Debris flow generation in volcanic zones in the southern Andes has
not been widely studied, despite the enormous economic and infrastructure damage
that these events can generate. The present work contributes to the
understanding of these dynamics based on a study of the 2017 Petrohué
debris flow event from two complementary points of view. First, a
comprehensive field survey allowed us to determine that a rockfall initiated
the debris flow due to an intense rainfall event. The rockfall lithology
corresponds to lava blocks and autobrecciated lavas, predominantly over 1500 m a.s.l. Second, the process was numerically modelled and constrained by in
situ data collection and geomorphological mapping. The event was studied by
back analysis using the height of flow measured on Route CH-255 with errors
of 5 %. Debris flow volume has a high sensitivity with the initial water
content in the block fall zone, ranging from
Landslide processes are among the most important natural hazards in developing countries due to their low resilience, generating damage to human life, property and engineering projects in all the mountainous areas of the world every year (Martha et al., 2010; Alimohammadlou et al., 2013; Sepúlveda et al., 2014; Fustos et al., 2020). Debris flows are an important type of mass wasting, described as one of the most dangerous of these processes due to their high velocity, the damage that they cause and the extensive areas affected (Jakob et al., 2005). Nevertheless, debris flows studies in volcanic zones are limited, where only primary volcanically originated processes like lahars have been studied. The present work evaluates the generation of debris flows, taking the 2017 Petrohué event as a case study. This event caused severe economic losses to one of the most popular tourist attractions in southern Chile (INE, 2018).
Debris flows are very destructive processes in active zones in the Andes,
especially in volcanic areas independent of their trigger (Sosio et al.,
2011). The northern Andes show examples like the 1985 Nevados del Ruiz
eruption. The volcanic activity triggered a lahar flow, which claimed at
least 25 000 lives (Naranjo et al., 1986). In December 1999, a mud and
debris flow in Venezuela caused the loss of 30 000 lives (Wieczorek et al.,
2000). Rock and soil movements, debris avalanches, debris, mudflows, and
the resulting floods destroyed about 40 km of the trans-Ecuadorian oil
pipeline and the only highway from Quito to Ecuador's north-eastern rainforests and oil fields. This phenomenon was caused by heavy rain and two
earthquakes in 1987 (Schuster et al., 1996). In 2017, a rainfall-induced
landslide event with more than 600 shallow landslides was triggered in
Colombia. Following the intense rainfall, landslides and the subsequent Mocoa
debris flow (MDF) event killed up to 333 people (García-Delgado et al.,
2019). Moreover, the central Andes have experienced massive debris flow
events like the ones in the Lastarria volcano (Rodríguez et al., 2020).
The collapse of part of the edifice triggered a 270 km h
In the southern Andes, volcanic edifices are covered by materials that could produce recurrent debris flows, moulding the relief. The debris flows reported currently in the literature on volcanoes are mainly the result of snow and ice melting during eruptions in the form of lahar (Johnson and Palma, 2015; Major et al., 2016; Thouret et al., 2020). Nevertheless, few works address the relation between the debris flows generated by stratigraphic conditions in volcanic systems and the morphology of the edifice. Their spatial and temporal extension has also been little studied; these are very important since the number of debris flows in volcanic systems has increased in recent years (Pierson, 1995; Aguilar et al., 2014; Korup et al., 2019; Thouret et al., 2020).
The present work seeks to advance our comprehension of the generation of debris flows in volcanic edifices in the southern Andes. An atypical debris flow event of 8 January 2017 (southern summer) on Osorno volcano (southern Andes) was assessed. The geomorphological and geological factors influencing the generation of the debris flow event were studied, estimating the conditions which triggered the event through back analysis using the r.avaflow model. Finally, we discuss whether the event might be recurrent in time or whether it is part of the normal cycle of the volcano; this knowledge will assist in assessing the risk of debris flows in the southern Andes.
