Andrea Cevasco

Assessment of shallow landslide risk mitigation measures based on land use planning through probabilistic modelling

On October 25, 2011, an extreme rainfall event affected a wide area along the coasts of Cinque Terre (eastern Liguria, northern Italy). Particularly, in the Vernazza catchment, the event triggered hundreds of shallow landslides and a debris flood that caused three casualties. Investigation of the slope stability after the event was carried out aiming at defining the most effective mitigation measures which may be adopted in future land use planning. To this objective, a susceptibility model was produced and a series of scenarios were simulated using probabilistic methods. The susceptibility model has provided information about landslide conditioning factors on which to act for reducing landslide occurrence and therefore the associated risk. The simulations have taken into consideration the following alternative types of mitigation measures: (1) restoration of abandoned terraces, (2) reforestation of abandoned terraces, (3) use of local structural measures over stretches of potentially unstable hillsides and (4) avoidance of any intervention. The advantages and the disadvantages of the proposed mitigation measures for shallow landslide risk are discussed considering the results of the simulations and taking into account their complex interaction with environmental, historical, cultural and socio-economic aspects. The results show that the most effective mitigation strategy for reducing landslide risk at short term consists of applying structural measures over potentially unstable slopes. However, a long-term programme promoting the development of agricultural or forestry practices on terraced slopes is necessary. In fact, the simulations indicate that if no measures are applied to avoid the degradation of the terraced areas, landslide areal frequency would inevitably increase.

Storminess and geo-hydrological events affecting small coastal basins in a terraced Mediterranean environment

This study was prompted by the occurrence of an extreme Damaging geo-Hydrological Event (DHE) which occurred on October 25th 2011 and which affected a wide area of the northern Mediterranean region. After analysing the storm by means of the precipitation time series, the study attempts to relate the October 25th 2011 DHE with a series of other DHEs that occurred in the period 1954-2012, assessed via the use of historical data and classified according to severity, with a Storm Erosivity Indicator (Ra). The annual mean of the Ra value (2582 MJ mm ha(-1) h(-1) y(-1)) confirmed that the study area is one of the European regions with the highest rainfall erosivity level. A shift in storminess during 1991-2012 with respect to 1954-1990 was observed. A return period of 1000 years was calculated for the single storm erosivity of October 25th, which contributed to 84% of the total annual storm erosivity of 2011 A quite good agreement was found comparing DHE distribution and severity with Ra anomalies over time. As a matter of fact, most of the low severity DHEs (62.5%) occurred in years in which the Ra was below the average value. Moreover, almost all DHEs (93%) ranging from medium- to very high-severity occurred in years for which the Ra exceeded the average value. With regard to the occurrence of the most severe DHE classes, a threshold of the Ra and a recurrence time of approximately 3300 MJ mm ha(-1) h(-1) y(-1) and 12 years, respectively, were identified. Finally, some evidences suggest that an increasing frequency of DHEs is expected in the forthcoming years. It is argued that understanding these issues is a major priority for future research in order to improve land and urban planning strategies for preserving people and the environment, leading ultimately to an effective risk reduction.

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.

Analysis of Site Amplification Phenomena: An Application in Ripabottoni for the 2002 Molise, Italy, Earthquake

The geophysical working group of the University of Genoa conducted a field experiment to analyze site amplification effects in Ripabottoni, a village in the Molise region of Italy. We used both noise and earthquake recordings, combined with detailed geologic and geotechnical surveys, to define site amplification phenomena. The site effects determination was obtained using the Nakamura technique and the H/V spectral analysis applied to earthquake recordings. The results were validated by applying a one-dimensional simulation code. The computed spectral ratios point out three different typologies of site effects: the southern sector of Ripabottoni is characterized by an absence of local amplification phenomena; the central sector of the village shows a local amplification phenomena with a fundamental frequency of 4–6 Hz; and the northeastern side of the village shows a site response at a fundamental frequency between 2–3 Hz.

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–Brown 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 “weighted average” 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.

Geo-hydrological Risk Mitigation Measures and Land-Management in a Highly Vulnerable Small Coastal Catchment

On 25 October 2011 very intense rainfall affected a wide area between eastern Liguria and northern Tuscany (NW Italy) causing thousands of landslides and floods. The Vernazza catchment (Cinque Terre) suffered very severe damages that affected both the road network and the historical village of Vernazza, where a disastrous debris flood occurred. After the disaster, in the framework of the land management of the Vernazza municipality, some local structural works aimed to secure the exposed buildings and roads, have been carried out by emergency procedure. This paper describes the different executed works, affecting only problematic slopes and stream segments. It is believed that geo-hydrological risk mitigation measures able to ensure long-term effects are needed at basin scale. Detailed studies aimed to hazard assessment and to the evaluation of vulnerable areas are required for the choice of proper risk mitigation strategies. Considering the high environmental and cultural value of Cinque Terre, characterized by a man-made terraced landscape, that currently is mainly abandoned, the issue of sustainable land management can be crucial both for risk mitigation and socio-economic development.

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.

Rapid debris volume estimation by LiDAR data derived DEMs: applications to the 25 October 2011 debris flood event at Vernazza (Cinque Terre, Italy)

This study describes an example of analysis of morphological changes by using high resolution DEMs derived from LiDAR data. The evaluation of eroded or deposition volumes is based on the comparison between topographical profiles derived from two time-series of high resolution DEMs.The proposed method has been applied to selected areas of the Vernazza basin (Cinque Terre, eastern Liguria) that suffered strong morphological changes due to the 25 October 2011 very intense rainfall event. Three human embankments areas located along channels, completely or partially destroyed during the event, were selected for calculating eroded volumes whereas five areas in the valley floor were investigated for estimating deposition volumes. The obtained results have shown the impact of human embankments in increasing the effects of very hazardous phenomena typical of hilly or mountain areas, such as debris flows and debris floods. The proposed method, although approximate, can be very useful when a rapid estimation of volume mobilized in specific areas is required.

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

(*) Università di Genova Dipartimento di Scienze della Terra, dell’Ambiente e della Vita Corso Europa, 26 16132 Genova, Italy (**) Università di Pisa Dipartimento di Scienze della Terra Via S. Maria, 53 56126 Pisa, Italy Corresponding author: cevasco@dipteris.unige.it PRELIMINARY ANALYSIS OF THE NOVEMBER 10, 2014 RAINSTORM AND RELATED LANDSLIDES IN THE LOWER LAVAGNA VALLEY (EASTERN LIGURIA)

Multi-method Evaluation of Denudation Rates in Small Mediterranean Catchments

The paper presents the results of the research tasks of the Quantitative Geomorphology Working Group (of the Italian Association of Physical Geography and Geomorphology) focused on multi-method evaluation of denudation rates in small catchments of Italy. Several study areas are compared with the goal of quantifying the morphodynamic evolution in different response times and with traditional and innovative techniques. The final aims are the direct erosion monitoring, the geomorphic analysis for the comprehension of drainage basin morphodynamics, up to the geomorphological hazard evaluation. The catchments are key Mediterranean areas particularly sensitive to climatic and anthropic modifications. The efforts of the Working Group are finalized to favour scientific collaboration activities among members with the aim of strengthen the potential of Quantitative Gomorphology in morphodynamic studies.