Ascanio Rosi

Statistical and environmental analyses for the definition of a regional rainfall threshold system for landslide triggering in Tuscany (Italy)

The aim of this work is the determination of regional-scale rainfall thresholds for the triggering of landslides in the Tuscany Region (Italy). The critical rainfall events related to the occurrence of 593 past landslides were characterized in terms of duration (D) and intensity (I). I and D values were plotted in a log-log diagram and a lower boundary was clearly noticeable: it was interpreted as a threshold representing the rainfall conditions associated to landsliding. That was also confirmed by a comparison with many literature thresholds, but at the same time it was clear that a similar threshold would be affected by a too large approximation to be effectively used for a regional warning system. Therefore, further analyses were performed differentiating the events on the basis of seasonality, magnitude, location, land use and lithology. None of these criteria led to discriminate among all the events different groups to be characterized by a specific and more effective threshold. This outcome could be interpreted as the demonstration that at regional scale the best results are obtained by the simplest approach, in our case an empirical black box model which accounts only for two rainfall parameters (I and D). So a set of thresholds could be conveniently defined using a statistical approach: four thresholds corresponding to four severity levels were defined by means of the prediction interval technique and we developed a prototype warning system based on rainfall recordings or weather forecasts.

Landslides triggered by rainfall: A semi-automated procedure to define consistent intensity-duration thresholds

In this paper, a methodology to automate and standardize the identification of rainfall intensity-duration thresholds for landslides triggering is presented. A newly developed software called MaCumBA (MAssive CUMulative Brisk Analyzer) can be used to analyze rain-gauge records, extract the intensities (I) and durations (D) of the rainstorms associated with the initiation of landslides, plot these values on a diagram and identify thresholds that define the lower bounds of the aforementioned I-D values. Because the methodology is automated, it is possible to process a relevant amount of data in short times, while allowing for user decision input. A back analysis using data from past events that did not trigger landslides can be used to identify the threshold conditions associated with the least amount of false alarms. We applied the methodology in two test sites. A validation procedure returned satisfactory results, demonstrating the potential utility of the proposed methodology in the development of landslide warning systems.

Updating EWS rainfall thresholds for the triggering of landslides

In this paper, the updating of rainfall thresholds for landslide early warning systems (EWSs) is presented. Rainfall thresholds are widely used in regional-scale landslide EWSs, but the efficiency of those systems can decrease during the time, so a periodically updating should be required to keep their functionality. The updating of 12 of the 25 thresholds used in the EWS of Tuscany region (central Italy) is presented, and a comparison between performances of new and previous thresholds has been made to highlight the need of their periodical update. The updating has been carried out by collecting ca. 1200 new landslide reports (from 2010 to March 2013) and their respective rainfall data, collected by 332 rain gauges. The comparison has been made by the use of several statistical indexes and showed a marked increasing in the performances of the new thresholds with respect to previous ones.

A Procedure to Map Subsidence at the Regional Scale Using the Persistent Scatterer Interferometry (PSI) Technique

In this paper, we present a procedure to map subsidence at the regional scale by means of persistent scatterer interferometry (PSI). Subsidence analysis is usually restricted to plain areas and where the presence of this phenomenon is already known. The proposed procedure allows a fast identification of subsidences in large and hilly-mountainous areas. The test area is the Tuscany region, in Central Italy, where several areas are affected by natural and anthropogenic subsidence and where PSI data acquired by the Envisat satellite are available both in ascending and descending orbit. The procedure consists of the definition of the vertical and horizontal components of the deformation measured by satellite at first, then of the calculation of the “real” displacement direction, so that mainly vertical deformations can be individuated and mapped.

Rainfall thresholds for rainfall-induced landslides in Slovenia

In Slovenia, rainfall-induced landslides lead to considerable damages, even causing human losses. In order to reduce the impact of this kind of landslide, several researchers analyzed rainfall-induced landslides in this country, but to date, no rainfall thresholds have been developed for a landslide warning system at national scale. In this paper, the definition of rainfall thresholds for rainfall-induced landslides in Slovenia is presented. The thresholds have been calculated by collecting approximately 900 landslide data and the relative rainfall amounts, which have been collected from 41 rain gauges. To define the thresholds, an existing procedure characterized by a high degree of objectiveness has been used. This approach is based on a software that was developed for a test site with very different characteristics (Tuscany, central Italy). At first, a single national threshold has been defined; subsequently, the country was divided into four zones, on the basis of the major river basins. The effectiveness of the thresholds has been verified by the use of several statistical parameters and it resulted in quite good performances, even if with some uncertainties, probably due to the quality of the available data. Besides the setting of a threshold system, usable for civil protection purposes at national scale, an additional outcome of this work was the possibility of applying, with good results, a methodology defined for another region, therefore testing its degree of exportability in different settings.

