1,720,961 research outputs found
Studio dei meccanismi di deposizione dei Debris Flow: integrazioni tra esperienze di laboratorio, analisi di campo e modellazioni numeriche
Get PDFThis research is related to the study and modelling of the depositional processes of debris flows and an understanding of the related rheological behaviour. The main aims of this study are to analyse the depositional features of a debris flow, to verify the possible contribution of laboratory tests, carried out by means of a tilting-plane rheometer, to determine the rheological parameters and to investigate the reliability and limitations of computer models employed for debris flow simulation and rheological parameter determination.
The laboratory tests were been carried out at the Institute for Hydrological and Geological Protection of the Italian National Research Council (CNR IRPI) in Padova. The physical model consists of a 2 x 1 m tilting plane with inclination from 0° to 38°, on which a steel tank with a removable gate has been installed. A fixed horizontal plane (1.5 x 1 m), with an artificial roughness to simulate natural basal friction, served as the deposition area. In total, 93 laboratory tests were carried out: 62 tests simulated the quasi-static formation of a fan (with the tank installed at the lower end of the tilting plane), the remaining 31 examined dynamic fan formation by the means of a flume. The steel tank, with a removable gate facing the deposition plane, is parallelepiped with a square base (15 x 15 cm and 33 cm high) having a maximum volume of 7 dm3. The laboratory tests were conducted using three different materials: plastic cylinders with a diameter and height of 3 mm; medium to fine gravel; and debris-flow matrix (with a diameter < 19 mm), varying amounts of water were added to the debris-flow matrix to test solid concentrations in a range between 0.45 and 0.67. Data analysis included the development of semi-empirical equations for runout distance, the maximum width of the deposit and total travel distance. An energy balance approach was tested in order to determinate the rheological parameters of the debris-flow matrix, this method is based on the comparison between the potential energy of the mass stored in the tank and the work made in the process of deposit formation.
The field sites related to the debris-flow events are located upstream of the town of Cortina d'Ampezzo (Fiames locality, Belluno, Italy), where an intense rainstorm triggered six debris flows during the afternoon of 5th July 2006. Immediately following the event, field surveys were carried out in the study area. These field surveys made it possible to measure several features, including the debris-flow deposits, main channels and initiation areas. Samples taken from the debris-flow deposits have been used for laboratory tests. Total travel distances and the runout distance on fans measured in the field were compared with formulas found in the literature (empirical/statistical and physically oriented) and also compared to the results of the laboratory tests. An estimation of shear stress from the field site was calculated using Johnson's (1970) formula.
The Fiames debris-flow event of 5th July 2006 and the laboratory tests (dynamic runs) were simulated using FLO-2D, while RAMMS (Rapid Mass MovementS) was used solely to simulate the Fiames event. FLO-2D (O'Brien, 2003) is a two-dimensional flood routing model with a rigid bed (debris-flow simulations) or a mobile bed (sediment transport simulations), it simulates water flows in wide rivers as well as non-Newtonian flows over alluvial fans. FLO-2D numerically routes a flood hydrograph while predicting the area of inundation, the maximum depth and the flow velocity in each cell of the square grid system derived from input topographic data. RAMMS was developed in 2005 by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Birmensdorf and the Swiss Federal Institute for Snow and Avalanche Research (SLF), Davos. RAMMS uses a one-phase approach based on Voellmy rheology (Voellmy, 1955; Salm et al. 1990). The input file combines the total volume of the debris flow located in a release area with a mean depth. The model predicts the area of inundation, the maximum depth and the flow velocity in each triangular-shape cell of the input DTM.
