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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1063
DOI: 10.4408/IJEGE.2011-03.B-115
DEBRIS-FLOW PROTECTION IN RECURRENT AREAS OF THE
PYRENEES. EXPERIENCE OF THE VX SYSTEMS FROM OUTPUT RESULTS
COLLECTED IN THE PIONEER MONITORING STATION IN SPAIN
R
obeRto
LUIS-FONSECA
(*)
, C
aRles
RAÏMAT
(*)
, m
aRCel
HÜRLIMANN
(**)
,
C
laudia
ABANCÓ
(**)
, J
osé
MOYA
(**)
& J
esús
FERNÁNDEZ
(***)
(*)
Geobrugg Ibérica S.A.U, Spain
(**)
Department of Geotechnical Engineering and Geosciences, Technical University of Catalonia, Spain
(***)
Forestal Catalana S.A., Spain
the debris flow. We have collected information related
with different events of small volume (< 100m
3
) that
had a direct correlation with the intensive rainfall in the
basin. During 2009, based on the knowledge gained
from the behaviour of the Erill basin, the first applica-
tions and designs of VX160 systems were created for
other sites. The work done in the Portainé gully is an
example of this. In 2008, this zone was affected by a
debris flow of more than 20.000 m
3
. The protection sys-
tem that was installed was made up of 9 VX160 trans-
versal protection lines, with a total retention capacity
of approx. 25.000 m
3
. Three months after they were
installed, the fences were completely full as a result
of two events caused by summer storms. The installed
solution costs 40% less than the traditional check dam
solution. This paper shows the importance of the re-
search, of these specific phenomena in the Pyrenees, to
the development of protection technology. The autono-
mous measurement equipment, together with the tested
protection system can be applied, with the correspond-
ing reduction in costs, to civil protection and hydrologi-
cal correction situations in urban and suburban zones
where debris flow is a recurring phenomenon.
K
ey
words
: monitoring, hydrological correction, debris-flow
protection, Pyrenees, ring net
INTRODUCTION
Potentially dangerous events are part of the nor-
mal dynamics of natural systems. Daily interaction
ABSTRACT
The south eastern part of the Pyrenees is currently
affected by debris flow phenomena. The combination
of Mediterranean and Continental climate, the orog-
raphy (up to altitudes of 3300 m), the glacial materi-
als on the slope’s surface, the lack of arboreal cover-
age compared with the rest of the Pyrenees, the high
seismicity and the increasing human occupation in the
valleys, put altogether is a very dangerous combination
and makes the debris flow management extremely dif-
ficult. The first monitoring system for debris flow phe-
nomena was installed in Spain in 2005. The aim was to
monitor the debris flow phenomenon’s behaviour in the
Erill basin, in the north of the province of Lerida. The
Erill location in the south of the Pyrenees is one of the
places where the debris flow phenomenon is common
and where, apart from the possible magnitude of any
event, the likely affected area includes the urban area of
the Erill village. Remote controlled autonomous moni-
toring equipment was installed in this location. It was
composed of an automatic meteorological station, a set
of geophones to activate the measuring and recording
systems, a VX160 barrier protection system, monitored
with load-cells, a camera and digital recording equip-
ment, all connected to a data logger with a GSM mo-
dem. The information provided by the detection system
was completed during 2009 by a topographic schema,
created by the LIDAR system, of the basin that gener-
ates the debris flow. The objective was to try to detect,
before the event, the specific deformations that cause
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
flow events while documenting the performance and
optimizing the design procedures using results from
the natural scale on-site tests.
GENERAL SETTING
The Pyrenees is the large mountain range that sep-
arates Spain and France and also is home to the coun-
try of Andorra. It ranges from east to west and reaches
an altitude of 3000m, with a combination of igneous,
metamorphic and sedimentary materials, amongst
which we highlight the glacial and a very important
condition of two major orogenies: the Ordovician and
Alpine. These two orogenies produce discontinuities
and the combination of these discontinuities creates
instability problems. Together with other areas, the
eastern Pyrenees was severely affected by flooding in
November 1982 (G
aRRido
, 1992). Significant human
and economic losses have required detailed defence
projects and hydrological correction, which are still
being implemented in the region.
The measures used have generally been the con-
struction of check dams across the flow of the streams
that were most severe in 1982 using either a stone,
concrete or metal structure or any combination of the
three materials. These projects inherited the calcula-
tion-design, implementation and construction meth-
odology of structures designed in the Pyrenees in the
late nineteenth and early twentieth century. The tradi-
tional techniques applied in modern times have had a
significant economic and environmental cost.
