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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
37
DOI: 10.4408/IJEGE.2011-03.B-005
RECONSTRUCTION OF MAGNITUDE AND FREQUENCY OF DEBRIS FLOWS IN
THE REBAIXADER TORRENT (EASTERN PYRENEES, SPAIN)
BY DENDROGEOMORPHOLOGICAL ANALYSIS
G
uillaume
G. CHEVALIER
(*)
, J
ose
MOYA
(*)
, m
aRCel
HÜRLIMANN
(*)
& a
llen
BATEMAN
(**))
(*)
Technical University of Catalonia (UPC) - Jordi Girona 1-3 (D2) - 08034 Barcelona, Spain, Tel. +34 93 401 69 28
e-mail: guillaume.chevalier@upc.edu
(**)
Technical University of Catalonia (UPC) - Jordi Girona 1-3 (D1) - 08034 Barcelona, Spain
develop in periglacial deposits and under a submedi-
terranean climate (Tordò creek). Now the method has
successfully been extended to assess the frequency-
magnitude relationship of debris flows in a different
setting, the Rebaixader creek, in which the mobi-
lizedmaterial is a glacial till and the local climate is a
high mountain one but with mediterranean influence.
Seven event deposits have been mapped and dated in
the Rebaixader creek, corresponding to debris-flows
events occurred during the last eighty years, involving
a depositional area 90,000 m
2
. A magnitude-frequency
relationship was prepared for the site and is compared
to that of the Tordó creek.
K
ey
words
: debris flows, dendrochronology, relative dating,
frequency, magnitude
INTRODUCTION
Mass movements are the manifestation of physi-
cal erosion’s processes actively shaping landscape.
Debris-flow occurrence in stream channels has been
reported in numerous mountainous environments
(C
oussot
& m
eunieR
, 1996; J
akob
, 2005) including
the Eastern Pyrenees (H
üRlimann
et alii, 2003; P
oR
-
tilla
et alii, 2010). Together with the vivacious re-
membrance of the 1996 Biescas debris flow (camping
site hit and 87 casualties suffered; a
lCoveRRo
et alii,
1999), it pledges for the existence of high-risk spots
in the region.
Debris flows leave traces in the landscape and on
ABSTRACT
Occurrence of debris flows has received little at-
tention in the Pyrenees, probably due to the small risk
shown by most of the debris-flow prone sites in this
mountain range. Nevertheless, the event of Biescas
(which occurred in August 1996 and that caused 87
casualties) demonstrates the existence of high risk
spots in the region.
Historical data on debris flow occurrence are
usually scarce or lacking in the Pyrenees, as in many
mountain ranges. This makes difficult the determi-
nation of their frequency. Dendrogeomorphological
techniques have shown their efficiency in reconstruct-
ing temporal series of debrisflow events from which
frequency can be assessed. The determination of
magnitude of an historic event can be done by dis-
tinguishing its deposits, but this is not a trivial task
in debris fans that accumulate deposits correspond-
ing to consecutive debris flows, especially if only a
conventional geomorphological analysis is carried
out. In catchments where debris-flow frequency is
moderate to high (i.e. return period of several years)
deposits corresponding to each event (event deposits)
are identifiable using a set of relative dating criteria,
like cross-cutting relationship, size and cover percent-
age of lichens on blocks, forest density and size of
trees colonizing the deposits. The event deposits can
be mapped and, subsequently, trees damaged by the
flows sampled for dating of events. This approach was
tested in a Pyrenean catchment where debris flows
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G.G. CHEVALIER, J. MOYA , M. HÜRLIMANN & A. BATEMAN
38
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
nian and Alpin orogenesis. Tills overtopping the bed-
rock were deposited during the Upper Pleistocene by a
glacier that occupied the Noguera Ribagorçana valley
(v
ilaPlana
, 1983). Deglaciation resulted in destabili-
zation of the steep slopes developed during the Last
Glacial cycle, giving rise to landslide activitiy.
.The catchment of Rebaixader lies between 2479
and 1225 meters above sea level (m asl). Its catch-
ment’s area has an extent o 0.7 km2, and its length 2.6
km. The mean slope of the catchment is 28 degrees.
