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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
921
DOI: 10.4408/IJEGE.2011-03.B-100
ANALYSIS OF THE DEBRIS-FLOW HAZARD ON THE RIOULONG
TORRENT (HAUTES-PYRéNéES, FRANCE) ALLUVIAL FAN
USING A SCENARIO-BASED APPROACH
d
ominiQue
LAIGLE
(*)
& C
HRistoPHe
PETEUIL
(**)
(*)
Cemagref, Unité de Recherche Erosion Torrentielle, Neige et Avalanches, BP 76 Domaine Universitaire,
F-38402 Saint-Martin-d’Hères Cedex, France. Tel.: +33476762805 - Email: dominique.laigle@cemagref.fr
(**)
Office National des Forêts, Service de Restauration des Terrains en Montagne de l’Isère, 9 quai Créqui,
F-38000 Grenoble, France
essary to evaluate the intensity (flow depth, velocity
and potential damage) and probability of occurrence
(once every 10 years, 100 years, 1000 years?) of events
likely to affect a given point in the area (a house, for
instance). It is therefore crucial to take this variability
into account. In this context, the scenario-based ap-
proach exemplified here can be considered a practical
method of hazard assessment. It consists first in a di-
agnosis of debris-flow triggering processes at work in
the catchment considered. Then the ranges of variation
of all the inputs of a debris-flow simulation model are
deduced from the diagnosis. Each of the input values
is then assigned a probability of occurrence, mostly
qualitative (frequent, rare, exceptional, etc.). Numeri-
cal simulations, covering these predefined ranges of
variation of input data are then carried out. Each simu-
lation result can subsequently be assigned a probabil-
ity of occurrence, once again mostly qualitative.
This method is here exemplified on the Rioulong
torrent (Loudenvielle, Hautes-Pyrénées, France) allu-
vial fan, with a few houses, a camp site and a local road
within the hazard zone. First, the catchment is diagnosed
considering erosion processes at work in the catchment.
This diagnosis means that the probability of debris-flow
triggering cannot be ruled out. It also provides an evalu-
ation of the magnitude of debris flows likely to affect the
alluvial fan. In the second part of this article, a scenario-
based analysis, using a numerical model dedicated to the
computation of debris-flow spreading on alluvial fans, is
carried out in order to assess the debris-flow hazard. In
ABSTRACT
A method of debris-flow hazard analysis, using
the Cemagref LAVE2D model, within a scenario-
based approach, is applied to a torrent of the French
Pyrenees. A preliminary analysis of the catchment
shows that some risk of debris-flow occurrence is
present and that scenarios should be constructed ac-
cordingly. Numerical simulations are carried out on
the basis of these scenarios. They produce maps of
maximum extension of debris flows in relation to
qualitative levels of probability of occurrence. Simu-
lations also provide information that is useful for the
design of a protection structure. Finally, the maximum
extension of debris flows resulting from the presence
of the protection structure is analyzed.
K
ey
words
: debris-flow, mudflow, erosion processes, hazard
assessment, numerical simulation, protection structures
INTRODUCTION
Mudflows and debris flows, like many other natu-
ral phenomena, vary greatly in their occurrence con-
ditions. This variability originates, of course, in the
occurrence of triggering rainfall conditions, but also
in the nature and availability of solid material present
in the debris-flow-prone catchment. These two factors,
once combined, will determine not only the volume of
each individual event, but also the mechanical proper-
ties, and more generally speaking, all flow conditions.
For people in charge of protection strategies, it is nec-
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D. LAIGLE & C. PETEUIL
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
steep, with mean slopes ranging from 0.3 m/m to 0.5
m/m (Figure 1). In this area, the stream cuts into ancient
morainic deposits. Covering about one-quarter of the
catchment, this sector resembles a large hanging humid
zone with more or less active instabilities. It is also the
confluence point of several gullies of the upper catch-
ment hydrographic network (Figure 2). Given the small
size of the basin, the steepness of slopes, gullies and
stream bed, the large number of gullies, and the pres-
ence of humid or impervious zones, the catchment is
likely to respond very quickly to any rainfall event.
practice, we use the Cemagref LAVE2D model (l
aiGle
et alii, 2003), but other models are able to compute the
spreading of debris flows as well, for instance, FLO2D
(o’b
Rien
et alii, 1993) could similarly be employed.