Research carried out since 2001 by WSL and
G
eobRuGG
AG in Switzerland (a
mmann
& v
olkwein
,
2006), at the debris-flow test field, has greatly ad-
vanced the concept of design and operation of protec-
tive measures against debris flow, with the knock-on
effect in the applicability of the designs.
In 2005, as a result of the research agreement
between the Forest Technology Centre in Catalonia,
Forestal Catalana and Geobrugg Ibérica, testing of the
VX-UX systems developed by Geobrugg AG (R
otH
,
2003) began in particularly active basins in the Pyr-
enees Mountains and the Coastal Mountain Range.
Two test sites were installed in the La Galera and Erill
gully (l
uis
f
onseCa
et alii, 2006), in order to compare
the effect of debris-flow phenomenon in two different
environments: the Axial Pyrenees and the Mediter-
ranean Coastal Range. The data collected so far has
already led to the design and construction of effective
with human activity generates increasing losses, that’s
why management is essential. Reports on natural
disasters created by the United Nations International
Strategy for Disaster Reduction (ISDR) and by large
insurance companies conclude that the social and
economic impact of natural hazards, both in devel-
oped and developing countries has been increasing
over recent years and shows the same trend for the
foreseeable future. The causes are attributed to the
severity of natural phenomena, together with the
physical and social vulnerability of the territory. Sev-
eral papers suggest that the vulnerability factor has
increased alarmingly.
In general, the lack of reliable and comparable data
makes it impossible to develop a detailed picture af-
ter the event, which would be extremely useful, both
for technical and scientific studies on the extent of the
phenomenon, and for the authorities responsible for
mitigating its effects. Field experience in the design and
location of flexible systems is still limited due to the
frequency with which these phenomena occur and the
uncertainty associated with the extrapolation of results
from laboratory to full scale events. In 2005, with the
support of the Swiss Federal Agency for the Promotion
of Innovation (KTI), a field trial was commissioned in
order to evaluate the performance of the flexible ROC-
CO® ring net against debris flows (Fig. 1).
Traditional measures of mitigation against debris-
flow have consisted of the construction of expensive
concrete dams or strong steel structures to retain the
sediment. The VX-UX flexible systems of ring net
barriers provide a new lower-cost alternative and
have already been successfully used for flows of up to
25,000 m
3
. The aim of this paper is to present the ap-
plication of this technique in areas affected by debris
Fig. 1 -- Evolution of debris-flow event. Experience in Ill-
graven (Switzerland)
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DEBRIS-FLOW PROTECTION IN RECURRENT AREAS OF THE PYRENEES. EXPERIENCE OF THE VX SYSTEMS FROM OUTPUT RE-
SULTS COLLECTED IN THE PIONEER MONITORING STATION IN SPAIN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1065
than 20,000 m
3
(P
oRtilla
et alii. 2010). In 2010, af-
ter the installation of nine VX type protections, two
events that coincided with summer storms caused the
accumulation in the barriers of more than 25,000 m
3
.
The basin has its highest point in the Orri Mountain,
at an altitude of 2439 m, with its lowest point flow-
ing into the Romadriu gully, at an altitude of 950 m,
implying a variation of almost 1500m in about 5.7
km with an average gradient of 16º, with some places
surpassing 26º in the steepest points (Fig. 3). This is
a watershed covered by grass above 2100 m, while
the rest is covered with trees and native shrub veg-
etation. Geologically, it is formed by metamorphic
materials, quartzite and slate in general, with a very
important fragmentation, which favours a sediment
cover that is aligned with the gradient slope.
DESCRIPTION OF DEVICES AND PRO-
TECTION SYSTEMS
ERILL TEST SITE
In the Erill gully in 2005 a set of auscultation
equipment was installed, the combination of data ex-
tracted should allow a betterunderstanding of the cor-
relation between local hydrology, local geology and de-
velopment of the event. A prototype of the VX barrier
was also installed, in order to retain the debris-flow. The
installation included a video camera in order to
monitor the events. In the spring of 2010 a LIDAR
topography scanner of the basin has been realized,
to detect possible premonitory deformations of mass
movements and to compare volumes displaced by each
event. Additionally wireless strain gauges were in-
stalled over support ropes of the VX flexible structure.