The head zone has a semi-circular scar culminating
at 1725 m asl (Fig. 1). The debris fan has the apex
at 1325 m asl and the bottom at 1225 m asl, in the
confluence with the river Noguera Ribagorçana. The
channel linking the source zone and the fan is 630 m
long. The fan has a radius of 370 m, an area of 0.082
km
2
and a mean slope just over 15 degrees; this last
value may seem intriguingly high for an alluvial fan
but d’a
Gostino
et alii (2010) report even greater val-
ues of depositional areas (fans) mean slope. The fan
was built up by debris flows and displays a complex
ensemble of deposits, which are mostly colonised by
a forest of Pinus Sylvestris. Rebaixader has recently
been monitored (summer 2010). Geophones and an
ultrasonic device, which have been set up to control
occurrence and discharge of debris flows, and a me-
teorological station to control rainfall conditions, are
also currently operating (H
üRlimann
et alii., 2011)
. No historical data on past events having affected
the study area have been encountered, safe a debris-
flow event recognised in 1963 to have developed in an
adjacent catchment (b
RoCal
, 1984).
METHODS DETERMINING THE DEBRIS-FLOw
MAGNITUDE
The magnitude of debris flows is conventionally
expressed as its volume of deposition. However, as-
sessing the volume of past debris flows from field
work can prove to be complicated. Past studies have
emphasized the relationship between volume and area
of debris-flow deposits (i
veRson
et alìì, 1998; b
eRti
& s
imoni
, 2007; s
CHeidl
& R
iCkenmann
, 2009). The
magnitude can thus be assessed by delimiting the
surficial extent of the deposit and extrapolating to an
estimate of the volume deposited.
Nevertheless, at sites where debris flows are re-
current, as on a debris cone, new debris-flow events
may remove evidences of earlier ones by eroding or
vegetation: landslide scars, channels, depositional ar-
eas, lateral levees, bent trees, wounded trees among
others. Thanks to dendrogeochronological techniques,
it is possible to date the traces left on trees cohorts
growing on the fan. Past studies have shown the ad-
equacy of such techniques when debris flows are in-
volved (s
toffel
et alii, 2006; s
toffel
& b
ollsCHweil
-
eR
, 2008; b
ollsCHweileR
et alii., 2008; C
oRominas
&
m
oya
, 2010).
Few attempts have been made so far in the es-
timation of the magnitude of debris-flow events us-
ing dendrogeomorphology. One approach combining
tree-ring and relative dating was tested in a Pyrenean
catchment where debris flows develop in periglacial
deposits and accumulate in a valley bottom (Tordò
creek, Eastern Pyrenees). Here, the method is extend-
ed to another Pyrenean site, the Rebaixader creek, in
order to start a regional study of magnitude-frequency
of debris flows and to check the method in a different
setting (a debris fan).
STUDY AREA
Rebaixader torrent is a tributary of the Noguera
Ribagorçanariver and lies in the high reach of the ba-
sin, about 10 km south of the atlantic-mediterranean
water divide (Fig. 1). The bedrock consists of the
Paleozoic meta-sedimentary rocks of the Axial Zone
of the Pyrenees, which were folded during the Hercy-
Fig. 1 - Aerial view of the Rebaixader creek. The inset
shows the location of the site. C: head zone; F:
debris fan; A : fan apex; NR: river Noguera Riba-
gorçana
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RECONSTRUCTION OF MAGNITUDE AND FREQUENCY OF DEBRIS FLOWS IN THE REBAIXADER TORRENT
(EASTERN PYRENEES, SPAIN) BY DENDROGEOMORPHOLOGICAL ANALYSIS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
39
In Rebaixader, assessing the size of past debris
flows is rendered very difficult due to the superimpo-
sition of depositional units. The magnitude of debris
flows is conventionally expressed in volume but can
also be determined in terms of area of event deposits.
The site dictated the use of area of deposits (of depo-
sitional units, more exactly) to express the debris-flow
magnitude; this is because the low relief in the fan sur-
face made very difficult the estimation of the deposits
thicknesses.
FREQUENCY
Once the number and extent of the depositional
units were determined for the site, the numerical dat-
ing was carried out by means of dendrochronology.
