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

Investigation on Farmland Abandonment of Terraced Slopes Using Multitemporal Data Sources Comparison and Its Implication on Hydro-Geomorphological Processes

This paper presents a quantitative multi\u2010temporal analysis performed in a GIS environment and based on different spatial information sources. The research is aimed at investigating the land use transformations that occurred in a small coastal terraced basin of Eastern Liguria from the early 1950s to 2011. The degree of abandonment of cultivated terraced slopes together with its influence on the distribution, abundance, and magnitude of rainfall\u2010induced shallow landslides were accurately analysed. The analysis showed that a large portion of terraced area (77.4%) has been abandoned over approximately sixty years. This land use transformation has played a crucial role in influencing the hydro\u2010geomorphological processes triggered by a very intense rainstorm that occurred in 2011. The outcomes of the analysis revealed that terraces abandoned for a short time showed the highest landslide susceptibility and that slope failures affecting cultivated zones were characterized by a lower magnitude than those which occurred on abandoned terraced slopes. Furthermore, this study highlights the usefulness of cadastral data in understanding the impact of rainfall\u2010induced landslides due to both a high spatial and thematic accuracy. The obtained results represent a solid basis for the investigation of erosion and the shallow landslide susceptibility of terraced slopes by means of a simulation of land use change scenarios

A new procedure for an effective management of geo-hydrological risks across the "Sentiero Verde-Azzurro" trail, Cinque Terre National Park, Liguria (North-Western Italy)

In recent years, Cinque Terre National Park, one of the most famous UNESCO sites in Italy, experienced a significant increase in tourist visits. This unique landscape is the result of the rough morphology of a small coastal basin with a very steep slope and a long-term human impact, mainly represented by anthropic terraces. This setting promotes the activation of numerous geo-hydrological instabilities, primarily related to heavy rainfall events that often affect this area. Currently, the main challenge for the administrators of Cinque Terre National Park is the correct maintenance of this environment along with the functional management of the hiking trail to ensure the safety of tourists. The definition of a methodology for effective management is mandatory for the sustainable administration of this unique site. We implement a new codified procedure based on the combined use of the Operative Monography and the Survey Form, focusing on the "Sentiero Verde-Azzurro" trail, for a proper description of the known landslides affecting the trail and the identification of damage and/or landslides activated by critical meteorological events. This guarantees effective geo-hydrological risk management, which is also applicable to other similar sites in a unique environmental and cultural heritage site such as Cinque Terre Park

Crack initiation and damage thresholds in some brittle metamorphic rock types

In this study 60 unconfined compression laboratory tests performed on three brittle metamorphic rock types were analysed in order to evaluate the stress thresholds associated with the onset of stable and unstable microcracking which are known in literature as the "crack initiation stress" and the "crack damage stress", respectively. The crack initiation stress was identified applying the lateral strain response (LSR) methodology while the crack damage stress was detected where total volumetric strain reversal occurs. Although the mineralogical and textural features were quite similar among the investigated rock types, the results of this study showed that the crack initiation and damage stress values were affected by variability. However, when the normalized stress values sci/sUCS and scd/sUCS were considered, the scatter in results decrease consistently suggesting that the stable and unstable crack growth begin at similar stress levels in the investigated rock types

The influences of geological and land-use settings on shallow landslides triggered by an intense rainfall event in a coastal terraced environment