Scenarios are constructed based on the diagnosis
outcomes. In the third part, the effect of a protection
levee is also analyzed within the scenario-based ap-
proach. This analysis provides useful elements for the
design of the protection structure. Finally, the conse-
quences of the presence of the protection levee on the
flow downstream, and thus on the hazard on the al-
luvial fan, are also analyzed.
PRELIMINARY ANALYSIS OF THE RIOU-
LONG CATCHMENT
MAIN FEATURES OF THE CATCHMENT
The Rioulong torrent is a tributary of the Neste du
Louron, a valley river located in the Hautes-Pyrénées
district in France (Fig. 1). This torrent catchment covers
1.72 km² and ranges from 1920 m to 960 m a.s.l. The
catchment is for the most part well covered by vegeta-
tion and partly subjected to anthropogenic effects. Six
per cent of the catchment area is impervious because
of the presence of the Val Louron ski resort in its up-
per part. The channel in the medium reach is rather
Fig. 1 - General overview of the Rioulong catchment
(photo: ONF - RTM 65-64)
Fig. 2 - Limits of the catchment and map of sediment production areas
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ANALYSIS OF THE DEBRIS-FLOW HAZARD ON THE RIOULONG TORRENT (HAUTES-PYRéNéES, FRANCE)
ALLUVIAL FAN USING A SCENARIO-BASED APPROACH
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
923
tive evaluation of the mobile material potential (Tab.
1). These assessments show very large uncertainties
but given the present state of the catchment, they can
be considered realistic.
Possible magnitude of debris flows
Even though the observed erosion activity is limit-
ed, the field surveys carried out in the catchment and the
analysis of the lengthwise profile of the stream show that
the risk of debris-flow triggering cannot be neglected.
Huge quantities of weathered and wet material,
with features that are similar to viscous debris-flow
deposits, are present in the vicinity of the channel and
gullies and are likely to mobilize. Furthermore, the
steep longitudinal slope of the channel and gullies in
this area are compatible with the triggering and propa-
gation of mudflows down to the alluvial fan.
When comparing the results of the application of
several evaluation methods of the volume of material
potentially mobilized, it appears pertinent to consider
at least three debris-flow scenarios associated with
a probability of occurrence qualified as follows: in-
frequent, rather rare and rare. The absence of recent
events, however, gives substantial uncertainty to the
estimations. We therefore considered the possible oc-
currence of an exceptionally rare event. Table 2 gives
the debris-flow volume values associated with these
qualitative terms and an order of magnitude of the an-
nual probability of occurrence of these volumes.
Three assumptions of peak discharge values were
considered and their probability of occurrence de-
scribed. These values were established based on the
catchment hydrology and the empirical relationship
between peak discharge and volume of a debris-flow
event proposed by R
iCkenmann
(1999). These values
Historically, several floods triggered by intense
rainfalls have been recorded (1875, 1885, 1891, 1909,
1929, 1936, and 1987, according to the unpublished ar-
chives of the ONF-RTM department who is in charge of
the torrent control). The consequences were severe for
human activities in the vicinity of the stream. Charac-
terized by huge sediment transport with gravels, blocs,
and mud of glacial origin, some of these floods probably
included debris flows. This is confirmed by the typical
shape of ancient deposits still present on the alluvial fan.
A preliminary protection strategy, mainly based
on small check-dams and revegetation, was defined as
early as the late 19
th
century and reinforced in 1945.
More recently, a camp site has been established on the
alluvial fan, thus increasing the vulnerability already
present in this area (several houses and a local road).
A drainage network and check dams were built dur-
ing the 1990s (unpublished archives of the ONF-RTM
department who is in charge of the torrent control).
