The characterization of triggering dynamics and remobilized volumes is crucial to the assessment of associated lahar hazards. We propose an innovative treatment of the cascading effect between tephra fallout and lahar hazards based on probabilistic modelling that also accounts for a detailed description of source sediments. As an example, we have estimated the volumes of tephra fallout deposit that could be remobilized by rainfall-triggered lahars in association with two eruptive scenarios that have characterized the activity of the La Fossa cone (Vulcano, Italy) in the last 1000 years: a long-lasting Vulcanian cycle and a subplinian eruption. The spatial distribution and volume of deposits that could potentially trigger lahars were analysed based on a combination of tephra fallout probabilistic modelling (with TEPHRA2), slope-stability modelling (with TRIGRS), field observations, and geotechnical tests. Model input data were obtained from both geotechnical tests and field measurements (e.g. hydraulic conductivity, friction angle, cohesion, total unit weight of the soil, and saturated and residual water content). TRIGRS simulations show how shallow landsliding is an effective process for eroding pyroclastic deposits on Vulcano. Nonetheless, the remobilized volumes and the deposit thickness threshold for lahar initiation strongly depend on slope angle, rainfall intensity, grain size, friction angle, hydraulic conductivity, and the cohesion of the source deposit.
Lahars, an Indonesian term to indicate volcanic debris flows and
hyper-concentrated flows with various amounts of volcanic solid content, can
cause loss of life and damage to infrastructure and cultivated lands; they
represent one of the most devastating hazards for people living in volcanic
areas (Pierson et al., 1990, 1992; Janda et al., 1996; Scott et al., 1996,
2005; Lavigne et al., 2000; Witham, 2005; De Bélizal et al., 2013). The
most destructive lahars are caused by the breakout of crater lakes or volcano
dammed lakes (e.g. Mt. Kelud in Indonesia; Thouret et al., 1998) and by the
interaction of hot pyroclastic density currents (PDCs) with glacial ice and
snow at ice-capped volcanoes (e.g. Nevado del Ruiz in Colombia; Pierson et
al., 1990). However, the most common lahars are those generated by heavy
rainfall on tephra fallout and PDC deposits emplaced on volcano slopes
(e.g. Casita Volcano, Nicaragua; Scott et al., 2005; Panabaj,
Guatemala; Charbonnier et al., 2018). For example, torrential rainstorms on
loose pyroclastic deposits produced by the 1991 eruption of Pinatubo
(Philippines) have generated hundreds of secondary lahars for years after
the end of the eruption (e.g. Janda et al., 1996; Newhall and Punongbayan,
1996). Despite their relatively small volumes, over 6 years, these lahars
have remobilized 2.5 km
Many studies exist that make use of various analytical and numerical models
to describe the potential inundation area of lahars (e.g. Procter et al.,
2012; Córdoba et al., 2015; Caballero et al., 2016; Mead and Magill, 2017;
Charbonnier et al., 2018). The associated outcomes are fundamental to the
development of risk reduction strategies; nonetheless, all inundation models
require the determination of the volume of potentially remobilized material
that is often approximated due to the lack of information. In fact, the
identification of lahar source areas and lahar initiation mechanisms is
crucial to the evaluation of lahar recurrence and magnitude. With “lahar
source areas” we refer to areas with pyroclastic material that can be
remobilized to form lahars; these areas are normally located on steep slopes
(
In order to determine the volume of potentially remobilized material, Volentik et al. (2009) and Galderisi et al. (2013) have already combined lahar-triggering modelling with probabilistic assessment of tephra deposition based on a static hydrological model (Iverson, 2000) and assuming total saturation of the deposit. In addition, Tierz et al. (2017) have compiled a probabilistic lahar hazard assessment through the Bayesian belief network “Multihaz” based on a combination of probabilistic hazard assessment of both tephra fallout and PDCs with a dynamic physical model for lahar propagation. Even though these three examples were pioneering in assessing the effect of cascading hazards, the associated description of lahar triggering was overly simplified (i.e. fundamental aspects such as hydraulic conductivity and friction angle were not taken into account) and the soil characteristics as well as the intensity and duration of the rainfall were not considered. In our paper we build on these first studies to show the importance of the application of physically based models in combination with the characterization of pyroclastic material for the determination of deposit instability. Our goal is to accurately predict the volume of tephra fallout that could be remobilized by a rainfall-triggered shallow landslide in association with various eruptive conditions at the La Fossa cone in order to compile a rain-triggered lahar susceptibility map. To achieve this task, we combine the shallow landslide model TRIGRS (Baum et al., 2002) with both probabilistic modelling of tephra fallout (for eruptions of different duration and magnitude) and field and geotechnical characterizations of tephra fallout deposits (i.e. grain size, hydraulic conductivity, soil suction, deposit density).
