
m
is the sliding friction coefficient, k is the “tur-
bulence coefficient, also called “mass to drag ratio”.
μ
m
and k should be defined by backanalysis, but typi-
cal values can be settled.
The stopping mechanism of the routing is assessed
by the following relationship between the reach angle
and the total debris flow volume (C
oRominas
, 1996):
where β is the reach angle, H is the gradient between
centre of mass of landslide and fan, Lmax is the travel
distance and V is the volume in m
3
of total amount of
mobilized sediment.
This volume may be is estimated trough the initia-
tion model introduced above. In Figure 3 the variables
involved in equation 7 are described
That method is extremely simple and has a very
short computing time, but the result is not deterministic
and does not includes the depth of deposit. However
the velocity, useful for hazard assessment, is estimated.
It is important to note that this methodology is
valid only in a natural environment, where the anthro-
pogenic modification of land is considered low
The 2D model proposed here is the FLATModel
(m
edina
et alii, 2008). FLATModel is a two-dimen-
sional finite volume code that has been validated
with analytical, experimental and real test cases. It is
a complete model that include basal entrainment of
sediments, stop and go phenomenon, dynamical cor-
rection of the evolution of fan slope and different fluid
models including laminar rheologies (Bingham, Her-
schel-Bulkley), granular flows (Coulomb, Voellmy)
(6)
(7)
Fig. 3 - scheme of a debris flow reach angle and variables
involved
T
ab. 1 - Multilevel approach at debris flow hazard assess-
ment.
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DEVELOPMENT OF PRELIMINARY ASSESSMENT TOOLS TO EVALUATE DEBRIS FLOW HAZARD
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
839
a two-dimensional shallow water flow model (FLAT-
Model). In order to run such model it is necessary
to estimate the rheological properties of solid-liquid
mixture in motion. The results of the computation are
the flow depth and velocity for each numerical cell of
the affected area. The result of the L_arge system is a
map with quantitative assessment of classes of debris
flow intensity corresponding to fixed return periods.
The parameters required for each proposed sys-
tem are shown in the Tab. 2.
HAZARD ASSESSMENT
In that case hazard is assessed by the probability
that a debris flow can invade a certain area. The prob-
ability is given by the combination of the initiation
and the propagation models. Depending on the model
adopted, the hazard is defined as below.
HAZARD IN THE S_MALL MODEL
A qualitative steady state stability model is adopt-
ed as in equations 1 in which the ratio q/T is defined.
Taking in account the works of d
ietRiCH
(1995),
G
uzzetti
(1999) and C
aRRaRa
(2008), three thresh-
olds, in term of logarithm, are defined as in Tab. 3.
It has to be remarked that values of Log(q/T)= 1 are
possible due to particular local values and truncation
errors. Such value has to be neglected.
After the definition of the three zones of land-
slides initiation, the propagation is carried out for each
zone and the invaded areas are assigned the same level
of hazard defined in Tab. 3.
HAZARD IN THE M_EDIUM MODEL
The quantitative steady state stability model is
adopted as in equations 3, where the safety factor Fs is
The S_mall system is suitable for preliminary risk
analysis or when the advanced systems cannot be ap-
plied, for the width of study area and for the lack of
input data. The compilation of input data files and the
running of the system are very fast. The assessment
of possible unstable areas is performed by the infinite
slope stability model in which the water pore pressure
is estimated, in a simplified way, by assuming steady-
state groundwater flow (d
ietRiCH
et alii, 1995) and
complete saturation of the soil. In this extremely sim-
plified approach only the morphological description
of the basin is needed and the result does not depend
on rainfall data. As a consequence it is not possible to
define a return period for the event. The given result
is simply a qualitative map of the area most prone to
debris flow initiation
The propagation model is performed by the sto-
chastic approach. The result of the S_mall system is
a map with qualitative assessment of classes of debris
flow intensity corresponding to hypothetical scenarios.
The complexity increases in the M_edium system
in which the water pressures are computed depending
on a given rainfall in the hypothesis of steady-state
groundwater flow (d
ietRiCH
et alii, 1995). With the
proposed method for the estimation of rainfall dura-
tion it is possible to obtain a map of instable area with
given return period..
As the S_mall system, the M_edium implements
a stochastic propagation model. The result of the M_
edium system is a map with qualitative assessment
of classes of debris flow intensity corresponding to
fixed return periods.
