World Landslide Forum Physically-based models for estimating rainfall triggering debris flows in Campania (southern Italy) --Manuscript Draft-Manuscript Number:
WLFO-D-16-00177R2
Full Title:
Physically-based models for estimating rainfall triggering debris flows in Campania (southern Italy)
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***Vol. 4 – Diversity of Landslide Forms***
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Vol. 4 - Session 2 - Rainfall-Induced Landslides
Funding Information:
Università degli Studi di Napoli Federico II Dr Elisabetta Napolitano (TEMASAV Post-doc Research Program) Università degli Studi di Napoli Federico II Dr Rita Tufano (MSc thesis) Università degli Studi di Napoli Federico II Dr Francesco Fusco (PhD Program (29th cycle) of the Department of Earth, Resources and Environmental Sciences)
Abstract:
The societal risk related to rainfall-triggered rapid debris flows is commonly managed in urbanized areas by means of early warning systems based on monitoring of hydrological parameters (such as rainfall or soil moisture) and analysis of thresholds values. In this paper are exposed results of physically-based modelling of ash-fall pyroclastic soil coverings involved in debris flows along mountain slopes nearby the Somma-Vesuvius volcano (Campania, southern Italy), which represent one of the major geohazards of Italy. The methods adopted combine deterministic approaches at the source area and distributed scales to estimate Intensity-Duration rainfall thresholds triggering debris flows. The first approach is based on the reconstruction of detailed physical models of ash-fall pyroclastic soil coverings in representative source areas of debris flows and on the related hydrological and slope stability modelling. The second is focused on a regional distribution model of ash-fall pyroclastic soils over mountain slopes surrounding the Somma-Vesuvius volcano, which takes into account both total thicknesses of pyroclastic coverings and variable stratigraphic settings. For both, effects of different initial antecedent hydrological conditions, associated with summer and winter, were considered.
Corresponding Author:
Pantaleone De Vita, Ph.D Universita degli Studi di Napoli Federico II Naples, ITALY
Corresponding Author Secondary Information: Corresponding Author's Institution:
Universita degli Studi di Napoli Federico II
Corresponding Author's Secondary Institution: First Author:
Pantaleone De Vita, Ph.D
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Pantaleone De Vita, Ph.D Francesco Fusco Elisabetta Napolitano Rita Tufano
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Physically-based models for estimating rainfall triggering debris flows in Campania (southern Italy) Pantaleone De Vita, Francesco Fusco, Elisabetta Napolitano and Rita Tufano Abstract The societal risk related to rainfall-triggered rapid debris flows is commonly managed in urbanized areas by means of early warning systems based on monitoring of hydrological parameters (such as rainfall or soil moisture) and analysis of thresholds values. In this paper are exposed results of physically-based modelling of ash-fall pyroclastic soil coverings involved in debris flows along mountain slopes nearby the Somma-Vesuvius volcano (Campania, southern Italy), which represent one of the major geohazards of Italy. The methods adopted combine deterministic approaches at the source area and distributed scales to estimate Intensity-Duration rainfall thresholds triggering debris flows. The first approach is based on the reconstruction of detailed physical models of ash-fall pyroclastic soil coverings in representative source areas of debris flows and on the related hydrological and slope stability modelling. The second is focused on a regional distribution model of ash-fall pyroclastic soils over mountain slopes surrounding the Somma-Vesuvius volcano, which takes into account both total thicknesses of pyroclastic coverings and variable stratigraphic settings. For both, effects of different initial antecedent hydrological conditions, associated with summer and winter, were considered. Keywords Ash-fall pyroclastic soils, debris flows, hillslope hydrological processes, deterministic rainfall thresholds. Introduction The societal risk related to rainfall-triggered rapid debris flows is commonly managed in urbanized areas by means of early warning systems based on monitoring of hydrological parameters (such as rainfall or soil moisture) and analysis of thresholds values (Guzzetti et al., 2007). The latter represent the principal issue due to the frequent lack of data about landslide occurrences and/or unreliability of hydrological records that usually affect empirical approaches (Caine, 1980; Crozier and Eyles, 1980). To overcome this problem, the hydrological and stability modelling of surficial deposits involved in debris flows is a promising way to define hydrological thresholds (Godt et al., 2008; Peres and Cancelliere, 2014). In this paper are exposed results of physically-based modelling of ash-fall pyroclastic soil coverings involved in debris flows along mountain slopes surrounding the Somma-Vesuvius volcano (Campania, southern Italy). Such a risky regional framework is well testified by the number of deadly debris flow events known from the chronicles since the 17th and especially in the 20th
centuries, with a total count of casualties exceeding 600. Several studies demonstrated that thickness of ashfall pyroclastic soils, slope angle and morphological discontinuities along slopes, such as knickpoints, abrupt increases of slope angle related to bedrock outcrops and man-made road cuts, play a relevant role in rising locally landslides susceptibility (Guadagno et al., 2005). In such a geomorphological framework several studies were carried out to estimate rainfall thresholds triggering debris flows by applying empirical approaches, which were based on landslide inventory and related rainfall records (Calcaterra, 2000; De Vita and Piscopo, 2002). Nevertheless, the reliability of this approach seems to be limited by several factors, which control the uncertainty of results. Among them, spatial and temporal variability of rainfall patterns, antecedent soil moisture status and reliability of rainfall recording potentially determine uncertainty of estimations. To overcome these difficulties, in this paper are presented estimations of Intensity-Duration rainfall thresholds obtained by physically-based approaches carried out
P. De Vita, F. Fusco, E. Napolitano, R. – Physically-based models for estimating rainfall triggering debris flows
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both at the landslide source areas and distributed scales. The first approach is based on the reconstruction of detailed physical models of ash-fall pyroclastic soil coverings in representative source areas and on hydrological and slope stability modelling. The second is based on a regional distribution model of ashfall pyroclastic soils over mountain slopes surrounding the Somma-Vesuvius volcano, which takes into account total thicknesses of the cover and variable stratigraphic settings. For both, different initial antecedent hydrological conditions, related to summer and winter, were considered.
or GP); 6) Bbbasal horizon, corresponding to a residual pyroclastic deposit, highly weathered by pedogenesis (SM), which is always present at the bedrock interface; 7) R horizon, fractured carbonate bedrock with open joints filled by the overlying paleosol. Differently, for the Lattari mountains the stratigraphic setting is simpler due to the existence of only one C horizon (pumiceous lapilli), formed by the 79 A.D. eruption. By the analysis of isopach maps of the principal Plinian eruptions of the Somma-Vesuvius and Phlaegrean Fields, a total theoretical thickness map of the ash-fall pyroclastic soil coverings was reconstructed (De Vita et al., 2006; De Vita and Nappi, 2013) (Fig. 1).