EROSION AND DEBRIS-FLOw MAGNITUDE
Erosion processes
In the catchment’s present state, the extension of
erosion processes is rather limited. The most active
phenomenon is a landslide in morainic terrains, drained
by the Paulède gully, a small tributary of the Rioulong
torrent. Consequently, sediment transport in the chan-
nel remains very limited for common discharge values,
occurring once a year on average. Nevertheless, for in-
tense flooding, bed destabilization is likely to occur in
the steep channel between the upper basin and the al-
luvial fan, involving at least 700 m of channel where
the bed grain size distribution appears very small com-
pared to the stream steepness. Only one-third of this
channel has been protected by check dams.
Also in this intermediate zone, an automatic analy-
sis using GIS, based upon instability criteria defined by
z
immeRmann
et alii (1997) and completed by detailed
field surveys, showed that landslides are likely to be
triggered under long and intense rainfall conditions.
These observations led us to map the most ero-
sion-sensitive areas (Fig. 2) and carry out a quantita-
Tab. 1 - Assessment of potential mobilized volume of mate-
rial considering erosion processes at work
Tab. 2 - Reference scenarios considered: qualitative probability of occurrence, volume and tentative value of the annual prob-
ability of occurrence
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D. LAIGLE & C. PETEUIL
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
laboratory experiments or records of real events, we
invite the reader to refer to l
aiGle
(1997), l
aiGle
et
alii (2003) and R
iCkenmann
et alii (2006).
CONSTRUCTING THE SCENARIOS
Since the topography of the alluvial fan is known
based on a precise survey (up to 1 elevation value per
square meter), using the LAVE2D model (l
aiGle
et
alii, 2006) required two additional input data: the de-
bris-flow hydrograph at the alluvial fan apex and the
rheological parameters of the flowing material.
INPUT HYDROGRAPH
The input hydrograph imposes the debris-flow dis-
charge versus time at the alluvial fan apex. With no data
on the shape of this hydrograph, we assume a linear evo-
lution of the discharge between its peak value at time t =
0 to a zero value at time t
1
. This time t
1
is easily comput-
ed from the peak discharge and the volume of the event.
The scenarios studied hereafter have been inferred from
the diagnosis presented previously. The hydrograph is
“injected” into the computation domain (Fig. 3) in the
channel located at the apex of the alluvial fan.
RHEOLOGICAL PARAMETERS
We assume that Rioulong debris flows are of the
viscous type, which is consistent with field observa-
tions (shape of ancient deposits, grain size distribu-
tion of soils present on the slopes and in the gullies).
are: 30 m
3
/s (infrequent occurrence), 100 m
3
/s (rare oc-
currence) and 180 m
3
/s (exceptionally rare occurrence).
Given the channel’s low capacity on the alluvial fan,
the risk of some overflow for any debris-flow occurrence
(including events with a low discharge value) is quite
high. Some spreading of the flow towards human set-
tlements on the alluvial fan is therefore likely to occur.