Two types of tree-ring dating elements were
found at the site: trees wounded by debris flows and
trees colonising the depositional unit’s surface. Dat-
ing of wounds by counting the number of rings hav-
ing straddled over the scar gives the number of years
passed since the tree was injured, and, therefore al-
lows an exact dating of debris flow occurrence (with
an accuracy of a growing season). The age of trees
colonising depositional units gave a minimum age of
the corresponding debris-flow events. There is a time
span between the new surface’s formation and the
overlapping them. As a consequence, several parts of
a debris-flow deposit can be found at present separat-
ed by younger deposits or by a channel scoured by a
more recent event. These remnants should be identi-
fied as corresponding o do this, sediments deposited
by debris flows were differentiated using relative dat-
ing criteria, and were grouped into relative-age classes
and, finally, assembled by correlation in depositional
units. The term depositional unit is used for a set of
deposits included in a relative-age class and can be
regarded as accumulated by a debris-flow event. The
definition of the depositional units is indispensable for
estimating debris-flow magnitude and age (C
oRomi
-
nas
& m
oya
, 2010) even though constraint inherent to
field works can render it time-consuming.
At the Rebaixader creek, the fan is the place of
preferential deposition of the debris flows developed
in the site. A series of criteria has been used in order to
distinguish between different depositional units (DU)
accounting for its development: maximum size of
trees (diameter and height), lichen’s cover on boulders
making up the DU and cross-cutting relationships (i.e.
in-channel deposits are younger than terrace deposits).
Each depositional has developed different cohorts of
trees and lichen development covering the fan to dif-
ferent extents (Fig. 2).
Fig. 2 - Examples of relative age characteristics shown by different depositional units in the Rebaixader debris fan. a) and b)
show DU 7 trees and lichen assemblages; c) shows lichens for DU 5; d) exhibits trees as found on DU 3.
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G.G. CHEVALIER, J. MOYA , M. HÜRLIMANN & A. BATEMAN
40
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
or small. The GLT is highly dependent of the local con-
ditions, as mentioned above, and has not been studied
for Pinus Sylvestris in the study site. P
ieRson
(2007)
found that for a mixed population of conifers, the GLT
had a mean value of 6 years. Although this gap has only
been taken as guidance for the Rebaixader creek, it was
not applied to obtain tree ages.
A minimum age for each depositional unit was
finally obtained after applying the age-corrections to
samples and using the age of the oldest tree colonis-
ing the unit.
.
RESULTS
DEBRIS FLOw DEPOSITIONAL UNITS AND
MAGNITUDE
Using these different features, seven depositional
units have been recognized on Rebaixader’s fan. Be-
low is their description:
• The first depositional unit recognised (DU 1)
corresponds to the currently active channel. No veg-
etation is found and lichens have not covered boulders
yet.
• DU 2 is made up of small trees scarcely distrib-
uted and its boulders are not colonised by lichens.
• DU 3 exhibits trees of medium size (up to hu-
man size) with an increase in the density, and boulders
covered by lichens; coverage estimated to range from
5 to 10 %.
• DU 4’s trees are between 2 and 3 meters high
with a diameter varying between 5 to 10 centimetres.
Boulders are covered with lichens (15-20%) and moss
(scarcely).
To be noted is the accumulation of organic matter
(twigs, spikes, leaves…) on the ground.
• DU 5 shows a dense cohort of trees with diam-
eter closing 10 cm and as high as 4 meters. Very simi-
lar to DU 4, the difference resides in the assemblage
of lichens found on the boulders. Lichens (white and
yellow) cover 30-40 % of them, and moss less than
5%. Ground remains covered with organic matter.
• DU 6’s trees exceed 4 m, and diameters range
from 15 to 25 cm. Their density decreases compared
to DU 5. The boulders are covered by 5-10 % of moss,
the rest being almost entirely covered by both lichens.
Organic matter is still found on the ground.
• DU 7, which is the oldest depositional unit en-
countered, has the biggest trees, reaching 50 cm in di-
ameter, with a high density. Yellow lichens disappear
colonisation by trees. The time for colonizing a newly
formed surface, ecesis time or colonization time gap,
is highly variable and depends upon a wide variety of
controlling factors (s
CHRodeR
, 1978).