Geomorphological and stratigraphic settings The Sarno and Lattari mountains ranges surround the Campanian Plain, which is a tectonic depression formed by the faulting of carbonate Mesozoic tectonic units occurred during Pleistocene. The structural depression is filled by shallow marine deposits, fluviolacustrine and marsh sediments as well as ash-flow and ash-fall volcanic deposits (Fig. 1) erupted by the volcanic centers of Ischia island (150 k-years to AD 1302), Phlaegrean Fields (39 k-years to AD 1538) and Somma-Vesuvius (25 k-years to AD 1944) rising at its middle. Ash-fall pyroclastic soils were distributed along these mountain ranges with different thicknesses, according to the distance from the eruptive center, which varies from about 12 to 25 km, and the orientation of dispersal axes of principal eruptions (De Vita et al., 2006). Eruptions had dispersion axes generally oriented eastward, thus mainly involving the Sarno mountains in the deposition of ash-fall deposits (Fig. 1). Just the A.D. 79 eruption had a dispersion axis oriented southward, thus affecting mainly the Lattari mountains. The theoretical maximum thickness of the ash-fall volcaniclastic series reaches about 7 meters on the Sarno mountains and about 2 meters on the Lattari mountains (De Vita et al. 2006; 2013). Along mountain slopes, volcaniclastic series are often found incomplete and laterally discontinuous due to denudational phenomena, which acted since their deposition. Stratigraphic settings are generally formed by an alternation of pedogenised and weathered pyroclastic soil horizons with unweathered pumiceous lapilli horizons. The latter mark the deposition of the principal Plinian eruptions of the Somma-Vesuvius. Considering both principal pedological horizons and the USCS classification system, a typical stratigraphic setting for the Sarno mountains can be sketched as follows: A horizon, consisting of abundant humus (Pt); B horizon, mainly characterized by pumiceous clasts (SM); 3) C horizon, constituted of unweathered coarse pumiceous pyroclasts (GW or GP); 4) Bb horizon or paleosol (SM); 5) Cb horizon, or buried C horizon (GW
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Fig. 1 - Geologic map of the area surrounding the Somma-Vesuvius volcano: (1) alluvial deposits; (2) travertine deposits; (3) incoherent ash-fall deposits; (4) mainly coherent ash-fall deposits; (5) lavas; (6) detritus and slope talus deposits; (7) Miocene flysch; (8) Middle Jurassic–Upper Cretaceous limestone; (9) Lower Triassic–Middle Jurassic dolomites and calcareous limestone; (10) outcropping and buried faults; (11) total isopachous line (meters) of the most important of the Mount Somma-Vesuvius eruptions (De Vita and Nappi, 2013); (12) study area for site-specific scale approach.
A theoretical distribution model of ash-fall pyroclastic soils along slopes was reconstructed in term of real thickness (z) by: z = z0 ∙ cosα
[1]
where: z0 is the theoretical thickness considered for a deposition on a horizontal surface (isopach map); α is the slope angle. This theoretical distribution along slopes was compared with thicknesses data measured
Proceedings of the 4th World Landslide Forum Ljubljana, Slovenia, May 29 – June 2, 2017
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in different test areas of the mountain range that surround the Campanian Plain, allowing to find results described following. For slope angle values lower than 28°, the actual thicknesses well match with the theoretical one [1]. Instead, above a slope angle value of about 28°, thickness of the ash-fall pyroclastic coverings reduces linearly down to its approximate annulment for slope angle greater than 50° (De Vita et al., 2006). A validation of this empirical model comes from the typical slope angle interval in the source areas of debris flows, which is normally between 31° and 43° (de Riso et al., 1999), and from a number of field observations. The decrease in thickness of the ash-fall pyroclastic soil mantle, as the slope angle increases, causes the progressive thinning and the reduction of the number of soil horizons. For slope angle values lower than 28°, two or, more frequently, three pumiceous lapilli horizons can be usually found along slopes of the Sarno mountains, and only one for the Lattari mountains. Instead, for values of slope greater than 40°, a single pumiceous horizon can be usually found. These stratigraphic settings exert a strong impact on hillslope hydrological processes because pedogenized and pumiceous horizons have very different hydraulic conductivities, according to their contrasting grain size (De Vita et al, 2013).
Data and methods In order to estimate Intensity-Duration rainfall thresholds (Caine, 1980) by deterministic approaches, hydrological and slope stability modelling were carried out at both site-specific (source area) and distributed scales. At the source area scale, three initial debris slides, which subsequently triggered debris flows on 56-May 1998, were identified in the Sarno mountains. These initial landslides were considered representative of geomorphological conditions which typically characterize initiation areas of debris flows, respectively corresponding to knickpoints (landslide 1), upslope rim of rocky cliffs (landslide 2) and above road cuts (landslide 3). A fourth test area, upslope of another initial debris slide, was identified for monitoring pore pressure head within the ash-fall pyroclastic soil mantle. For each initiation area, a detailed engineering geological model was reconstructed by means of field investigations, consisting in exploratory pits, dynamic penetration tests and topographic surveys. The physical models of initial landslides were completed with laboratory geotechnical and hydraulic (unsaturated and saturated) characterizations of pyroclastic soil horizons (Bilotta et al., 2005; De Vita et al., 2013; Napolitano et al., 2016). Moreover, the Van Genuchten model (1980) was used to estimate unsaturated hydraulic conductivity.
