Corrado Cencetti

A preliminary method for the evaluation of the landslides volume at a regional scale

Dealing with the evaluation of the risk connected to the formation of landslide dams at regional scale, it is important to estimate the volume of the depleted material that can reach the riverbed. This information, combined with other elements (river dimensions, valley width, landslide velocity, etc.) allows making predictions on the possibility of river blockage. One of the problems of this approach is the lack of data concerning the shape and position of the sliding surface; this does not permit us to estimate the volume of the landslide material. The IFFI (Inventario dei Fenomeni Franosi in Italia, i.e. Landslide Inventory in Italy) project furnishes information, at different levels of precision, on nearly totality of the landslides existing in Italy. The first level of the IFFI (compiled for all slides) does not contain information on the depth of the sliding surface but contains data regarding the type and the activity of the slope movement. Along with this information the IFFI project also furnishes vector maps containing the boundary of each landslide and the main sliding direction. This paper describes the implementation of an algorithm aimed to define, with an adequate approximation, the 3D geometry of the sliding surface of rotational slides for which, on the basis of geologic maps available at regional scale, some geotechnical parameters can be known or estimated. The work also required the creation of a computer code useful for the 3D analysis of slope stability (3D safety factor) using the simplified Janbu method. All computer code has been created on a GNU-Linux OS and using shell scripting, based on GRASS GIS and R statistical software.

Geoinformatics in morphological study of River Paglia, Tiber River basin, Central Italy

This work reports the results of a geomorphological study on the River Paglia (Tiber basin, Central Italy), to analyse the historical evolution of the riverbed/floodplain system in its lower valley reaches, upstream the confluence of River Tiber. A morphological–sedimentological approach has been used, starting with an historical analysis and a survey of the forms and sediments in the riverbed/alluvial plain system, in order to reconstruct the current evolutionary trend and to identify the causes and processes leading to changes in natural trends. This approach is not in contrast to the hydrologic–hydraulic one, normally applied in engineering fields, but it is integrated with it in a complementary and parallel manner, in order to achieve the complete knowledge of the river system, result of an investigation multidisciplinary, aimed to define its physical state. Historical data (cartographic documents, digital terrain models—DTM, topographic surveys, aerial photographs, etc.) indicate clear-cut narrowing and deepening of the active channel, manifested after the Second World War and due to the considerable changes which affected the entire fluvial system (from the recovery of land for agriculture near the river, to sediments taken from its bed over the past few decades). The result is a state of great disequilibrium, which is shown in the passage from a braided morphology, observed in the 1950s, to a single channel with low sinuosity (wandering type). This type of morphology is certainly more unstable: in fact, the narrowing and deepening of fluvial sections provide a better hydraulic efficiency for the upstream, so the peak discharge does not overflow. Such peak discharge arrives downstream, next to the confluence with the Tiber River, not laminated, causing serious flooding effects. An increased vulnerability is observed in the last reach, in the areas pertaining to the river, where most of the industrial-economic activities are concentrated and where transportation infrastructures of nationwide importance (Italy’s most important motorway, the A1, and the Rome–Florence–Milan high-speed rail links) are located. The evolution of the Paglia (which is not substantially different from that of many streams in Italy and in general throughout Europe) is affected by severe anthropic constraints and intense exploitation of resources, which have produced a state of disequilibrium approaching irreversibility in an already vulnerable system. Finally, some suggestions for the correct management of the river system are pointed out, with the aim of mitigating the flood risk in the lower Paglia valley.

The hermitage of Cerbaiolo (Tuscany, Italy): stability conditions and geomorphological characterization

