Rainfall-induced shallow landslides in Northern Tuscany (Italy): geotechnical characterization and rainfall thresholds

In this preface we introduce the PhD Thesis on rainfall-induced landslides entitled “Rainfall-induced shallow landslides in Northern Tuscany (Italy): geotechnical characterization and rainfall thresholds”. Rainfall-induced landslides deserved a large interest in international literature in the last decades. The literature on this matter shows a markedly interdisciplinary approach, with contributions from different fields, such as engineering geology, soil mechanics, hydrology and geomorphology. The subject is of high interest for many practical and scientific reasons. In fact, rainfall is the most relevant factor for triggering shallow landslides, and rainfall analysis is the most frequently adopted approach in forecasting the occurrence of such phenomena. Moreover, the estimation of the properties of materials mainly involved by shallow landslides is fundamental in understanding their triggering mechanism. With the aim of contributing in studying the rainfall-induced shallow landslides in Northern Tuscany, different topics were considered: the analysis of the rainstorms of 2009-2010 period and of their consequences, the geotechnical characterization of the source areas of shallow landslides, and the determination of critical rainfall thresholds for triggering such phenomena. For these reasons, the manuscript was structured in three different but complementary papers. The papers are briefly introduced, examining their contribution in understanding of the rainfall-induced shallow landslides, as follows. The first paper “The 2009 and 2010 meteorological events in Northern Tuscany (Italy): Characteristics and effects on slope stability” describes characteristics and main effects on slope stability of the rainstorms which hit the Northern Tuscany in December 2009 (north-western Tuscany), June 2010 (Serchio River Valley) and October 2010 (Massa-Carrara province). The rainfall events were analyzed in terms of rainfall amount, intensity and duration. Moreover, the antecedent rainfall related to such events was analyzed, focusing on its role in causing instability conditions of slopes. The paper highlights again that the shallow landslides commonly occurred in peculiar geologic and geomorphologic environments: colluvium/debris thin slope cover, semi-permeable or impermeable bedrock, hollow shaped slope, high slope gradient. Moreover, it highlights that the shallow landslides in Northern Tuscany mainly involve the soils covering the arenaceous formations, such as the Macigno Fm. In this context, the second paper “Geotechnical characterization of source areas of shallow landslides by dynamic penetration tests in Northern Tuscany (Italy): first results and perspectives” aims at contributing to the characterization of the typical source areas of the shallow landslides in the Serchio River Valley, by means of dynamic penetration tests (Dynamic Probing, DP). In fact, these tools are particularly suitable to obtain geotechnical properties of the soils in difficult access slopes. Original data, coming from inspections of existing databases or expressly performed tests, are presented and discussed. Relative density and friction angle were determined processing the results of DP tests by means of empirical methods. Moreover, the soil properties obtained by DP tests were compared with those obtained by Standard Penetration Tests (SPT) and direct shear tests. The comparison suggests that the DP tests can be an effective tools in geotechnical characterization of potentially unstable soil slope covers. Finally, the last paper “Critical rainfall thresholds for triggering shallow landslides in the Serchio River Valley (Tuscany, Italy)” proposes the critical rainfall thresholds for triggering shallow landslides in Middle Serchio River Valley. The rainfall data recorded by three rain gauges in the 1935-2010 period were analyzed and compared with the occurrence of shallow landslides. The rainfall thresholds were defined in terms of mean intensity, rainfall duration and normalized using the mean annual precipitation. Some attempts were also carried out to analyze the role of rainfall prior to the damaging events. Moreover, the rainfall thresholds obtained for the study area were compared with the local, regional and global thresholds proposed by various authors. The results of these analysis suggest that in the Middle Serchio River Valley, and in general in Northern Tuscany, landslides activity initiation requires a higher amount of rainfall and greater intensity than elsewhere. At present, further research is necessary in order to reach a zonation of the shallow landslide hazard in the study area. The methodological approach used for the characterization of the source areas of the shallow landslides may be improved in several aspects (e.g. performing of direct shear tests on undisturbed soils, calibration of DP tests results and performing of empirical relations specifically calibrated on the considered soils). Other important parameters, such as slope gradient and soil thickness, may be considered. The critical rainfall thresholds obtained in this work may be tested and compared with those obtained by statistical or deterministic approaches. The evolution of this research is towards performing of susceptibility maps and comparison with rainfall thresholds for triggering shallow landslides. This is crucial for the preparation of landslide hazard maps and scenarios for different rainfall amounts in an area particularly prone to shallow landslides

Critical rainfall thresholds for triggering shallow landslides in the Serchio River Valley (Tuscany, Italy)

Abstract. The Serchio River Valley, in north-western Tuscany, is a well-known tourism area between the Apuan Alps and the Apennines. This area is frequently hit by heavy rainfall, which often triggers shallow landslides, debris flows and debris torrents, sometimes causing damage and death. The assessment of the rainfall thresholds for the initiation of shallow landslides is very important in order to improve forecasting and to arrange efficient alarm systems. With the aim of defining the critical rainfall thresholds for the Middle Serchio River Valley, a detailed analysis of the main rainstorm events was carried out. The hourly rainfall recorded by three rain gauges in the 1935–2010 interval was analysed and compared with the occurrence of shallow landslides. The rainfall thresholds were defined in terms of mean intensity I, rainfall duration D, and normalized using the mean annual precipitation. Some attempts were also carried out to analyze the role of rainfall prior to the damaging events. Finally, the rainfall threshold curves obtained for the study area were compared with the local, regional and global curves proposed by various authors. The results of this analysis suggest that in the study area landslide activity initiation requires a higher amount of rainfall and greater intensity than elsewhere