Osorno is a stratovolcano that is part of the active volcanic arc of the
southern Andes, called the Southern Volcanic Zone (SVZ). The SVZ is a
1400 km long continuous volcanic arc, extending from
33.3
The area has a total population of 44 578 people with a density of 11 people per square kilometre. The topography makes Route CH-255 the only connection between Petrohué and Ensenada, and regular aerotransport is impossible except for a few flights by helicopters, unavailable for the local population. Therefore, Route CH-255 becomes a critical infrastructure for local development. This has led to the route being called “El Solitario Pass” (Lonely Pass) Finally, note that the village of Petrohué has a population of 193 people, surpassing 3000 in summer. The village does not have the capacity for autonomous subsistence, depending on the food and services of Ensenada.
Osorno volcano is a mainly basaltic Pleistocene to Holocene composite volcano (Fig. 1). Its part of an SW–NE volcanic alignment along with three other volcanoes: La Picada, Puntiagudo and Cordón Cenizos (Moreno et al., 2010), oriented obliquely to the main volcanic arc and the Liquiñe–Ofqui Fault System (LOFS). The chain orientation suggests that the area is an active transtensional zone of the crust, with mafic magma extrusion during the Quaternary (Cembrano and Lara, 2009; Moreno et al., 2010). The surrounding area shows pre-LGM (Last Glacial Maximum) Pleistocene volcanic rocks including tuffs, breccias and lava originating from the northern zone of the volcano (Moreno et al., 1985).
Alluvial fans in Osorno volcano are composed of unselected sandy rich matrix polymictic gravels, organised in metres-thick banks. These form the current filling of the gullies on Osorno volcano, together with alluvial deposits generated by the re-working of moraine deposits and old lahar fans. Debris flows have also been recorded, triggered by snowmelt and intense rainfall in the zone (Moreno et al., 2010). Alluvial deposits exist that are associated with debris flows and rockfalls in the zone. The granulometry ranges from sand to gravel, and the deposits extend to the shore of Lake Todos los Santos (Garrido, 2015).
The zone has suffered recurrent debris flow events during periods of
variable precipitation, so the factors which triggered these events are
strictly unknown (Garrido et al., 2017, 2018). For example,
debris flows and mudflows occurred on 2 June 2015, associated with a
front of intense and prolonged precipitation over CH-255 (El Solitario
Pass). These flows damaged five houses and four barns as well as destroying
one water tank and some pipework of the second water tank of the drinking
water supply network of the village of Petrohué (Garrido, 2015). On
8 January 2017, large debris flows occurred again in the eastern sector
of the volcano's southern flank, during a front of intense precipitation. A total of 94 mm
of precipitation in a period of 24 h was recorded in Ensenada, with the
0
Geological map of Osorno volcano based on Moreno et al. (2010).
The 2017 Petrohué event was studied to understand the factors which control the occurrence of debris flows in the Osorno volcano. We implement a methodological approach based on comprehensive numerical modelling constrained by field data and laboratory analysis (Fig. 2). We considered geomorphological factors that influenced debris flow generation in fieldwork, and we defined the release zones. The field results were used as border conditions in numerical modelling of the event by back analysis, taking flow heights recorded in technical reports and photographs to define zones of comparison.
Short methodology.
The mechanisms which generated the 2017 Petrohué debris flow event were
studied in the area, approaching by El Solitario Pass
(41.1943
Due to the extensiveness of the area, elevation and channel gradient data are derived from the two different digital elevation models (DEMs), SRTM and ASTER. Additional information regarding distance measurements such as side slopes, channel depth and the maximum width of the landslide was evaluated in the field using metric rulers. Specifically, the width of the landslide is identified and georeferenced using a handle GPS to constrain the numerical modelling results. On Route CH-255, the final height of debris flows is established as 1.5 m by Garrido et al. (2017). Moreover, debris flow deposits identified in the field allowed the understanding of the rheology of these events (non-Newtonian flows). We evaluated debris flow initiation zones close to the volcano summit and the physical weathering of rock and soil. Scarps with potential rockfalls of unstable blocks were identified, measured and georeferenced. We established these scarps as initiation debris flow zones (or release zones) in the following numerical model. Finally, debris flow runout was estimated by measuring the channel distance between the liberation zone and the CH-255 limit using navigation GPS.