A new appraisal of the Ancona landslide based on geotechnical investigations and stability modelling

On the night of 13 December 1982, Ancona experienced the catastrophic reactivation of an old and large landslide located along the coast to the west of the city. The outcomes of past and new geotechnical investigations and the data from the 30 year readings of the monitoring instruments have been integrated to redefine and update the actual location of the sliding surfaces. According to the new analysis, the landslide involves four main sliding surfaces with different extents and depths. The deepest surfaces converge at depth in a shear band and their toes are positioned near or beyond the coast. Numerical and analytical modelling of the landslide has been carried out using the newly derived sliding surface geometries. The numerical modelling has allowed a qualitative assessment of the deformation pattern, confirming the geometry of the sliding surfaces derived from the geotechnical investigations. The stability analyses have been performed applying the limit equilibrium method to quantify the instability conditions of the landside. The analyses have been carried out for five stratigraphic–geotechnical scenarios. All of these scenarios show a stability condition near the limit equilibrium.

Subsidence mapping at regional scale using persistent scatters interferometry (PSI): The case of Tuscany region (Italy)

Abstract In this paper the mapping of subsidences in Tuscany (Italy) is presented. To achieve our goal satellite SAR data processed with persistent scatters interferometry (PSI) technique have been used. Several subsiding areas have been identified and three of them have been analyzed in detail and subsequently compared with literature work both to validate the results and to analyze the evolution of the identified subsidences. In general, this comparison confirmed the quality of the analyses and, furthermore, lead to the identification of an important ground uplift close to Prato city, an area that was historically affected by subsidence.

The new landslide inventory of Tuscany (Italy) updated with PS-InSAR: geomorphological features and landslide distribution

In this paper, the updating of the landslide inventory of Tuscany region is presented. To achieve this goal, satellite SAR data processed with persistent scatter interferometry (PSI) technique have been used. The updating leads to a consistent reduction of unclassified landslides and to an increasing of active landslides. After the updating, we explored the characteristics of the new inventory, analysing landslide distribution and geomorphological features. Several maps have been elaborated, as sliding index or landslide density map; we also propose a density-area map to highlight areas with different landslide densities and sizes. A frequency-area analysis has been performed, highlighting a classical negative power-law distribution. We also explored landslide frequency for lithology, soil use and several morphological attributes (elevation, slope gradient, slope curvature), considering both all landslides and classified landslide types (flows, falls and slides).

A Regional Real Time Landslide Warning System Based on Spatially Variable Rainfall Thresholds

In the Tuscany region (23,000 km2, Central Italy) landslides triggered by rainfall are a recurring phenomenon. We set up a regional warning system for the prediction and monitoring of the occurrence of landslides, which is based on statistical intensity–duration rainfall thresholds. Since a single regional threshold would be affected by a too large uncertainty, the region was partitioned into 25 alert zones and for each of them an independent set of thresholds was defined analyzing with an automated and objective procedure the rainfall measurements connected to the triggering of 2,132 past landslides.

Combination of Rainfall Thresholds and Susceptibility Maps for Dynamic Landslide Hazard Assessment at Regional Scale

We propose a methodology to couple rainfall thresholds and susceptibility maps for dynamic landslide hazard assessment at regional scale. Both inputs are combined in a purposely-built hazard matrix to get a spatially and temporally variable definition of landslide hazard: while statistical rainfall thresholds are used to accomplish a temporal forecasting with very coarse spatial resolution, landslide susceptibility maps provide static spatial information about the probability of landslide occurrence at fine spatial resolution. The test site is the Northern part of Tuscany (Italy), where a recent landslide susceptibility map and a set of recently updated rainfall thresholds are available. These products were modified and updated to meet the requirements of the proposed procedure: the susceptibility map was reclassified and the threshold set was expanded defining additional thresholds. The hazard matrix combines three susceptibility classes (S1, low susceptibility; S2 medium susceptibility; S3 high susceptibility) and three rainfall rate classes (R1, R2, R3), defining five hazard classes, from H0 (null hazard) to H4 (high hazard). A key passage of the procedure is the appropriate calibration and validation of the matrix, letting the hazard classes have a precise meaning in terms of expected consequences and hazard management. The employ of the proposed procedure in a regional warning system brings two main advantages: (i) it is possible to better hypothesize when and where landslide are expected and with which hazard degree, thus fostering a more effective hazard and risk management (e.g. setting priorities of intervention); (ii) the spatial resolution of the regional scale warning system is markedly refined because from time to time the areas where landslides are expected represent only a fraction of the alert zone.