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Evidence of bedform superimposition and flow unsteadiness in unit-bar deposits, South Saskatchewan River, Canada
No full textUnit-bar deposits are ubiquitous components of river-channel deposits and strongly influence their hydrological properties, yet they are not easy to interpret. This paper concerns details of the internal structures of six unit bars from the South Saskatchewan River, Canada, that were investigated using trenches, epoxy resin peels, and 900 MHz ground-penetrating radar (GPR) profiles. The composition of unit bars depends on flow unsteadiness and superimposed bedforms. Flow unsteadiness causes changes in the mean grain size of the sediment in transport, but is expressed primarily as a change in the type and direction of migration of smaller ripple- and dune-scale bedforms superimposed on unit bars. Superimposed bedforms with heights that exceed 25% of the host bedform height reduce their host's slope and generate inclined sets. Host bedforms with smaller superimposed bedforms form angle-of-repose cross strata with a visible pre-sorting pattern. The relationship between the formative host- and superimposed bedforms and such pre-sorted cross strata can be used to interpret numerous aspects of the three-dimensional geometries of the bedforms. Such detailed interpretations rely on: (i) regularly spaced fine-grained drapes deposited during the passage of troughs of superimposed bedforms, (ii) grain-size sorting patterns generated by sorting within the superimposed bedforms before deposition, (iii) grain-size sorting during deposition by grainfall and intermittent grainflows down the bar lee-slope, and (iv) the presence and nature of low-angle reactivation surfaces.The detailed interpretations revealed that the deposits of dam-related floods were significantly smaller than the deposits of individual unit bars in this study. The unsteady flow and sediment transport conditions are reflected by changes in the structural composition of the unit bars. Reactivation surfaces associated with flow unsteadiness, as opposed to those formed by large superimposed bedforms, were characterized by changes in mean grain size, the buildup of sediment at the base of the bar lee slope, and changes in structures formed by superimposed bedforms. Reactivation surfaces generated by large superimposed bedforms indicate that bedform preservation is likely increased in areas of flow deceleration. The trough deposits of exposed unit bars contained bubble sand, planar laminae due to upper-stage plane beds, and low-angle inclined laminae due to antidunes. These structures indicate that unit-bar troughs can act as ephemeral channels. The composition of the investigated unit bars thus emphasizes the interplay between flow unsteadiness and bed morphology in the formation of sedimentary structures in river channels.Sediment surfaces generate visible GPR reflections where the thickness and contrast in electrical properties of the sediment layers are sufficiently large. Most cross-stratified sets were represented only by reflections from the base of the set, and not by high-angle inclined reflections from the cross strata. The local absence of reflections from cross strata is attributed to the lack of contrast in electrical properties of the well-sorted cross strata and their limited thickness relative to the radar wavelength. In contrast to cross strata, reactivation surfaces formed by large superimposed bedforms and flow unsteadiness were commonly associated with distinct inclined reflections
Influence of superimposed bedforms and flow unsteadiness on formation of cross strata in dunes and unit bars
No full textFormation of angle-of-repose cross strata on the lee side of dunes and unit bars is dependent on three main grain-sorting mechanisms: (1) presorting of sediment that arrives at the lee side of the bedform, related to superimposed bedforms and longer-term variations in water flow and sediment transport; (2) sorting due to differential deposition of sediment on the lee side and associated grain flows, and; (3) movement of sediment on the lee side by the water currents in the lee side flow-separation zone. Although most emphasis has been put on mechanism (2), recent field and experimental studies of the dynamics of sandy and gravelly dunes and unit bars show that mechanism (1) is at least as important.Bedforms superimposed on the backs of dunes and unit bars are ubiquitous, and are composed of size sorted sediment. These superimposed bedforms travel faster than the host bedform and overtake it. If the overtaking bedform has a relatively large height, the lee side of the host bedform becomes reduced in slope and decelerates as the superimposed bedforms overtakes. This results in a “reactivation surface” that is commonly lined with relatively fine-grained sediment. However, if an overtaking superimposed bedforms has a relatively small height (e.g., a ripple or bedload sheet), the lee side of the host bedform remains at the angle-of-repose. The sediment within the overtaking bedform forms a relatively thick cross stratum on the lee side of the host bedform. Arrival of the trough of the next superimposed bedform at the crest of the host bedform results in deposition of a relatively thin stratum. The thickness of the cross stratum couplet formed in this way is related to the height and length of the superimposed bedform and the height of the host bedform, which are related to flow depth and dimensionless bed shear stress on the back of the host bedform. The grain size of the sediment in the superimposed bedforms varies with their height, and this is reflected in the grain size of successive cross strata. The formation of cross-stratified open-framework gravel is associated with size sorting of sand and gravel in superimposed bedforms, and further size sorting associated with flow-separation on the lee side of large host bedform
Influence of bedform superimposition and flow unsteadiness on the formation of cross strata in dunes and unit bars — Part 2, further experiments
No full textThis experimental investigation examines the processes that control the grain-size sorting and geometry of cross strata in dunes and unit bars, in particular the effects of bedform superimposition and flow unsteadiness. Cross stratification formed by dunes and unit bars is the most common sedimentary structure in river-channel deposits, and is common in many other depositional environments. Previous work by Reesink and Bridge (2007) shows that the grain-size variation that allows cross strata to be recognized is determined by three main factors: (1) pre-sorting by superimposed bedforms and flow unsteadiness; (2) sorting during deposition on the lee slope, and; (3) reworking of the slope by currents in the lee of the bedform. Differences in the relative importance of these three factors cause cross strata to be more varied in their geometry, grain-size sorting and permeability than commonly realized, and provide information for detailed quantitative interpretation of river-channel deposits.The experiments presented in this paper cover a wide range of steady and unsteady flow conditions with different combinations of host and superimposed bedforms. Lee-side deposition was highly variable at the time required for the buildup of cross strata, but systematic variations related to bed morphology provided good indicators of flow conditions. Water currents in the lee-side flow separation zone affect initial deposition on the lee slope and can re-distribute sediment on the lee slope. Centimeter-scale turbulent eddies were observed to increase the tangential shape, decrease the (vertical) grain-size separation, and reduce the thickness of the cross strata. These effects of turbulent eddies were found to increase with increasing flow velocity, decreasing grain size, and decreasing bedform height.Flow unsteadiness was primarily expressed in a change of host and/or superimposed bedform type. The pre-sorting pattern formed by the superimposed bedforms is lost if the pre-sorting involves less sediment than the processes that re-sort the sediment on the lee slope (e.g. grainflows). If the pre-sorting pattern of the superimposed bedforms can be identified, the cross strata they form can be used to interpret host and superimposed bedform geometries. The presence of reactivation surfaces indicates that superimposed bedform heights (Hs) exceed 25% of the host bedform height (H; Hs/H > 0.25). Pre-sorting by superimposed bedforms is recognizable as cross stratification where Hs/H < 0.25, and comprises: (i) drapes of fine-grained sediment that settles from suspension during the passage of the superimposed trough, and; (ii) a body of coarse-grained bedload with a cross-sectional area that equals the cross-sectional area of the formative superimposed bedform. The distinctiveness and lateral extent of fine-grained drapes represent the distinctiveness and lateral extent of their formative superimposed troughs. The lateral extent and internal grain-size sorting of the thick, coarser-grained body represents the lateral extent and internal grain-size sorting of the body of the superimposed bedform. The plan view shape of the host lee slope controls the cross-stream geometry of cross strata. Thus, cross strata can be used to interpret type, size, and geometry of the host and superimposed bedforms, hence flow and sediment transport condition on the back of the host bedfor
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
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