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Hydraulics of subaqueous ash flows as deduced from their deposits
No full textSubaqueous ash flows are gravity currents consisting of a mixture of sea water and ash particles. Also called
volcaniclastic turbidity currents (VTCs), they can be generated because of remobilization of pyroclastic fall
deposits, which are emplaced into the sea around a volcanic island, as well as far away, during an explosive
eruption. The VTC upper part is the turbulent transport system for the flow, whereas the viscous basal one is
the depositional system. Typical sequences of VTC deposits are characterized by cross-laminations, planar
and convolute laminations, and massive beds, which reflect the stratified nature of the flow. Here, the analysis
of some VTC hydraulic parameters is presented in order to depict flow behavior and sedimentation during
deposition. A reverse engineering approach is proposed, which consists of calculating hydraulic
parameters by starting from deposit features. The calculated values show that a VTC is homogeneouslyturbulent
for most of the thickness, but is viscous at its base. First, cross-laminations are directly acquired
over the rough pre-existing seafloor, then planar or convolute laminations aggrade over the newly formed
substrate. Finally, fine-grained suspended particles gently settle and cap the flow deposit
Hydraulics of subaqueous ash flows as deduced from their deposits: 2. Water entrainment, sedimentation, and deposition, with implications on pyroclastic density current deposit emplacement
No full textTurbidity currents are the most common flows of water, and suspended and bed load sediment occurring in the relatively deep sea and lakes, and act as large systems of sediment distribution. Due to the turbid nature of the flow, they may impact subaqueous infrastructures, as well as ecosystems, during motion, so quantifying the turbidity degree is important to predict the current impact. A particular type of these currents is the so-called volcanidastic turbidity currents or subaqueous ash flows, which are mostly composed of fine-grained volcanic particles, and are used here as synonymous (sensu hydraulic) with the meaning of secondary current.
In this paper, a method to estimate the water entrainment (column condition model), as well as the sedimentation and deposition rates (conveyer model), in volcaniclastic turbidity currents is proposed, by starting from the physical features of the deposits or inverse procedure. Some criteria of sediment mechanics are used to approximate the flow hydraulic parameters needed to quantify the water entrainment, as well as the shear velocity in volcaniclastic turbidity currents.
The deposits used as case study are the impressive, meters thick, well-sorted rhyolitic ash turbidites of Late Pliocene cropping out in Southern Italy, particularly in the Craco area, Matera. The water entrainment coefficient of the currents is calculated in a range of particle concentration and slope angle, whereas the slope angle giving the sedimentation rate is calculated in a range of flow shear velocity, which in turns gives the deposition rate.
The results have a general validity for depositional turbidity currents laden with well-sorted sediment, and they show that the water entrainment is low for relatively dense, slow-moving, subcritical flows, but it increases as the Richardson number decreases for relatively dilute, fast, supercritical flows. Moreover, the sedimentation and deposition rates are high for relatively intense flows moving over gentle slopes, but they decrease for relatively weak flows moving over steep slopes. The deposit sedimentary structures thus infer sedimentation, and particularly depositional processes.
Finally, the conveyer model is extended conceptually to pyroclastic density currents, when approximating, but not exclusively, the column condition model in the flow above the flow boundary zone
Pyroclastic density currents: state of the art and perspectives
No full textPyroclastic density currents (PDCs) are mixtures of twocomponents, namely solid particles and fluid (gas) phase.
Theymacroscopically behave as dense,multiphase gravity currents (flowing pyroclastic mixtures of particles and
gas) immersed in a less dense, almost isotropic fluid (the atmosphere). As for other natural phenomena, their
study needs a multidisciplinary approach consisting of direct observations, analysis of the associated deposits,
replication through laboratory experiments, and numerical simulations. This review deals with the description
of the current state of the art of PDC physics, and combines analysis of data from various methodologies. All of
the above-mentioned approaches have provided significant contributions to advancing the state of the art;
in particular, laboratory experiments and numerical simulations deserve a special mention here for their tumultuous
growth in recent years.
A paragraph of the review is dedicated to the puzzling behaviour of large-scale ignimbrites,which are (fortunately)
too rare to be directly observed; they cannot be easily reproduced through laboratory experiments, or investigated
by means of numerical simulations.
The final part is dedicated to a summary of the whole discussion, and to a comment on some perspectives for
future developments of PDC studies
Numerical analysis of the effect of topography on deposition from dilulte pyroclastic density xurrents
No full textPyroclastic density currents (PDCs) vary between two end members, concentrated and dilute. When a PDC interacts with an uneven topography, the flow field variables (velocity, pressure, bulk density, particle concentration) may drastically change near the flow-substrate boundary. These changes may significantly affect the sediment flux and the resulting deposits can record the effects in their facies architecture. Here we show, by means of numerical simulations, how a dilute pyroclastic density current interacts with four different types of simple topography, namely: flat, one hill, one valley and two hills. Our numerical scheme treats the very fine particles as being in full thermo-mechanical equilibrium with the volcanic gas, i.e. a dusty gas. A dusty gas-air mixture is defined as a mixture of dusty gas and atmospheric air. The trajectories of the coarser particles or discrete phase (three grain-size classes of 1 mm, 5 mm and 10 mm and density of 1500 kg/m(3)) are tracked as Lagrangian particles that interact with the dusty gas-air mixture through two-way momentum and energy coupling. Numerical results are used to analyze the local effects of topography on the deposition of the Lagrangian particles, by monitoring with time and space the local changes at the boundary between the current and the substrate. The results show that the sediment flux in the flow boundary zone increases near the stoss sides of hills and in the valleys, relative to the flat reference case, whereas it decreases along the lee flanks and on top of the hills. We use the sediment flux in the flow boundary zone and the grain-size distribution of the Lagrangian particles as proxies of the deposit features, and by these parameters we qualitatively compare simulations with deposits of known eruptions
Aerodynamics of stratovolcanoes during multiphase processes
No full textPyroclastic density currents (PDCs) are gas-particle flows generated during explosive eruptions, which are often erupted over the flanks of stratovolcanoes. These volcanoes may have different shapes, which can affect the flow aerodynamics and hence the depositional processes. Here, multiphase numerical simulations are carried out in order to define semiquantitative relationships among the PDC behavior, particle response, and deposit formation. Three stratovolcano shapes are used: straight, convex and concave, and, by means of numerical simulations, their effects both on the flow structure and depositional processes are highlighted. The current starts moving as a homogeneous flow, and then it rapidly evolves to a turbulent boundary layer moving in contact with the ground, overlaid by a companion wake region. Results show that thin boundary layers produce thick deposits of massive layers, whereas thick boundary layers produce thin laminated deposits. Moreover, concave wake regions would produce thick massive deposits of fine ash, whereas convex wake regions would produce thin ash deposits. Copyright 2012 by the American Geophysical Union.Peer Reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
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