Silvia Luciani

Catalogue of Rainfall Events with Shallow Landslides and New Rainfall Thresholds in Italy

In Italy, rainfall-induced shallow landslides are frequent and harmful phenomena. The prediction of their occurrence is of social significance for civil protection purposes. For the operational prediction of rainfall-induced shallow landslides empirical rainfall thresholds based on the statistical analysis of past rainfall conditions that triggered slope failures are commonly used. The paper describes a catalogue of 1981 rainfall events, which caused 2408 shallow landslides in Italy in the period 1996–2012. Information on rainfall-induced landslides was collected searching chiefly online newspaper archives, blogs, and fire brigade reports. For each documented failure, we reconstructed the triggering rainfall conditions (rainfall duration D and cumulated rainfall E) using national and regional rain gauge networks. We analysed the rainfall conditions to determine new ED rainfall thresholds for Italy. The calculated thresholds can be implemented in a landslide forecasting system to mitigate landslide hazard and risk.

Rainfall Thresholds for Possible Occurrence of Shallow Landslides and Debris Flows in Italy

In mountain regions worldwide, rainfall-induced landslides and associated debris flows erode slopes, scour channels, and contribute to the formation of alluvial fans that may harm humans and destroy buildings. Rainfall-induced slope failures are frequent and widespread in Italy, where individual rainfall events can result in single or multiple slope failures in small areas or in very large regions. Most of the harmful failures were rainfall-induced, and several were shallow slides or debris flows. In the 60-year period 1950–2009, casualties due to landslides were at least 6,349, an average of 16 harmful events per annum. The large number of harmful events indicates the considerable risk posed by rainfall-induced shallow landslides and debris flows to the population of Italy (Guzzetti et al. 2005a; Salvati et al. 2010).

TRMM satellite rainfall estimates for landslide early warning in Italy: preliminary results

Early warning systems can predict rainfall-induced landslides by comparing rainfall data with landslide rainfall thresholds. These systems are based on empirical rainfall thresholds defined using rain gauges data. Despite quantitative satellite rainfall estimates are currently available, limited research has compared satellite estimates and rain gauge measurements for the forecasting of possible landslide occurrence. In this work, we validate satellite estimates obtained for Italy by the NASA Tropical Rainfall Measuring Mission (TRMM) against rainfall measurements from the Italian rain gauge network (< 1950 rain gauges), in the period from 1 September 2009 to 31 August 2010. Using cumulative rainfall measurements/estimates, we: (i) evaluate the correlation between the rain gauge measurements and the satellite estimates in different morpho-climatological domains, (ii) analyse the distributions of the ground-based measurements and the satellite estimates using different statistical approaches, and (iii) compare rainfall events derived automatically from satellite and rain gauge rainfall series. We observe differences between satellite estimates and rain gauge measurements in different morpho-climatological domains. The differences are larger in mountain areas, and collectively reveal a complex relationship between the ground-based measurements and the satellite estimates. We find that a power law correlation model is appropriate to describe the relation between the two rainfall data series. We conclude that specific rainfall thresholds must be defined to exploit satellite rainfall estimates in existing landslide early warning systems.

Probabilistic Prediction of Landslides Induced by Rainfall

Landslides are widespread and cause every year casualties and extensive damages. Predicting their spatial and temporal occurrence of landslides is a problem of scientific and societal interest. Empirical threshold model approaches proposed in the literature have limitations related to the heuristic identification of rainfall conditions triggering landslide, to the subjective choice of threshold model, to the biased probability estimation related to the classical empirical threshold model, and to limited use of rainfall events not associated to landslides. A new probabilistic empirical prediction schema is proposed to overcome these limitations. The model was applied successfully in Umbria region considering rain gauge measures and satellite rainfall estimate.

TXT-tool 2.039-1.1 Italian National Early Warning System

In Italy rainfall-induced slope failures occur every year, claiming lives and causing severe economic disruptions. We have designed and implemented a warning system, named SANF (an acronym for national early warning system for rainfall-induced landslides), to forecast the possible occurrence of rainfall-induced landslides. The system is based on: (i) rainfall thresholds for possible landslide occurrence, (ii) sub-hourly rainfall measurements obtained by a nationwide network of 1950 rain gauges, and (iii) quantitative rainfall forecasts. All system components exploit Open Source software. Twice a day the system compares the measured and the forecasted rainfall amounts against pre-defined thresholds, and assigns to each rain gauge a probability of landslide occurrence. This information is used to prepare synoptic-scale maps showing where rainfall-induced landslides are expected. The system outputs are delivered to the National Civil Protection Authorities in different formats. Spatial outputs are published as standard OGC (Open Geospatial Consortium) web services (WMS, WFS, WCS) by the IRPI Spatial Data Infrastructure (IRPI SDI). A password protected WebGIS interface facilitates the use of the system by the Civil Protection personnel and gives access to current and past forecasts. In addition, bulletins containing the system information can be generated automatically and sent via e-mail to the Civil Protection personnel. In a more recent implementation, the system calculate hourly-based forecast using new regional rainfall thresholds and combine landslide forecasts with landslide susceptibility information available at synoptic scale in the national territory. Improvements of the validation procedures and of the landslide susceptibility layer are currently underway.

Topographic and Pedological Rainfall Thresholds for the Prediction of Shallow Landslides in Central Italy

In Italy, rainfall induced shallow landslides are frequent phenomena that cause casualties every year. At the national and regional scales, empirical rainfall thresholds can predict the occurrence of single or multiple rainfall-induced shallow landslides. In this work, we updated a historical catalogue listing 553 rainfall events that triggered 723 landslides in the Abruzzo, Marche and Umbria regions, central Italy, between February 2002 and March 2011. For each event, the rainfall duration (D) and the cumulated event rainfall (E) responsible for the failure are known, together with the exact or approximate location of the landslide. To analyse the influence of topography and soil characteristics on the occurrence of rainfall induced shallow landslides, we subdivided the study area in three topographic divisions, and eight soil domains. We analysed the (D, E) rainfall conditions that resulted in the documented shallow landslides in each topographic division and regional soil domain, and we defined ED rainfall thresholds at 5 % exceedance probability level for the two subdivisions. We expect that the new topographic and pedological thresholds will contribute to forecast shallow landslides in central Italy, and in areas characterized by similar morphological and soil settings.