MODEL PRESENTATION
LAVE2D (l
aiGle
et alii, 2003) is a numerical
model dedicated to the computation of the unconfined
free-surface spreading of materials with complex rhe-
ology. It is based upon the 2D steep-slope-shallow-wa-
ter-equations which are solved by using a finite volume
technique. It takes into account viscous dissipation in-
side the flowing material, assumed homogeneous, by
the use of the wall shear stress expression. This expres-
sion from C
oussot
(1994) is based upon the assump-
tion of a visco-plastic behavior which can be repre-
sented by a Herschel-Bulkley model mainly applying
to mudflows or so-called viscous debris flows. Apart
from values of the rheological parameters, model in-
puts are: boundary conditions (imposed discharge ver-
sus time at the point where the flow enters the zone
of spreading), a computation mesh combined with a
digital elevation model of the alluvial fan and a set of
numerical parameters (i.e. stability criterion of the nu-
merical scheme, simulation duration). For more details
about this model and its evaluation by comparison to
Fig. 3 - Maximum simulated flow depth for a debris-flow volume of 10 000 m
3
, a peak discharge of 100 m
3
/s and a τ
c
/ρ = 0.5 m
2
/s
2
ratio
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ANALYSIS OF THE DEBRIS-FLOW HAZARD ON THE RIOULONG TORRENT (HAUTES-PYRéNéES, FRANCE)
ALLUVIAL FAN USING A SCENARIO-BASED APPROACH
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
925
Following C
oussot
(1994), we consider that their
rheological properties are properly described by a Her-
schel-Bulkley model. These material properties are de-
termined by two parameter values: the yield-stress to
density ratio, τ
c
(m
2
/s
2
), and the consistency to yield-
stress ratio, k/τ
c
(s
1/3
). With no means to evaluate these
parameter values for the Rioulong torrent, we made
the following assumptions. The value of parameter k/
τ
c
is taken equal to 0.3 s
1/3
, following C
oussot
(1996)
who proposes this value as a first approximation. The
value of parameter τ
c
is considered parametrically
and three values were considered for the simulations:
- An average value τ
c
= 1.0 m
2
/s
2
, considering,
based on our experience, that a value on this or-
der of magnitude is frequently observed on many
torrents. Its probability of occurrence is conside-
red high. Furthermore, the observation of the an-
cient deposits on the Rioulong alluvial fan tends
to confirm this assumption.
- A high τ
c
= 2.0 m
2
/s
2
value, with a low probabi-
lity of occurrence, but which is useful to consider
in the parametric approach because it gives high
flow and deposit thickness values (considered
here as the highest thickness likely to be observed
for a given set of discharge and volume values).
- A low τ
c
= 0.5 m
2
/s
2
value, with a low probability
of occurrence, but which is useful to consider in a
parametric approach because it gives high flow ve-
locities and extensions (considered here as the hi-
ghest velocity and extension likely to be observed
for a given set of discharge and volume values).
SYNTHETIC PRESENTATION OF SCENARIOS
CONSIDERED
The simulations presented below are the results
of scenarios based upon the assumptions summarized
in Table 3. They do not cover all possible cases, for
practical purposes unlimited in number. For each of
the cases considered, perturbing phenomena are likely
to modify the result substantially. As an example of
Tab. 3 - Synthetic presentation of all scenarios considered in the study. Not all of them are illustrated in the present paper
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D. LAIGLE & C. PETEUIL
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
This is consistent with previous results established on
other torrents (Rickenmann et alii, 2006). However,
one can see an important trend on Rioulong: the lat-
eral overflow at the alluvial fan apex of all the mate-
rial coming from upstream with a distribution between
the right and the left bank. This trend is confirmed by
simulations presented below.
INFLUENCE OF THE PEAk DISCHARGE
The goal of the following simulations is to evalu-
ate the influence of the peak discharge. These simula-
tions are carried out considering a given volume of
10 000 m
3
and rheological characteristics correspond-
ing to a mean viscosity (τ
c
= 1.0 m
2
/s
2
). Three peak
discharge assumptions were considered: 30 m
3
/s, 100
m
3
/s and 180 m
3
/s. These simulations show that a vari-
ation in the peak discharge moderately influences the
maximum extension of the flow. The peak discharge
value essentially influences the flow velocities and
to a lesser extent the flow depth, mainly in the chan-
nelized area at the apex of the alluvial fan.
INFLUENCE OF THE VOLUME
The goal of following simulations is to evaluate
the influence of the debris-flow volume. These simu-
lations were carried out considering a peak discharge
of 100 m
3
/s and an average viscosity material (τ
c
=
1.0 m
2
/s
2
). The volume assumptions considered are: 5
these perturbing phenomena, we considered one as-
sumption of log jamming occurring in the channel at
the apex of the alluvial fan. Two protection levee as-
sumptions are also considered.
