Yuri Galanti

Shallow landslides susceptibility assessment in different environments

ABSTRACT The spatial distribution of shallow landslides is strongly influenced by different climatic conditions and environmental settings. This makes difficult the implementation of an exhaustive monitoring technique for correctly assessing the landslide susceptibility in different environmental contexts. In this work, a unique methodological strategy, based on the statistical implementation of the generalized additive model (GAM), was performed. This method was used to investigate the shallow landslide predisposition of four sites with different geological, geomorphological and land-use characteristics: the Rio Frate and the Versa catchments (Southern Lombardy) and the Vernazza and the Pogliaschina catchments (Eastern Liguria). A good predictive overall accuracy was evaluated computing by the area under the ROC curve (AUROC), with values ranging from 0.76 to 0.82 and estimating the mean accuracy of the model (0.70–0.75). The method showed a high flexibility, which led to a good identification of the most significant predisposing factors for shallow landslide occurrence in the different investigated areas. In particular, detailed susceptibility maps were obtained, allowing to identify the shallow landslide prone areas. This methodology combined with the use of the rainfall thresholds for triggering shallow landslides may provide an innovative tool useful for the improvement of spatial planning and early warning systems.

The influence of geological–morphological and land use settings on shallow landslides in the Pogliaschina T. basin (northern Apennines, Italy)

ABSTRACT On 25 October 2011, the eastern Liguria (Vara Valley and Cinque Terre area) and northwestern Tuscany (Magra Valley) were affected by an extreme rainstorm (almost 600 mm/24 h) that caused floods, thousands of shallow landslides, 13 casualties and damage to villages and infrastructure. This study aims at analysing the main features of the 25 October 2011 shallow landslides occurred in the Pogliaschina Torrent basin (25 km2 wide, Vara Valley), in order to investigate the influence of specific predisposing factors (land use, geological and structural setting, plan and profile curvature, slope angle and aspect) on landslide occurrence. For this purpose, both a landslide inventory map and a geology map (scale 1:10,000) were prepared. In addition, a database including the main geological, geomorphological, structural and land use features of the landslide source areas was implemented. The relationship between landslide source areas and the main predisposing factors was evaluated through spatial and statistical analysis.

Fragility of Territory and Infrastructures Resulting from Rainstorms in Northern Tuscany (Italy)

In mountainous urbanized areas, shallow landslides cause significant, often unpredictable, hazard conditions. Landslides may involve and destroy infrastructures, kill people. Source areas are often located along the road network, representing a recurring situation also during not heavy rainstorms in Northern Tuscany. The landslides are generally first time debris slide-flows, occurring in peculiar environments: colluvium/debris slope cover (0.5–2 m thick), semipermeable-impermeable bedrock, hollow shaped slope, high slope gradient. Despite little size, they cause damage and deaths owing to their high velocity and erosion power. The source area along the road network is generally associated to the lack of efficient drainage systems. In fact, the concentration of uncontrolled runoff downslope creates an “unnatural” increase in pore pressure that the only rainfall should not have produced. This generates reaching and exceeding of the local critical rainfall threshold, making landslide hazard assessment more difficult. As example, in October 2010, almost 60 % of the landslides source area in the Massa area was located along the road network.

Shallow Landslides Triggered by the 25 October 2011 Extreme Rainfall in Eastern Liguria (Italy)

A very heavy rainstorm hit Eastern Liguria (Vara Valley and Cinque Terre area) and North-western Tuscany (Magra Valley) on 25 October 2011. This event produced floods and hundreds of shallow landslides, causing 13 casualties and severe damage to villages, infrastructures and roads. In the Vara Valley the Brugnato rain gauge recorded 468.8 mm in 6 h, with a maximum rainfall intensity of 143.4 mm/h. A landslide inventory map was carried out, together with a database including the main features of the source areas. At present, the database is complete for the Pogliaschina Torrent basin (Vara Valley), where at least 658 shallow landslides (mainly soil slip-debris flows and debris flows) were triggered. The shallow landslides induced by the 25 October 2011 event were analysed, together with geological, geomorphological and land use features of their source areas, with the aim of identifying common triggering conditions. This paper shows preliminary results of the comparison between the landslide inventory and the main slope features of the Pogliaschina Torrent basin.