First, we describe the physical characteristics (e.g. grain size, hydraulic conductivity, angle of friction) of selected tephra fallout deposits associated with both a long-lasting Vulcanian eruption (i.e. the 1888–1890 eruption) and a subplinian eruption (PAL-D eruption of the Palizzi sequence; Di Traglia et al., 2013). Second, we characterize the lahar deposits associated with the 1888–1890 eruption, which provide insights into lahar source areas, flow emplacement mechanisms, and inundation areas of future lahars. The physical characteristics of tephra fallout deposits are used in combination with a probabilistic modelling of tephra fallout (Biass et al., 2016a) to estimate the unstable areas based on the shallow landslide model TRIGRS (Baum et al., 2002). This approach provides the first integrated attempt to quantify the source volume of lahars as a function of probabilistic hazard assessment for tephra fallout (with TEPHRA2), numerical modelling of lahar triggering (with TRIGRS), field observations (including primary tephra fallout deposits, geology, geomorphology, and precipitation), and geotechnical tests of source deposits. Finally, we propose a new strategy to map syn-eruptive lahar susceptibility as a critical tephra fallout deposit thickness resulting in unstable conditions, which could represent a valuable tool for contingency plans. Here the term syn-eruptive is used in the sense of Sulpizio et al. (2006) to indicate lahars that originated during volcanic eruptions or shortly after, while post-eruptive lahar events are generated long (i.e. a few to several years) after an eruption. This is also in agreement with the current definition of primary (syn-eruptive) and secondary lahars (post-eruptive or unrelated to eruptions) provided by Vallance and Iverson (2015) and Gudmunsson (2015). To sum up, this study explores new strategies for volcanic multi-hazard assessment and offers an innovative treatment of the cascading effects between tephra fallout and lahar susceptibility.
Overview of the study area.
Number of observed eruptions for the different types of activity in the last 1000 years on Vulcano island. Values of frequencies and the detailed events for each type are also reported. In bold are the selected scenarios used in Sect. 3.3; based on data from Di Traglia et al. (2013) and De Astis et al. (1997, 2013).
The island of Vulcano, the southernmost island of the Aeolian archipelago,
consists of several volcanic edifices whose formation overlapped in time and
space beginning 120 kyr ago. The most recently active volcano is the La Fossa
cone, a 391 m high active composite cone that began to erupt 5.5 kyr ago
(Frazzetta et al., 1984) and whose erupted products vary in composition from
latitic to rhyolitic, with minor shoshonites (Keller, 1980; De Astis et al.,
1997; Gioncada et al., 2003). The stratigraphy of the La Fossa cone has been
described in detail in several studies (Keller, 1980; Frazzetta et al.,
1983; Di Traglia et al., 2013; De Astis et al., 2013). The last period of
eruptive activity (younger than 1 ka) has been recently divided into two
main eruptive clusters, further separated into eruptive units (Di Traglia et
al., 2013; Fig. 1). The activity of the Palizzi and Commenda eruptive
units (PEU and CEU, respectively) is grouped into a single eruptive period
(Palizzi–Commenda eruptive cluster, PCEC) lasting approximately 200 years
(11th to 13th century). The following Pietre Cotte cycle, with
the post-1739 CE and 1888–1890 CE activity form the Gran Cratere eruptive
cluster (GCEC, 1444–1890 CE; Di Traglia et al., 2013). The stratigraphic
sequence of PEU displays a large variety of eruptive products and a wide
spectrum of magma compositions, including cross-stratified and
parallel-bedded ash layers (e.g. PAL-A and PAL-C in Di Traglia et al.,
2013), pumiceous tephra fallout layers of rhyolitic (PAL-B) and trachytic
composition (PAL-D), several lava flows intercalated in the sequence (e.g.
the rhyolitic obsidian of Commenda and the trachytic lava flows of Palizzi,
Campo Sportivo, and Punte Nere), and, finally, several ash layers and widely
dispersed PDC deposits (CEU) associated with the hydrothermal eruption of
the Breccia di Commenda eruptive unit (Gurioli et al., 2012; Rosi et al., 2018),
which closes the PCEC. The GCEC includes the recent products of the Pietre
Cotte eruptive unit (ash and lapilli layers from Vulcanian activity),
rhyolitic pumiceous tephra fallout layers, and the rhyolitic 1739 CE Pietre Cotte
lava flow. The uppermost part of the GCEC is represented by the products of
the 1888–1890 CE eruption, consisting of latitic spatters, trachytic and
rhyolitic ash, lapilli layers, and the characteristic bread-crust bombs.
