1,721,238 research outputs found
Field investigation of bedform morphodynamics under combined flow
No full textFieldwork was conducted at the Red Cliff sandbar located in the upper Humber Estuary in order to investigate bedform dynamics under different hydrodynamic conditions related to combinations in tidal and wave-generated currents. A fixed mooring was deployed to obtain current flow and wave properties across the sand bar site at both spring and neap tidal flows and during conditions of high wind generated waves. A terrestrial laser scanner was used to scan the sandbar during low water across the various forcing conditions, acquiring detailed information of ripple geometries. Under spring tide and calm wave conditions, two-dimensional asymmetrical ripples with straight crest lines were generated on the sandbar. Surveys were also conducted during strong winds. Prior to high-wind-waves, two-dimensional symmetrical and washed-out ripples were observed, during spring and neap tidal conditions respectively. The lengths of these ripples were almost the same as those generated under current only conditions, but their heights were relatively smaller. After the strong wind periods, 2D current-induced ripples were replaced by flatbed conditions and 2D symmetrical wash-out ripples, which indicates wave-induced bed shear stress is enhanced by the presence of a tidal current. Most pertinently our study reveals discrepancies between field observations and existing predictions of bed configurations that are largely based on laboratory investigations. This is likely due to a larger grain size distribution in field conditions and due to turbulence dampening by high concentrations of suspended clay particles. The study thus highlights a need to extend a range of field investigations that explore the current deficiencies in our abilities to predict bedforms and bedform dynamics in estuarine systems
Hummocky sedimentary structures within rippled beds due to combined orbital waves and transverse currents
No full textHummocky cross-stratification is commonly observed in the marine offshore transition to lower shoreface environments. However, to date, the origins of hummocky cross-stratification and its associated hummocky bedforms and hydrodynamic processes remain controversial and enigmatic. In the present study, a large-scale flume experiment was conducted to study the formation of hummocky bedforms. In the central test area of the flume, combined flow with water waves, with period of 2 s and velocity of 0.34 m s-1, progressed at a right angle to a current with velocity of 0.17 m s-1, whereas a wave alone condition pertained upstream and downstream of the test area. The combined-flow ripples in the test area had smaller dimensions than wave ripples, but their cross-section geometries were very similar. Most importantly, the experimental results, for the first time, revealed that humps occur with lengths up to approximately 40 mm beneath combined flow ripples. The formation of these structures appears to relate to the enhanced turbulence for the combined flow, because the turbulence kinetic energy for combined flow was ca 50% higher than that under the wave-only condition. Moreover, the observed small-scale humps in the present experiment had comparable cross-section geometries with hummock-like bedforms previously reported in laboratories and fields under storm conditions. Additionally, these humps could scale up to large-size hummocks under waves with longer period and faster velocity than the present conditions. The present experiment indicates that waves perpendicular to a current do generate hummock-like structures in association with ripples and therefore provide a new perspective for future study of the origins of hummocky cross-stratification
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
Modulation of sediment transport rates and hydraulic resistance by increasing mud-to-sand ratios
No full textBiologically-mediated muds and sand-mud sediment mixtures are prevalent in lowland rivers, coastal, marine, and estuarine environments. These systems are highly sensitive to ongoing sea-level rise and environmental change. Effective management of these environments and adaptation to future changes, including mitigation to flood risk, requires accurate prediction of how flow and bed morphology changes over time, which has recently been shown to strongly depend upon substrate composition and the mud-to-sand ratios. Mud is cohesive and helps stick granular sediment together, potentially reducing sediment transport rates and bedform growth, which impacts hydraulic resistance and thus the fluid flow. We examined the co-evolution of bedform growth (morphodynamics) and hydraulic resistance (hydrodynamics) in muddy, shallow coastal environments subject to the simultaneous action of waves and currents (combined-flow) through controlled physical experiments in the Total Environment Simulator at the University of Hull. We conducted experiments with combined flow (regular waves plus a steady current in 0.4 m water depth) over 1.5 m wide channels constructed within the experiment basin (11 m long). The channels were each filled with a homogeneous sediment mixture of kaolin clay (D50 = 8 microns) and medium sand (D50 = 390 microns) in mud-to-sand ratios ranging between 0% (clean sand, baseline) and 16% by mass, to a substrate depth of 0.10 m. We ran the experiments to equilibrium conditions whereby steady-state bedform dimensions were approached with respect to the flow conditions. As such, longer experimental run-times were required for beds with higher mud-to-sand ratios. We quantified bedform formation and evolution, and flow velocities with a suite of acoustic sensors. With the 3D flow velocity data, we quantified turbulent fluctuations to assess the flow dynamics and estimate shear characteristics of the flow. We used these data to quantify hydraulic resistance. Our results show that there is a mud-content