The limitations of the numerical simulations relating to the laboratory tests were investigated to verify to what extent rheological parameters could be determined using this indirect method. The analysis of the Fiames event enabled understanding of the rheological behaviour related to the dolomitic debris-flows and the influence exerted by the input parameters on the final results. Finally, some improvements to the RAMMS model were proposed to obtain simulations in keeping with the events observed in the field
Ricostruzione delle portate di piena mediante indicatori di campo: analisi comparata di alcuni torrenti della provincia di Belluno
No full textLa ricostruzione delle portate di piena per mezzo delle tracce lasciate dall’evento ha assunto notevole importanza negli ultimi anni a causa del ripetersi di eventi alluvionali estremi. Si è accresciuta di conseguenza la richiesta di una maggiore comprensione dei fenomeni e di una stima quantitativa degli eventi occorsi, al fine di progettare e gestire interventi sia conservativi che di prevenzione. L’obiettivo del presente lavoro è di testare l’affidabilità di alcuni metodi di stima paleoidrologica in alcuni tratti di torrenti montani. Gli indicatori di campo utilizzati sono le tracce lasciate sulle sponde, il diametro intermedio dei massi più grossi depositati dalla piena, la geometria delle buche di scavo e la lunghezza d’onda degli step della morfologia a gradinata. E’ stata condotta una campagna di rilievi su cinque tratti di corsi d’acqua montani della Provincia di Belluno: i torrenti Cordevole e Boite ed il fiume Piave. I dati sono stati elaborati applicando le seguenti metodologie di stima: il metodo slope-area, l’ipotesi di criticità della corrente, il metodo dei diametri competenti, i parametri di scavo, la geometria delle forme di fondo e le condizioni di moto in regime di forte scabrezza. Mediante un confronto tra i picchi di piena stimati e quelli misurati dalle stazioni idrometriche regionali, si è concluso che i valori stimati sono ragionevoli e si collocano tra l’ordine di grandezza ed il dato puntuale
Reologia e distanza di arresto dei debris flow: sperimentazioni su modello fisico a piccola scala
No full textLo studio di campo delle caratteristiche di movimento delle colate fangose e fangoso-detritiche
(debris flow) presenta delle notevoli difficoltà tecniche e operative a causa dell’imprevedibilità temporale del fenomeno, della sua breve durata e dell’elevata forza
distruttiva del flusso. Per questa ragione l’uso di modelli fisici a scala ridotta può considerarsi un valido strumento di indagine per l’analisi reologica dei debris flow o
quanto meno della loro componente medio-fine (ghiaie, sabbie, limi, argille), che ne condiziona fortemente gli sforzi deformativi di tipo viscoso, frizionale e collisionale. In questo ambito si sono svolte alcune prove sperimentali mediante un reometro a piano inclinato, costituito da una canaletta, lunga 2 m, seguita da un piano orizzontale che
funge da zona di deposizione. La canaletta è dotata di un serbatoio per il carico delle miscele fangoso-granulari, dal quale vengono poi rilasciati impulsivamente, a mezzo di
una paratoia, flussi a concentrazione volumetrica nota. Sono state testate alcune miscele costituite da sedimenti campionati da recenti depositi di colata: due campioni
(uno indisturbato ed uno rimaneggiato) provengono da un debris flow verificatosi in zona Dolomitica (falde del massiccio del Pomagagnòn, Cortina d’Ampezzo, Belluno) e
sono caratterizzati da diverse percentuali dei sedimenti coesivi, un terzo campione è composto da materiale prevalentemente sabbioso prelevato dalla colata del rio
Gabbiolo (affluente del torrente Sarca di Val Genova, Trento), un ultimo campione (rio Val degli Schivi, Trento) presenta un discreto assortimento di sedimenti a grana grossa e a grana fine. Le prove sperimentali hanno portato a determinare delle relazioni piuttosto strette tra la pendenza media di attrito dei vari flussi gravitativi e la loro
concentrazione volumetrica, mettendo in luce come la composizione granulometrica dei sedimenti ne influenzi grandemente la mobilità. Le sperimentazioni hanno inoltre
suggerito un metodo per la stima della distanza di arresto, che è stato verificato mediante il confronto con eventi reali
Basal shear stress of debris flow in the runout phase
No full textA laboratory device is proposed to assess the basal shear stresses generated by debris-flow
mixtures during their runout phase. The device consists of an inclinable box with a gate facing a deposition
plane. The box is filled with a selected debris-flow mixture, and after sudden opening of the gate, the
features of the dam-break deposit can be measured. Based on some simplified assumptions of the energy
balance, a methodology is proposed to assess basal shear stresses. The device has been tested using sediment
samples from debris-flow deposits generated by two catchments of the Dolomites (Cortina d'Ampezzo,
Belluno, Italy) by carrying out runout tests for different sediment concentrations by volume. The results
show how the static Coulomb friction law is valid in the runout phase, with friction angles on the order of the
angle of repose of the same material in dry conditions. The data elaboration also yields an innovative
constitutive equation for shear stresses. This relation merges the Coulomb mixture approach with the concept
of a one-phase flow with a certain rheology. This integration offers a useful insight into the weaknesses of
the rheological approach if it is not properly scaled up to the ambient pressure of interest
Comparison between FLO-2D and RAMMS in debris-flow modelling: a case study in the Dolomites
No full textThis paper presents a comparison of the results obtained through the use of two numerical models for debris flow simulation. FLO-2D and RAMMS were used to carry out a back analysis of a well documented debris-flow event, which occurred on July 2006 in the Dolomites (Fiames locality, Belluno, Italy). The performances of FLO-2D and RAMMS are tested in terms of adaptation degree to the observed field data
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