Historical chronicles (Mercalli and Silvestri, 1891; De Fiore, 1922),
archaeomagnetic data (Arrighi et al., 2006; Zanella et al., 1999; Lanza and
Zanella, 2003), and stratigraphic investigations (Di Traglia et al., 2013; De
Astis et al., 1997, 2013) concur in indicating that in the past 1000 years at
least 20 effusive and explosive eruptions have occurred. Among the explosive
eruptions, the Vulcanian cycles represent the most important events in terms
of recurrence (at least five long-lasting episodes corresponding to annual
frequencies of
Vulcano island has a typical semi-arid Mediterranean climate (De Martonne,
1926) with annual rainfall between 326 and 505 mm, falling mostly during
autumn and winter seasons (Fig. 2a). Based on Arnone et al. (2013),
rainfall trends in Sicily island can be classified in three intensity-based
categories: light precipitation (0.1–4 mm d
Both syn-eruptive and post-eruptive lahar events induced the progressive
erosion of the tephra deposits that cover the La Fossa cone. The
tephra fallout deposit associated with the most recent Vulcanian eruption
(1888–1890) has been almost completely removed from the upper slopes and
accumulated at the foot of the cone, where the stratigraphic sections show a
succession of lahar deposits with thicknesses between 0.1 and 1 m (Ferrucci
et al., 2005). The entire tephra sequence of the Gran Cratere eruptive cluster
(including the 1888–1890 Vulcanian eruption) lies on top of thinly stratified,
reddish, impermeable ash layers (varicoloured tuffs or “tufi varicolori”;
Frazzetta et al., 1983; Capaccioni and Coniglio, 1995; Dellino et al., 2011),
which concluded the Breccia di Commenda phase. In most parts of the cone,
the dark grey tephra of the Vulcanian cycles is almost completely eroded and
the tufi varicolori are exposed. A rill network developed on the
impermeable fine-grained tuffs that conveys water to a funnel-shaped area,
where a main gully defines a drainage basin (Ferrucci et al., 2005). The
main gullies start where the loose grey interdigitated tephra fallout and
lahar deposits crop out (Fig. 1). Lahar volumes and travel distance
strongly depend on both the availability of pyroclastic material in the
source area and on the characteristics of the rainfall events (intensity and
duration). Ferrucci et al. (2005) estimated volumes between 20 and 50 m
The field characterization of both tephra fallout primary deposits in the lahar initiation zones and lahar deposits was carried out during two field campaigns in 2017 and 2018. Eight undisturbed samples of tephra fallout primary deposit were collected for geotechnical tests from four outcrops on the La Fossa cone and Palizzi valley for the 1888–1890 and the PAL-D units (yellow circles in Fig. 1b and d). In particular, PAL-D tephra fallout primary deposit was sampled at locations V3 (one sample) and V4 (one sample) (Fig. 1b), while the 1888–1890 tephra fallout primary deposit was sampled at locations V1 (four samples) and V2 (two samples) (Fig. 1d). In addition, five samples for grain size analysis were collected vertically every 6 cm at two 1888–1890 tephra fallout primary deposit outcrops (yellow circles V1 and V2 in Fig. 1d). We consider the tephra fallout primary deposit of V1 and V2 as representative of the lahar initiation zone on the S and NW flanks of the La Fossa cone, respectively. In contrast, the lahar source area associated with the PAL-D primary deposit is either covered by new eruptive products or eroded. As a result, PAL-D had to be sampled at the base of the cone (V3 and V4 in Fig. 1b). Deposit sampling for geotechnical tests was performed by inserting a steel tube with a height of 30 cm and a diameter of 10 cm into the ground (see Appendix A, Fig. A1). A basal support was then inserted, and the tube was extracted from the deposit with minimum disturbance of the internal stratigraphy. The tube was then covered on both ends to preserve the deposit for further laboratory analysis (see Sect. 3.2). Due to the sampling apparatus, most geotechnical tests could only be carried out on the top 30 cm of each deposit location. As a comparison, in V1 the 30 cm tube was inserted after having eliminated the top 30 cm of deposit to analyse the central part of the outcrop. Given the characteristics of the deposit, we consider 30 cm of sampling to be representative for the main characteristics of both the 1888–1890 (which is thinly laminated across the entire section) and the PAL-D (which is mostly massive) deposits. Soil suction measurements were carried out in situ on the 1888–1890 tephra fallout deposit with a soil moisture probe (“Quick Draw” model 2900FI) (Fig. 1 and Appendix A). The saturated hydraulic conductivity was estimated in the field with a single-ring permeameter on PAL-D primary deposits (V3 and I3 in Fig. 1) and on 1888–1890 primary deposits (I1 and I2, near the V2 location in Fig. 1) (see Appendix A, Fig. A3). The field description and sampling of syn-eruptive lahar deposits associated with the 1888–1890 eruption were performed on the NW volcano flanks, in the Palizzi valley, and in the Porto Plain (red squares in Fig. 1; V5 to V12). At total of 11 samples of lahar matrices were sampled on the S cone flank (V5), on the NW cone flank (V8–V12), in the Palizzi valley (V6), and in the Porto Plain (four samples in V7).
