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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
645
DOI: 10.4408/IJEGE.2011-03.B-070
LANDSLIDES DAMS INDUCED BY DEBRIS FLOWS
IN QUEBRADA DEL TORO (PROVINCE OF SALTA, ARGENTINA)
C. CENCETTI
(*)
& f.R. RIVELLI
(**)
(*)
Università degli Studi di Perugia, Dipartimento di Ingegneria Civile e Ambientale - Via Goffredo Duranti 93 - 06125 Perugia, Italia
(**)
Escuela de Geologia, Universidad Nacional de Salta - Bolivia 5150, 4400 Salta, Argentina
ing the ‘80s to protect the road (two meters in height
over the river bed), now are completely buried due
to the floods produced by the debris flows from the
Quebrada del Candado. A solution to the problems of
the Río Toro in this reach, related to the processes of
debris flows, could consist both in the construction of
works intercepting the solid transport, and above all,
defense works from the erosion of the slopes, to pre-
vent the source of mobilization of the detrital material
within the catchment of the Río Candado.
K
ey
words
: Argentina, debris flows, risk
INTRODUCTION
The Quebrada del Toro is the main valley of Río
Toro, a stream located southwest of Salta City, in
northwestern Argentina (Fig. 1). The Quebrada is a
tectonic, narrow and deep valley, and it is one of the
most characteristic areas of the morphostructural re-
gion named Cordillera Oriental (Argentine Andes). In
this area of the chain the processes due to gravity have
a decisive role in the shaping and evolution of the re-
lief and, even more, of the hydrographical system that
is the object of the present study.
In fact the Quebrada del Toro is covered, for all
its length, by the homonymous stream (Río Toro),
an important river carrying its water to the artificial
basin of Cabra Corral, having received the supply of
several minor tributaries. Among these lateral que-
bradas
, an important contribution is represented by
ABSTRACT
The Quebrada del Toro is the valley of Río Toro,
located southwest of Salta City, in northwestern Ar-
gentina. During the summer months, in the catchment
of its tributaries, debris flows occur with relative fre-
quency. Such processes of mass transport bring im-
portant volumes of materials inside of the main collec-
tor (Río Toro) and its alluvial plain, and they produce
the narrowing, or sometimes the occlusion (landslide
dam) of the main riverbed.
These processes frequently occur, for example, in
the Río Candado, a tributary on the left of the Río Toro.
The consequence is an important sediment supply, com-
ing from such a tributary, which has produced a marked
modification of the longitudinal profile of the Río Toro,
due to the systematic aggradation of the reach situated
close to the confluence between the two streams.
A direct consequence is affecting of the bridge
of “Ruta Nacional 51”, situated in the immediate vi-
cinities of the confluence, where often in the period
of summer rains (when the water discharge increases)
the overflowing of Río Toro occurs, with floods that
produce the interruption of this important road. In
fact, the aggradation of the river bed due to the ac-
cumulation of detrital material (that usually cannot be
eroded by low water discharge), when the discharge in
the Río Toro is increasing, during the summer period,
produces easily the overflow of the stream, flooding
the lower Ruta Nacional 51.
The available data show that the defense built dur-
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C. CENCETTI & F.R. RIVELLI
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
processes, it is moderately dry, with important ther-
mal range between day and night that produce deg-
radational physical processes in the rocks (thermal
stress weathering).
CHARACTERISTICS OF THE RELIEF
In order to consider the debris flow processes oc-
curring inside the catchment of Río Candado, we need
to linger over the characteristics of the relief.
We are in a mountainous environment with pres-
ence of inner, narrow valleys, bounded by generally
not much steep slopes.
In the specific case, the catchment of Río Can-
dado measures 11 km
2
in area, so that it represents the
most important subcatchment of Río Toro, consider-
ing the extension. Moreover, this tributary is the only
one, among those supplying the main stream, which
have a water discharge during all the year, so that it
assures a continuous flow in the Río Toro (this is an
important aspect regarding to the generally dry condi-
tions of the whole area).
The extension of the basin and its water availabil-
ity represent very important elements from a geomor-
phologic point of view, in particular regarding to the
occurring of gravitational mass movements.