HAZARD ASSESSMENT: ANALYSIS OF MA-
XIMUM EXTENSION OF DEBRIS FLOWS
Since debris flows are likely to generate major
damage at any point of their spreading area, we con-
sidered their maximum extent as the most pertinent
criterion for the hazard analysis on the Rioulong al-
luvial fan. These extensions are mapped for each of
the scenarios considered. However, rather than con-
sidering only the extensions it is more interesting to
consider their variation related to any variation of the
model’s input parameters. This is why the results are
presented as a sensitivity analysis.
INFLUENCE OF THE YIELD-STRESS VALUE
The goal of the following simulations is to evalu-
ate the influence of rheological properties of the flow-
ing material. We consider a given volume of 10 000
m
3
and a given peak discharge of 100 m
3
/s. Three as-
sumptions on the material properties are considered:
fluid (τ
c
= 0.5 m
2
/s
2
) (Figure 3), average (τ
c
= 1.0
m
2
/s
2
) (Figure 4) or viscous (τ
c
= 2.0 m
2
/s
2
).
These simulations show the substantial influence
of the material viscosity on the final deposit extension.
Fig. 4 - Maximum simulated flow depth for a debris-flow volume of 10 000 m
3
, a peak discharge of 100 m
3
/s and a τ
c
/ρ = 1.0 m
2
/s
2
ratio
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ANALYSIS OF THE DEBRIS-FLOW HAZARD ON THE RIOULONG TORRENT (HAUTES-PYRéNéES, FRANCE)
ALLUVIAL FAN USING A SCENARIO-BASED APPROACH
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
927
currence of a fluid material (τ
c
= 0.5 m
2
/s
2
), which
tends to spread more easily but whose deposits are
less thick. The simulations are carried out considering
a peak discharge of 100 m
3
/s. The volume assump-
tions considered are 10 000 m
3
(Fig. 3), 15 000 m
3
(Fig. 6) and 25 000 m
3
.
The simulations show that beyond a debris-flow
volume ranging from about 15 000 to 20 000 m
3
and
under the assumption of a fluid material, the houses
(represented as black rectangles in the figures) located
000 m
3
, 10 000 m
3
(Fig. 4), 15 000 m
3
and 25 000 m
3
(Fig. 5). The simulations show that the assumed vol-
ume has a substantial influence on the flow extension.
This result is coherent with previous results obtained
on other torrents (R
iCkenmann
et alii, 2006).
ANALYSIS OF THE MAXIMUM EXTENSION OF
A FLUID MATERIAL
The goal of the following simulations is to evalu-
ate the maximum flow extension for the unlikely oc-
Fig. 5 - Maximum simulated flow depth for a debris-flow volume of 25 000 m
3
, a peak discharge of 100 m
3
/s and a τ
c
/ρ = 1.0 m
2
/s
2
ratio
Fig. 6 - Maximum simulated flow depth for a debris-flow volume of 15 000 m
3
, a peak discharge of 100 m
3
/s and a τ
c
/ρ = 0.5 m
2
/s
2
ratio
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D. LAIGLE & C. PETEUIL
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
on the right bank of the channel, downstream of the
alluvial fan, are likely to be reached by debris flows.
Additionally, as for the previous simulations, the risk
is high on the left bank of the channel (notably the
camp site area).
EXAMPLE OF THE INFLUENCE OF LOG
JAMMING
Given that the catchment is covered by timber and
the channel is narrow, it is pertinent to consider the as-
sumption of log jamming. It is unrealistic to consider
all possible assumptions of log jamming, since this
phenomenon is likely to occur almost anywhere in the
channel and the resulting plug dimensions can vary
greatly. Consequently, we illustrate this phenomenon
using an example. The assumption is that the log jam-
ming occurs at the alluvial fan apex, where previous
simulations show that the flow tends to spread later-
ally. Simulations are based on a peak discharge of 100
m
3
/s and an average viscosity. Two volume assump-
tions are considered: 10 000 m
3
(Fig. 7) and 25 000
m
3
. The log jamming is numerically represented as a
wall, 9 m wide, indefinitely high, clogging the channel
at the alluvial fan apex.