Geomechanical properties of the mobilised material were characterised using
geotechnical testing. Three unaltered geotechnical soil samples were
extracted for a direct shear test (Fig. 1). The
samples were extracted from a depth of 40 cm (41.1843
Properties obtained by direct shear test. Geotechnical results incorporated into the r.avaflow model as constraints.
Representing debris flows is currently a challenge due to the possible changes in phase, which may occur during the process from generation to stabilisation. Therefore, the r.avaflow model was used to evaluate the fall of blocks in saturated and unsaturated zones and their subsequent evolution as non-Newtonian flow (Mergili et al., 2017, 2018b). Measurements of soil water content into the fall of blocks are not available. Therefore, various water content scenarios were carried out.
The r.avaflow model was applied by back analysis, taking as constraints the
cohesion data and the angle of internal friction obtained from the
undisturbed soil samples collected in the field
(Fig. 1). The back analysis considered a final
height of 1.5 m on Route CH255, according to reports of the National Geological
and Mining Survey (SERNAGEOMIN in Spanish). Moreover, volumes of the liberation
zone were integrated from the evidence collected in the field. The model
used a two-phase parameterisation based on Pudasaini (2012). First, the solid
phase corresponds to the lava and autobreccia fall; meanwhile, the second
phase is associated with the debris flow generation under
saturated and non-saturated water conditions. During the solid phase, a
Mohr–Coulomb plasticity approach allowed it to be estimated for the stress.
The fluid stress was modelled as a solid-volume-fraction-gradient-enhanced
non-Newtonian viscous stress (Pudasaini, 2012; Mergili et al., 2017). Let
Surface features such as slopes are important input data for debris flow
modelling (Qin et al., 2013). DEM errors introduce uncertainty in terrain
representation, leading to a poor estimation of the numerical solutions.
Given the uncertainty in the DEM before the 2017 Petrohué event, two
DEMs were used as references to assess the sensitivity of changes in
elevation. The SRTM and ASTER models were used separately, with a
spatial resolution of 30 m due to data availability limitations. The
possible release zones or areas of origin of these debris flows were
established from the geomorphological evidence found in the field. The
results of the back analysis were compared with photographs provided by the
Chilean Geology and Mining Survey (SERNAGEOMIN) based on El Solitario
(
Model parameters used in r.avaflow.
A systematic study has been carried out to represent the debris flow. Mergili et al. (2018a) established that parameters with high sensitivity correspond to the basal friction angle, fluid friction coefficient and environmental drag coefficient. We used reference values previously considered in the zone (Somos-Valenzuela et al., 2020). Moreover, geotechnical laboratory tests allowed us to represent the friction angle and cohesion values adequately. Sources of uncertainty were attributed to surface representation and initial water content in the head of the block fall. A wide range of initial water content was considered, constrained by the geomorphological evidence at the site, to calibrate the model to observations in the field. Since the initial proportion of water when the hyper-concentrated flow was generated is unknown, we assumed a water content in the initial volume from 40 % up to 70 %, considering the high porosity of the material involved. Hence, we calibrate the flow runout taking control points of the height of the flow measured on the main road minutes after the event. The percentual error was calculated using the height simulated with the measured height in El Solitario Pass. A percentual difference was used between the simulated value and the measured height divided by the measured height. We also assessed the quality of the simulations using possible release volumes based on field evidence.
Finally, possible scenarios evaluating the impact of new debris flows in the area were analysed. Therein, we defined new unstable release zones identified visually in the field. We identify areas with intense rain erosion, hanging blocks or fractured rocks. This enabled us to estimate the potential volume transported and thus to understand the impact of different debris flows generated in zones that were very close together but with release at different altitudes.