On the basis of such physical models, numerical hydrological modeling was carried out by the VS2DTI finite-difference numerical code (Hsieh et al., 2000), which was calibrated by time series of pore pressure head. The numerical models were set considering the ground surface as a flux boundary condition corresponding to infiltration with constant intensity values of 2.5, 5, 10, 20 and 40 mm/h and the contact with the carbonate bedrock as a gravity drainage boundary. Hydrological antecedent conditions for summer and winter were assessed by statistical analysis of the monitored pressure head time series within the ash-fall pyroclastic cover (Napolitano et al., 2016). Finally, finite slope stability analyses were carried out to identify Intensity-Duration thresholds. Driving forces were computed along the failure surfaces, which were considered as corresponding to the currently exposed rupture surfaces (Fig. 2) due to the negligible effect of erosional phenomena. A variable unit weight (γ) of pyroclastic soil horizons depending on water content (θ) was considered. Shear strength forces were set considering also the contributions of the apparent cohesion (ca), according to the suction stress model (Lu and Likos, 2004). To obtain pressure head (P) and water content (θ) values for each time step of the simulation, different observation points were set into the VS2DTI models, at various depths and locations along the slope. These data allowed estimating the Factor of Safety (FOS), related to constant rainfall intensities (2.5, 5, 10, 20 and 40 mm/h) and variable duration. At the distributed scale, given the finding of an empirical relationship between the actual total thickness of ash-fall pyroclastic deposits and slope angle (De Vita et al., 2006), a regional distributed model of pyroclastic mantle thickness was developed (De Vita and Nappi, 2013). By such a model, slope hydrological and stability modeling were generalized to different theoretical conditions, depending on the slope angle. A variability of stratigraphic settings was also taken into account by an empirical analysis of field data. Geotechnical and unsaturated/saturated hydraulic properties for each pyroclastic soil horizon were set according with the same results of characterization used for the analyses performed for the site-specific scale. To find the critical failure conditions, slope stability analyses were carried out by the infinite slope approach and taking into account results from hydrological simulations such as water content (θ), pressure head (p) and saturation degree (S). In addition, shear strength developed along the main scarp was considered of tensile type due to its nearly perpendicularity to the orientation of driving forces (parallel to the failure surface).
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P. De Vita, F. Fusco, E. Napolitano, R. – Physically-based models for estimating rainfall triggering debris flows
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Results Site-specific scale modelling At the site-specific scale, or source areas scale, engineering geological models of the three initial debris slides were reconstructed by field surveys and laboratory analyses (Fig. 2). For landslides 1 and 2, slopes have convex longitudinal profiles, which progressively steepen from 8° - 13° in the upper part to 45° near the toe of landslide 1, and to vertical at the rock outcrop at the toe of landslide 2. Rupture surfaces cut shallower pyroclastic soil horizons with a high angle of inclination that becomes tangent to the basal paleosol (Bbbasal) cropping out in the terminal part of landslide scars. Volumes of the initial debris slides were estimated in about 178 m3 and 81 m3 for landslides 1 and 2, respectively. By longitudinal cross sections reconstructed for landslides 1 and 2, the total thickness of the ash-fall pyroclastic soil mantle can be recognized decreasing as the slope angle increases, thus determining a thinning of the mantle in the case of landslide 1 and an abrupt interruption in the case of landslide 2. The total thickness of the ash-fall pyroclastic cover varies from 5 to 6 m in the upper slope, up to 2 or 3 m in the depletion zones. Stratigraphic reconstructions, carried out upslope of the three representative initial debris slides, show a volcaniclastic series more complete than those observed in the main scarps. In landslide main scarps, pyroclastic series was observed thinner with the lack of the deepest C-horizon (Ottaviano eruption). Moreover, stratigraphic correlations across the longitudinal landslide axes show the progressive thinning of each horizon and the pinching out of the deepest C horizon. The deepest horizon is the basal paleosol, Bbbasal, which wraps the carbonate bedrock. For the landslide 3, the longitudinal shape of the slope is rectilinear and is inclined with a mean angle of about 35° (Fig. 2). Stratigraphic reconstruction indicates a volcaniclastic series, approximately 3 m thick and characterized by two C horizons associated to the Avellino and Pollena eruptions. Volume of the initial slide was estimated in about 600 m3. On the basis of these engineering geological models (Fig. 2), numerical hydrological and slope stability models were carried out allowing estimating IntensityDuration rainfall thresholds (Fig. 3). The seasonal effect on the antecedent hydrological conditions was taken into account by the analysis of pressure head time series obtained by field hydrological monitoring carried out in a fourth sample area (Fusco and De Vita, 2015; Napolitano et al., 2016).
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Fig. 2 Engineering geological models of initial debris slides reconstructed at site-specific scale by dynamic penetrometric tests (De Vita et al., 2013).
Distributed scale modelling At the distributed scale, the physical model of ash-fall pyroclastic soil mantle was based on total thickness and stratigraphic data, which were measured in several test areas along the Sarno and Lattari mountains. Empirical correlations (Fig. 4) between slope angle and total thickness of the pyroclastic mantle as well as between slope angle and thickness of each pyroclastic soil horizon were identified (Tufano et al., 2016). Empirical models, identified by the upper envelope of field data (Fig. 4), confirmed the approximately correspondence between field and theoretical data for slope angle below 28°. Instead, a progressive divergence between observed and theoretical data was recognized for values of slope angle greater than 28° as well as the approximate annulment for slope angle greater than 50°. In this slope angle range, the stratigraphic setting is also varying with slope angle, due to downslope thinning and pinching out of pyroclastic soil horizons. In the case of C horizons, a pinching out is observed approximately for slope angle values between 42° and 45°, for both Sarno and Lattari mountains.
Proceedings of the 4th World Landslide Forum Ljubljana, Slovenia, May 29 – June 2, 2017
100.0 Salerno (October 1954)
i (mm h-1)
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10.0
Fig. 4 Empirical correlations between slope angle and total thickness of the pyroclastic mantle as well as between slope angle and thickness of each pyroclastic soil horizon for the Sarno mountains (Tufano et al, 2016).