The L_arge system is the most complex compu-
tational level. The initiation model is the same as in
the M_edium system while the propagation model is
Tab. 2 - Data input required by the different proposed
methods. DEM, Digital Elevation Model; ,, inter-
nal friction angle; c’, cohesion; k, hydraulic con-
ductivity; z, soil thickness; q, rainfall intensity; D,
rainfall duration
Tab. 3 - Definition of hazard in the S_mal model
T
ab. 4 - Definition of hazard in the M_edium model
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computed for each return period.
When Fs<1, the instability is recognized. In Tab.
4 the hazard is defined depending on the return period.
HAZARD IN THE L_ARGE MODEL
In the large model the classical definition of haz-
ard could be done and is explained in Tab. 5. The
definition of the intensity is done in Tab. 6 following
G
aRCia
et alii (2005).
IMPLEMENTATION OF ALGORITMS
The proposed methodologies and algorithms are
implemented using the Java language, and declared as
GNU/GPL open source code. Most of the algorithms
require basic common Geographic Information Sys-
tem (GIS) tools (i.e. slope, curvature, aspect). These
tools are provided through the SEXTANTE GIS library
(G
imenez
& o
laya
, 2008). Other new libraries are im-
plemented by GITS team to carry out the presented
job. Algorithms themselves are included inside SEX-
TANTE to be available to extern applications. Notice
that SEXTANTE is not a GIS but a library that could
be accessed from different open source as well as com-
mercial GIS. A command line application to use SEX-
TANTE library without GIS is developed in that study.
The data exchange formats of information are ESRI
ASCII for raster and ESRI shapefile for vectorial.
APPLICATION OF METHODOLOGIES
The methodologies are applied in the Upper part
of Llobregat River Basin in the region of Cataluña
(Spain). The outlet is located immediately upstream of
the Baells Reservoir in the municipality of Berga. The
catchment considered has an area of about 350 km
A digital elevation model (DEM) of 30x30 meters
of resolution is used as the best DEM that covers all
the world is a 30x30 meters, coming from the database
of ASTER_GDEM (2008).
S_MALL MODEL APPLICATION
Here a unique value of cohesion, thickness and
internal friction angle is chosen:
• c' = 770 Pa
• z = 1 m
• Z = φ.48 rad
The saturated bulk density chosen for the calcula-
tion is [ = 1700 kg/m
3
.
It must be emphasized that the model could work
even without any value of cohesion and thickness,
but only with the internal friction angle. In that case
the internal friction angle should be increased till
450 to counteract the absence of cohesion (d
ietRiCH
at alii, 1994).
In Figure 5.a, it is showed the result after the
qualitative steady state model simulation for the
slope stability. It is a common result that that tech-
nique is overestimating zones of failure (as also re-
ported in previous studies as C
aRRaRa
et alii, 2008).
In Figure 5.b, it is shown the result after running the
Tab. 5. - Definition of hazard in the L_arge model
Tab. 6 - Definition of inensity in the L_arge model (G
ArciA
et alii, 2005)
Fig. 4 - Upper Llobregat basin location
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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
841
installed a monitoring system to study the mass
movement and the local weather condition (H
üR
-
limann
et alii, 2010). In the framework of the same
project, also a rather good number of field studies have
been done, including the reconnaissance of initiation
points, total volume involved and depositional area
through standard studies as well as dendrochronology
studies on affected trees. The event of 2006 is taken in
account for the present simulation (Fig. 7)
Starting from de initiation model, the initiation
points are selected consistently with field studies..
The propagation is performed with the FLAT-
Model that gives as outputs the velocity and the water
depth of the flow..
In particular here are reported the maximum ve-
locity (Fig. 8) and depth (Fig. 9) over the simulation.
The volume of initiation is estimated to be 2000 m3.
The Voellmy rheology for granular flow is used and
the rheological parameters are estimated by back anal-
ysis and settled as below:
• dry friction coefficient: μ = 0.25
• turbulent friction term: C = 10 m
0.5
/s.
The analysis of intensity of a single return period
is reported asan example in Figure 10.
The model’s agreement with the depositional area
witnessed on the field is satisfactory, even if a bifurca-
tion of flow at the end of the fan is registered. That
trend seems to be possible in the future, due to the
reconnaissance of a new flooded path emphasized by
recent events.
stochastic model of propagation. The result is given
qualitatively and not in term of return period..
M_EDIUM MODEL APPLICATION
In that case spatial distributed values of cohesion,
thickness, permeability and internal friction angle are
estimated from geological maps. In that region no soil
map is available and a reclassification of a geological
map has been done. The use of geotechnical parameters
coming from geology maps is not appropriate for the
methodologies presented (v
an
w
esten
, 2008). In that
case, the use of such maps shows to influence the results.
The saturated bulk density chosen for the calcula-
tion is r = 2200 kg/m
3
.