Grain size analyses were carried out at the University of Geneva for three
tephra fallout sections (11 samples) and for 11 lahar deposit matrix samples
(fractions between
Natural water content and shear strength were measured on undisturbed
samples at the University of Salerno. The natural water content (
The saturated hydraulic conductivity
The saturated soil diffusivity
In order to best describe the cascading effect between tephra deposition and
lahar triggering susceptibility in the context of multi-hazard assessments,
the tephra fallout deposits considered in our analysis are those
probabilistically modelled by Biass et al. (2016a). Based on the stratigraphy
of the last 1000 years of La Fossa (Di Traglia et al., 2013), Biass et al. (2016a) defined three eruption scenarios for tephra fallout including the following: (i) a
long-lasting Vulcanian eruption scenario (plume heights: 1–10 km a.s.l.;
total mass: 1.9–
Based on the physical characteristics of the tephra fallout deposits (high permeability) and on the high intensity of rainfall events, we assume that the most probable rain-triggered lahar initiation mechanism on the La Fossa cone is shallow landsliding. Shallow landsliding is produced by an increase in water pore pressure due to rainfall infiltration on tephra deposits, which causes a slope failure. Several slope-stability models have been used to predict lahar initiation processes as shallow landslides in volcanic areas (e.g. Cascini et al., 2010; Frattini et al., 2004; Crosta and Dal Negro, 2003; Sorbino et al., 2007, 2010; Cascini et al., 2011; Cuomo and Iervolino, 2016; Cuomo and Della Sala, 2016; Mead et al., 2016, Baumann et al., 2018). Among those, the Fortran programme TRIGRS (Baum et al., 2002) can be used for computing transient pore pressure and the related changes in the factor of safety due to rainfall infiltration. Here, TRIGRS is used to investigate the timing and location of shallow landslides in response to rainfall in large areas (e.g. Baumann et al., 2018). Baum et al. (2002) extended the method of Iverson (2000) by implementing the solutions for a complex time sequence of rainfall intensity, an impermeable basal boundary at infinite depth, and an optional unsaturated zone above the water table. TRIGRS is applicable for unsaturated initial conditions, with a two-layer system consisting of a saturated zone with a capillary fringe above the water table overlain by an unsaturated zone that extends to the ground surface. The unsaturated zone acts like a filter that smooths and delays the surface infiltration signal at depth. The model uses the soil water characteristic curve for wetting of the unsaturated soil proposed by Gardner (1958) and approximates the infiltration process as a one-dimensional vertical flow (Srivastava and Yeh, 1991; Savage et al., 2004). The reader is referred to the vast literature published on the application of this model for more details (e.g. Baum et al., 2002, 2008; Savage et al., 2003; Salciarini et al., 2006; Cuomo and Iervolino, 2016). Briefly, the infiltration models in TRIGRS for wet initial conditions are based on Iverson's (2000) linearized solution of the Richards equation and its extension to that solution (Baum et al., 2002; Savage et al., 2003, 2004). The solution is valid only where the transient infiltration is vertically downward and the transient lateral flow is relatively small.
Following Iverson (2000), slope stability is calculated using an
infinite slope-stability analysis. Incipient failure of infinite slopes is
described by an equation that balances the downslope component of
gravitational driving stress against the resisting stress due to basal
Coulomb soil friction and the influence of groundwater (Iverson, 2000). The
factor of safety (FS) is calculated at a depth
TRIGRS for unsaturated initial conditions was applied on the
probabilistic isopach maps described in Sect. 3.3 for the long-lasting
Vulcanian, the subplinian VEI 2, and the subplinian VEI 3 eruption scenarios.
For each eruption scenario, probabilistic isopach maps are computed for
probabilities of occurrence of 25 % and 75 % (Biass et al., 2016a). The
eruption associated with the subplinian scenario is considered to be short
lived (
We logged two sections of the PAL-D tephra fallout deposit at outcrops
located in the Palizzi valley (point V3 and V4; Fig. 1b). The isopach map of
Di Traglia (2011) shows the associated southward dispersal (Fig. 1b). The
PAL-D section at V3 is a 1 m thick, massive, grain-supported, and well-sorted
pumice deposit between two subunits of the Palizzi cycle (Fig. 3a): PAL-C
deposit at the base (alternation of black lapilli and ash) and the rhyolitic
white ash of the Rocche Rosse eruption from Lipari and the Breccia di Commenda
at the top (Di Traglia et al., 2013; Rosi et al., 2018). The mean saturated
hydraulic conductivity of the PAL-D deposit measured in the field at V3
(Fig. 1b) is
Two stratigraphic sections of the 1888–1890 tephra fallout deposit were logged
in the upper part of the La Fossa cone S flank (V1 in Fig. 1d; Fig. 3c) and
at the base of the NW flank (V2 in Fig. 1d; Fig. 3d). The isopach map for
the 1888–1890 primary tephra fallout deposit shows an almost circular dispersal
(Fig. 1d). The V1 section overlies several older tephra fallout and lahar
units. It is a 1 m thick deposit consisting of an alternation of thin ash
and lapilli layers overtopped by 0.2 m of reworked tephra. The whole
sequence shows an inclination of 30
Stratigraphic sections in gullies and small channels on the NW flank of the
La Fossa cone show several massive to laminated, remobilized deposits
covering the 1888–1890 primary tephra fallout deposit. Unfortunately, no map
exists that describes the distribution of lahars on Vulcano, mostly due to
the difficulty of correlating the exposed deposits across the different
gullies. The thickness of each lahar layer, representing different flow
pulses, varies between
The Md
Md
The natural water content (
Geotechnical parameters for the subplinian tephra fallout deposit (PAL-D) and the Vulcanian tephra fallout deposit associated with the 1888–1890 eruption (Vulc).