Sarno (May 1998)
1.0 1
10
100
1000
Duration (h) Landslide 01 (Winter) Landslide 02 (Winter) Landslide 03 (Winter) Blue Ridge (Wieczorek et al., 2000) Puerto Rico (Larsen & Simons, 1993) Peri-vesuvian area (Guadagno, 1991) Winter threshold (I = 287.8*D^-1.089)
Landslide 1 (Summer) Landslide 02 (Summer) Landslide 3 (Summer) Seattle (Godt et al., 2006) WW (Caine, 1980) Debris flow events of highest magnitude Summer threshold (I = 400.3*D^-0.774)
Fig. 3 Intensity-Duration thresholds reconstructed by a physically-based approach for site-specific scale (Napolitano et al., 2016).
The Bb horizons were detected only in the Sarno Mountains with a pinching out approximately around a slope angle value of 45°. Thickness of the Bbbasal horizons resulted not correlated to the slope angle due to its filling of fractures and irregularities of the carbonate bedrock surface. On the basis of such empirical model, three representative stratigraphic settings related to slope angle values of 35°, 40° and 45° were modeled for both the Sarno and Lattari mountain ranges (Fig. 5). These values were considered significant because including slope angle interval of initial debris slides occurred in May 1998 (de Riso et al., 1999). Hydrological and stability simulations for all slope models, using different constant rainfall intensities (2.5, 5, 10, 20 and 40 mm/h), under typical winter and summer hydrological antecedent condition allowed estimating durations of rainfall to slope failure. Consequently, deterministic Intensity-Duration rainfall thresholds were reconstructed.
Fig. 5 Conceptual physically-based models of the ashfall pyroclastic cover for the Sarno Mountain Range (a) and the Lattari Mountain Range (b). The models represent theoretical slopes with angle varying from 35°, to 40° and to 45°. The distribution of pyroclastic deposits in terms both of total thickness and stratigraphic settings is also shown. Several interesting hints can be recognized by the obtained results. Among them, the most important are relevant differences in Intensity-Duration thresholds
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P. De Vita, F. Fusco, E. Napolitano, R. – Physically-based models for estimating rainfall triggering debris flows
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between the Sarno and Lattari mountains, which appear being controlled by the difference in ash-fall pyroclastic soil thickness and stratigraphic setting (Tab. 1). Sarno Mountains Slope angle
35°
40°
Lattari Mountains 45°
35°
40°
45°
n.f. n.f. 25.3 13.5 8.0
22.2 11.0 5.3 2.7 1.3
Summer initial conditions Intensity (mm/h) 2.5 5.0 10.0 20.0 40.0
Time to failure (h) n.f. n.f. 107.8 n.f. n.f. n.f. 54.3 n.f. n.f. 66.2 27.8 n.f. n.f. 36.3 14.8 n.f. n.f. 19.2 7.8 n.f. Winter initial conditions
Intensity Time to failure (h) (mm/h) 2.5 n.f. n.f. 20.2 n.f. n.f. 22.2 5.0 n.f. 32.8 11.2 n.f. n.f. 11.0 10.0 n.f. 13.8 6.7 n.f. 25.3 5.3 20.0 n.f. 7.3 4.0 n.f. 13.5 2.7 40.0 n.f. 4.8 2.3 n.f. 8.0 1.3 Tab. 1 Durations (hours) to slope failure of rainfall events under different rainfall intensities and for summer and winter antecedent hydrological conditions (n.f. means no failure).
As expected, the increase of slope angle plays a fundamental role for the slope stability: as the ash-fall pyroclastic covering thins, the rainfall duration necessary for slope instability is shorter. Also, the initial hydrological conditions of pyroclastic cover affect relevantly critical durations of rainfall. Moreover, the most critical failure surface was always found for depths shallower than the Bbbasal paleosol, which is in direct contact with the carbonate bedrock. Results point out the relevance of the stratigraphic and geotechnical models, in fact for slope angle value of 35°, slope stability conditions were found. Intensity-Duration rainfall thresholds estimated at the site-specific and distributed scales show a good match for the case of the Sarno mountains and for slope angle of 40°, which corresponds to the mean angle of rupture surfaces (Fig. 2).
Conclusions The deterministic Intensity-Duration rainfall thresholds obtained by hydrological and stability modelling of ash-fall pyroclastic covers, show the
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spatial variability of the critical conditions which can activate initial landslides triggering debris flows in the peri-Vesuvian area. The proposed model unifies the effect of slope angle on both ash-fall pyroclastic thicknesses and stratigraphic settings of the Sarno and Lattari mountains as well as its control on IntensityDuration rainfall thresholds. Moreover it highlights the significant and not negligible control of antecedent hydrological conditions. The obtained results identify an alternative approach to the empirical one for taking into account different hazard conditions related to seasonality of hydrological processes inside the ash-fall pyroclastic soil mantle. They could be implemented in an early warning system for the Sarno and Lattari mountains area.