In the catchment of Candado, the minor alti-
tude of the divide located in the eastern portion of
the same, allows the humid masses of air proceed-
ing from East to discharge a larger volume of water
as rainfalls, with respect to the others subcatch-
ments located around. All this explains the pres-
ence of water during all the year in the riverbed of
Candado.
Río Candado, not only for its water discharge, but
also for its solid transport due to debris flow pro-
cesses originated inside its catchment that frequently
reach the confluence with Río Toro and produce
landslide dams in the alluvial plain of the main
stream (C
enCetti
& R
ivelli
, 2008).
The aim of the paper is to highlight the impor-
tance of debris flows produced along the Río Can-
dado, as an important factor of control in the fluvial
dynamics of Río Toro, due to formation of landslide
dams at their confluence.
GENERAL SETTING
STUDY AREA
The confluence between Río Candado and Río
Toro is easily reachable during the year from Salta
City, through the Ruta Nacional 51 (Fig. 1) and is lo-
cated at an height of about 1950 m above sea level.
The catchment of the Río Candado is located in an
arid region from a climatic point of view, with average
yearly rainfalls included between 200 and 250 mm,
and concentrated in few events, with torrential rains
which occur almost exclusively during the summer
months. Moreover, important snowfalls don’t occur:
they are very unusual and not much significant.
The lithological characteristics of the basin are
very homogeneous, because the only outcropping
bedrock is constituted by low grade metamorphic
rocks of Precambric age, diffusely fractured and
weathered with, at the top, recent sediments of fluvial
or gravitational origin.
With regard to climate, which plays a decisive
role in the development of the present morphogenetic
Fig. 1 -Location of study area
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LANDSLIDES DAMS INDUCED BY DEBRIS FLOWS IN QUEBRADA DEL TORO (PROVINCE OF SALTA, ARGENTINA)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
647
of the stream.
The debris flows, at present time, are the most im-
portant morphogenetic process, both for the shaping
of relief, and for the secondary effects produced out of
the limits where they were generated.
Considering the abundance of detrital material
accumulated at the borders and the bottom of all lat-
eral valleys of Candado, the trigger of debris flows is
very easier, above all when torrential rainfalls occur,
responsible of a quick saturation and destabilization of
the loose material.
On the field, you can see superficial evidences
(lobate forms) of recent phenomena; on the cuts of
ancient outcropping sediments in Candado, you can
deduce that the flows occur from a long time, con-
sidering also the volume of accumulated material and
remobilized by new events (Figs. 2-3).
The presence of lobate forms at the present raised
with respect to the riverbed, due to the intense and fast
tectonic activity, points out a constant movement of the
GEOMORPHOLOGICAL CHARACTERISTICS
The evolution of relief allows clearly to identify
the processes responsible for the shapes that charac-
terize itself.
Above all tectonic activity was and it is the main
responsibility of the conformation at ”juxtaposed
blocks” which can be observed in the region and con-
stitutes the main structure of the relief; moreover, the
exogenous processes have effect on these, incessantly
changing their shapes.
With regard to the geomorphological processes,
we need to distinguish the present processes, typical of
dry environment, from the past ones, occurred during
a colder and more humid climatic period, revealed by
the abundance of detrital material covering the bedrock.
During this more humid climatic period, indeed,
the larger volume of rainfalls supported fluvial activ-
ity and sheet floods more intense and continuous, al-
lowing the sedimentation of wide blankets of debris
constituted by not consolidated material, collected
inside the different hollows of the catchment.
Such material is, at the present time, the main
source of sediments which are mobilized in the trig-
gering and routing phases of debris flows.
With respect to degradational meteoric processes,
we have already discuss that the thermal stress weather-
ing is the dominant process; it produces a considerable
volume of loose material that can be affected by mass
movement developing in debris flows. Many times, in-
deed, the mass movements (prevalently slides) obstruct
the channel of Río Candado, producing small tempo-
rary dams that accumulate water until the break occur,
mobilizing the debris and triggering the debris flows.
They reach the confluence with Río Toro, often produc-
ing the occlusion of the main stream.
The specific fluvial process in the small catch-
ment of Río Candado has not great significance, due
to the reduced volume of rainfalls that doesn’t allow,
considered the consequent low water discharge, either
erosional activity, or solid transport of fluvial origin,
important in order to shape the relief.