Assessing shallow landslide susceptibility by using the SHALSTAB model in Eastern Liguria (Italy)

On 25 October 2011 a heavy rainstorm hit Eastern Liguria (Vara Valley and Cinque Terre) and North-western Tuscany (Magra Valley), causing floods and hundreds of shallow landslides.This study aims at assessing the shallow landslide susceptibility using the physically based model SHALSTAB (Shallow Landslide Stability Model) in the Pogliaschina Torrent basin (Vara Valley). The susceptibility map elaborated with SHALSTAB was compared with the landslide inventory map, which confirmed the good performance of this model for the study area.The implementation of the SHALSTAB model provided a preliminary shallow landslide susceptibility map of the Pogliaschina T. basin and quite promising results on the shallow landslide spatial prediction.

Estimation of soil properties of shallow landslide source areas by dynamic penetration tests: first outcomes from Northern Tuscany (Italy)

In Northern Tuscany (Italy) rapid, shallow landslides often cause casualties and severe damage. Aimed at contributing to the characterisation of the source areas of rapid, shallow landslides, this paper deals with the geotechnical parameterisation of the mainly involved soil by means of dynamic penetration tests. The source areas are usually located in difficult access sites, where boring and undisturbed sampling are very hard and onerous. Therefore, the results of 177 dynamic penetration tests were analysed, including dynamic probing (DP) tests and standard penetration tests (SPT). The results of these tests were related to relative density Dr and friction angle Φ′ of the soil by means of empirical equations. The distribution and variability of these parameters were analysed and related to soil type, test type and probing depth. The Dr and Φ′ values coming from the DP and SPT tests were found to be comparable. The Φ′ values coming from DP tests and direct shear tests showed an acceptable correlation, confirming the utility of the DP tests. The DP test can thus be an effective tool in the estimation of the properties of potentially unstable soil slope covers.RésuméDans la Toscane septentrionale (Italie) des glissements de terrain superficiels rapides provoquent assez souvent de considérables dégâts et de victimes. Ces glissements s’amorcent fréquemment dans les zones où le difficile accès ne permet pas de recueillir des échantillons intacts pour être testés en laboratoire. Ainsi pour la caractérisation géotechnique des terrains ont été utilisés les résultats de 177 tests de pénétration dynamique, ci-inclus les Dynamic Probing Tests (DP) et les Standard Penetration Tests (SPT). Du résultat de ces tests, par des corrélations empiriques ont été estimées les valeurs moyennes de la densité relative et l’angle de frottement du terrain étudié. Distribution et variabilité des résultats obtenus ont été analysées et corrélées avec le type de sol et le type et la profondeur de la preuve. L’étude a révélé que les valeurs moyennes de la densité relative et l’angle de frottement, obtenues par des tests DP, sont comparables à ceux obtenus à partir de SPT à la même profondeur. De plus, les angles de frottement, obtenues par de tests DP, montrent une bonne corrélation linéaire avec ceux déterminés par des essais de cisaillement directs réalisés en laboratoire sur des échantillons intacts. En conclusion, les comparaisons montrent que le pénétromètre dynamique continu peut être considéré comme un instrument utile pour l’estimation de certains paramètres géotechniques des sols en pentes potentiellement instables.

Developing and testing a data-driven methodology for shallow landslide susceptibility assessment: preliminary results

In this work a data-driven methodology for shallow landslide susceptibility assessment is presented. The procedure is based on the Generalized Additive Model (Hastie and Tibshirani, 1990) and it is developed to be applied in different contexts, using terrain attributes, land use and lithological data. The application of the method in three different contexts in Italy shows the good forecasting capability of the model. The implementation of this method allows for building landslide susceptibility maps, which are a fundamental basis in hazard and risk assessment.