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E-mail:
[email protected] Francesco Fusco University of Naples Federico II, PhD Program (29th cycle) of the Department of Earth, Resources and Environmental Sciences. Via Mezzocannone 8, Napoli - 80134 , Italy E-mail:
[email protected] Elisabetta Napolitano CNR-IRPI, Perugia, Via Madonna Alta, 126-06128, Perugia, Italy E-mail:
[email protected] Rita Tufano University of Naples Federico II, CIRAM. Via Mezzocannone 16, Napoli - 80134 , Italy E-mail:
[email protected]
Pantaleone De Vita ( ) University of Naples Federico II, Department of Earth, Resources and Environmental Sciences. Via Mezzocannone 8, Napoli - 80134 , Italy
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Physically-based models for estimating rainfall triggering debris flows in Campania (southern Italy) Pantaleone De Vita, Francesco Fusco, Elisabetta Napolitano and Rita Tufano Abstract The societal risk related to rainfall-triggered rapid debris flows is commonly managed in urbanized areas by means of early warning systems based on monitoring of hydrological parameters (such as rainfall or soil moisture) and analysis of thresholds values. The latter represent the principal issue due to the frequent lack of data about landslide occurrences and/or unreliability of hydrological records that usually affect the empirical approaches. To overcome this problem, the hydrological and slope stability modelling of surficial deposits involved in debris flows can be conceived as a promising way to define hydrological thresholds. In this paper are exposed results of physically-based modelling of ash-fall pyroclastic soil coverings involved in debris flows along mountain slopes nearby the Somma-Vesuvius volcano (Campania, southern Italy), which represent one of the major geohazards of Italy. The methods adopted combine approaches at the scale of single landslide source areas and at the distributed scale to estimate deterministic Intensity-Duration rainfall thresholds. The first approach is based on the reconstruction of detailed physical models of ash-fall pyroclastic soil coverings in representative source areas of debris flows and on the related hydrological and slope stability modelling. The second is focused on a regional distribution model of ash-fall pyroclastic soils over mountain slopes surrounding the Somma-Vesuvius volcano, which takes into account both total thicknesses of pyroclastic coverings and the related variable stratigraphic settings. For both, effects of different initial antecedent hydrological conditions, associated with summer and winter, were considered. Results obtained by both approaches converge to identify Intensity-Duration rainfall thresholds triggering initial slope instabilities, and the subsequent debris flows, thus proving a new possible method for assessing rainfall thresholds. Keywords Ash-fall pyroclastic soils, debris flows, hillslope hydrological processes, deterministic rainfall thresholds. Introduction The societal risk related to rainfall-triggered rapid debris flows is commonly managed in urbanized areas by means of early warning systems based on monitoring of hydrological parameters (such as rainfall or soil moisture) and analysis of thresholds values (Guzzetti et al., 2007). The latter represent the principal issue due to the frequent lack of data about landslide occurrences and/or unreliability of hydrological records that usually affect empirical approaches (Caine, 1980; Crozier and Eyles, 1980; Govi and Sorzana, 1980). To overcome this problem, the hydrological and stability
modelling of surficial deposits involved in debris flows is a promising way to define hydrological thresholds (Godt et al., 2008; Peres and Cancelliere, 2014). In this paper are exposed results of physically-based modelling of ash-fall pyroclastic soil coverings involved in debris flows along mountain slopes surrounding the Somma-Vesuvius volcano (Campania, southern Italy). Such a risky regional framework is well testified by the number of deadly debris flow events known from the chronicles since the 17th and especially for the 20th centuries, with a total count of casualties exceeding 600. The awareness of this geohazard achieved in the last decades led researchers to study initiation and
P. De Vita, F. Fusco, E. Napolitano, R. – Physically-based models for estimating rainfall triggering debris flows
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propagation mechanisms as well as early warning systems based on the identification of rainfall thresholds (Guadagno, 1991; Calcaterra et al., 2000; De Vita et al., 2013; Napolitano et al., 2015). Several studies demonstrated that thickness of ashfall pyroclastic soils, slope angle and morphological discontinuities along slopes, such as knickpoints, abrupt increases of slope angle related to bedrock outcrops and road cuts, play a relevant role in rising locally landslides susceptibility (Di Crescenzo and Santo, 2005; Guadagno et al., 2005). Moreover, among the principal outcomes of these studies is the recognition that these landslides have a complex style that is characterized by three fundamental consecutive evolutionary stages: a) the first is debris slide (Cruden and Varnes, 1996), or soil slip, that generally involves a volume of pyroclastic debris (gravel content > 20%) from few tens to hundreds of cubic meters; b) the second is debris avalanche, propagating on open slopes (Hungr et al., 2001) by a dynamic liquefaction mechanism and causing the increase of the amount of soil involved and of the velocity of the mass; c) the last stage is debris flow, which occurs when the flow-like mass movement is channeled into the hydrographic network (Hungr et al., 2001). The first stage always exists and occasionally can evolve directly in the third one or can be limited to the second one, depending on the slope morphology, and on the availability of pyroclastic soils along its downward path. In such a geomorphological framework several studies were carried out to estimate rainfall thresholds triggering debris flows by applying empirical approaches, which were based on landslide inventory and related rainfall records (Calcaterra, 2000; De Vita and Piscopo, 2002). Nevertheless, the reliability of this approach seems to be limited by several factors, which control the uncertainty of results. Among them, spatial and temporal variability of rainfall patterns, antecedent soil moisture status and reliability of rainfall recording potentially determine uncertainty of estimations. To overcome these difficulties, in this paper are presented estimations of Intensity-Duration rainfall thresholds obtained by physically-based approaches carried out at the scale of landslide source areas and at the distributed scale. The first approach is based on the reconstruction of detailed physical models of ash-fall pyroclastic soil coverings in representative source areas and on hydrological and slope stability modelling. The second is based on a regional distribution model of ashfall pyroclastic soils over mountain slopes surrounding the Somma-Vesuvius volcano, which takes into account both total thicknesses of the cover and variable stratigraphic settings. For both, different initial antecedent hydrological conditions, related to summer and winter, were considered.