Several factors (including slope morphology, geological and structural settings, the mechanical properties of soils, weathering, hydrological and hydrogeological conditions and land use changes) can affect the origin and development of shallow landslides induced by rainfall. On 25 October 2011, an intense rainfall occurred in a wide area of Italy between eastern Liguria and northern Tuscany, triggering thousands of shallow landslides and causing widespread flooding. Floods and landslides caused severe damage to villages and infrastructure and caused 13 fatalities. This study was conducted to investigate the shallow landslides triggered by the rainfall in the Vernazza basin (Cinque Terre in eastern Liguria), where landslides and debris floods caused 3 fatalities and damage of approximately \u20ac130 million. The Vernazza basin, which is characterised by steep slopes, geological heterogeneity and unusual land use settings (approximately 50 % of the slopes have been terraced for centuries), was chosen as the focus of this study, conducted to assess the influences of several factors on landslide occurrence. Field surveys and engineering geological investigations, including laboratory and in situ tests, were carried out at various sites to assess the stratigraphic and geotechnical features of the soil slope covers. A basic engineering geological zoning was carried out, taking into account the bedrock lithology, the geotechnical properties of the soils and the land use settings. On the basis of a detailed inventory of the shallow landslides that occurred, the distribution of the landslides was analysed with respect to the slopes, the bedrock lithology, the land uses and the engineering geological setting. Terraced areas on impermeable bedrock were identified as the main landslide-prone areas. The results of the study also showed that the time since abandonment of terraces plays an important role in the occurrence of shallow landslides induced by rainfall

Geomechanical characterization of a highly heterogeneous flysch rock mass by means of the GSI method.

Over the last years, several researchers have investigated heterogeneous rock mass behaviour using the Geological Strength Index (GSI) classification system in conjunction with the Hoek\u2013Brown criterion. Because of the variable composition of such rock masses, the availability of both petrographic and mechanical features of the intact rock is essential in order to carry out geomechanical studies. Starting from the detailed evaluation of the strength and deformability of the intact rock, the GSI method was applied to the Sanremo flysch formation, which is widely outcropping in Western Italy and involved in many engineering projects. It is characterized by both a remarkable lithological heterogeneity and a structural complexity. Petrographic analysis and laboratory tests allowed the definition of the main constituent lithotypes and the identification of their mechanical properties, whereas geomechanical surveys were performed in order to estimate the GSI values and the strong/weak ratio. Based on the frequency of strong and weak layers, a \u2018\u2018weighted average\u2019\u2019 of intact rock values was used in estimating the rock mass parameters. Three main rock mass classes were identified, and a basic engineering geological map of the study area was prepared

First observations on the internal erodibility of some soil-like weathered rocks by means of Hole Erosion Tests

Concentrated leak erosion involves the initiation and evolution of soil erosion along a pre-existing crack or micro-fissure due to seeping water and it can be often source of severe safety issues in man-made hydraulic infrastructures. In laboratory, this peculiar internal soil erosion mechanism can be simulated using the Hole Erosion Test (HET). The erodibility of highly weathered rock materials is poorly investigated in literature, although these materials often interact with embankment dams or levees. In this study, a series of HET tests were performed on some soil-like weathered rock specimens, coming from a project-site located in Africa, to investigate their susceptibility to develop internal erosion in concentrated leaks. Although affected by a certain degree of variability, the first outcomings have highlighted that some samples are potentially susceptible to be affected by internal erosion with rates from moderately to extremely rapid. The results of this research provide useful insights in the implication of weathering processes in modifying hydraulic properties of rock materials. Eventually, this study testifies that the Hole Erosion Test can be suited to study the progressive erosion of highly decomposed rocks

Variability of intact rock mechanical properties for some metamorphic rock types and its implications on the number of test specimens

Intact rock strength and stiffness properties are commonly used in rock mass mechanical characterization, and their evaluation is usually based on laboratory tests. Due to the variability that affects strength and stiffness parameters, the determination of the number of laboratory- tested specimens required to obtain a reliable reference value is very useful. However, many studies reported in apposite literature focused only on the variability of strength parameters. This study investigates the variability of some of the most important strength and stiffness properties (unconfined compressive strength, indirect tensile strength, tangent and secant Young\u2019s moduli, Poisson\u2019s ratio) by applying statistical methods (statistical decision theory and statistical inference theory). A data set of 451 laboratory tests was used, performed on three rock types. The statistical analyses were applied with the aim of assessing how closely intact rock laboratory data follow a normal distribution and determining the minimum number of specimens required to obtain a reliable average value of the parameters in relation to a targeted precision index for a confidence level of 95 %. The results indicate that the minimum number of samples needed varies depending on rock and test types. Among the stiffness properties, tangent Young\u2019s modulus has a lower variability than both the secant modulus and the Poisson\u2019s ratio, whereas in terms of strength parameters, unconfined compressive strength is subject to greater variability than indirect tensile strength