To obtain the age of colonising trees two tech-
niques have been used, depending on the apparent age
of the trees:
1) For old or mature trees, a core of the biggest
(likely to represent the oldest) trees was extracted (45)
and the rings counted. The minimum age for the event
which produced the surface (in our case DU) is given
by the germination date of the oldest tree.
2) For young trees, conifers younger than 30 years
old, another technique (more direct and easily applica-
ble) was used. It is based on the fact that the number of
branch whorls of a young conifer indicates the
tree age (s
CHweinGRubeR
, 1990; 371 young pines were
dated by this method).
Obtaining the tree age form cores has several
shortcomings. First, the sampling height should be
as close to ground level as possible in order to hit
the oldest tree-ring of the tree (e.g. G
Rissimo
-m
ayeR
,
2003). The higher the sampling height, the greater the
number of tree-rings formed before the tree reaches
this height. This is called the Sampling Height Growth
Time (SHGT). A sampling height common for all
trees is hardly achievable as it depends on the access
to the tree. In this study sampling heights ranged from
0.5 to 1.4 metres. Second, cores may not reach the pith
(which is the oldest part of a tree radius). Thus, results
given by this technique are subject to approximations,
as s
CHRodeR
(1978) emphasized.
In order to get the most exact age, a series of cor-
rections has been realised. When the pith was not hit
and inner growth ring arcs were visible, the geometric
model proposed in d
unCan
(1989) was used to esti-
mate the number of rings between the last visible ring
and the virtual pith. Details and limits of the method
are not discussed here and can be found in the refer-
ence above. This method is not applicable to all cores
as arcs are not always visible. 23 cores are concerned
by the method. The colonization time gap (CTG) can be
defined as including the Germination Lag Time (GLT)
and the Breast Height Growth Time (BHGT) (follow-
ing P
ieRson
, 2007) or the Sampling Height Growth
Time (SHGT) if the samples were taken close to the
ground. Because the cores were sampled as close from
the ground as possible, the BHGT is considered as null
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RECONSTRUCTION OF MAGNITUDE AND FREQUENCY OF DEBRIS FLOWS IN THE REBAIXADER TORRENT
(EASTERN PYRENEES, SPAIN) BY DENDROGEOMORPHOLOGICAL ANALYSIS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
41
DU3. The comparison of minimum and exact ages for
these units suggests that Pinus Sylvestris needs at least
3 years to colonise a new debris-flow deposit.
The tree-ring record at Rebaixader fan spans the
last 70 years, during which seven units were deposited.
From these numbers, an average return period for de-
bris flows of about 9 years can be obtained. The area
of the deposits ranged from 600 to 6200 m
2
. However,
the frequency of debris flows seems to be higher for the
last 25 years (Table 1) with a return period of 5 years.
Figure 4 shows the magnitude-cumulative fre-
quency (MCF) relationship of debris flows at Rebaix-
ader fan. Landslide MCF relationships is usually fitted
by a right line (in logarithmic scales) (C
oRominas
&
m
oya
, 2008). The MCF curve fit to the Rebaixader
data is, however, strongly nonlinear, or more exactly
bilinear. Certainly, the points corresponding to the five
smaller depositional units can be fit with a line of a
gentler slope than the three bigger ones. Frequency
for theses smaller magnitudes is notoriously lower in
Rebaixader than in other site studied in the Pyrenees
(Tordó torrent). A decrease in the slope of the MCF
curve is also commonly reported for other sites and
for different types of landslides (C
oRominas
& m
oya
,
2008). This flattening of the curve, so-called rollover
effect, is usually considered as the consequence of an
under-recording of small events. In the Rebaixader
fan, deposits of some small-magnitude debris flows
can have been totally buried by younger events of
larger magnitude. In fact, low debris-flow frequency
leaving the boulders covered by moss (25-30%) and
white lichens. Organic matter still covers the ground,
and in places has grown over boulders.
The establishment of the different DU encoun-
tered on site has permitted to evaluate the minimum
extent of a series of debris flows occurred in Rebaix-
ader (Fig. 3 and Tab. 1).
.
