1,721,017 research outputs found
A field evaluation of the formative conditions, wavelengths and heights of alternate bars in alluvial channels
No full textThis study investigates the fluvial dynamics of straight natural stream channels. In particular, this experimental field study quantitatively assesses: (1) six experimentally derived formulas that predict the conditions for alternate bar formation and equilibrium bar dimensions, and (2) a physically-based nonlinear mathematical theory of alternate bar formation under unsteady natural flow conditions within a straight alluvial stream (Tubino 1991). The study site is an artificially-straightened section of the Embarras River located approximately 16 km south of Champaign, Illinois. Data were collected on channel form, gradient, alternate bar dimensions, and flow conditions over a two-year study period.Experimental flume studies suggest that alternate bars form in wide, straight, shallow streams have wavelengths between 4 and 15 channel widths, and have heights that are roughly equal to the average depth of flow. Both linear and nonlinear steady flow hydrodynamic theories suggest that alternate bars are critical to the process of meander development. Tubino (1991) suggests that bar evolution for a flood hydrograph can be divided into three parts: (1) a period of limited bar growth during the rising stage of the flood, (2) a stage of modest bar decay near the peak of the flood, and (3) a stage of nonlinear bar growth during the prolonged falling stage of the flood.Quantitative evaluations of: (1) the experimentally-derived formulas of bar development under steady flow, and (2) Tubino's model of bar development under unsteady flow were performed using data on flow stage, bed topography, and bed sediment size data from a straight alluvial channel. Bar formation under unsteady flow conditions can be accurately predicted by the steady-flow formulas, but these formulas fail to successfully predict bar dimensions. Furthermore, although bars developed during the falling limb of a hydrograph, they exhibited sequential development rather than the uniform growth along the reach predicted by Tubino's model. Further work should focus on attempting to isolate the physical mechanisms responsible for alternate bar formation in straight natural streams with heterogeneous bed material and flashy hydrologic flow regimes through either additional analytical or empirical research.Made available in DSpace on 2011-05-07T13:46:24Z (GMT). No. of bitstreams: 2
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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
Objectively identifying spatial variability in bankfull stage
Full text linkThe bankfull flow concept has occupied a prominent role in geomorphological theory about process dynamics in alluvial meandering river systems for decades. As the flow that defines the transition between channel and floodplain and marks the level of incipient flooding, bankfull flow represents an important geomorphic and hydrological threshold. However, most field methods of bankfull identification rely on subjective judgement which can result in varying measurements of the same location. The focus of this study is the development of an innovative, objective method for extracting bankfull elevation data from high resolution LiDAR data at different spatial scales. The capability of this method is demonstrated on three different alluvial lowland meandering rivers to provide a range of scale and geomorphic and hydrologic conditions. Selection criteria are defined to maximize objectivity in identification of bankfull surfaces. Bankfull elevations are extracted at any spacing along the river that the user desires, providing the basis for high-resolution mapping of longitudinal variability and the exploration of geomorphic factors that produce this variability. Two-dimensional hydrodynamic modeling is employed to assess the effects of bankfull variability on inundation patterns. The results of this work demonstrate the considerable variability inherent in the study rivers at large and small scales. The presence of geomorphic features, such as sloughs, secondary channels, and meanders, has been shown by this method to affect bankfull elevation. Modeling results display changing spatial patterns of floodplain inundation with increasing flow. The findings have implications for river management efforts related to the natural configuration of channel banks. Additionally, the results illustrate the role of variability in the hydrological complexity of channel-floodplain connectivity.Submission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2021-12-01The student, Evan Lindroth, accepted the attached license on 2019-12-12 at 17:17.The student, Evan Lindroth, submitted this Thesis for approval on 2019-12-12 at 17:21.This Thesis was approved for publication on 2019-12-13 at 08:56.DSpace SAF Submission Ingestion Package generated from Vireo submission #14810 on 2020-02-28 at 17:24:29Made available in DSpace on 2020-03-02T22:18:20Z (GMT). No. of bitstreams: 2
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Previous issue date: 2019-12-13Embargo set by: Seth Robbins for item 113945
Lift date: 2022-03-02T22:18:25Z
Reason: Author requested U of Illinois access only (OA after 2yrs) in Vireo ETD systemU of I Only Restriction Lifted for Item 113945 on 2022-03-03T10:15:25Z
High-resolution spatial and temporal evolution of two-dimensional flow structure at a small stream confluence
Full text linkConfluences are recognized as locations within rivers where interaction between incoming flows produces complex hydrodynamic conditions characterized by distinct spatial patterns of mean or turbulent flow. Despite recognition of this hydrodynamic complexity, few studies have mapped in detail spatial patterns of flow at confluences and variation in these patterns over time. Recent developments in Large-Scale Particle Image Velocimetry (LSPIV) have created novel opportunities to explore the spatial and temporal dynamics of flow patterns at confluences. This study uses LSPIV to map two-dimensional flow structure at the water surface and to examine variation in this structure over time. Results show that flow within the confluence is characterized by a large region of flow stagnation develops at the junction apex, a region of low velocities at the downstream junction corner, and a region of merging of the two flow along a mixing interface within the center of the confluence. Interaction between the incoming flows varies over time in the form of episodic pulsing in which one of the two tributaries flows first decelerates and then subsequently accelerates into the confluence. The cause of this pulsing remains uncertain, but it seems to reflect changes in the water-surface pressure-gradient field as the two flows compete for space within the confluence. No large-scale vortices are evident within the mixing interface for the particular flow conditions documented in this study, but such vortices do occur along the margins of the stagnation zone where shearing action between fast-moving and slow-moving fluid is strong. The results of the study provide insight into the time-dependent dynamics of the spatial structure of flow at stream confluences.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2022-12-01The student, Sadia Sabrina, accepted the attached license on 2020-12-08 at 11:35.The student, Sadia Sabrina, submitted this Thesis for approval on 2020-12-08 at 11:38.This Thesis was approved for publication on 2020-12-09 at 13:43.DSpace SAF Submission Ingestion Package generated from Vireo submission #16083 on 2021-03-04 at 16:33:50Made available in DSpace on 2021-03-05T21:47:34Z (GMT). No. of bitstreams: 2
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Previous issue date: 2020-12-09Embargo set by: Seth Robbins for item 117341
Lift date: 2023-03-05T21:47:41Z
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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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
The Influence of Bank Vegetation on Flow Structure Within a Small Agricultural Stream: Implications for Channel Stability
No full textVegetation growing on river banks can have significant effects on the flow, sediment transport, and ultimately channel form. Although past studies in fluvial
geomorphology have largely ignored the complicated role ofinstream and bank
vegetation, some recent studies have attempted to characterize flow in vegetated environments. There is, however, still a need to examine the influence of bank
vegetation on flow structure and channel stability. This study seeks to investigate the
influence of dense overhanging bank grasses on the three-dimensional flow structure within a sinuous reach of the Spoon River, a small inset channel at the bottom of a
drainage ditch in East Central Illinois. The three-dimensional velocity data analysed in this study was collected at seven cross sections using an acoustic Doppler velocimeter.
The analyses of contour and vector plots of velocity, turbulent kinetic energy, and shear stress for each cross section has revealed that the dense overhanging grasses growing on the banks have effectively shifted the high-velocity core away from the outer banks of the bends. Along with the channel geometry at this site, the grasses have contributed to the existence of a zone of stagnant and recirculating flow along the outer banks, with the high shear layer adjacent to this zone. This has protected the outer banks from much of the
forces of erosion, thereby increasing the stability of this meandering inset channel
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