The most important effect of the fluvial (even if
slight) erosional processes, appears in the destabiliza-
tion of terraced fluvial sediments that are undermined
at the base; it favours the action of the flows when
these ones move towards the lower sections of the ba-
sin, because they can, at their turn, incorporate a larger
volume of material produced by the erosional activity
Fig. 3 - A detail of the sediments produced by debris flows
in the catchment of Río Candado
Fig. 2 - Ancient sediments of debris flows outcropping
along the riverbed of Río Candado
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
cubic meters of material; this one is placed on the riv-
erbed of the main stream, producing temporary lakes.
By considering the classification proposed by
Harvey (H
aRvey
, 2002), both the accumulated mate-
rial on the original cone and inactive at present time
(coinciding with a more humid period than the present,
characterizing this area during the past time), and the
present sediments, correspond to the so named aggra-
dational fans
(composite deposition) by the Author.
CONSEQUENCES OF THE FLOwS OF CAN-
DADO
The debris flows produced in the watershed of
Candado, except for the loss of areas used by agri-
culture (even if any real information exists about the
agricultural surface eliminated by the debris flows),
don’t produce enormous problems or inconveniences;
not so the ones reaching the riverbed of Río Toro.
In the area of confluence between the two streams,
Río Toro has a riverbed measuring about 200 metres in
width. The debris flows coming from Río Candado are
constituted by cobbles having mean diameter between
15 and 30 cm, with a matrix that has a variable grain
size distribution (from minute gravel until clay). The
debris flows build an alluvial fan with marked slopes
next to the apex of cone (20%), gradually lower pro-
ceedings towards the front which enlarges in the riv-
erbed of Río Toro for about 200 metres in lenght. The
main body of the cone modifies noticeably the gradient
of the same Río Toro, due to the accumulated material
in the riverbed. So the gradient is reduced upstream
(5‰), due to the aggradational process occurring when
the obstacle striving for obstructing the water flow;
after, the gradient progressively increases next to the
main body of the cone (10‰), until to reach, down-
stream, the stable value of about 15-16‰ (Fig. 6).
Referring to the gradients assumed by Río Toro,
we can understand that the cone built by debris flows
coming from Río Candado is strongly asymmetric: in-
deed, with respect to its central longitudinal axis, there
is a larger volume of material downstream, along Río
Toro, that is where the debris flows are channelled
(see sketch in Fig. 6).
The total blockage of the riverbed of Río Toro,
often produced when intense debris flows occur and
reach the right rocky bank of the riverbed, usually has
short life (not more of about ten days). After this pe-
riod, the overflowing of Río Toro occurs.
flows from a long time ago, even if, unfortunately, spe-
cific studies are not in order to date these sediments and
so allow a greater accuracy in the study of the evolution
of the processes and of the resulting sediments.
This consideration is corroborated by the observa-
tion of the cuts in the terraced sediments located at the
hollows in the minor valleys of Candado, where it is
possible observe levels with blocks of rocks about 1 m
in diameter, irregularly arranged, combined with ma-
terial having smaller dimensions, alternated with thin
layers set in order, of fluvial facies, with a stratified
subhorizontal structure. Due to the energy of debris
flows along the Quebrada del Candado, they can en-
train a lot of material from the riverbed or from the
terraced sediments, already eroded by fluvial current.
So, flows develop reaching the alluvial plain of Río
Toro (Figg. 4-5), often producing the occlusion of the
riverbed by means of landslide dam (this phenomenon
is not infrequent along this river, also during ancient
periods - t
RautH
& s
tReCkeR
, 1999; v
ieRa
& C
enCetti
,
2008) and building fans that accumulate until 240,000
Figs. 4-5 - The debris flows occurring along Río Candado
reach the riverbed of Río Toro, often producing its
temporary occlusion
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LANDSLIDES DAMS INDUCED BY DEBRIS FLOWS IN QUEBRADA DEL TORO (PROVINCE OF SALTA, ARGENTINA)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
649
on left bank, is at the present time at the same eleva-
tion of the riverbed.
In order to avoid interruptions of the road during
summer months, due to the flooding of Río Toro, a
longitudinal defence work was built in 1980, about
400 meters in lenght (Fig. 8), raised with respect to
riverbed of about 2 meters.