For next year, it is planned to implement a set of string
piezometers to determine the degree of saturation that
exists before the mass movements (Tab. 2).
PORTAINÉ GULLY
In the Portainé gully, eleven VX protections were
projected to be installed in 2009, in order to prevent
protections which have low economic and environ-
mental costs such as the Portainé gully.
ERILL TEST SITE AND PORTAINé GULLY SITE
The Erill site in the Pyrenees was selected as a
Test Site due to its well known high debris flow ac-
tivity. While the most significant volumetric events
have coincided with other major events in this re-
gion, its constant economic activity has resulted in
important investment which has not eliminated the
risk of further debris flow. This is a very small basin
(<0.5 km
2
) with a pronounced slope incline (>16º),
with almost no plant coverage due to the continued
erosion and loam sediment and gravel-loam depos-
it of glacial origin that lies on slate and Devonian
quartzite. These conditions are applicable to many
other basins in this area (Fig. 2).
The Portainé gully has been one of the most ac-
tive in the Pyrenees since 1982 where it is estimated,
based on the incisions and retrospective measures
and simulations carried out, that over 50,000 m
3
has
been displaced. In 2008, the access road to the ski
resort was destroyed and the Hydroelectric Station,
was significantly damaged, by the movement of more
Fig. 3 - Profile overview of the Portainé Job Site
Fig. 2 - Profile overview of the Erill test site
Tab. 2 - Devices installed in Erill test site
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R. LUIS-FONSECA , C. RAÏMAT, M. HÜRLIMANN, C. ABANCÓ , J. MOYA & J. FERNÁNDEZ
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
storm reached the Portainé gully one hour later. Ac-
cording to eyewitnesses an impressive debris-flow
event cut the access road to the ski resort of Portainé at
22:30 hours. The Erill’s auscultation system recorded
an event for a period of 17 minutes. The record shows
future damage similar to that caused in 2008. In the
winter and spring of 2010, nine VX protections of the
eleven planned were installed (Tab. 3 and 4). The ba-
sic objective was to gradually retain the material pro-
vided by the potential debris-flow events, creating a
mid-term hydrological correction. The works associ-
ated needed to ensure minimal environmental impact,
such as the prohibition to open access paths during the
installation. The client also requested that the works
maintained the fluvial dynamics characteristic of a
high mountain torrent river and that it should be use-
ful only for catastrophic events.
JULY 22
ND
EVENTS. PRELIMINARY DATA
Since the installation of the auscultation system
in the Erill gully, there have been several episodes
of debris-flows, but without doubt the one that oc-
curred on July 22
nd
, which affected within minutes
of each other the Erill basin and the Portainé gully,
has marked an important milestone in our research.
The arrival of cold wind at altitude from the north and
warm wind pushed from the east is symptomatic of a
volatile weather pattern, with the potential for heavy
localized rains (Figg. 4 and 5). Therefore the forecast
is for a summer storm in the Pyrenean area
An evening summer storm progresses from west
to east reaching the Erill gully (Figg. 8 and 9) at 19:00
hours. At 20:38 a warning system was activated au-
tomatically sending a signal, to our call centre op-
erations, via GSM “Debris-Flow in Erill”. The same
T
ab. 3 - Devices installed and position in Portainé test site
T
ab. 4 - .Devices installed and position in Portainé test
site
Fig. 7 - Geophone data registered in Erill’s debris flow on
July 22
nd
Fig. 6 - Geophone data registered in Erill’s debris flow on
July 22nd.
Fig. 4 - Rainfall data registered in Erill’s debris flow on
July 22nd.
Fig. 5 - Rainfall data registered in Portainé’s debris flow
on July 22nd
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DEBRIS-FLOW PROTECTION IN RECURRENT AREAS OF THE PYRENEES. EXPERIENCE OF THE VX SYSTEMS FROM OUTPUT RE-
SULTS COLLECTED IN THE PIONEER MONITORING STATION IN SPAIN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1067
the constituent elements of the VX system and convert
them to load measurements.
At the same time, one of the largest debris-flow
events in the Pyrenees was happening. Nine VX flex-
(Figg. 6 and 7)
- different flow waves evolve through the channel aus-
cultation by 4 geophones.
- the high current flows recorded in geophone 1 are less
evident when they get to the position of the VX sys-
tem where the geophone number 4 is located.
- a constant background vibration is recorded by all the
geophones once the event reaches the position of
geophone number 1.
