The influence of the geological and geomorphological settings on shallow landslides. An example in a temperate climate environment: the June 19th, 1996 event in north-western Tuscany (Italy)

On June 19, 1996, an extremely heavy rainstorm hit a restricted area in the Apuan Alps (northwestern Tuscany, Italy). Its max intensity concentrated over an area of about 150 km2 astride the Apuan chain, where 474 mm was recorded in about 12 h (21% of the mean annual precipitation, with an intensity up to 158 mm/h). The storm caused floods and hundreds of landslides and debris flows, which produced huge damage (hundreds of millions of Euros), partially destroyed villages and killed 14 people. This paper reports the results obtained from a detailed field survey and aerial view interpretation. In the most severely involved area, 647 main landslides were investigated, mapped and related to the geologic, geomorphic and vegetational factors of the source areas. This was in order to define the influence of these factors and contribute to an evaluation of the landslide hazard in the study area. An assessment was also made of the total area and volume of material mobilised by landsliding. The study area, about 46 km2 wide, includes three typically mountainous basins, characterised by narrow, deep cut valleys and steep slopes, where many rock types outcrop. Most of the landslides were shallow and linear, referable to complex, earth and debris translational slide, which quickly developed into flow (soil slip – debris flow). Usually, they involved colluvium and started in hollows underlain by metamorphic rock (metasandstone and phyllite), often dipping downslope. Therefore, bedrock lithology and impermeability appeared to be important factors in the localisation of the landslide phenomena. The investigation of the geomorphic and land use features in the source areas also frequently highlighted a rectilinear profile of the slope, a high slope gradient (31–45j) and dense chestnut wood cover. In the area, about 985,000 m2 (2.1% of 46 km2) was affected by landsliding and about 700,000 m2 of this area was covered by chestnut forest. The landslides removed about 7000 trees. The volume of mobilised material was about 1,360,000 m3; about 220,000 m3 remained on the slopes, while the rest poured into the streams. In addition, about 945,000 m3 was mobilised by the torrential erosion in the riverbeds. D 2004 Elsevier B.V. All rights reserved

Preliminary analysis of the November 10, 2014 rainstorm and related landslides in the lower Lavagna valley (eastern Liguria)

On the evening of November 10, 2014, eight rainfall-induced shallow landslides were triggered on a slope in the lower Lavagna valley (eastern Liguria, Italy). Most of the shallow landslides were channelled as flows into steep hollows and reached the toe of the slope, where some sparse houses were built. One of these landslides impacted and destroyed a building located just at a steep channel outlet, causing two fatalities. Damage affected also agricultural terracing as well as some other buildings and a road running at the toe of the slope, which was buried for long tracts by landslide deposits. Since a few days after the landslides occurrence, various activities were carried out, with the aim of better understanding both the triggering and predisposing factors of landslides. These activities included field surveys, rainfall data analysis, topographic/thematic maps, DEM and aerial photo analyses, preliminary laboratory tests on soil samples. From the analyses performed, it seems that, in addition to the rainfall characteristics of the November 10, 2014 event, the antecedent rainfall may have played an important role as landslides predisposing factor. Other relevant predisposing factors can be referred to slope steepness, presence of hollows, stratigraphic and structural settings at the source areas and lack of maintenance of terracing. Investigations are still in progress to achieve a complete geotechnical and hydraulic characterization of soils. Furthermore, it is also expected to extend the analyses performed to the whole area affected by shallow landslides. However, we believe the results of this study can be helpful in shallow landslide modelling, hazard assessment and planning of appropriate risk mitigation measures

Preliminary analysis of the November 10, 2014 rainstorm and related landslides in the lower Lavagna valley (eastern Liguria)

On the evening of November 10, 2014, eight rainfall-induced shallow landslides were triggered on a slope in the lower Lavagna valley (eastern Liguria, Italy). Most of the shallow landslides were channelled as flows into steep hollows and reached the toe of the slope, where some sparse houses were built. One of these landslides impacted and destroyed a building located just at a steep channel outlet, causing two fatalities. Damage affected also agricultural terracing as well as some other buildings and a road running at the toe of the slope, which was buried for long tracts by landslide deposits. Since a few days after the landslides occurrence, various activities were carried out, with the aim of better understanding both the triggering and predisposing factors of landslides. These activities included field surveys, rainfall data analysis, topographic/thematic maps, DEM and aerial photo analyses, preliminary laboratory tests on soil samples. From the analyses performed, it seems that, in addition to the rainfall characteristics of the November 10, 2014 event, the antecedent rainfall may have played an important role as landslides predisposing factor. Other relevant predisposing factors can be referred to slope steepness, presence of hollows, stratigraphic and structural settings at the source areas and lack of maintenance of terracing. Investigations are still in progress to achieve a complete geotechnical and hydraulic characterization of soils. Furthermore, it is also expected to extend the analyses performed to the whole area affected by shallow landslides. However, we believe the results of this study can be helpful in shallow landslide modelling, hazard assessment and planning of appropriate risk mitigation measures