The Italian cultural heritage is one of the most noticeable in the world; an important part of it is subjected to natural hazard and risk conditions. Religious buildings, due to their location on impervious sites are particularly exposed to landslides. In this paper we study the hermitage of Cerbaiolo (Tuscany, central Italy). The hermitage, founded in the 8th century as a Benedictine monastery is now a Franciscan Women Institute seat. The buildings are on the edges of a limestone plate overlapped to a clayey formation. Falls, toppling and slides are present along the limits of the plate due to the geomechanics properties of the rock and to the contrast between the two lithological complexes. In order to identify the main joint sets affecting the rock mass a structural-geological survey was carried out and four sets are identified. The rock plate was classified according to Bieniawski, Barton and Geological Strength Index approaches. The outcomes confirm the predisposing factors to mass movements, corresponding to the intensity and characteristics of fracturing of the rock mass. With regard to the slope stability analysis, a failure mechanism involving sliding along a single plane (plane failure) was assumed on the first approximation. The analyses take into account the presence of tension cracks as an indicator factor for the instability phenomenon. The results of stability analyses, performed in static and seismic conditions, indicate a widespread instability condition. The dip slope direction, the properties and type of discontinuities and the local variation of composition influence the hazard assessment.

A statistical test for drainage network recognition using MeanStreamDrop analysis

This paper provides a new statistical test to evaluate the threshold of validity for the Mean Stream Drop analysis. In the case of a constant area threshold, the method aims to provide a unique threshold value to extract the drainage network through a statistical test more efficient than those widely used. The proposal starts from the assumption that a minimum threshold value exists suitable for drainage network extraction. Then, the method proceeds with Horton–Strahler ordering of the network and statistically analysing the network geometry. This procedure is repeated for all the threshold values in the set under investigation, using a statistical permutation test, called APTDTM (Adjusted Permutation Test based on the Difference between Trimmed Means). Statistical significance is evaluated by p-values adjusted to account for multiple comparisons. As a final result of the statistical analysis, the right threshold value for the specific basin is identified. Classical procedures are based on a set of two sample t-tests. However, this method relies on the assumptions of normality and homogeneity of variance, which are unlikely to hold in practice. The APTDTM test presented here provides accurate p-values even when the sampling distribution is not close to normal, or there is heteroskedasticity in the data.

Evaluation of landslide dams hazard and risk: an application in Upper Tiber Valley (central Italy)

The note presents the results of a project, carried out by the Engineering Geology Group of Department of Physics and Geology of University of Perugia and funded by the Umbria Region, aimed at evaluating the hazard of occlusion of riverbeds by landslide (landslide dams) in the Upper Tiber Valley. The authors explain the used methodology that, starting from the data available in the catalogs of IFFI (Inventario dei Fenomeni Franosi in Italia, i.e. Italian Landslides Inventory) and PAI (Piano di Assetto Idrogeologico, i.e. Hydrogeological Assessment Plan), concerning the active and quiescent landslides, selects, through different steps (preliminary identification of landslides for which it is assumed the interference with riverbeds; photointerpretation; geomorphological survey; simulation of the total occlusion of riverbed; simulation of the collapse of the natural dam) the landslides for which is considered realistically possible the occlusion of riverbed due to a reactivation or persistence of the mass movement. The methodology also provides for a first evaluation of conditions of risk related to the complex phenomenon of interference between landslide and dynamics of riverbeds, for both areas located upstream of the dam, at risk of drowning due to the formation of the dam lake, both for those located downstream, where it is possible the transit of an abnormal flood wave, resulting from the eventual collapse of the natural dam and the rapid emptying of the lake.

Use of aerial multispectral images for spatial analysis of flooded riverbed-alluvial plain systems: the case study of the Paglia River (central Italy)

ABSTRACT Image processing and classification techniques are widely used for land use definition. They can also provide interesting applications in fluvial geomorphology, for outlining morpho-sedimentary features (bars, channels, banks and floodplain) at various temporal stages, in order to monitor the evolution of river systems. Frequent monitoring is especially important for streams, in terms of flood risk in urban areas. This study shows how techniques of supervised analysis can be applied to river systems, also under particular conditions, like after flood events (when large portions of riverbed and alluvial plain are covered with mud). The procedure starts from the classical photogrammetric techniques, based on multispectral classification, and goes on with post processing operations of pixel aggregation and shadow treatment. The classification also uses the elevation information provided by Digital Surface Model produced by photogrammetry. This paper introduces a new technique of remote sensing in fluvial areas that allows for both the identification and classification of the fluvial features in a post flooding condition. Application of the procedure over time permits the evolution of the fluvial dynamics to be monitored in an accurate and inexpensive way, particularly for flood event conditions which lead to major changes in the dynamics of riverbeds.