- the geophone number 4 installed in vertical section
of the VX barrier, recorded impulses by the effect
of vibration of the flexible protection system itself.
The event caused a build-up in the VX system of
680 m
3
. Trial pits carried out in the sediment and sedi-
mentary structures showed that 320 m
3
was caused by
hyperconcentrated flow, while the rest of the materials
have been transported by turbulent flow and turbu-
lentlaminar.
Although at the time of the event the load cells
were not installed due to maintenance-related issues, a
calculation has been made from the experimental data
at our disposal with brake rings and ring nets, in which
we can measure plastic deformation (Figg. 10-11) in
Fig. 9 - Lateral view of the barrier filled in Erill
Fig. 8 - Frontal view of the barrier filled in Erill
Fig. 11 - Force-path diagram for the breaking rings
Fig.10 - Energy absorption of the single brake rings
Fig. 12 - Pictures of a VX in the Portainé gully before and
after the event
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
ible structures were filled (Fig. 12) and exceeded by
a debris flow of approximately 25,000 m
3
.The design
of the protection system was based on data obtained
from the last event recorded and registered in Por-
tainé in mid 2008.
It was made with the VX-160 model with heights
of between 4 and 6 meters.
In the same way that was done for the Erill gully,
we can determine the energy consumed by each of the
fences and the entire hydrological correction system
as a whole, implemented in the Portainé gully.
With the topography records taken three months
earlier, during the construction of the fences, an ac-
curate comparison could be made of the volumes held
by the VX protections (Fig.13).
Due to the fact that the volume of mobilized ma-
terial is known and that we have a good knowledge
of the upwelling zone and/or erosion of the contribu-
tions, them we can make an accurate calculation of the
energy generated by the event.
PEAk DISCHARGE
Several studies proved that the peak discharge
of a debris flow is correlated to its volume. There are
different relations for granular debris flows and mud
flows. m
izuyama
et alii (1992) propose for a granular
debris flow (debris avalanche) the following empiri-
cal relationship between peak discharge and debris
flow volume:
Q
P
= 0.135 V
DF
0.78
(1)
where: Q
P
: flow peak discharge, V
DF
: average volume
of the material The following equation represents the
corresponding relationship for mud flows:
Q
P
= 0.0188 V
DF
0.79
DETERMINATION OF FLOw VELOCITY
By using the peak discharge it is possible to es-
timate the average flow velocity v at the front of the
flow. R
iCkenmann
(1999) proposes a regime condi-
tion for the relation between velocity, peak discharge
and slope inclination (friction considered). S refers
to the gradient of the torrent (tangent of the slope
inclination in degrees).Typical values are S=0.18
(10°), S=0.36 (20°) or S=0.58 (30°).
v = 2.1 Q
P
0.33
S0.3
3
(2)
Japanese guidelines (m
uRaisHi
, 1997) suggest
a Manning- Strickler equation to determine the av-
erage flow velocity nd refers to a pseudo-manning
value which is typically between 0.05 s/m
1/3
and 0.18
s/m
1/3
, while the values for granular debris flows lay
between 0.10s/m
1/3
y 0.18 s/m
1/3
. We could also arrive
at this conclusion using literature such as the one be-
low; in this case we know the maximum slope of the
course (26º) in the area where the VX barriers have
been installed.
v =1/ n
d
H
0.67
S
0.5
The flow depth H is calculated by using the cross
section and the peak discharge.
n
d
is the Manning coefficient, the value is be-
tween 0.05 s/m
1/3
and 0.18 s/m
1/3
H = Qp / v b.
It’s recommended to use both equations and
compare the results.
ACTIVE MASS OF MATERIAL MOVED BY THE
FLOw
As a result of the composition of the flow and the
permeability of the barrier, a wash occurs during the
impact; therefore not all the mass that is in the flow is
stopped. The actual weight is determined under the as-
sumption that only this part of the flow works dynami-
cally and that the debris barrier is filled with debris
Fig. 13 - Lateral view of the full VX system and graphic
with the total material retention in the Portainé
gully after the 22
nd
July event
background image
DEBRIS-FLOW PROTECTION IN RECURRENT AREAS OF THE PYRENEES. EXPERIENCE OF THE VX SYSTEMS FROM OUTPUT RE-
SULTS COLLECTED IN THE PIONEER MONITORING STATION IN SPAIN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1069
energy divided by the breaking distance
F
QS
= 2 . E
K
/ d
(9)
where:
F
QS
: quasi-static force;
E
K
: kinetic energy;
d: maximum deflection.