Although the four specimens of the 1888–1890 primary tephra fallout deposit are
consolidated at three different total stresses, all specimens exhibit a
slight hardening associated with a dilative behaviour. The shear stress
envelope exhibits high friction angle (
The mean saturated hydraulic conductivity of the 1888–1890 tephra fallout
deposit measured in the laboratory (
The SWRC of the 1888–1890 primary tephra fallout deposit exhibits an air entry
value equal to 1 kPa, while the water content at saturation (
The saturated soil diffusivity of the 1888–1890 and PAL-D tephra fallout
deposits are 1 order of magnitude higher than their saturated soil
conductivity (Table 2). The 1888–1890 tephra fallout deposit shows a value of
Based on the local weather pattern (Sect. 2.2), TRIGRS simulations were
run using one high-intensity rainfall scenario of 6.4 mm h
Slope map for the La Fossa cone and surrounding areas. Northwestern and southern lahar source areas are indicated with a black contour. The NW and S upper catchments are indicated with a black contour. SCA: summit cone area; VPP: Vulcano Porto Plain.
Probabilistic isopach maps (converted from the
probabilistic isomass maps of Biass et al., 2016a, based on deposit density)
and corresponding instability maps compiled with TRIGRS for a Vulcanian
eruption with the following:
Input parameters for the subplinian (
Unstable areas (FS
TRIGRS simulations assume the following: (i) a water table is located at the bottom of
the tephra fallout sequence (lower boundary); and (ii) the tephra fallout
sequence lies on an impermeable layer. These assumptions are supported by
the exposure of the consolidated and impermeable tufi varicolori unit on the
upper part of the La Fossa cone (Frazzetta et al., 1983; Capaccioni and
Coniglio, 1995; Dellino et al., 2011). Although lahars have been initiated
all around the La Fossa cone in the past, here we analyse the slope and the
stability condition of the tephra deposits in two selected potential lahar
source areas (Fig. 6, black lines). The first NW source area represents a
direct threat to the populated Porto village, while the second S source area
is downwind of the prevailing wind and presents the highest probabilities of
tephra accumulation (Sect. 1.2; Biass et al., 2016a). The percentage of
slope angle ranges (Fig. 6) for the NW and S lahars source area are the following:
18 % and 22 % for a slope angle between 6
Figure 7 shows the probabilistic isopach maps (for a probability of
occurrence of 25 %) and the instability maps for eruption durations of 6,
12, 18, and 24 months. For an eruption duration of 6 months, only 4.8 %
percent of the NW and 6 % of the S lahar source areas, respectively, are
unstable due to the small tephra fallout deposit thickness (between 6 and 12 cm) (Fig. 7a). For an eruption duration of 12 months, the unstable areas are
significantly higher: 69 % for the NW and 81 % for the S lahar source
areas, respectively (tephra fallout deposit thickness between 14 and 22 cm;
Fig. 7b and Table 4). For an eruption duration of 18 months the percentage
of unstable areas for the NW is also very high (89 %) and reached a value
of 66 % for the S area (Fig. 7c). The percentage of unstable area
decreases for an eruption duration of 24 months, with 52 % for the NW and
22 % for the S; the tephra fallout deposit accumulation is more than
35 cm in the case of the S source area (Fig. 7d, Table 4). Figure 8 shows the
unstable volumes as a function of the eruption durations described above
calculated for the two single upper catchments with the same area (4665 m
Unstable tephra fallout volume for the S and NW upper catchments obtained with TRIGRS for eruption durations of 3, 6, 9, 12, 18, and 24 and for probabilities of occurrence of 25 % and 75 %. UC: upper catchment (see Fig. 6).
The same two lahar source areas were used for investigating the instability
of the subplinian deposits. Four probabilistic isopach maps were considered
(VEI 2 and VEI 3 with a 25 % and 75 % probability of occurrence) and
combined with hydraulic conductivities measured in the field and
derived from the literature (i.e.
Unstable tephra fallout volume for the S and NW upper
catchments obtained with TRIGRS for the subplinian scenarios VEI 2 and
VEI 3 with
Total and unstable volumes of primary tephra deposits for
long-lasting Vulcanian and subplinian eruptions (VEI:
In order to characterize the minimum tephra fallout deposit thickness necessary to trigger lahars on Vulcano during or just after either a Vulcanian cycle or a subplinian eruption, we carried out TRIGRS simulations using the characteristics of the 1888–1890 and PAL-D tephra fallout deposits, with increasing deposit thicknesses from 0.1 to 1.1 m and an interval of 0.05 m. In these simulations, the deposit thickness was considered constant over the whole NW and SE source areas. The same rainfall scenario was applied. The percentage of unstable areas for the NW and S source areas (Fig. 10) shows that the tephra fallout deposit thickness generating the largest instability for the 1888–1890 eruption tephra fallout deposit is between 20 and 30 cm. For PAL-D tephra fallout deposits using the lowest hydraulic conductivity, the unstable percentage area decreases rapidly with an increase in deposit thickness, with virtually the entire deposit being stable beyond a 45 cm thickness. In contrast, when the PAL-D tephra fallout deposit is simulated with high hydraulic conductivity, almost all of the lahar source area is unstable (98 %) for deposit thickness between 10 and 65 cm, after which the fraction of unstable area decreases with increasing deposit thickness.