Probabilistic rainfall thresholds for triggering debris flows in a human-modified landscape

Abstract In the Carrara Marble Basin (CMB; Apuan Alps, Italy) quarrying has accumulated widespread and thick quarry waste, lying on steep slopes and invading valley bottoms. The Apuan Alps are one of the rainiest areas in Italy and rainstorms often cause landslides and debris flows. The stability conditions of quarry waste are difficult to assess, owing to its textural, geotechnical and hydrogeological variability. Therefore, empirical rainfall thresholds may be effective in forecasting the possible occurrence of debris flows in the CMB. Three types of thresholds were defined for three rain gauges of the \CMB\ and for the whole area: rainfall intensity–rainfall duration (ID), cumulated event rainfall–rainfall duration (ED), and cumulated event rainfall normalized by the mean annual precipitation–rainfall intensity (EMAPI). The rainfall events recorded from 1950 to 2005 was analyzed and compared with the occurrence of debris flows involving the quarry waste. They were classified in events that triggered one or more debris flows and events that did not trigger debris flows. This dataset was fitted using the logistic regression method that allows us to define a set of thresholds, corresponding to different probabilities of failure (from 10% to 90%) and therefore to different warning levels. The performance of the logistic regression in defining probabilistic thresholds was evaluated by means of contingency tables, skill scores and receiver operating characteristic (ROC) analysis. These analyses indicate that the predictive capability of the three types of threshold is acceptable for each rain gauge and for the whole CMB. The best compromise between the number of correct debris flow predictions and the number of wrong predictions is obtained for the 40% probability thresholds. The results obtained can be tested in an experimental debris flows forecasting system based on rainfall thresholds, and could have implications for the debris flow hazard and risk assessment in the CMB

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Analysis of fragmented piezometric levels records: the ARTE (Antecedent Recharge Temporal Effectiveness) approach

In contrast to climatic data, piezometric records are often fragmented both in time and space continuity, despite their crucial importance in groundwater studies. This work presents a new method for analysis of groundwater level vs. recharge processes relation from fragmented piezometric data, called Antecedent Recharge Temporal Effectiveness (ARTE). The ARTE method was tested on 5 year-long (2016-2020) water table level datasets measured by three automatic piezometers located in the Lucca plain (Tuscany, Italy). For each piezometric level time series, measurements were extracted every 30, 60, and 120 days, and randomly, obtaining fragmented records inlcuding less than 3% of the complete time series. As for recharge processes of the monitored aquifer, rainfall and riverbed infiltration were selected. Hence, daily rainfall and daily mean river stage time series were acquired from different automatic raingauges and hydrometers respectively. The relationship between these recharge processes and the variation of the piezometric level from the artificially fragmented datasets were evaluated with the ARTE method. The ARTE method was potentially able to identify maximum correlation time intervals, for which the recharge processes are most likely to influence the groundwater level. Based on the analysis conducted on the fragmented piezometric datasets, the reconstruction of each piezometric time series was attempted for the study period. The simulated daily groundwater level records have RMSE values between 0.21 m and 0.73 m and NRMSE values between 0.08 and 0.16, which are satisfactory results when compared with other more complex simulation procedures, in which the training datasets are increasingly larger

Human exposure to thallium through tap water: A study from Valdicastello Carducci and Pietrasanta (northern Tuscany, Italy)

A geological study evidenced the presence of thallium (Tl) at concentrations of concern in groundwaters near Valdicastello Carducci (Tuscany, Italy). The source of contamination has been identified in the Tl-bearing pyrite ores occurring in the abandoned mining sites of the area. The strongly acidic internal waters flowing in the min- ing tunnels can reach exceptional Tl concentrations, up to 9000 μg/L. In September 2014 Tl contamination was also found in the tap water distributed in the same area (from 2 to 10 μg/L). On October 3, 2014 the local authorities imposed a Do Not Drink order to the population. Here we report the results of the exposure study carried out from October 2014 to October 2015, and aimed at quantifying Tl levels in 150 urine and 318 hair samples from the population of Valdicastello Carducci and Pietrasanta. Thallium was quantified by inductively coupled plasma — mass spectrometry (ICP-MS). Urine and hair were chosen as model matrices indicative of different time periods of exposure (short-term and long- term, respectively). Thallium values found in biological samples were correlated with Tl concentrations found in tap water in the living area of each citizen, and with his/her habits. Thallium concentration range found in hair and urine was 1–498 ng/g (values in unexposed subjects 0.1–6 ng/g) and 0.046–5.44 μg/L (reference value for the European population 0.006 μg/L), respectively. Results show that Tl levels in biological samples were significantly associat- ed with residency in zones containing elevated water Tl levels. The kinetics of decay of Tl concentration in urine samples was also investigated. At the best of our knowledge, this is the first study on human contamination by Tl through water involving such a high number of samples

shallow landslides susceptibility assessment in different environments

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

Estimation of the susceptibility of a road network to shallow landslides with the integration of the sediment connectivity