Figure 6.a illustrates the result after the qualitative
steady state model simulation for the slope stability.
In Figure 6.b it is shown the result after running the
stochastic model of propagation. In that case the result
is given in term of return period.
A validation of the model is in progress, but since
the results are too much depending on the geotechni-
cal parameters, it would be more interesting to move
in a basin where those parameters are well known
L_ARGE MODEL APPLICATION
For that application is decided to work on the
catchment of Ensija Creak, a small sub-catchment of
Llobregat that is suffering debris flow activity. In that
spot the Technical University of Catalonia, thanks to
the National Research Project DEBRISCATCH, has
Fig. 5 - S_mall model application. a) Result of the qualita-
tive steady state initiation model; b) hazard after
the propagation with the stochastic model
Fig. 6 - M_edium model application. a) Result of the
quantitative steady state initiation model; b) haz-
ard after propagation with the stochastic model
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F. BREGOLI, A. BATEMAN, V. MEDINA, F. CIERVO, M. HÜRLIMANN & G. CHEVALIER
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The approaches proposed in this study allow the users
to choose the most appropriate method according to
their data and needs. A toolbox has been developed to
facilitate users in the application of the methodologies
for their test beds.
Through the project, a stepwise prescription for ob-
taining hazard maps for debris flow has been provided:
The first step of the process is the assessment of event
intensity and consequent hazard; it requires math-
ematical and numerical modelling of the debris flow.
DISCUSSION AND CONCLUDING RE-
MARKS
Different methodologies have been proposed for
the evaluation and determination of risk areas for de-
bris flow and flash floods. Methodologies of differ-
ent level of accuracy have been developed, requiring
different level of elaboration, manual work and data
accuracy. The project has shown that hazard assess-
ment delineation is a large computational process sea-
soned with an important manual effort for the user.
F
ig. 7 - Initiation and depositional areas of debris flow for
the event of 2006 in the Ensija’s Catchment
Fig. 8 - FLATModel simulation result in the Ensija’s
Catchment. Maximum velocity recorded du-
ring the simulation
Fig. 9 - FLATModel simulation result in the Ensija’s
Catchment. Maximum depth recorded during
the simulation
F
ig. 10 - Result of the intensity analysis using the output of
FLATModel
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DEVELOPMENT OF PRELIMINARY ASSESSMENT TOOLS TO EVALUATE DEBRIS FLOW HAZARD
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
843
the current basin. As discussed before, the results are
too much depending on the geotechnical parameters
and it would be more interesting to move in a basin
where those parameters are known.
Concerning the L_arge model, some studies and
validation cases are available (i.e. m
edina
et alii, 2008,
H
üRlimann
et alii, 2008) and the case of studies pre-
sented, as reported below, show a good agreement be-
tween the model and the field studies.
ACKNOWLEDGEMENTS
The study was financially supported by Euro-
pean Community, trough the project IMPRINTS
(IMproving Preparedness and RIsk maNagemenT
for flash floods and debriS flow events), SEVENTH
FRAMEWORK PROGRAMME THEME 6.1.3.3,
ENVIRONNEMENT: Preparedness and risk manage-
ment for flash floods including generation of sediment
and associated debris flow, Grant agreement n°: FP7-
ENV-2008-1-226555.
The present study is also supported by the project
“DEBRIS FLOW” (Agreement CGL 2009-13039) of
the Spanish Ministry of Education.
A multilevel approach is defined with increasing
complexity, data request and computational effort.
Obviously the quality of the output depends on
data input availability.
Resuming, the three levels of system are, in order
of increasing complexity:
• The S_mall system
• The M_edium system
• The L_arge system
The choice of the proper system may be done de-
pending on the requested result quality, the size of the
study area and the computational effort; the simple
models (S_mall and M_edium) well meet requirements
of early warning system, while the complex models,
like the L_arge, are more useful for the compilation of
detailed hazard maps to be used for territorial planning
As the validation of the methodologies used is in
progress, the results here shown are only preliminary.
The S_mall model is known from previous similar
studies, which is overestimating the zone of failure
(C
aRRaRa
et alii., 2008) due to the low accuracy of the
initiation model; while for the M_edium model, an ac-
curate study has to be done to validate the methodol-
ogy. The main issue, in that case, is the lack of data in
REFERENCES
C
aRRaRa
a., C
Rosta
G., f
Rattini
P. (2008) - Comparing models of debris-flow susceptibility in the alpine environment. Geomor-
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F. BREGOLI, A. BATEMAN, V. MEDINA, F. CIERVO, M. HÜRLIMANN & G. CHEVALIER
844
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
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