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Geomorphological and stratigraphic settings The Sarno and Lattari mountains ranges surround the Campanian Plain, which is a tectonic depression formed by the faulting of carbonate Mesozoic tectonic units occurred during Pleistocene. The structural depression is filled by shallow marine deposits, fluviolacustrine and marsh sediments as well as ash-flow and ash-fall volcanic deposits (Fig. 1) erupted by the volcanic centers of Ischia island (150 k-years to AD 1302), Phlaegrean Fields (39 k-years to AD 1538) and Somma-Vesuvius (25 k-years to AD 1944) rising at its middle. The mountain ranges surrounding the Campanian Plain were covered by ash-fall pyroclastic soils, erupted mainly by the nearest and youngest Somma-Vesuvius volcano. Ash-fall pyroclastic soils were distributed along these mountain ranges with different thicknesses, according to the distance from the eruptive center, which varies from about 12 to 25 km, and the orientation of dispersal axes of principal eruptions (De Vita et al., 2006). Since their deposition, landsliding and other erosional processes affected ashfall pyroclastic coverings along mountain slopes. Therefore, along slopes, volcaniclastic series are often found laterally discontinuous and incomplete due to denudational phenomena which acted since their deposition. A complete pyroclastic series was recognized in the southwestern footslope of the Sarno mountains, which was subdivided into two principal pyroclastic complexes (Rolandi et al., 2000). The Ancient Pyroclastic Complex (APC) is composed mainly by ash-flow deposits belonging to the Campanian Ignimbrite erupted 39 k-years B.P. by the Phlaegrean Fields. The younger pyroclastic deposits, named Recent Pyroclastic Complex (RPC), is mainly composed by ashfall pyroclastic products of the Somma-Vesuvius volcano, attributed to the Sarno, Ottaviano, Avellino and Pollena eruptions, respectively dated 17 k-year B.P., 8 k-year B.P., 3.7 k-year B.P. and A.D. 472. These eruptions had dispersion axes generally oriented eastward, thus mainly involving the Sarno mountains in the deposition of ash-fall deposits (Fig. 1). Just the A.D. 79 eruption had a dispersion axis oriented southward, thus affecting mainly the Lattari mountains. The theoretical maximum thickness of the ash-fall volcaniclastic series reaches about 7 meters on the Sarno mountains and about 2 meters on the Lattari mountains (De Vita et al. 2006; 2013). Notwithstanding erosional processes, relevant ashfall pyroclastic soil thicknesses still exist along mountain slopes (De Vita and Nappi, 2013) as well as a high debris flows hazard under heavy and prolonged rainfall. Pyroclastic covers along carbonate slopes are formed by an alternation of pedogenised and weathered pyroclastic soil horizons with unweathered
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pumiceous lapilli horizons. The latter mark the deposition of the principal Plinian eruptions of the Somma-Vesuvius. Considering both principal pedological horizons and the USCS classification system, a typical stratigraphic setting for the Sarno mountains can be sketched as follows: A horizon, consisting of abundant humus (Pt); B horizon, mainly characterized by pumiceous clasts highly subjected to pedogenetic processes (SM); 3) C horizon, constituted of pumiceous pyroclasts, weakly weathered (GW or GP); 4) Bb horizon, corresponding to a B horizon buried by a successive depositional event and thus considerable as a paleosol (SM); 5) Cb horizon, representative of a buried C horizon (GW or GP); 6) Bbbasal horizon, corresponding to a residual pyroclastic deposit, highly weathered by pedogenesis (SM), which is always present at the bedrock interface; 7) R horizon, fractured carbonate bedrock with open joints filled by the overlying paleosol for the first few meters. Differently, for the Lattari mountains the stratigraphic setting is simpler due to the existence of only one C horizon (pumiceous lapilli), formed by the 79 A.D. eruption. By the analysis of isopach maps of the principal Plinian eruptions of the Somma-Vesuvius, a total theoretical thickness map of the ash-fall pyroclastic soil coverings was reconstructed (De Vita et al., 2006; De Vita and Nappi, 2013) (Fig. 1). A theoretical distribution model of ash-fall pyroclastic soils along slopes was reconstructed in term of real thickness (z) by: z = z0 ∙ cosα
[1]
where: z0 is the theoretical thickness considered for a deposition on a horizontal surface (isopach map); α is the slope angle. This theoretical distribution along slopes was compared with thicknesses data measured in different test areas of the mountain range that surrounding the Campanian Plain, allowing to find results described following. For slope angle values lower than 28°, the actual thicknesses well match with the theoretical one [1]. Instead, above a slope angle value of about 28°, thickness of the ash-fall pyroclastic coverings reduces linearly down to its approximate annulment for slope angle greater than 50° (De Vita et al., 2006). Precisely, for slope angle values higher than 50°, the A and B horizons are thinly and discontinuously present before the complete outcropping of the carbonate bedrock. A validation of this finding comes from the typical slope angle interval in the source areas of these landslides, which is normally between 31° and 43° (de Riso et al., 1999), and from a number of field observations.
Fig. 1 - Geologic map of the area surrounding the Somma-Vesuvius volcano: (1) alluvial deposits; (2) travertine deposits; (3) incoherent ash-fall deposits; (4) mainly coherent ash-fall deposits; (5) lavas; (6) detritus and slope talus deposits; (7) Miocene flysch; (8) Middle Jurassic–Upper Cretaceous limestone; (9) Lower Triassic–Middle Jurassic dolomites and calcareous limestone; (10) outcropping and buried faults; (11) total isopachous line (meters) of the most important of the Mount Somma-Vesuvius eruptions (De Vita and Nappi, 2013); (12) study area for site-specific scale approach.
The decrease in thickness of the ash-fall pyroclastic soil mantle, as the slope angle increases, causes the progressive thinning and the reduction of the number of soil horizons. For slope angle values lower than 28°, two or, more frequently, three pumiceous lapilli horizons can be usually found along slopes of the Sarno mountains, and only one for the Lattari mountains. Instead, for values of slope greater than 40°, a single pumiceous horizon can be usually found. Finally, in the slope angle interval between 45° and 50°, pumiceous lapilli horizons pinch out downslope, determining the direct contact between the B and Bbbasal horizons. These stratigraphic settings exert a strong impact on hillslope hydrological processes because pedogenized and pumiceous horizons have very different hydraulic conductivities, according to their contrasting grain size. This observation suggests that an increase of pore pressure and the instability of the ash-fall pyroclastic covering are more likely to occur where C horizons pinch out (De Vita et al, 2013).