Summary description of outcomes of Figs. 11 and 12 showing the
relation between tephra fallout deposit thickness and the factor of safety (FS)
based on various key parameters (i.e. tephra fallout properties, slope
angle, rainfall intensities). Unstable deposit thickness is shown in bold.
The ratio between rainfall intensity and hydraulic conductivity (
In order to investigate the combination of multiple parameters (FS, deposit
thickness, pore pressure, slope, rainfall intensity), we have carried out
dedicated simulations for a smaller area (100 pixels only on the NW source
area) (Fig. 11 for the 1888–1890 eruption and Fig. 12 for PAL-D). Three slope
angles (38, 35.4, and 30.1
Median grain size, hydraulic conductivity, and infiltration capacity on tephra fallout (TF) and PDC deposits measured near volcanic vents and the lahar volumes of selected examples.
Md
Percentage of unstable area for the NW and S lahar
source areas simulated with TRIGRS for tephra fallout deposit thicknesses of 0.1–1.1 m and a rainfall intensity of 6.4 mm h
Rain-triggered lahars associated with both tephra fallout and PDC deposits are associated with a variety of precipitation, grain size, hydraulic conductivity, and infiltration capacity (Table 7). It is important to note that infiltration capacity and hydraulic conductivity can be considered similar parameters for the sake of this comparison. In fact, the infiltration capacity is a measure of the rate at which soil is able to absorb water (Horton, 1945), while the hydraulic conductivity measures the ease with which water will pass through a porous medium (Darcy, 1856). Infiltration capacity typically decreases through time and converges to a constant value, which is the hydraulic conductivity. Infiltration capacity is more easily measured in the field, while hydraulic conductivity is more easily measured in the laboratory. Examples of lahar generation enhanced by fine-grained deposits include Mount St. Helens in 1980 (Leavesley et al., 1989), Chaitén in 2008 (Pierson et al., 2013), and Cordón Caulle in 2011 (Pistolesi et al., 2015) (Table 7). In contrast, the grain size of the Vulcano 1888–1890 tephra fallout deposit is closer to Mt. Unzen PDCs and the Pinatubo tephra fallout deposit. Hydraulic conductivity associated with the Vulcano 1888–1890 tephra fallout deposit is 44 times higher than the infiltration capacity of the PDC in Shultz Creek (Mount St. Helens). Infiltration capacity is also low in the case of Mt. Unzen but is higher for the PDCs of Mt. Pinatubo. If we also compare the lahar volumes of Mount St. Helens in 1980, Pinatubo in 1990, Chaitén in 2008, and Vulcano in 1888–1890, we observe that the volumes are in the range of millions of cubic metres for the first three volcanoes, while in the case of Vulcano the larger events were only in the range of thousands of cubic metres. An important difference between the deposits studied in this paper and the other deposits is the climatic conditions (Table 7). In fact, Vulcano is characterized by semi-arid, poorly vegetated regions with nonpermanent streams and limited annual rainfall (500 mm), while all other cases are characterized by forested area with permanent streams draining the volcano flanks and annual precipitation between 1000 mm and 4300 mm.
Total pressure head and factor of safety versus
tephra fallout deposit thicknesses between 0.1 and 0.55 m for Vulcanian
tephra fallout deposits (Table 3) and a rainfall intensity of the following:
Lahar triggering is clearly influenced by the hydraulic conductivity and
infiltration capacity of the primary deposits, which in turn are strongly
related to deposit grain size. The highest hydraulic conductivities
(
The effect of grain size on runoff has also been investigated based on
laboratory experiments. As an example, Jones et al. (2017) investigated
the behaviour of two tephra fallout samples with contrasting grain size (a
fine-grained sample from the Chaitén 2008 eruption (
Total pressure head and factor of safety versus
tephra fallout deposit thicknesses between 0.1 and 0.55 m for subplinian
tephra fallout deposits with
The duration of a long-lasting eruption plays an important role in the
pattern of remobilization of tephra fallout deposits. Different unstable
volumes calculated with TRIGRS were obtained for durations of Vulcanian
eruptive cycles between 3 and 24 months without considering remobilization
between the eruptive cycles. The results show that for an eruption time
of 18 months and a probability of occurrence of 25 % (corresponding to a
tephra fallout deposit thickness between 17 and 33 cm) the unstable areas,
and therefore the remobilized volumes from the lahar source areas, reached
a maximum (1105 m
The morphology of the middle and the upper part of the La Fossa cone shows a strong remobilization of the 1888–1890 eruption tephra fallout deposit. The coarse ash grain size range and medium permeability of the 1888–1890 tephra fallout deposits in combination with the impermeable deposits at the base of the sequence (i.e. tufi varicolori) make this deposit easily remobilized by rainfall through a shallow landslide initiation mechanism. Deep channels due to the continuous remobilization of this deposit can be observed on the cone (Fig. 1). Because of the short transport distance (200–400 m) the lahar deposits on the La Fossa cone have almost the same grain size as the 1888–1890 tephra fallout deposits (Fig. 5). The same relation between the primary pyroclastic deposits and the lahars has been described for the la Cuesta succession (Valentine et al., 1998).
