Rinaldo Genevois

Field observations of a debris flow event in the Dolomites

A debris flow event occurred in June 1997 in the Dolomites (Eastern Alps, Italy). The phenomenon was directly observed in the field and recorded by a video camera near its initiation area. The debris flow originated shortly after an intense rainstorm (25 mm in 30 min) whose runoff mobilised the loose coarse debris that filled the bottom of the channel in its upper part. The analysis of the steep headwater basin indicates a very short concentration time (9–14 min) that fits the quick hydrological response observed in the field. The debris flow mobilisation was not contemporaneous with the arrival of the peak water discharge in the initiation area probably due to the time required for the saturation of the highly conductive channel-bed material. Channel cross-section measurements taken along the flow channel indicate debris flow peak velocity and discharge ranging from 3.1 to 9.0 m/s and from 23 to 71 m3/s, respectively. Samples collected immediately after deposition were used to determine the water content and bulk density of the material. Channel scouring, fines enrichment and transported volume increase testify erosion and entrainment of material along the flow channel. Field estimates of the rheological properties based on open channel flow of Bingham fluid indicate a yield strength of 5000±400 Pa and relatively low viscosity (60–326 Pa s), probably due to a high percentage of fines (approx. 30%).

Terrestrial laser scanner to detect landslide displacement fields: a new approach

A terrestrial laser scanner (TLS) allows the generation of a detailed model of a landslide surface. In this way, when two or more georeferenced models obtained by multi‐temporal scans are available, the landslide displacement field can be computed. Nevertheless, such a computation is a relatively complex task because the recognition of correspondences among the multi‐temporal models is required. The Iterative Closest Point (ICP) algorithm allows the alignment of two 3D objects having a common part by iterative shape matching. A new method for the automatic calculation of a landslide displacement field is presented here. It is based on a piecewise application of the ICP algorithm and is made possible by the robustness of this algorithm against noise and small morphological modifications. After a series of numerical experimentations, this method was successfully applied to two test sites located in the North‐Eastern Italian Alps affected by high‐risk landslides of the slump type (Perarolo di Cadore and Lamosano) with very different observational conditions.

Debris flow monitoring in the acquabona watershed on the Dolomites (Italian alps)

In 1997 a field monitoring system was installed in Acquabona Creek in the Dolomites (Eastern Italian Alps) to observe the hydrologic conditions for debris flow occurrence and some dynamic properties of debris flow. The monitoring system consists of three remote stations: an upper one located at the head of a deeply-incised channel and two others located downstream. The system is equipped with sensors for measuring rainfall, pore pressures in the mobile channel bottom, ground vibrations, debris flow depth, total normal stress and fluid pore-pressure at the base of the flow. Two video cameras record events at the upper channel station and one video is installed at the lowermost station. During summer 1998, three debris flows (volumes from less than 1000 m3 up to 9000 m3) occurred at Acquabona. The following results were obtained from a preliminary analysis of the data: 1) All of the flows were triggered by rainfalls of less than 1 hour duration, with peak rainfall intensities ranging from 4.8 to 14.7 mm / 10 minute. 2) Debris flows initiated in several reaches of the channel, including the head of the talus slope. 3) The initial surges of the mature flows had a higher solid concentration and a lower velocity (up to 4 m/s) than succeeding, more dilute surges (more than 7 m/s). 4) Total normal stress and pore fluid pressures measured at the base of the flow. (mean depth about 1.1 m) were similar (about 15 kPa), indicating a completely liquefied flow. 5) Peak flows entrained debris at a rate of about 6 m 3/m of channel length and channel bed scouring was proportional to the local slope gradient and was still evident in the lower channel where the slope was 7°.

A 3D GEOLOGICAL MODEL OF THE 1963 VAJONT LANDSLIDE

non-planar geometry is affected by the interference pattern of two regional-scale fold systems. The landslide is partitioned into two distinct and internally continuous rock masses with a distinct kinematics, which are characterised by a very limited internal deformation during the slide. The continuity of these two large blocks points to a very localized deformation, occurring along a thin, continuous and weak cataclastic horizon.

Ground-based monitoring of high-risk landslides through joint use of laser scanner and interferometric radar

Terrestrial laser scanner (TLS) and interferometric synthetic aperture radar (InSAR) allow the acquisition of data on an observed surface with high spatial sampling rate. The data provided by TLS observation of a landslide ground surface can be used to generate a very detailed digital model of this surface, and multitemporal observations with TLS or continuous or multitemporal observation with InSAR can provide a reliable displacement map. In order to acquire useful information about the analogies, differences, and capabilities, as well as limitations of these techniques, a joint experimentation of TLS and InSAR was performed over two years in various sites in the Italian Alps. The results have indicated that these techniques can provide high‐quality data, can be very useful in the monitoring intended for the mitigation of hydrogeological risk in a wide range of cases, and must be supported by a topographical georeferenced network.

Variation in the Occurrence of Rainfall Events Triggering Landslides

We analyze the climatic features of the Vicenza Province (NE Italy) and the characteristics of the exceptional rainfall event that hit the area in November 2010, triggering a huge number of landslides. Our analysis aims at identifying the hydrological variable related to the triggering of the recorded instabilities and the recent variation in the occurrence of rainfall events inducing landslides.

