Infiltration, seepage and slope instability mechanisms during the 20-21 November 2000 rainstorm in Tuscany, central Italy

International audienceOn 20?21 November 2000, a storm of high intensity, with a estimated return period of more than 100 years, triggered over 50 landslides within the province of Pistoia in Tuscany (Italy). These failures can be defined as complex earth slides- earth flows. One of the documented landslides has been investigated by modelling the ground water infiltration process, the positive and negative pore water pressure variations and the effects of these variations on slope stability during the rainfall event. Morphometric and geotechnical analyses were carried out through a series of in-situ and laboratory tests, the results of which were used as input for the modelling process. The surface infiltration rate was initially simulated using the rainfall recorded at the nearest raingauge station. Finite element seepage analysis for transient conditions were then employed to model the changes in pore water pressure during the storm event, using the computed infiltration rate as the ground surface boundary condition. Finally, the limit equilibrium slope stability method was applied to calculate the variations in the factor of safety during the event and thereby determine the critical time of instability. For the investigated site the trend of the factor of safety indicates that the critical time for failure occurs about 18 h after the storm commences, and highlights the key role played by the soil permeability and thickness in controlling the response in terms of slope instability

Composition of supralittoral sediments bacterial communities in a Mediterranean island

Marine coasts represent highly dynamic ecosystems, with sandy beaches being one of the most heterogeneous. Despite the key importance of sandy beaches as transition ecosystems between sea and land, very few studies on the microbiological composition of beach sediments have been performed. To provide a first description of microbial composition of supralittoral sediments, we investigated the composition of bacterial communities of three sandy beaches, at Favignana Island, Italy, using metagenetic approaches (Terminal-Restriction Fragment Length Polymorphism, sequencing of 16S rRNA genes by Illumina-Solexa technology, functional genes detection, and quantitative Real-Time PCR). Results showed that the investigated beaches are harboring a rich bacterial diversity, mainly composed by members of classes Alphaproteobacteria, Gammaproteobacteria, Flavobacteria and Actinobacteria. The metagenetic analysis showed profiles of decreasing beta diversity and increasing richness, as well as a differentiation of communities, along the sea-to-land axis. In particular, members of Firmicutes and Proteobacteria displayed contrasting profiles of relative abundance (to decrease and to increase, respectively) along the sea-to-land axis of the beach. Finally, a search for the presence of genes related to the nitrogen and carbon biogeochemical cycle (nifH, nosZ, pmoA/amoA) detected the presence of ammonia monoxygenase sequences (amoA) only, suggesting the presence of bacterial ammonia oxidation to some extent, probably due to members of Nitrospira, but with the lack of nitrogen fixation and denitrification

Analysis of infiltration, seepage processes and slope instability mechanisms during the November 2000 storm event in Tuscany

On the days 20-21 November 2000, a storm of exceptional intensity triggered over 50 landslides within the province of Pistoia in Tuscany (Italy). These failures are mostly of complex type, originating as rotational or translational landslides, and transforming into flows. Two of these landslides were investigated in this paper by modelling the ground water infiltration process, the pore water pressure variations, both positive and negative, and the effects of these variations on slope stability during the rainfall event. Morphometric and geotechnical analyses were carried out for both sites through a series of in-situ and laboratory tests, the results of which were used as input for the modelling process. In a first step the surface infiltration rate was simulated using a modified Chu (1978) approach for the Green and Ampt (1911) equations in case of unsteady rainfall together with a surficial water balance. A finite element seepage analysis for transient conditions was then employed to model the changes in pore water pressure during the event, using the computed infiltration rate as the ground surface boundary condition. Finally, once again using the data from the previous step as input, the limit equilibrium Morgenstern-Price (1965) slope stability method was applied to calculate the variations in the factor of safety during the event and thereby determine the most critical time of instability. In both sites this method produced a curve for the factor of safety that indicated that the most critical time for failure occurred a few hours after the peak of rainfall