Field evidence for post-PAL-D remobilization and lahar deposits is not
recorded in the stratigraphic record (Di Traglia, 2011). This is consistent
with our modelling results with
The tephra remobilization model used in our study assumes rainfall-induced
shallow landslides caused by the infiltration of rain on the slope surface.
These shallow landslides can eventually transform into lahars depending on
the availability of water, slope morphology, and characteristics of tephra
deposits. In particular, we studied cases in which rainfall intensity
(
The relationship between unstable areas and deposit thicknesses suggests a
significant influence of the hydraulic conductivity on the model outcomes
and on the resulting estimation of unstable volumes (Fig. 7 and Tables 4 and 5). Our results better explain the parameter values affecting slope
instability (Fig. 10 and Tables 4 and 5). In fact, the tephra fallout
deposit thickness of 21–33 cm, associated with the largest unstable volumes
for the Vulcanian scenarios (18- and 24-month durations), correlates well
with the thickness of 20–30 cm shown in Fig. 10. Similarly, the
tephra fallout deposit thickness associated with the largest unstable
volumes for the VEI 2 and 3 scenarios (with
For the 1888–1890 tephra fallout deposit, results suggest that cohesion leads
to a critical minimum landslide depth size (lower-limit deposit thickness
for instability) dependent on the slope angle (Fig. 11b, d; Table 6). Using
a model for natural slopes, Milledge et al. (2014) found a critical depth in
cohesive soil, resulting in a minimum size for failure. For cohesionless
material, such as the primary PAL-D, the lower thickness limit is not
defined as most small deposit thicknesses are unstable and become progressively
stable with a deposit thickness increase depending on rainfall intensity,
duration, and slope angle (Fig. 12b, d; Table 6). The different behaviour
shown by the different tephra fallout deposits modelled in our study could
relate to the fact that rainfall-induced slope failure can occur by two
mechanisms (Li et al., 2013): (1) rainfall infiltration that produces a rise
in groundwater, generating positive pore pressure and adding weight on the
slope (Cho and Lee, 2002; Crosta and Frattini, 2003; Soddu et al., 2003); and (2) rainfall that results in the propagation of a wetting front, causing an increase
in water content and pore pressure (loss in matric suction) (Ng et al.,
2001; Collins and Znidarcic, 2004; Rahardjo et al., 2007). First, in the
case of subplinian tephra fallout deposit with
The results obtained with the TRIGRS model show the potential for the evaluation of transient pore water pressure stability conditions and lahar (landslide) source areas during rainfall (Godt et al., 2008), even though the role of suction in unsaturated conditions, which plays a fundamental role for the pore pressure regime, is not included in the model (Sorbino et al., 2010). Matrix suction between 24 and 27 kPa was measured in 1888–1890 primary tephra deposits in May 2018 (at the beginning of the dry season), but further seasonal matrix suction variation needs to be performed to evaluate the role of suction in potentially unstable areas and the most critical period for slope stability (Pirone et al., 2016). Finally, our deposit-stability analysis could be largely strengthened by validation with the volume of observed lahar deposits, which is unfortunately difficult to obtain for the 1888–1890 eruption due to complex deposit correlation.
Remobilized tephra fallout volume was calculated with TRIGRS for two
different catchments with same area, one located on the NW flank and the
other on S flank of La Fossa volcano; different values were obtained for
the same eruption scenarios (Figs. 8 and 9). Two main factors are
responsible for these differences in volume. The first factor is that the
tephra deposit is thicker for the S flank due to the prevailing wind
direction to the SE, and therefore it inhibits the formation of lahars as
it requires more water to be remobilized (which is not frequently available
in the Vulcano area). In fact, there is a thickness threshold for
instability depending on rainfall intensity and tephra fallout deposit
properties (Figs. 11 and 12). An additional factor influencing the deposit
stability is the slope morphology. Steep slopes (
We presented a detailed analysis of the volume of tephra fallout deposit that could be potentially remobilized by rainfall as a result of two likely eruptive scenarios of the La Fossa volcano, the main volcanic system on Vulcano island: a long-lasting Vulcanian eruption (i.e. using the 1888–1890 eruption as the reference event) and a short-lived eruption (VEI 2 and VEI 3; using the PAL-D eruption as the reference event) (Fig. 1 and Table 1). The great novelty of this work is the assessment of compounding hazards (tephra fallout deposits and lahar triggering) based on both numerical modelling and field and geotechnical characterization of the source deposit. In fact, volumes of tephra fallout deposit that could be remobilized by rain-triggered lahars were analysed by combining a tephra sedimentation model (TEPHRA2) and slope-stability model (TRIGRS) along with field observations and geotechnical tests.