The conditions that generate debris flows were evaluated in an active volcanic zone, with reference to the 2017 Petrohué event. Information collected in the field was assessed and compared with numerical modelling using back analysis with r.avaflow.
The debris flow in the distal zone is characterised by poorly sorted
volcanic material. The deposited material is supported by a medium-coarse
sand matrix (2 mm) along with
Field evidence showed that debris flows are generated by the fracture of basaltic lava over volcanic deposits in the high-altitude zone of the study area. Rockfalls occurring above 1500 m a.s.l. were identified (Figs. 1 and 4). This zone presents numerous scarps with pronounced slopes overhanging fluvial drain channels (Fig. 3). The remains of the debris flow identified in the field are associated with transported blocks of basaltic lava and primary lahar deposits.
The incision is favoured by the presence of very thick lahar deposits (Fig. 3), which facilitate the removal and contribution of material to the main channel. A sequence of lahar deposits was observed, overlain by lava flows in blocks up to 1.5 m thick. These occur in regular sequences, leaving alternate levels of erosion and hanging blocks, facilitating the collapse of the lava levels, and generating rockfalls. The material is characterised by lava with base autobrecciation no more than 1 m thick (Fig. 3). The autobrecciated zone is also heavily weathered (Fig. 4); as a result, it can be easily removed, exposing the centre of the lava flow. The lava flow forms a hanging block that can easily fracture and break off. There is evidence of broken-off blocks associated with the central part of the lava flow due to the instability of the base autobrecciation, with dimensions of up to 2 m. The blocks fracture continuously in the lava runs perpendicular to the slope of the main channel where these debris flows occur. This breaking-off of material due to weathering contributes to the main channel, generating powerful debris flows as evidenced by the deposits further down the slope as a consequence of the continuous rockfalls. The intermediate zone has narrow drainage channels and an increase in the incision to a depth of around 15 m (Fig. 3).
A 15 m high scarp showing the stratigraphy of the volcano at this point, composed of alternating volcanic and lava deposits.
Set of scarps at approximately 1500 m a.s.l.; the base breccia of the lava flows is heavily eroded.
To understand the scope of the runout generated in the release zones of
material recognised in the field, the flow was modelled in r.avaflow.
Different initial water content into the simulations and DEM differences
allowed the understanding of the uncertainty in the main initial input. The results of
the back analysis, restricted by geotechnical soil data, showed that the
model that presented the smallest mean error was the simulation using the
SRTM DEM and 70 % water content (5 % error). The results show that in both
simulations, the flow covers a large part of Route CH-255, to a 1.59 m
depth with the SRTM DEM and 1.58 m depth with the ASTER DEM
(Fig. 5). Our results indicate that the ASTER DEM
presented a larger underestimation of the height of
The area affected by the debris flow was estimated at between 7.5 and
Back analysis results.
Simulation in the El Solitario sector, taking the release zone from the
results obtained in the field.
In addition to the zone identified as the source of the Petrohué event
of 2017, three zones with unstable autobrecciated lava were catalogued as
possible debris flow generation zones (Figs. 1 and
4). The information collected in the field
showed that the gullies in these zones are severely weakened, so debris
flows produced by rockfalls can be expected imminently. Additional release
zones to those in r.avaflow were estimated with calibration based on the
parameters of the back analysis model. Our results indicate that the area
potentially affected by the debris flow varies between 3.1 and
According to the back analysis, a rockfall with 50 % water content is
capable of transporting a potential volume of 138 628 m
Impact of different debris flows.
Finally, our results indicate the existence of events that will not generate debris flows even if there is a fall of lava blocks in the channel. This can be seen in zone 1, identified by the evidence collected in the field as an area in which debris flows are generated. On the other hand, a hypothetical scenario of a debris flow generated in zone 2 could lead to debris flows with larger volumes than those observed in previous events (Fig. 7). Likewise, it can generate greater flow heights than values recorded to date, leading to more catastrophic events in populated zones. This risk has not been considered to date and needs to be assessed with care.