The Ligosullo (UD, Italy) Landslide, Revisiting of Past Data and Prospects from Monitoring Activities

The paper focuses on the analysis of a landslide, located in the northern sector of the Friuli Venezia-Giulia Region (North-Eastern Italy) affecting the village of Ligosullo. Field surveys, geophysical investigations and interferometric analyses, financed by the Geological Survey of Friuli Venezia-Giulia Region in the last 15 years let to recognize a sliding surface up to 70 m deep, causing the mobilization of 7 millions m3 of material. A new phase of studies including geological and geomechanical surveys and monitoring activities has been recently undertaken by the University of Padua (Department of Geoscience) and NHAZCA S.r.l. (Spin-off of “Sapienza University of Rome”). The first results we obtained and the future goals are discussed in this paper.

A Geomechanical Evaluation of the Rock Mass Involved in the 1963 Vajont Landslide (NE Italy)

The Vajont landslide (NE Italy) occurred on the 9th October 1963, when approximately 270 million m3 of rock slid down into the existing reservoir at 20–30m/s. The movement involved limestones and marls of the Fonzaso and Soccher Formations and occurred along a chair-like sliding plane. The landslide has been object of numerous studies but a comprehensive explanation of its triggering and dynamics remains elusive. Here we report the results of new structural and geomechanical data from conventional mapping on 88 field stations within and outside the landslide. Each station includes joints orientation, spacing, persistence and roughness, Geological Strength Index and Schmidt Hammer tests. The characterization of the landslide deposit was completed by the description of seven boreholes drilled by RODIO Company in 1965 down to the sliding surface. Representative rock samples were collected for laboratory measurements of the unconfined compressive strength, Young modulus and Poisson’s ratio. The new data represent an important tool to reconstruct a detailed 3D geological and geomechanical model of the landslide.

A multidisciplinary approach to landslide modelling: the case of Campodenno, north-eastern Alps, Italy

Campodenno village, laying in the central-western part of the Trentino Region (Northern Italy), is affected by composite landslide phenomena that cause heavy damages to buildings and public facilities. The purpose of the present work was the definition of a complete geological model, to be used to perform stability analyses of sliding slopes. In order to carry out a valid slope stability analysis a complete and reliable geological and geotechnical model of the landslide slope must be performed. All the previous available data, then, have been collected and re-interpreted. The integration of all these data allowed the definition of a preliminary geological and geotechnical model of the slope that, however, resulted to be affected by some uncertainties as regards mainly the spatial distribution of different stratigraphical levels. Because of these uncertainties, some geophysical surveys have been carried out in order to investigate the lateral continuity and hydrogeological properties of the stratigraphic layers and also the thickness of the sliding material. Geoelectrical, seismic and magnetotelluric surveys have been carried on and the interpreted results compared and integrated with the already existing data. In that way, reliable geological models of the Campodenno slopes have been realized to be used for stability analysis purposes. RIASSUNTO: L’abitato di Campodenno, localizzato nella porzione centro-occidentale della regione Trentino (Italia settentrionale) su di una collina allungata in direzione NW-SE, è caratterizzato dalla presenza di fenomeni di instabilità compositi su entrambi i fianchi della collina, che provocano danni piuttosto importanti a costruzioni ed infrastrutture. La geologia locale è caratterizzata dalla presenza di un bedrock costituito da marne eoceniche fittamente stratificate, al di sopra delle quali si trova una spessa sequenza di depositi quaternari formata da tre livelli con differenti caratteristiche litologiche e, di conseguenza, geotecniche ed idrogeologiche. Le differenti proprietà dei suoli quaternari e la loro distribuzione spaziale danno origine ad una complessa e non completamente chiara struttura dei corpi idrici sotterranei. Lo scopo del presente lavoro è stato la definizione di un modello geologico e geotecnico completo ed affidabile, che potrà essere successivamente utilizzato per l’esecuzione delle analisi di stabilità dei versanti in studio. Tutti i dati disponibili sono stati quindi raccolti, analizzati e reinterpretati. L’integrazione di tali dati ha consentito la definizione di un modello geologico-tecnico preliminare dell’area in esame che, tuttavia, risulta affetto da notevoli incertezze legate principalmente alla distribuzione spaziale dei differenti livelli stratigrafici. A causa di tali incertezze, sono stati realizzati nuovi rilievi mediante metodi geofisici principalmente con il fine di investigare la continuità laterale dei corpi stratigrafici e lo spessore del materiale mobilizzabile dai fenomeni franosi in atto. I risultati dei sondaggi geoelettrici, sismici e magnetotellurici realizzati sono stati, quindi, confrontati ed integrati con i dati già esistenti. In questo modo è stato possibile ricostruire il modello geologico dell’area instabile da poter utilizzare nell’analisi delle condizioni di stabilità e nella scelta degli interventi sistematori. Key terms: geological modelling, landslide, geophysics methods, hydrogeology Termini chiave: modello geologico, frane, metodi geofisici, idrogeologia