threshold of approximately 8-11% (depends on hydrodynamic conditions) below which clean sand ripples form once the finer sediment is winnowed out, leading to similar ripple heights as those measured for clean sand conditions at equilibrium. This in turns results in comparable hydraulic resistance (friction) to the low mud or sand-only substrates. However, increasing clay content suppresses bedform dimensions (shorter and smaller ripples), and thus reduces hydraulic resistance. Above the mud-content threshold, ripples are inhibited and sand transport rates are insignificant, resulting in minimal form drag and subdued skin friction. Our results suggest that hydraulic resistance predictors for muddy-, shallow-coastal environments need to account for the presence of mud and its modulating effects in sediment transport and friction, which ultimately affects flow properties and associated flood risks
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
Changes in ripple migration rates and hydraulic resistance with increasing mud-to-sand ratios
No full textCoasts, estuaries, and lowland river environments are some of the most sensitive systems to climate-induced environmental change. In order to manage these systems, and adapt to future changes, we need to be able to predict how they might change. However, most available models have formulations based on assumptions that these systems are composed of only non-cohesive sands, even if mud is the most common sediment on Earth. Therefore, we need to find ways to incorporate the effect of sticky mud in predictors of flow resistance, sediment transport rates, and bedform geometries associated with form drag. In this paper, we show results from seven experiments conducted with different mud contents (0% - 12.6%) under the combined action of waves and currents. Experiments were conducted over a time period that was greater than the time needed for the bedform ripples to be at equilibrium with the imposed flow conditions. Our results reveal a mud content threshold above which friction coefficients are 4-6 times smaller than for clean sand and lower mud content counterparts. Below this threshold, flow resistance decreases with increasing mud content even though ripple migration rates are similar. Near the threshold, ripple migration rates increase as shorter ripples form and travel faster; the associated bedload transport rates also increase near the threshold and decrease beyond it. The threshold mud content, likely to vary with substrate composition and hydrodynamic conditions, is key to better estimating the behavior of different systems and needs further studies to properly incorporate it into predictors
Water and suspended sediment discharges for the Mekong Delta, Vietnam (2005-2015)
No full textThis dataset describes hourly time series of discharge and suspended sediment flux at four sites in the Vietnamese Mekong Delta (Chau Doc, Tan Chau, Can Tho and My Thaun) for the period 2005 – 2015. This data was calculated from historic Acoustic Doppler Current Profiler (aDcp)data obtained as part of routine flood monitoring conducted by the Vietnamese Hydrological Agency. The data were collated by the authors.
The data were processed to back out sediment fluxes through the delta through calibration of the acoustic backscatter signal to suspended sediment concentrations collected in Chau Doc (May 2017) and Can Tho (September 2017). For each aDcp instrument acoustic backscatter signal was calibrated to observed suspended sediment concentrations (SSCs). These concentrations values were then matched to measured acoustic backscatter values (dB) from the depth at which each sample was taken to generate power law calibration curves. To generate daily fluxes, the point specific ADCP fluxes were used to generate sediment ratings curves between sediment flux (kg/s) and discharge (m3/s). These ratings curves were then propagated over recorded daily discharge values measured by the Vietnamese hydrological agency to provide daily fluxes over the period of record.
The work was funded through NERC grant reference NE/P008100/1 - Deciphering the dominant drivers of contemporary relative sea-level change: Analysing sediment deposition and subsidence in a vulnerable mega-delta</span
Anger inhibition potentiates the association of high end-tidal CO2with blood pressure in women
No full textObjective: High resting end-tidal CO2(PetCO2) has been shown to be an independent predictor of systolic blood pressure (SBP) in women, particularly older women. The study reported in this article investigated whether the tendency to experience, express, and/or suppress anger contributes to the association of PetCO2and SBP in women and in men. Methods: The Spielberger Anger Expression Inventory was administered to 403 healthy male and female participants in the Baltimore Longitudinal Study on Aging. Resting PetCO2was obtained by means of a respiratory gas monitor, and resting blood pressure was obtained with an oscillometric device. The associations of resting PetCO2and the anger scales with SBP and diastolic blood pressure (DBP) were investigated using multivariate regression analyses. Results: PetCO2, as well as age and body mass index, was an independent predictor of SBP in women with low, but not high, trait anger and in women with low, but not high, anger-out. PetCO2was not an independent predictor of SBP in men with either high or low anger. In addition, PetCO2was not an independent predictor of DBP in either men or women. Conclusions: The results of this study indicate that inhibition of anger potentiates the relationship of high PetCO2with SBP in women but not men. Additional studies are needed to determine the origins of the observed gender differences and the psychophysiological pathways by which high resting PetCO2contributes to elevated resting blood pressure in women
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
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