AGE OF DEPOSITIONAL UNITS AND MAGNI-
TUDE FREQUENCY
Table 1 shows the dating results corresponding to
the depositional units. The reduced number of wounded
trees that has been sampled provided the exact age of
only two DU (DU2 and DU3). For the older units (DU4
to DU7) a minimum age was obtained. Minimum ages
from colonising trees were also obtained for DU2 and
Fig. 3. - Map of debris-flow depositional units at the Re-
baixader fan
Tab. 1 - Area and age of depositional units of Rebaixader
fan
Fig. 4 - Magnitude-Cumulative Frequency curve (loga-
rithmic scaled axes) for the Rebaixader creek. The
curve corresponding to another site studied in the
Eastern Pyrenees (Tordó Torrent) is also shown
(data from c
oromiNAS
& m
oyA
, 2010).
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G.G. CHEVALIER, J. MOYA , M. HÜRLIMANN & A. BATEMAN
42
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
constant ring width, when nothing proves it is.
• The colonisation time gap (CTG) of Pines in the
Pyrenees has not been investigated. 3 years was
chosen based on our field data. This value needs
further studies.
• DU4 has benefited from several dating tech-
niques used herein. Using branch whorls, a maximum
age of 21 years was found, and an average over 60
trees was calculated (13.5 years). Furthermore the big-
gest tree found on this DU was cored. A minimum age
of 13 years was determined. Coring the biggest tree
does not mean to date the oldest.
• Debris flows can remain undetected by the ap-
proach used here if they occur with a high frequency
(greate than on event per year)
This study of debris-flow frequency and magnitude
represents a step forward the understanding of Pyrenean
debris-flow hazard. Its assessment is actually expressed
in terms of susceptibility and frequency-magnitude re-
lationships. Therefore, coupled with the study of the
susceptibility of Pyrenean landscapes to debris flows
the work presented herein would fulfil the requirements
of a debrisflow hazard assessment in the Pyrenees.
Little information is available on the subject, at
both catchment and regional scales. More case stud-
ies are needed in order to better compare the current
dataset and provide accurate analysis and knowledge
to communities and stakeholders whose interests are
related to debris flows.
ACKNOWLEDGEMENTS
This research was financially supported by the
European programme IMPRINTS (IMproving Pre-
paredness and RIs maNagemenT for flash floods and
debriS flow events - EC FP7 project - contract ENV-
2008-1-226555) and by the Spanish Ministry of Sci-
ence and Innovation (DEBRIS-CATCH project, con-
tract CGL2008-00299/BTE).
obtained for the period older than the year 1983-84
(Tab. 1) can result from an overlapping of small-mag-
nitude debris flows. It should be noted that the two
older depositional units (DU6 and DU7) have also the
maximum extent. But such a “double trend”(Fig4) to
be confirmed by the study of more debris-flow events.
A shortcoming inherent to tree-ring dating can
also explain an under-recording of small debris flows.
Accuracy of tree-ring dating is usually of a year, al-
though obviously lower if only minimum ages are
obtained. This means that events occurring within a
same year cannot be differentiated, and that the as-
sessed depositional units may actually be formed by
several debris flows. Relative dating does not permit
to distinguish events occurred in a same year, unless
cross-cutting relationships were produced.
DISCUSSION AND CONCLUDING RE-
MARKS
It was decided to work out the frequency and the
magnitude of past debris flows having left a subse-
quent trace on Rebaixader’s fan. Within the last 70
years, at least 7 debris flows were great enough to
create new colonisable surfaces for vegetation. And in
the time study, several debris flows of small intensity
occurred (H
uRlimann
et alii, 2011).
When compared to another Pyrenean site, it ap-
pears that Rebaixader’s activity is lesser than expected
by the aspect of the fan. However this study suffers
limitations and estimates that are likely to bias the
analysis:
• The area making up the DU’s area is to be under-
stood as the DU’s area visible in 2009. No extrapola-
tion has been done, therefore clearly underestimating
this parameter.
• The trees ages require the pith to be hit when
coring. Because it is not always achieved, we have
used a geometric model to add the rings missing be-
tween the sample and the pith. This method implies a
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(EASTERN PYRENEES, SPAIN) BY DENDROGEOMORPHOLOGICAL ANALYSIS
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