Quantitative Assessment of Riverbed Planform Adjustments, Channelization, and Associated Land Use/Land Cover Changes: The Ingauna Alluvial-Coastal Plain Case (Liguria, Italy)

The active-channel planform adjustments that have occurred along the Centa, lower Arroscia and lower Neva rivers since 1930, along with the riverbed channelization processes and the land-use and land-cover changes in disconnected riverine areas, were investigated through a multitemporal analysis based on remote sensing and geographical information systems (GIS). These watercourses flow through the largest Ligurian alluvial-coastal plain in a completely anthropogenic landscape. This research is based on the integrated use of consolidated and innovative metrics for riverbed planform analysis. Specific indices were introduced to assess active-channel lateral migration in relation to the active-channel area abandonment and formation processes. The Arroscia and Neva riverbeds experienced narrowing, progressive stabilization, and braiding phenomena disappearance from 1930 to the early 1970s, and then slight narrowing up to the late 1980s. Subsequently, generalized stability was observed. Conversely, the Centa was not affected by relevant planform changes. Recently, all rivers underwent a slight to very slight width increase triggered by the November 2016 high-magnitude flood. The active-channel adjustments outlined in this paper reflect the relevant role in conditioning the river morphology and dynamics played by channelization works built from the 1920s to the early 1970s. They (i) narrowed, straightened, and stabilized the riverbed and (ii) reduced the floodable surface over the valley-floor. Thus, large disconnected riverine areas were occupied by human activities and infrastructures, resulting in a progressive increase in vulnerable elements exposed to hydrogeomorphic hazards. The outlined morphological dynamics (i) display significant differences in terms of chronology, type, and magnitude of active-channel planform adjustments with respect to the medium- and short-term morphological evolution of most Italian rivers and (ii) reflect the widespread urbanization of Ligurian major valley floors that occurred over the 20th century. The outcomes from this study represent an essential knowledge base from a river management perspective; the novel metrics enlarge the spectrum of available GIS tools for active-channel planform analysi

Relation between crack initiation-damage stress thresholds and failure strength of intact rock

The analysis of a wide literature dataset of mechanical parameters related to intact rocks from more than 480 unconfined compression tests, coupled with new laboratory tests on 132 specimens, is proposed herein with the aim of analyzing the mechanical behavior of a great variety of rock types, mainly focusing on their crack initiation (\u3c3ci) and crack damage (\u3c3cd) stress levels. These thresholds can be employed as warning indicators for rock mass damage and breakouts and represent important input parameters for numerical models. International literature lacks in a detailed analysis on the mutual dependence existing between the main mechanical properties of intact rocks and their crack stress thresholds. In this paper, the study of the correlation between crack initiation-crack damage stress levels and the failure strength of sedimentary, metamorphic and igneous rocks is carried out through single and multiple regression approaches aimed at finding reliable prediction models, which can be useful when time-consuming laboratory experimental procedures need to be avoided. The correlation between predicted and measured values demonstrates that defined models represent a good tool for the empirical estimation of \u3c3ci and \u3c3cd, and can be useful for preliminary engineering design dealing with stressinduced brittle fracturing, especially when the definition of warning indicators for rock mass damage and breakouts is needed. In fact, it is known that underground instability mainly depends on the redistribution of stresses around the excavation, which can produce induced stress concentrations, resulting in sudden release of stored energy and causing stress-induced brittle failure phenomena