We have considered 12 probabilistic isopach maps with different eruption
durations and probabilities of occurrence of 25 % and 75 % in the case of
the Vulcanian eruptive scenario. We have also considered four probabilistic
isopach maps for two short-lived eruptions of VEI 2 and 3 and the same
probabilities of occurrence as in the case of the Vulcanian eruptive
scenario. In addition, a parametric analysis was performed with TRIGRS to
determine the tephra fallout thickness thresholds required to trigger lahars
for a given rainfall event. Two basins of same area were identified on the
NW and S flank of the volcano to analyse the effect of different morphology
and different accumulation related to the prevailing wind direction. The
results of unstable volumes for the two basins show the following:
for the Vulcanian scenario, the largest unstable volume is reached for an eruption
duration of 18 months and a 25 % probability of occurrence scenario, with a volume
of 1105 m for the subplinian scenario, the largest unstable volume (2455 m for the subplinian scenario with
For a tephra fallout deposit with features associated with a Vulcanian
eruption we observe an unstable window of deposit thickness, suggesting that
particle cohesion leads to a critical minimum landslide depth, which is
dependent on the slope angle; an increase in rainfall intensity enlarges the
windows of thickness instability (Table 6). In contrast, for cohesionless
material such as the primary PAL-D, a low thickness limit of instability is
not reached, and the deposit becomes stable with thickness increase
depending on rainfall intensity and slope angle (Table 6). In particular,
the parametric analysis with variable tephra fallout thickness and slope, with
two rainfall intensities of 6.4 mm h
for a tephra fallout deposit with features associated with a Vulcanian eruption, the thickness generating the largest instability is between 20 and 27 cm for a rainfall intensity of 6.4 mm h for a tephra fallout deposit with features associated with a subplinian eruption with for a tephra fallout deposit with features associated with a subplinian eruption with
The results modelled with TRIGRS show that shallow landsliding is an
effective process for eroding both Vulcanian-type and subplinian-type (with
Most of the relevant data are made available in the main tables and the Supplement. Additional data are available upon request based on a collaborative agreement.
Undisturbed tephra fallout deposit is sampled for testing the properties in the laboratory without disturbing structure texture, density, natural water content, and stress condition (Figs. A1a and b). Sampling was performed by inserting a steel tube 3 mm thick with a height of 30 cm and a diameter of 10 cm into the ground (Fig. A1a). After that, we cleaned the entire deposit around the tube to extract it with minimum disturbance. Then, a support was inserted at the base of the cylinder, and the tube was extracted from the deposit. Finally, the tube was covered on both ends with a plastic cover and plastic tape to preserve the deposit during transport (Fig. A1b).
Soil suction measurements were carried out in situ on the 1888–1890 tephra fallout deposit with a soil moisture probe (“Quick Draw” model 2900FI) (Fig. A2b). The first step in taking a reading with the probe is to core a hole by pushing the coring tool into the soil (Fig. A2a). After removing the coring tool, we have a proper sized hole to insert the probe. The second step is to insert the probe in the soil and wait approximately 1 min (equalization time assessed for such soils). Finally, the suction can be read on the dial gauge (Fig. A2b). The soil suction is created by water capillary pressure that each soil particle applies to the soil. The moisture probe has a porous ceramic sensing tip at the end of the tube. The soil suction reading is obtained when a small amount of water transfers between the sensing tip of the probe and the soil.
The saturated hydraulic conductivity was estimated in the field with a single-ring permeameter for both deposits (Figs. A3a and b). The apparatus for the measurements consists of a steel ring with a diameter of 21 cm and height of 12 cm, as well as a plastic cover with a hole to insert a Mariotte bottle (Fig. A3b). In the field, we put the ring on a horizontal plane surface on the tephra fallout deposit. Then, the first 6 cm was pushed into the ground. Finally, we filled the Mariotte bottle with water and inserted it on the tape turned upside down. The water first formed a 1 cm layer on the tephra fallout deposit and then started to infiltrate the deposit. For the infiltration rate measurements, the readings were done every minute in the case of the 1888–1890 eruption deposit and every 30 s in the case of the PAL-D deposit. The duration of the measurements was 40 min for the 1888–1890 deposit and 3.2 min for the PAL-D deposit.
The supplement related to this article is available online at:
VB, CB, and SC conceived the study. VB, CB, SB, MP, and AG were involved in the fieldwork. VB carried out sample characterization both in the field and in the laboratory. SB carried out the tephra fallout modelling. SC and MM carried out the geotechnical tests in the laboratory. VB, SC, and MM carried out the slope-stability modelling. VB analysed the results and compiled the figures with input from the other authors. VB and CB prepared the paper with contributions from all co-authors. All the authors read, reviewed, and approved all versions of the paper.
The authors declare that they have no conflict of interest.
The authors are grateful to Irene Manzella, Michel Jaboyedoff, and Mario Sartori for valuable discussion and thank the NHESS editor, Jenni Barclay, and an anonymous reviewer for detailed and constructive comments.
This research has been supported by the Swiss National Science Foundation (grant no. 200021_163152) and the project Progetti di Ricerca di Ateneo (grant no. PRA 2018-19).
This paper was edited by Giovanni Macedonio and reviewed by Jenni Barclay and one anonymous referee.