The geomorphology of the Osorno volcano is characterised by the alternation of basaltic lava flows overlain by large volcanic deposits (Fig. 3). The fractures identified in the lava flows were probably generated by gravitational effects. Water can then enter the rock and soil fractures, making them more likely to break off and transporting the material in the form of debris flow. Conditions are therefore favourable for chain processes culminating in debris flows when the soil is saturated by high-altitude rainfall. Erosion of the deposits exposes the lava flows; material breaks away and is transported by gravity and/or swept down by the force of the water. Furthermore, the autobrecciation of the lava flows increases the instability of the rock faces in the release zones due to the high porosity of the material (Vezzoli et al., 2017; Schaefer et al., 2018).
The evidence collected in the field showed a heterogeneous distribution of lava and slopes close to the debris flow release zones. In this way, the magnitude and force of the landslide processes may be affected by the spatial distribution of volcanic products, principally lava flows. The results show that above 600 m a.s.l., there are many exposed lava layers at the base of the principal fluvial channels. A bedrock channel could increase the velocity of the flow in comparison to an alluvial-type channel. This characteristic suggests that the base could act as a sliding surface for the material (Dufresne et al., 2019), which is very common in stratovolcanoes in the southern Andes. It could be a smooth surface with lower friction, especially under rainy conditions. This could have a critical influence on the velocity and acceleration of the flow from the higher reaches of the edifice. Lavigne and Suwa (2004), Sheridan et al. (2005), and Aaron and Hungr (2016) suggest that the dynamic of debris flows depends upon the friction of the base surface. Having said this, the distribution of the lava and its autobrecciation play an important role in the generation of landslides, rockfalls and debris flows in the study zone (Fig. 8).
Our results represented the dynamic of the 2017 Petrohué debris flow
with variable errors in the back analysis. Our models were consistent with
results obtained in the field, showing a strong influence on the initial
water content. The simulations present high sensitivity to the water content
previous to the generation of the event; all the simulations in which the
water content in the release zone was higher than 45 % reached populated
zones. The calibration parameters played an important role in the
sensitivity of the numerical model. In the present study, the drag
coefficient was established at 0.020, based on Zwinger et al. (2003) and
Oyarzún (2019); this value was adopted due to the degree of optimisation
in the back analysis. A larger or smaller coefficient could produce large
deviations in the final height and direction of the flow (Mergili et al.,
2018a). The angle of internal friction was determined by a geotechnical
study. However, the heterogeneity of the zone could produce substantial
changes in
The results show that the scope of the debris flow is proportional to the initial water content. Thus the water content available during the collapse of material at the 1500 m contour allows events to occur which will reach populated zones. Our results show that debris flows are dangerous if the collapse happens with saturation over 50 %. We propose that the presence of water in the release zone is explained by local hydrometeorological conditions, i.e. rainfall at high altitude; this is consistent with similar events at Villa Santa Lucía (Garrido et al., 2018; Somos-Valenzuela et al., 2020).
The differences between the initial and final volumes suggest the incorporation of material into the debris flow due to the erosion, which causes movement of the flow (Fig. 9). This is supported by evidence in the field, which showed that movable material is available between the distal and proximal zones (Fig. 8b). Retreating scarps were observed, which continuously add material to the drainage networks, and this material is available when debris flows occur. Our field results indicate that the largest volume of the material comes from volcanic deposits (Fig. 8a).
Conceptual model of chain processes in Osorno volcano.
Finally, water-rich mass flows are distinguished by material type, water
content, the presence of excess pore pressure or liquefaction at the source
(Calhoun and Clague, 2018). We controlled possible sources of uncertainty
such as lithology and topography through a terrain analysis that allowed
us to reduce degrees of freedom. Therefore, the initial water content becomes an
important independent variable due to the unknown value during the main
event. The gap of soil moisture stations close to the liberation zone does
not allow us to constrain the numerical solution with precise detail. Our
results show debris flow generation over 50 % of initial water content.