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P. De Vita, F. Fusco, E. Napolitano, R. – Physically-based models for estimating rainfall triggering debris flows
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Data and methods In order to estimate Intensity-Duration rainfall thresholds (Caine, 1980) by deterministic approaches, hydrological and slope stability modelling were carried out at both site-specific and distributed scales. At the site-specific scale, three initial debris slides, which subsequently triggered debris flows on 5-6-May 1998, were identified in the Sarno mountains. These initial landslides were considered representative of geomorphological conditions which typically characterize initiation areas of debris flows, respectively corresponding to knickpoints (landslide 1), upslope rim of rocky cliffs (landslide 2) and above road cuts (landslide 3). A fourth test area, upslope of another initial debris slide, was identified for monitoring pore pressure head within the ash-fall pyroclastic soil mantle. For each initiation area, a detailed engineering geological model was reconstructed by means of field investigations, consisting in exploratory pits, dynamic penetration tests and topographic surveys. The physical models of initial landslides were completed with laboratory geotechnical and hydraulic (unsaturated and saturated) characterizations of pyroclastic soil horizons (Crosta and Dal Negro, 2003; Bilotta et al., 2005; De Vita et al., 2013; Napolitano et al., 2015). Moreover, the Van Genuchten model (1980) was used to estimate unsaturated hydraulic conductivity. On the basis of such physical models, numerical hydrological modeling was carried out by the VS2DTI finite-difference numerical code (Hsieh et al., 2000), which was calibrated by time series of pore pressure head. The numerical models were set considering the ground surface as a flux boundary condition corresponding to infiltration with constant intensity values of 2.5, 5, 10, 20 and 40 mm/h and the contact with the carbonate bedrock as a gravity drainage boundary. Hydrological antecedent conditions for summer and winter of the ash-fall pyroclastic cover were assessed by statistical analysis of the monitored pressure head time series (Napolitano et al., 2015). Finally, finite slope stability analyses were carried out to identify Intensity-Duration thresholds. Driving forces were computed along the failure surfaces, which were considered as corresponding to the actually exposed rupture surfaces (Fig. 2) due to the negligible effect of erosional phenomena. A variable unit weight (γ) of pyroclastic soil horizons depending on water content (θ) was considered. Shear strength forces were set considering also the contributions of the apparent cohesion (ca), according to the suction stress model (Lu and Likos, 2004). To obtain pressure head (P) and water content (θ) values for each time step of the simulation, different observation points were set into the VS2DTI models, at different depths and locations along the slope. These data allowed estimating the
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Factor of Safety (FOS), related to constant rainfall intensities (2.5, 5, 10, 20 and 40 mm/h) and variable duration. At the distributed scale, given the finding of an empirical relationship between the actual total thickness of ash-fall pyroclastic deposits and slope angle (De Vita et al., 2006), a regional distributed model of pyroclastic mantle thickness was developed (De Vita and Nappi, 2013). By such a model, slope hydrological and stability modeling were generalized to different theoretical conditions, depending on the slope angle. A variability of stratigraphic settings was also taken into account by an empirical analysis of field data. Geotechnical and unsaturated/saturated hydraulic properties for each pyroclastic soil horizon were set according with the same results of characterization used for the analyses performed for the site-specific scale. In order to find the critical failure conditions, slope stability analyses were carried out by the infinite slope approach. Slope stability analyses were performed through the determination of the FOS, depending on results derived by hydrological simulations such as water content (θ), pressure head (p) and saturation degree (S). In addition, shear strength developed along the main scarp was considered of tensile type due to its nearly perpendicularity to the orientation of driving forces (parallel to the failure surface).
Results Site-specific scale modelling At the site-specific scale, engineering geological models of the three initial debris slides were reconstructed by field surveys and laboratory analyses (Fig. 2). For landslides 1 and 2, slopes have convex longitudinal profiles, which progressively steepen from 8° - 13° in the upper part to 45° near the toe of landslide 1, and to vertical at the rock outcrop at the toe of landslide 2. Rupture surfaces cut shallower pyroclastic soil horizons with a high angle of inclination that becomes tangent to the basal paleosol (Bbbasal) cropping out in the terminal part of landslide scars. Volumes of the initial debris slides were estimated in about 178 m3 and 81 m3 for landslides 1 and 2, respectively. By longitudinal cross sections reconstructed for landslides 1 and 2, the total thickness of the ash-fall pyroclastic soil mantle can be recognized decreasing as the slope angle increases, thus determining a thinning of the mantle in the case of landslide 1 and an abrupt interruption in the case of landslide 2. The total thickness of the ash-fall pyroclastic cover varies from 5 to 6 m in the upper slope, up to 2 or 3 m in the depletion zones. Stratigraphic reconstructions, carried out upslope of
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the three representative initial debris slides, show a volcaniclastic series more complete than those observed in the main scarps. Below the B horizon, a complete series consisting of the alternation of up to three C horizons with interposed Bb horizons (paleosols) was found. The deepest horizon is the basal paleosol, Bbbasal, which wraps the carbonate bedrock. In landslide main scarps, pyroclastic series was observed thinner with the lack of the deepest C-horizon (Ottaviano eruption). Moreover, stratigraphical correlations across the longitudinal landslide axes show the progressive thinning of each horizon and the pinching out of the deepest C horizon. For the landslide 3, the longitudinal shape of the slope is rectilinear and is inclined at an average angle of about 35° (Fig. 2). Stratigraphic reconstruction indicates a volcaniclastic series, approximately 3 m thick and characterized by two C horizons associated to the Avellino and Pollena eruptions. Volume of the initial slide was estimated in about 600 m3.
out in a fourth sample area (Fusco and De Vita, 2015; Napolitano et al., 2015). 100.0 Salerno (October 1954)
i (mm h-1)
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10.0
Sarno (May 1998)
1.0 1
100
10
1000
Duration (h) Landslide 01 (Winter) Landslide 02 (Winter) Landslide 03 (Winter) Blue Ridge (Wieczorek et al., 2000) Puerto Rico (Larsen & Simons, 1993) Peri-vesuvian area (Guadagno, 1991) Winter threshold (I = 287.8*D^-1.089)
Landslide 1 (Summer) Landslide 02 (Summer) Landslide 3 (Summer) Seattle (Godt et al., 2006) WW (Caine, 1980) Debris flow events of highest magnitude Summer threshold (I = 400.3*D^-0.774)
Fig. 3 Intensity-Duration thresholds reconstructed by a physically-based approach for site-specific scale (Napolitano et al., 2015).