Abstract. Landslides cause severe damage to the road network of the hit zone, in terms of both direct (partial or complete destruction of a road or blockages) and indirect (traffic restriction or the cut-off of a certain area) costs. Thus, the identification of the parts of the road network that are more susceptible to landslides is fundamental to reduce the risk to the population potentially exposed and the financial expense caused by the damage. For these reasons, this paper aimed to develop and test a data-driven model for the identification of road sectors that are susceptible to being hit by shallow landslides triggered in slopes upstream from the infrastructure. This model was based on the Generalized Additive Method, where the function relating predictors and response variable is an empirically fitted smooth function that allows fitting the data in the more likely functional form, considering also non-linear relations. This work also analyzed the importance, on the estimation of the susceptibility, of considering or not the sediment connectivity, which influences the path and the travel distance of the materials mobilized by a slope failure until hitting a potential barrier such as a road. The study was carried out in a catchment of northeastern Oltrepò Pavese (northern Italy), where several shallow landslides affected roads in the last 8 years. The most significant explanatory variables were selected by a random partition of the available dataset in two parts (training and test subsets), 100 times according to a bootstrap procedure. These variables (selected 80 times by the bootstrap procedure) were used to build the final susceptibility model, the accuracy of which was estimated through a 100-fold repetition of the holdout method for regression, based on the training and test sets created through the 100 bootstrap model selection. The presented methodology allows the identification, in a robust and reliable way, of the most susceptible road sectors that could be hit by sediments delivered by landslides. The best predictive capability was obtained using a model in which the index of connectivity was also calculated according to a linear relationship, was considered. Most susceptible road traits resulted to be located below steep slopes with a limited height (lower than 50 m), where sediment connectivity is high. Different land use scenarios were considered in order to estimate possible changes in road susceptibility. Land use classes of the study area were characterized by similar connectivity features. As a consequence, variations on the susceptibility of the road network according to different scenarios of distribution of land cover were limited. The results of this research demonstrate the ability of the developed methodology in the assessment of susceptible roads. This could give the managers of infrastructure information about the criticality of the different road traits, thereby allowing attention and economic budgets to be shifted towards the most critical assets, where structural and non-structural mitigation measures could be implemented

Linee guida per l’elaborazione del modello idrogeologico concettuale

Introduzione. Il presente quaderno tecnico affronta il tema del modello idrogeologico concettuale, ovvero di quel passaggio dalle categorie storicamente qualitative, naturalistiche della geologia classica, a quelle di un inquadramento stringente, coerente e quantificato dell’idrogeologia applicata e dei suoi corollari numerici. Vengono di seguito descritti metodi, strumenti e concetti per una corretta elaborazione del modello concettuale, ovvero come conglobare l’informazione meteo-climatica, idrochimica, geochimica, agronomica, litologica, petro- chimica ed idrologica in uno schema coerente che consenta la sintesi olistica della dinamica delle acque sotterranee e di quanto esse veicolano, in un quadro comprensibile anche ad altre categorie tecnico-professionali. Un modello concettuale, correttamente concepito, agevola la comprensione dei fenomeni accaduti e la previsione di cosa potrà succedere anche, ma non solo, in conseguenza di azioni antropiche in atto o in programmazione. Si è quindi affrontato anche il tema delle normative che trattano la materia in riferimento alla programmazione ter- ritoriale, alla progettazione di manufatti ed opere ed alla tutela ed al risanamento dell’ambiente. Il modello concettuale è base imprescindibile per ogni successiva valutazione quantitativa. In assenza di un modello verificato e robusto, non solo il passaggio all’implementazione di un modello numerico è velleitario, ma anche i semplici calcoli deterministico-analitici, più ampiamente diffusi ed applicati, rischiano di perdere ogni significato pratico. Ovviamente il quaderno non ha alcuna pretesa di esaustività né di formulare codici, ma cerca semplicemente di for- nire ai colleghi una traccia utile, ancorché insufficiente se non corredata dagli approfondimenti che ciascuno di noi è sempre tenuto a fare, per soddisfare in scienza e coscienza le esigenze dei committenti. Il paragrafo bibliografia e l’annesso glossario servono proprio a questo