The final volume variation in the debris flow varies between 4.8 and
Our projections indicate that the danger to populated areas is strongly dependent on the release zone of debris flows (Fig. 7). This suggests that debris flows may repeatedly occur, which are not observed because they are remote from populated areas, increasing the structural destabilisation of the volcano in the long term. The release volumes calculated in the present study were defined according to the current stability conditions observed in the field; however, more intense precipitation could lead to more significant rainfall erosion of transportable material (Fig. 8b), favouring increasingly violent debris flows. Such scenarios require the appropriate authorities to propose road design projects as a matter of urgency to evacuate the flows quickly and efficiently. This will improve the mitigation efforts to prevent the population from becoming cut off from the rest of the country. Finally, the geomorphology of the Osorno volcano is not unique in the southern Andes. For example, Villarrica and Llaima volcanoes show similar conditions to Osorno. We thus have firm grounds for assuming that unexpected debris flows could occur elsewhere in southern Chile, with the difference that these volcanoes are in more densely populated zones, exposing the population to even more danger.
The 2017 Petrohué event was studied by back analysis to understand the impact of debris flows occurring on active volcanoes in the southern Andes. We used comprehensive in situ data to constrain a numerical model for understanding the debris flow event. We evaluate the liberation zone for Osorno volcano, showing that debris flows occur due to the collapse of autobrecciated lava flows above 1500 m a.s.l. for the first time (Fig. 9a). Geomorphological in situ data determined that the debris flow was a combination of factors such as fluvial erosion and the composition of the volcano, both of which together led to a loss of stability (Fig. 9b). The succession of volcanic deposits and autobrecciated lava flows generates conditions for the development of debris flows. Geomorphological evidence has shown that block falls and slips occurring mainly above 1500 m a.s.l. become debris flows, increasing the volume transported to the base of the volcano (Fig. 9c).
Simulations in r.avaflow showed that the debris flow volume varies between
464 564 and 544 903 m
Finally, our results report for the first time a case of debris flow on a stratovolcano in the southern Andes from its generation, using data collection in situ and modelling with sensitivity analysis using a two-phase model. Our conclusion is to urge the scientific community to focus efforts on generating scenarios for debris flows in the stratovolcanoes of the southern Andes. The population density around Osorno volcano is low but receives a high number of tourists every year. However, the areas of stratovolcanoes like Villarrica and Calbuco contain higher population densities, and these volcanoes must not be ignored in future territorial plans. The present study shows evidence that the debris flows identified are recurrent events, even though they do not always reach populated areas. Our results show that this threat is inherent to volcanic activity, so any future risk analysis must consider debris flows. This will allow better risk management in nearby population centres and a safer coexistence with the volcanic structures of the southern Andes.
The code is available at
The datasets used in this study are available in the paper. The ALOS-PALSAR DEM is publicly accessible at
The supplement related to this article is available online at:
IJFT and BMV contributed to the conceptualisation and methodology of the research and performed the formal analysis, visualisation and validation. IJFT and MSV were involved in the funding and supervision of the paper. IJFT and PMY contributed with the supervision, review and editing of the paper. RMR, IRA and NC provided input in terms of methodology and the review and editing of the paper.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This was made possible thanks to the “Agencia Nacional de Investigación y Desarrollo (ANID)” of the Chilean government (grant no. PII180008) and the “Fondecyt Iniciación” programme (grant no. 11180500). We acknowledge the Dirección de Investigación of the University of La Frontera for their English editing support.We acknowledgements to the reviewers for their exhaustive review and constructive comments that allow improving this study significantly.
We appreciate the support of Mauricio Hermosilla, Gonzalo Maragaño and Elizabet Lizama for their support in fieldwork and geotechnical analysis.
This research has been supported by ANID (grant nos. PII180008 and Fondecyt 11180500).
This paper was edited by Giovanni Macedonio and reviewed by Roberto J. Marin and one anonymous referee.