Rainfall events with shallow landslides in the Entella catchment, Liguria, northern Italy

Abstract. In recent decades, the Entella River basin, in the Liguria Apennines, northern Italy, was hit by numerous intense rainfall events that triggered shallow landslides and earth flows, causing casualties and extensive damage. We analyzed landslide information obtained from different sources and rainfall data recorded in the period 2002–2016 by rain gauges scattered throughout the catchment, to identify the event rainfall duration, D (in h), and rainfall intensity, I (in mm h−1), that presumably caused the landslide events. Rainfall-induced landslides affected the whole catchment area, but were most frequent and abundant in the central part, where the three most severe events hit on 23–24 November 2002, 21–22 October 2013 and 10–11 November 2014. Examining the timing and location of the slope failures, we found that the rainfall-induced landslides occurred primarily at the same time or within 6 h from the maximum peak rainfall intensity, and at or near the geographical location where the rainfall intensity was largest. Failures involved mainly forested and natural surfaces, and secondarily cultivated and terraced slopes, with different levels of maintenance. Man-made structures frequently characterize the landslide source areas. Adopting a frequentist approach, we define the event rainfall intensity–event duration (ID) threshold for the possible initiation of shallow landslides and hyper-concentrated flows in the Entella River basin. The threshold is lower than most of the curves proposed in the literature for similar mountain catchments, local areas and single regions in Italy. The result suggests a high susceptibility to rainfall-induced shallow landslides of the Entella catchment due to its high-relief topography, geological and geomorphological settings, meteorological and rainfall conditions, and human interference. Analysis of the antecedent rainfall conditions for different periods, from 3 to 15 days, revealed that the antecedent rainfall did not play a significant role in the initiation of landslides in the Entella catchment. We expect that our findings will be useful in regional to local landslides early warning systems, and for land planning aimed at reducing landslide risk in the study area

Quantitative comparison between two different methodologies to define rainfall thresholds for landslide forecasting

This work proposes a methodology to compare the forecasting effectiveness of different rainfall threshold models for landslide forecasting. We tested our methodology with two state-of-the-art models, one using intensity-duration thresholds and the other based on cumulative rainfall thresholds. The first model identifies rainfall intensity-duration thresholds by means of a software program called MaCumBA (MAssive CUMulative Brisk Analyzer) (Segoni et al., 2014a) that analyzes rain gauge records, extracts intensity (I) and duration (D) of the rainstorms associated with the initiation of landslides, plots these values on a diagram and identifies the thresholds that define the lower bounds of the I-D values. A back analysis using data from past events is used to identify the threshold conditions associated with the least number of false alarms. The second model (SIGMA) (Sistema Integrato Gestione Monitoraggio Allerta) (Martelloni et al., 2012) is based on the hypothesis that anomalous or extreme values of accumulated rainfall are responsible for landslide triggering: the statistical distribution of the rainfall series is analyzed, and multiples of the standard deviation (\u3c3) are used as thresholds to discriminate between ordinary and extraordinary rainfall events. The name of the model, SIGMA, reflects the central role of the standard deviations. To perform a quantitative and objective comparison, these two models were applied in two different areas, each time performing a site-specific calibration against available rainfall and landslide data. For each application, a validation procedure was carried out on an independent data set and a confusion matrix was built. The results of the confusion matrixes were combined to define a series of indexes commonly used to evaluate model performances in natural hazard assessment. The comparison of these indexes allowed to identify the most effective model in each case study and, consequently, which threshold should be used in the local early warning system in order to obtain the best possible risk management. In our application, none of the two models prevailed absolutely over the other, since each model performed better in a test site and worse in the other one, depending on the characteristics of the area. We conclude that, even if state-of-the-art threshold models can be exported from a test site to another, their employment in local early warning systems should be carefully evaluated: the effectiveness of a threshold model depends on the test site characteristics (including the quality and quantity of the input data), and a validation procedure and a comparison with alternative models should be performed before its implementation in operational early warning systems

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

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

A tool for the automatic calculation of rainfall thresholds for landslide occurrence

Abstract Empirical rainfall thresholds are commonly used to forecast landslide occurrence in wide areas. Thresholds are affected by several uncertainties related to the rainfall and the landslide information accuracy, the reconstruction of the rainfall responsible for the failure, and the method to calculate the thresholds. This limits the use of the thresholds in landslide early warning systems. To face the problem, we developed a comprehensive tool, CTRL–T ( C alculation of T hresholds for R ainfall-induced L andslides− T ool) that automatically and objectively reconstructs rainfall events and the triggering conditions responsible for the failure, and calculates rainfall thresholds at different exceedance probabilities. CTRL−T uses a set of adjustable parameters to account for different morphological and climatic settings. We tested CTRL−T in Liguria region (Italy), which is highly prone to landslides. We expect CTRL−T has an impact on the definition of rainfall thresholds in Italy, and elsewhere, and on the reduction of the risk posed by rainfall-induced landslides