Andrea Cevasco

Slow-moving landslides are very slow or extremely slow landslides that often affect urbanized slopes, involving a wide range of soil and rock materials. Often they can exhibit sudden changes in velocity related to local environmental changes, passing through slow (within 1 m/year) to rapid (more than 1 m/s) displacement. In built environments, the kinematic behaviour of these slope instabilities can lead to significant damage and even fatalities. Therefore, in active slow landslides, the prediction of movement acceleration is a crucial issue in the frame of landslide hazard and risk assessment for the design of warning systems and potential damage management. It is of great importance to investigate the factors that can drive velocity changes within unstable landslide bodies.In this contribution, we focused on the role of hydrological preparatory and triggering factors (e.g., rainfall and groundwater level variations) on the unstable mass mobility. It is known that deep-seated slow-moving landslides are driven by pore-water pressure fluctuations that can result from infiltrating precipitation and/or snowmelt. However, the relationship between precipitation, hydrological responses and movement is not straightforward, primarily due to the complexity of the processes governing the recharge of groundwater in response to the rainfall regime, which can be influenced by many factors, both external (e.g., temperature, evapotranspiration, vegetation cover) and internal (e.g., layering, cracks, fissures). Therefore, including hydrological processes and their variability in landslide modelling is of paramount importance.Here we present preliminary insights on the application of a simple physically-based model for quantifying groundwater fluctuations in response to discrete precipitation time-series in two reactivated slow-moving mass movements located in Liguria region (NW Italy): the Fontane landslide, in the Northern Apennines (eastern Liguria, Genoa Province) and the Mendatica landslide, in the Ligurian Alps (western Liguria, Imperia Province). Both landslides are rainfall-induced and affect small villages which have suffered damage in the past. The research activities are carried out in the framework of the RETURN (multi-Risk sciEnce for resilienT commUnities undeR a changiNg climate) project funded by the Italian MUR and the European Union Next-GenerationEU.Long-term hydro-geotechnical monitoring data series (e.g., groundwater table levels) available for the two selected landslides and meteorological data (e.g., rainfall and temperature) from nearby measuring stations were collected and analyzed for two significant periods in order to grasp the seasonal fluctuations of the water table and the response to rainfall events. During the modelling, each period was split into two sub-periods: one, for the calibration phase, in which meteo-hydro-geotechnical data were used to estimate the parameters needed for the simulated water table to best approximate the measured one; the second, for the validation phase, in which the goodness of the model is verified. The outcomes of this study may represent an initial basis for gaining insights about the processes that influence groundwater table variations and defining models for the simulation/prediction of quantitative scenarios related to the hydrologic preparatory processes that influence the kinematic behaviour of the two selected slow-moving landslides. Indeed, the results of the groundwater model may be used as input data for predicting the landslide displacements.

This paper deals with the investigation of factors influencing the movement patterns of a reactivated slow-moving landslide situated in the eastern Liguria region (NW Italy) through the analysis of extensive ground-based hydrological and geotechnical monitoring data. Subsurface horizontal displacement and pore water pressure data were acquired simultaneously by means of automatic sensors positioned at pre-existing and localized failure zones. The joint examination of field measurements enabled us to explore the connections between rain, pore water pressure, and displacements. The results of continuous displacement monitoring showed that the landslide kinematics involved phases of extremely slow movements alternated with periods of relative inactivity. Both stages occurred prevalently at seasonal scale displaying similar durations. The slow-motion phases took place at relatively constant pore water pressure and were ascribed to mechanisms of viscous shear displacements along failure surfaces. Inactive phases entailed no significant deformations, mostly corresponding to prolonged dry periods. The two motion patterns were interrupted by episodic sharp deformations triggered by delayed (preparation periods from 4 to 11 days) rainfall-induced pore water pressure peaks, which were ascribed to sliding mechanisms taking place through rigid-plastic frictional behaviour. During these deformation events, hysteresis relationships between pore water pressure and displacement were found, revealing far more complex hydro-mechanical behaviour.

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 analysis.

Rainfall-induced shallow landslides represent a serious threat in hilly and mountain areas around the world. The mountainous landscape of the Cinque Terre (eastern Liguria, Italy) is increasingly popular for both Italian and foreign tourists, most of which visit this outstanding terraced coastal landscape to enjoy a beach holiday and to practice hiking. However, this area is characterized by a high level of landslide hazard due to intense rainfalls that periodically affect its rugged and steep territory. One of the most severe events occurred on 25 October 2011, causing several fatalities and damage for millions of euros. To adequately address the issues related to shallow landslide risk, it is essential to develop landslide susceptibility models as reliable as possible. Regrettably, most of the current land-use and urban planning approaches only consider the susceptibility to landslide detachment, neglecting transit and runout processes. In this study, the adoption of a combined approach allowed to estimate shallow landslide susceptibility to both detachment and potential runout. At first, landslide triggering susceptibility was assessed using Machine Learning techniques and applying the Ensemble approach. Nine predisposing factors were chosen, while a database of about 300 rainfall-induced shallow landslides was used as input. Then, a Geographical Information System (GIS)-based procedure was applied to estimate the potential landslide runout using the “reach angle” method. Information from such analyses was combined to obtain a susceptibility map describing detachment, transit, and runout. The obtained susceptibility map will be helpful for land planning, as well as for decision makers and stakeholders, to predict areas where rainfall-induced shallow landslides are likely to occur in the future and to identify areas where hazard mitigation measures are needed.

This work presents the outcomes of the geomorphological investigation and mapping carried out within the Cinque Terre National Park (38 km²), an outstanding example of a human-modified landscape. Over the centuries, the natural landscape of Cinque Terre has been almost completely modified by slope terracing for agricultural purposes. Extensive field surveys, topographic maps examination and aerial photographs interpretation have led to the production, within a GIS environment, of a new geomorphological map at 1:18,000 scale which outlines the main genesis and related landforms and deposits: (i) gravity-induced features, (ii) fluvial and runoff features, (iii) coastal features and (iv) anthropogenic features. Special attention has been given to the mapping of terraced slopes, which at present are extremely vulnerable to gravity-driven processes and running water because of extensive farmland abandonment. The output map is a fundamental basis for future activities of hazard assessment and zonation and the definition of land management strategies.

In this work, experiences from the use of bioengineering techniques as geo-hydrological risk mitigation measures within the territory of Cinque Terre National Park (Eastern Liguria, Italy) after an extreme rainfall event that occurred on 25 October 2011 are described. This rainstorm was responsible for intense erosive processes and triggered numerous shallow landslides, causing severe structural and economic damage. After this disastrous event, many bioengineering interventions were planned to stabilize the most unstable slopes and the most problematic streams. Based on multidisciplinary studies and field surveys, an inventory of the executed bioengineering works was compiled. Subsequently, on the basis of expert judgement, both the efficiency and effectiveness of the works three years after their construction were examined. Furthermore, the compliance of the executed works with the design requirements was analysed. This study revealed that some of the investigated works lack post-intervention maintenance and require the adoption of remedial measures aimed at improving the biotechnical functions of live materials, which are often ineffective. This case study highlights the importance of technical aspects that should be considered during the design phase of bioengineering works, especially when implemented within protected areas. Specifically, it would be greatly helpful to define instructions for post-intervention maintenance and monitoring and to perform vegetational studies. Considering the great cultural and natural heritage of the study area, the obtained results are expected to provide useful information for the definition of guidelines for the best practices to be adopted when future bioengineering works are planned for geo-hydrological risk management purposes.