When compared to previous simulations car-
ried out under the same assumptions, the presence
of the log jamming tends to increase the overflow
towards the left bank of the channel. The location of
this overflow is not fundamentally modified. Conse-
quently, this plausible phenomenon tends to increase
the volume of material driven to the left part of the
alluvial fan and increase the risk in this sector, no-
tably in the camp site area. This phenomenon can
explain the numerous old flow traces still visible in
this area.
A PROTECTION LEVEE AGAINST OVER-
FLOW: DESIGN ELEMENTS
COMPARING TwO POSSIBLE ORIENTATIONS
OF THE PROTECTION LEVEE
The most vulnerable area (the camp site) is locat-
ed on the left part of the alluvial fan. Consequently,
the RTM service, in charge of controlling the torrent,
considered of building a protection levee within a
more general protection strategy. The objective of the
levee, located at the apex of the alluvial fan where the
flow tends to diverge, is to eliminate the overflow risk
towards the left bank. We examine two orientations of
the levee. The first orientation (type A levee in Fig-
ure 8) presents a 45° angle with the upstream channel
and a second one (type B levee in Fig. 8) presents a
15-20° angle with the upstream channel. The follow-
ing assumptions are considered for the comparison of
these orientations: an average viscosity (τ
c
= 1.0 m
2
/
Fig. 7 - Maximum simulated flow depth for a debris-flow volume of 10 000 m
3
, a peak discharge of 100 m
3
/s and a τ
c
/ρ = 1.0
m
2
/s
2
ratio assuming log jamming of the stream at the apex of the alluvial fan
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ANALYSIS OF THE DEBRIS-FLOW HAZARD ON THE RIOULONG TORRENT (HAUTES-PYRéNéES, FRANCE)
ALLUVIAL FAN USING A SCENARIO-BASED APPROACH
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
929
CINITY OF THE LEVEE
Only a type B protection levee is considered here:
the simulations have two objectives:
- analyze the flow in the vicinity of the levee in
order to determine the maximum flow depth and
velocity under several assumptions.
- analyze the consequences of the presence of the
levee in terms of risks on the alluvial fan.
To do this, we consider:
- one scenario based upon the assumption of a fluid
material and a high discharge value. Simulations
carried out under this assumption will provide in-
formation on maximum flow velocities likely to
occur in the vicinity of the levee.
- one scenario based upon the assumption of a vi-
scous material and a high discharge value. Simu-
lations carried out under this assumption will pro-
vide information on maximum flow depths likely
to occur in the vicinity of the levee (as well as
elements for the future design of the levee).
- two scenarios based upon the assumption of an
exceptionally rare volume and a high discharge
value. Simulations carried out under these assum-
ptions will provide information on the maximum
flow extension in presence of the levee.
The volume has a very limited influence on the
flow features in the vicinity of the levee. The assump-
tion leading to the maximum velocity (peak discharge
s
2
), a volume of 10 000 m
3
and a peak discharge of
100 m
3
/s. The aim of this comparison is to establish
which of these orientations is best in terms of effi-
ciency and then in terms of the required dimensions.
Levees are numerically considered to be cells imper-
vious to any flow.
Simulations show that the hydraulic behavior of
the type B levee is better than the type A. In terms
of flow extension, both types of have similar effects.
However, flow depths and velocities in the vicinity of
the type B levee are lower than for the type A levee
(maximum flow depth: 2.8 m and maximum veloc-
ity: 5.6 m/s for type A and maximum flow depth: 1.5
m and maximum velocity: 1.0 m/s for type B). This
presents two advantages in favor of type B: for simi-
lar efficiency, the type B levee can be lower and the
risk of impact and erosion is also lower because of
a lower velocity. Furthermore, the type A levee cre-
ates a bottle-neck for the channel flows immediately
downstream of the levee, producing local acceleration
of the flows (maximum local velocity under the as-
sumptions considered: 8.2 m/s). The type B levee does
not generate this bottle-neck effect and thus no local
acceleration of the flow (maximum local velocity un-
der considered assumptions: 5.3 m/s). A type B levee
is therefore preferred.