Fig. 2 Engineering geological models of initial debris slides reconstructed at site-specific scale (De Vita et al., 2013). On the basis of these engineering geological models (Fig. 2), numerical hydrological and slope stability models were carried out allowing estimating IntensityDuration rainfall thresholds (Fig. 3). The seasonal effect on the antecedent hydrological conditions was taken into account by the analysis of pressure head time series obtained by field hydrological monitoring carried
Distributed scale modelling At the distributed scale, the physical model of ash-fall pyroclastic soil mantle was based on total thickness and stratigraphic data, which were measured in several test areas along the Sarno and Lattari mountains. Empirical correlations (Fig. 4) between slope angle and total thickness of the pyroclastic mantle as well as between slope angle and thickness of each pyroclastic soil horizon were identified (Tufano et al., 2016). Empirical models, identified by the upper envelope of field data (Fig. 4), confirmed the approximately correspondence between field and theoretical data for slope angle below 28°. Instead, a progressive divergence between observed and theoretical data was recognized for values of slope angle greater than 28° as well as the approximate annulment for slope angle greater than 50°. In this slope angle range, the stratigraphic setting is also varying with slope angle, due to downslope
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P. De Vita, F. Fusco, E. Napolitano, R. – Physically-based models for estimating rainfall triggering debris flows
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thinning and pinching out of pyroclastic soil horizons. In the case of C horizons, a pinching out is observed approximately for slope angle values between 42° and 45°, for both Sarno and Lattari mountains.
Fig. 4 Empirical correlations between slope angle and total thickness of the pyroclastic mantle as well as between slope angle and thickness of each pyroclastic soil horizon for the Sarno mountains (Tufano et al, 2016).
The Bb horizons were detected only in the Sarno Mountains with a pinching out approximately around a slope angle value of 45°. Thickness of the Bbbasal horizons resulted not correlated to the slope angle due to its filling of fractures and irregularities of the carbonate bedrock surface. On the basis of such empirical model, three representative stratigraphic settings related to slope angle values of 35°, 40° and 45° were modeled for both the Sarno and Lattari mountain ranges (Fig. 5). These values were considered representative because including slope angle interval of initial debris slides occurred in May 1998 (de Riso et al., 1999). Hydrological and stability simulations for all slope models, using different constant rainfall intensities (2.5, 5, 10, 20 and 40 mm/h), under typical winter and summer hydrological antecedent condition allowed estimating durations of rainfall to slope failure. Consequently, deterministic Intensity-Duration rainfall thresholds were reconstructed. Several interesting hints can be recognized by the obtained results. Among them, the most important are relevant differences in Intensity-Duration thresholds between the Sarno and Lattari mountains, which appear being controlled by the difference in ash-fall pyroclastic soil thickness and stratigraphic setting (Tab. 1).
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Fig. 5 Conceptual physically-based models of the ashfall pyroclastic cover for the Sarno Mountain Range (a) and the Lattari Mountain Range (b). The models represent theoretical slopes with angle varying from 35°, to 40° and to 45°. The distribution of pyroclastic deposits in terms both of total thickness and stratigraphic settings is also shown. Sarno Mountains Slope angle
35°
40°
Lattari Mountains 45°
35°
40°
45°
n.f. n.f. 25.3 13.5 8.0
22.2 11.0 5.3 2.7 1.3
Summer initial conditions Intensity (mm/h) 2.5 5.0 10.0 20.0 40.0
Time to failure (h) n.f. n.f. 107.8 n.f. n.f. n.f. 54.3 n.f. n.f. 66.2 27.8 n.f. n.f. 36.3 14.8 n.f. n.f. 19.2 7.8 n.f. Winter initial conditions
Intensity Time to failure (h) (mm/h) 2.5 n.f. n.f. 20.2 n.f. n.f. 22.2 5.0 n.f. 32.8 11.2 n.f. n.f. 11.0 10.0 n.f. 13.8 6.7 n.f. 25.3 5.3 20.0 n.f. 7.3 4.0 n.f. 13.5 2.7 40.0 n.f. 4.8 2.3 n.f. 8.0 1.3 Tab. 1 Durations (hours) to slope failure of rainfall events under different rainfall intensities and for summer and winter antecedent hydrological conditions (n.f. means no failure).
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As expected, the increase of slope angle plays a fundamental role for the slope stability: as the ash-fall pyroclastic covering thins, the rainfall duration necessary for slope instability is shorter. Also, the initial hydrological conditions of pyroclastic cover affect relevantly critical durations of rainfall. Moreover, the most critical failure surface was always found for depths shallower than the Bbbasal paleosol, which is in direct contact with the carbonate bedrock. Results point out the relevance of the stratigraphic and geotechnical models, in fact for slope angle value of 35°, slope stability conditions were found. Intensity-Duration rainfall thresholds estimated at the site-specific and distributed scales show a good match for the case of the Sarno mountains and with slope angle of 40°, which corresponds to the mean inclination angle of rupture surfaces (Fig. 2).
Conclusions The deterministic Intensity-Duration rainfall thresholds obtained by hydrological and stability modelling of ash-fall pyroclastic cover, show the spatial variability of the critical conditions which can activate initial landslides triggering debris flows in the periVesuvian area. The proposed model unifies the effect of slope angle on both ash-fall pyroclastic thicknesses and stratigraphic settings of the Sarno and Lattari mountains as well as its control on Intensity-Duration rainfall thresholds. Moreover it highlights the significant and not negligible control of antecedent hydrological conditions. The obtained results identify an alternative approach to the empirical one for taking into account different hazard conditions related to seasonality of hydrological processes inside the ash-fall pyroclastic soil mantle. They could be implemented in an early warning system for the Sarno and Lattari mountains area.
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P. De Vita, F. Fusco, E. Napolitano, R. – Physically-based models for estimating rainfall triggering debris flows
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Pantaleone De Vita ( ) University of Naples Federico II, Department of Earth, Resources and Environmental Sciences. Via Mezzocannone 8, Napoli - 80134 , Italy
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E-mail:
[email protected] Francesco Fusco University of Naples Federico II, PhD Program (29th cycle) of the Department of Earth, Resources and Environmental Sciences. Via Mezzocannone 8, Napoli - 80134 , Italy E-mail:
[email protected] Elisabetta Napolitano CNR-IRPI, Perugia, Via Madonna Alta, 126-06128, Perugia, Italy E-mail:
[email protected] Rita Tufano University of Naples Federico II, CIRAM. Via Mezzocannone 16, Napoli - 80134 , Italy E-mail:
[email protected] 