It is recommended to multiply by a safety factor
to calculate the maximum expected force for the flow
of detritus.
F
max
= F
QS
. FS
(10)
FS: Safety Factor, default 1.5
With these combinations of values of kinetic en-
ergy E
K
and maximum force F
max
the following results
have been obtained (Fig. 14 and 15):
CONCLUSIONS AND OUTLOOK
The need for protection against debris-flow
phenomenon in the area of the Pyrenees is growing
considering the frequency and strength with which
they are hitting the urban and suburban areas in the
last decade.
The combination of topography, sparse vegeta-
tion, extreme weather and lack of regional planning are
parameters that, combined together can create a very
dangerous area. The investigation on the debris-flow
between the moment of contact and the time when the
maximum deflection of the ring net occurs.
The density γ
DF
of the material is based on em-
pirical values and is about 18-23 kN/m
3
. The material
being moved is usually heterogeneous and of variable
density, which can be taken as an average value meas-
ured 22 kN/m
3
as density.
As a result of the dewatering of the debris flow
during the impact on a permeable barrier not all the
mass that is in the flow is stopped, but only a relevant
length or mass. The relevant mass M
DF
is determined as
follows: It’s assumed that only this part of the flow is
acting dynamically and that the debris barrier is filled
with debris between the moment of contact and the time
when the maximum deflection of the ring net occurs. In
the Oregon tests (d
e
n
atale
et alii, 1996) with volumes
of 10 m
3
the Timp was about 1s. Real debris flows are
expected to be much larger and therefore the braking
time will also last longer. Timp is estimated to be be-
tween 1 s and 4 s.
T
imp
= 1s - 4s
(depending on velocity and barrier length)
In this case the selected value is 1s
The relevant mass is consequently calculated as
follows:
M
DF
= γ
DF
. Q
DF
. t
(3)
IMPACT LOADING
With the values of active mass and velocity, we can
determine the kinetic energy of the flow in contact with
the barrier. The total energy is determined by using the
law of kinetic energy.
E
K
= 0.5 . M
DF
v
2
Also, as a result of testing and experience in differ-
ent locations, it is known that the maximum deflection
is reached in the order of 2- 4m. Thus we can estimate
the quasi-static load to be borne by the barrier:
According to Newton's second law:
F = M
DF
· a
(5)
The distance is obtained from the product of the
speed and time
D = v · t
(6)
Speed is the product of acceleration and time
V = a · t
(7)
From (6) and (7) it obtains:
a = v
2
/d
(8)
Using equations (4), (5) and (8) we can deduce
that the quasistatic force is equal to twice the kinetic
Fig. 14 - ROCCO
®
ring net and the braking elements, in the
Portainé Torrent after the 22
nd
July event
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R. LUIS-FONSECA , C. RAÏMAT, M. HÜRLIMANN, C. ABANCÓ , J. MOYA & J. FERNÁNDEZ
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
of a landslide and those created from excavation in the
riverbed itself, since the design of protection may be
significantly different.
VX Systems has demonstrated its support bearing
capacity to provide support when faced with loads of
up to 25,000 m
3
from combined events.
ACKNOWLEDGEMENTS
This research was supported by the Forest Tech-
nology Centre of Catalonia, Forestal Catalana, the De-
partment of Geotechnical Engineering and Geoscienc-
es, Technical University of Catalonia and Geobrugg,
with the technical help of Swiss Federal Research
Institute, WSL.
phenomenon that recently began in the Pyrenees is pro-
viding important data for the determination of param-
eters which can be triggers for different areas of study.
The knowledge of critical parameters should en-
able us to clarify the protections’ requirements (main
forces at the anchorage points and in the ring net). It
should also allow the design and proper use of new
technology. The new VX concept clearly implies
the transmission of traction forces on the side of the
river, the reduction of the water pressure on the flex-
ible structure and has the additional advantage of im-
proved environmental integration.
However, experience shows that it is very impor-
tant todistinguish in the design phase between the ba
sins - debris-flowgenerating areas created as a result
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DEBRIS-FLOW PROTECTION IN RECURRENT AREAS OF THE PYRENEES. EXPERIENCE OF THE VX SYSTEMS FROM OUTPUT RE-
SULTS COLLECTED IN THE PIONEER MONITORING STATION IN SPAIN
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