Non-dimensional analysis for rock slope plane failure in seismic (pseudostatic) conditions

This study examines plane failure analysis of rock slopes and discusses its effect on seismic actions in a non-dimensional form, incorporating external loads transferred by anchors and stability indicators such as the depth of tension cracks and the failure plane dip. The results of limit equilibrium analyses, in terms of attained level of safety, are summarized in stability charts which can be used for design and assessment. Following guidelines established by Eurocode EC7, the stability is assessed through the R/E ratio, i.e. the ratio of the resisting shear forces (R) acting on the sliding surface and those promoting sliding (E). The proposed stability charts can be applied to various common practical situations, in order to provide a preliminary - but quantitative - assessment of the rock slope stability. As an example of an application, the paper focuses on the stability of a rock mass located at the bottom of the southern slope of the “Sasso di Pale” mountain (Central Italy), at the outskirts of the village of Pale. As many other historical towns in Central Italy, due to their location close to rock masses, this site is particularly exposed to instability phenomena such as plane failure and rock wedge failure. The structural-geological survey allowed us to identify the main joint sets affecting the behavior of the rock mass and, accordingly, we examined the plane failure mechanism involving sliding along a single plane. Stability verifications in pseudostatic conditions have been performed by adopting the proposed charts, consistent with Eurocodes prescriptions.

An evaluation of sediment yield reduction induced by small irrigation reservoirs.

A GIS Based method has been applied for the determination of the reduction in fluvial sediment transport, induced by small agricultural reservoirs. The method involves the Revised Universal Soil Loss Equation (RUSLE) for the estimation of the soil erosion (gross soil erosion) and the concept of sediment delivery ratio to route surface erosion from each cells to the catchment outlet. The SDR is often used coupled with RUSLE in order to reduce the over estimation of soil erosion provided by the RUSLE method itself. The process of sediment delivery from grid cells to the catchment outlet is represented by the topographical index derived from Digital Elevation Model (DEM). This procedure has been applied in the Upper Tiberian Valley (Tuscany-Umbria boundary, central Italy) in order to evaluate the pitfall effect of numerous small reservoirs present in this area.

Spatial analysis of critical fluvial areas from aerial multispectral images: an application to River Paglia (central Italy)

The paper presents the first results of a work aimed to develop automatic procedures, and/or semi-automatic, for the morphological and sedimentary analysis of riverbeds (made through multispectral aerial photographs) and to define the morphological changes (plano-altimetric) of the river, by comparison with previous surveys. The methodology has been applied to an area of study that includes the lower valley of River Paglia (a tributary on right of the River Tiber, in central Italy) where a flood, occurred in November 2012, has produced important morphological changes of the riverbed-floodplain system.

The Landslide Dam of Ventia Creek (Umbria, Central Italy)

L’occlusione d’alveo per frana del T. Ventia (Umbria, Italia centrale).La nota descrive la frana di C.se Sterpaiolo, in Umbria, che nel 1963 occluse l’alveo del T. Ventia, un affluente del F. Tevere.La frana, di tipo complesso (scivolamento traslazionale – colamento), ha prodotto uno sbarramento d’alveo, tuttora presente, assolutamente stabile e tale da permettere la formazione di un lago permanente a monte dello sbarramento stesso. Infatti, a causa dell’eterogeneita granulometrica del corpo di frana, costituito da blocchi di arenarie e calcareniti immersi in una matrice marnosoargillosa, la soglia di tracimazione appare oggi “corazzata” e cio ne ha impedito l’approfondimento e la completa erosione.I dati cartografici e fotogrammetrici a disposizione, precedenti il movimento franoso, sono relativamente scarsi. Tuttavia, l’elaborazione tramite GIS, con procedure di map algebra, ed il loro confronto con i dati riferiti alla situazione attuale hanno permesso di valutare con buona approssimazione le caratteristiche del movimento, del corpo di sbarramento e dei volumi in gioco.La metodologia e particolarmente utile in tutti i casi di antiche frane, come quello in esame, in cui la situazione precedente l’evento risulta particolarmente carente per quanto riguarda i dati a disposizione.