Geology and tectonic setting of the Fornovolasco area, Alpi Apuane (Tuscany, Italy)

ABSTRACT The study area is located in the Alpi Apuane (Tuscany, Italy), and extends between N 44°0′49.883″–E 10°20′23.467″ (SW corner) and N 44°2′53.403″–E 10°23′19.175″ (NE corner). The area shows a pile of tectonic units belonging either to the Apuane metamorphic complex, and Tuscan Nappe, stacked during the Alpine orogeny. The latter is represented by a sedimentary Triassic-Miocene succession, and it is characterized by a large-scale east-dipping monocline, with local excision of stratigraphic terms due to the occurrence of intra-unit low-angle normal faults. The Apuane metamorphic complex is formed by Paleozoic phyllites, post-Variscan magmatic rocks (Fornovolasco Metarhyolite Fm.), and Mesozoic-Tertiary metasedimentary formations. The 1:5000 scale geological map and the cross-sections illustrate the tectonic setting of the Fornovolasco area, resulting from the Tertiary tectono-metamorphic events. In the Fornovolasco area, small Tl-rich pyrite + magnetite ore bodies occur close to the contacts between the Paleozoic basement and the cover metasedimentary formations.

Rainfall Intensity-Duration Thresholds for Triggering Shallow Landslides in the Eastern Ligurian Riviera (Italy)

The Eastern Ligurian Riviera, including the famous Cinque Terre, is frequently hit by heavy rainfall, which often induces shallow landslides and floods, causing damage and sometimes death. In this context, the assessment of the rainfall thresholds for shallow landslides initiation is very important in order to improve forecasting and to arrange efficient warning systems. With this purpose, a detailed analysis of the main rainstorms was carried out. The hourly rainfall recorded from 1967 to 2006 was analysed and compared with the occurrence of shallow landslides. Critical threshold curves were defined, in terms of duration and intensity, applying statistical techniques (logistic regression) in order to separate rainfall events that induced failures and events that did not. The rainfall thresholds obtained in this work were compared with the local and regional curves proposed by various authors. The results of this analysis suggest that in the study area landslide activity initiation requires higher amount of rainfall and intensity than elsewhere.

Evidence of Permian magmatism in the Alpi Apuane metamorphic complex (Northern Apennines, Italy): New hints for the geological evolution of the basement of the Adria plate

Abstract The occurrence of metavolcanic rocks within the Paleozoic basement of the Alpi Apuane metamorphic complex has been known since long time. Among them, some massive porphyritic tourmaline-bearing rocks cropping out in the southern sector of the Alpi Apuane present some distinctive and peculiar features, differing from the better known middle Ordovician metarhyolites of the “Porfiroidi e scisti porfirici” Fm. The porphyritic tourmaline-bearing rocks belong to the recently proposed Fornovolasco Metarhyolite Fm. They are granular to porphyritic, with phenocrysts of quartz (often with magmatic embayment), pseudomorphosed feldspars, and mica (both biotite and muscovite), in a groundmass formed by quartz, white mica, albite, and K-feldspar. Tourmaline (schorl-dravite in composition) is an abundant accessory mineral, in some cases forming cm-sized spots. The studied rocks plot into the rhyolite field of the Total Alkali vs Silica classification diagram. They show a peraluminous nature, having an Alumina Saturation Index ranging from 1.3 and 3.2. Their trace-element signature is that typical of highly evolved orogenic magmas. Laser ablation-ICP-MS U—Pb datings on zircon suggest a Permian crystallization age (weighted average ages of the four samples ranging from 292 and 271 Ma), thus relating these rocks to a post-Variscan magmatism. This new dating represents the very first evidence of a Permian magmatism in the pre-Triassic basement of the Northern Apennines. The potential relationships between Permian felsic magmatism and the ore genesis in the Alpi Apuane metamorphic complex are also discussed.