This one, due to the accumulation of sediment,
showed reaches completely covered by material, so
that in 1995 a new level about 1 meter in height was
built. At the present time, in some parts, it needs a new
increase in height in order to provide effective protec-
tion of the road.
Due to the systematic accumulation of mate-
rial coming from Candado, the road is still in a very
critical situation which needs intervention, even if
complementary to the execution of the longitudinal
defense work.
Considering the characteristics of the catchment
But another important consequence of the tem-
porary occlusions produced in the riverbed of Río
Toro occurs. We already have discussed about the ac-
cumulating of material upstream the landslide dam:
this phenomenon, particularly during the rainy period,
increases the already difficult problem of the flooding
affecting the nearby bridge (Fig. 7).
This fact, indeed, forces to carry out periodically
the cleaning of the riverbed, in order to regain the flow
cross-section under the bridge. It causes a high cost of
maintenance of the road, that has vital and strategic
importance, both for the region, and the access road to
the harbours of Pacific.
The consequences of the occlusions occurring in
Río Toro affect also the reaches located downstream.
Indeed, when the dam break occurs, the volume of
water suddenly flowing produces the mobilization of
a lot of material that is distributed along a reach 4 kms
in length, until the confluence with another danger-
ous creek, Río Capillas, increasing the already critical
problem of the flooding of this reach (Fig. 6).
Due to material accumulating in this reach of Río
Toro, the sedimentation is more rapid and this fact
negatively affects the road and others present infra-
structures. The sedimentation produces the aggrada-
tion of the riverbed, affecting the cross-section under
the bridge. The gap between the riverbed and the
lower part of the bridge (about 4 meters) has reduced
to 2 meters after the supply of Candado, so that the re-
moval of material has needed, with high cost because
the same needs to be moved elsewhere, due to the im-
possibility to accumulate it nearby.
Due to the aggradation of the riverbed of Río
Toro, the stretch of Ruta Nacional 51, which is located
Fig. 6 - Sketch of the reach of Río Toro affected by the
debris flows coming from Río Candado. 1) aggra-
dation of the reach upstream the confluence, due
to the landslide dam; 2) aggradation of the reach
downstream the confluence, due to the redistribu-
tion of material mobilized by the debris flows com-
ing from Río Candado
Fig. 7 - The bridge located upstream the confluence of
Río Candado with Río Toro, frequently affected by
floods during the rainy periods
Fig. 8 - The longitudinal defense work built on the left
bank of Río Toro, downstream the confluence with
Río Candado. It needs frequently to be raised, due
to the aggradation of the riverbed
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C. CENCETTI & F.R. RIVELLI
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
of Candado and the influence of the landslide dams
in the behaviour of Río Toro (that turns into the de-
scribed problems affecting Ruta 51), interventions
need to avoid the trigger of debris flows so that the
development of landslide dams can be averted.
The most suitable solution, due to the cost of execu-
tion, convenience, gettable results and low environmen-
tal impact, seems to be the building of transverse works
devoted to detain as well as possible the sediments and
to reduce the slope and the stream power, in order to pre-
venting the transport and the consequent accumulation
of material due to the occurring debris flows.
All this could be accompanied by the building of
works for the control of erosional processes along the
slopes (mattresses, fences), suitable for prevent the
mobilization of material along the slopes during the
the raining periods.
CONCLUSIONS
Debris flows occurring in the Quebrada del Can-
dado cause serious damages over all along the final
reach, where they produce landslide dams on the con-
fluence with Río Toro, and various problems in dif-
ferent portions of Ruta 51 which needs high costs of
permanent maintenance.
The risk mitigation and the reduction of the damag-
es cannot be solved by means of building of passive de-
fenses, but they need to provide the building of works
devoted to reduce the possibility of triggering the flows.
The intervention could be extended at the basin
scale and it could affect the upper portion of the catch-
ment, by means of building of transverse works and
works for the control of erosional processes along the
slopes. By reducing the gradient and retaining the ma-
terial, this could mitigate the effects of flooding which
systematically occurs in the lower part, close to the
confluence with Río Toro.
ACKNOWLEDGEMENTS
This research was produced within the agree-
ment, active since 2002, between Nacional University
of Salta (Argentina) and University of Perugia (Italy),
regarding the study of gravel-bed rivers in different
geographic and morphoclimatic conditions.
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enCetti
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ivelli
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aRvey
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