ANALYSIS OF FLOw SIMULATIONS IN THE VI-
Fig. 8 - Maximum simulated flow depth for a debris-flow volume of 10 000 m
3
, a peak discharge of 100 m
3
/s and a τ
c
/ρ = 1.0
m
2
/s
2
ratio assuming the presence of a protection levee showing a 45° angle with the channel axis
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D. LAIGLE & C. PETEUIL
930
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
of 180 m
3
/s and a fluid material with τ
c
= 0.5 m
2
/s
2
)
gives simulated velocities on the order of 10 m/s and
very locally 13 m/s in the vicinity of the downstream
edge of the levee (Fig. 9). The assumption leading to
the maximum flow depth (peak discharge of 180 m
3
/s
and a fluid material with τ
c
= 2.0 m
2
/s
2
) gives simu-
lated flow depths on the order of 2.5 m in the vicin-
ity of the levee (Fig. 10). Even though these figures
should be considered with caution, they can contribute
to the future design of the levee.
CONSEQUENCES OF THE PRESENCE OF THE
LEVEE
The levee, driving all the flow volume towards
the right bank of the channel, intensifies the flows
and subsequent extensions on this part of the al-
Fig. 9 - Maximum simulated flow velocity for a debris-flow volume of 10 000 m
3
, a peak discharge of 180 m
3
/s and a τ
c
/ρ = 0.5
m
2
/s
2
ratio assuming the presence of a protection levee showing a 15° angle with the channel axis
Fig. 10 - Maximum simulated flow depth for a debris-flow volume of 10 000 m
3
, a peak discharge of 180 m
3
/s and a τ
c
/ρ = 2.0
m
2
/s
2
ratio assuming the presence of a protection levee showing a 15° angle with the channel axis
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ANALYSIS OF THE DEBRIS-FLOW HAZARD ON THE RIOULONG TORRENT (HAUTES-PYRéNéES, FRANCE)
ALLUVIAL FAN USING A SCENARIO-BASED APPROACH
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
931
complete set of flood scenarios was developed.
Flow simulation in this scenario-based approach
is of particular interest, as the alluvial-fan area af-
fected by debris-flow hazards can be predicted. It
also provides practical information on the design of
a protection levee, including guidelines on the levee’s
orientation and dimensions based on modeled maxi-
mum flow depths and velocity and on evaluation of
the potential changes to the debris-flow hazard area on
the alluvial fan due to the presence of the levee.
This scenario-based approach, as applied to the
Rioulong torrent, provides a simple method to assess
the variability of debris-flow hazards on an alluvial fan
without the costs of more rigorous approaches. Such
rigorous approaches do exist, however they all require
the assessment of statistical properties for model input
parameters which is difficult to achieve in practice.
luvial fan (Fig. 11), notably on the road and two
houses located downstream. However, the houses
are potentially hit in only one of the two scenarios
considered with a volume of 25 000 m
3
and a τ
c
=
0.5 m
2
/s
2
ratio. This scenario has a very low prob-
ability of occurrence.
CONCLUSION
In this study of debris-flow hazards on the Rioulong
torrent alluvial fan, we first present a susceptibility anal-
ysis of the torrent catchment for producing debris flows.
This analysis was a necessary step prior to the numeri-
cal modeling for the scenario-based analysis employed
in the second phase of this study. This analysis made it
possible to define flow volume and discharge scenarios
and to assign a qualitative probability of occurrence. In
conjunction with the assumptions of flow rheology, a
Fig. 11 - Maximum simulated flow depth for a debris-flow volume of 25 000 m
3
, a peak discharge of 180 m
3
/s and a τ
c
/ρ = 1.0
m
2
/s
2
ratio assuming the presence of a protection levee showing a 15° angle with the channel axis
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
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