1,721,051 research outputs found
Bed roughness over vegetated beds: sonar imaging techniques and effect on unidirectional currents
No full textSmall scale roughness of the seafloor is of direct relevance to a range of interests, including boundary layer hydrodynamics, sediment transport and high-frequency acoustic scattering. Despite its importance, only few studies have quantitatively resolved seafloor height at the relevant scales. In particular, characterisation of roughness over vegetated beds is needed to better understand hydrodynamics and sediment transport in the coastal zone. A new Benthic Roughness Acoustic Device (BRAD) has been developed to define microtopographical roughness through high-resolution imagery of the seabed. BRAD, composedof a profiling sonar – the Sediment Imager Sonar (SIS) – and a motor, both mounted on a frame, enables measurements of the seabed elevation over an area of 1.7 m2. A thresholdmethod was established to detect the seabed from the SIS raw data. Laboratorydeployments were carried out in order to assess the system accuracy over known targetsand its ability to discriminate sediment sizes. Field deployments at 6 sites enabled the imaging of a variety of seabed types; in particular bioturbated fine sand and mud, seagrass canopies, gravelly sand and ripple fields. Spectral analysis applied on the seabed elevations was used to characterise roughness type. Seagrasses are flowering plants that have adapted to the submerged marine environment.They develop extensive underwater meadows in coastal areas around the world, forming complex, highly productive ecosystems. The SIS was used together with a towed videocamera system to survey a seagrass (Zostera marina) bed in Calshot, UK. A method was developed to assess Z. marina presence from the SIS data and its results were tested against video data. The SIS proved to be a useful tool for seagrass surveying and the use of the SIS and the video yielded a preliminary map of the seagrass bed. Seagrass canopies exert strong effects on water flow inside and around them. The influenceof Zostera marina canopies on flow, turbulence, roughness and sediment movement was evaluated through laboratory experiments. Numerous runs were carried out in an annular and a straight, recirculating flume using live Z. marina and a mobile sand layer. Flow was greatly decelerated inside the canopy while turbulence was increased. The Turbulent Kinetic Energy was observed to be maximal at the canopy/water interface. This was hypothesised to be related to the canopy ‘wavy’ motion. Sediment movement was observed within the canopy as scour around the stems. Ripples formed downstream of the canopy at velocities lower than the sediment threshold of motion. Intermittent turbulence associated with the burst phenomenon is thought to be responsible for this
Wave-induced sand re-suspension at dredged gravel pits based upon hydrodynamic measurements (Tromper Wiek, Baltic Sea)
No full textABSTRACTGravel pits created by anchor hopper dredging may affect regional sediment transport patterns by trapping sediments. In turn, this may cause -or enhance- erosion at the adjacent coastline. Reliable assessment of such impacts requires a good understanding of the hydro-sediment dynamic processes acting at dredged pits. This paper examines the processes for sand re-suspension from pressure, current and turbidity data collected inside and outside a single dredged pit, in a non-tidal environment (Tromper Wiek, Baltic Sea). The data confirm the generally weak sediment dynamics in the area, with waves being the main hydrodynamic agent for sediment re-mobilization. Comparisons with historical data indicate a small number of sediment re-suspension events (<15%), over a 37 months-long period, without significant difference inside and outside the pit. Suspended sediment concentration profiles are predicted inside the studied pit by a simplistic model, tuned to over-estimate sediment re-suspension. The results suggest that the depth of the excavation should be very shallow (<1 m) for the bed material to be frequently extracted out by waves, and redistributed over the area. With pits up to 7 m-deep within the extraction zone, we conclude that a significant fraction of sediment is trapped over the long-term period (years)
Bedform properties, Weser Estuary, 2009-2014
No full textBedform (dune) properties calculated from multibeam bathymetric survey from the Weser Estuary between 2009 and 2014 are given. This includes the BEP bedform properties (crest position, date of survey, mean depth of bedform, length, height, bedform asymmetry, mean and maximum angles of the flood and ebb lee sides, normalised horizontal and vertical positions of the maximum angles, height, length and mean angle of the ebb and flood steep faces (if present) and normalised position of the beginning and end of the steep faces), and the crestline properties (positions of the crestline middle point, crestline length, mean and standard deviation of crestline orientation, standard deviation of crestline height, and non-dimensionnal span)
Summary of numerical model results investigating the influence of dune lee side shape on time-averaged velocities and turbulence
No full textA total of 106 numerical simulations were done with Delft3D to investigate the influence of lee side shape on time-averaged flow properties and turbulence. This dataset presents a summary of the key parameters calculated from the numerical simulations: the bed smoothness, the mean lee side angle, the steep portion angle, the steep portion position, the dune length, the dune height, the steep portion height, the flow separation zone length, the maximum and mean Turbulent Kinetic Energy and the length of the wake
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
Dune morphology and hysteresis in alluvial channels during long-duration floods revealed using high temporal-resolution MBES bathymetry
No full textIn natural rivers, flow discharge may fluctuate across a range of time scales – from diurnal to seasonal - but is often most pronounced during hydrographs that may encompass long-duration floods. Under these varying flows, bedforms can be created and modified by the flow without achieving any ‘equilibrium’ state. A lag between changes in flow and the morphological response of the bedforms, termed bedform hysteresis, is commonly present. Importantly for channel management and navigation, since dunes may grow larger during floods, but often experience a lagged decay in size during lowering water levels, critical water depths may be reached for inland shipping. There is also a consensus that dunes possess a more flattened shape, and lower leeside angle, than previously assumed in large rivers and that such dunes do not exhibit a region of permanent flow separation downstream of the dune. This different leeside shape thus questions traditional ideas of flow interactions with dunes, where flow separation in the steep dune lee side leads to energy loss (form drag) that increases flow resistance and energy expenditure within the flow. This paper quantifies dune hysteresis in the River Waal, Netherlands, by investigating how dune morphology changes through flood hydrographs, using high spatial- and temporal- resolution bathymetric data and robust computational analysis methods to produce probability density functions of dune morphology. This quantification aims to provide a better understanding of dune hysteresis in large rivers. The analysis examines several large data sets of river bathymetry from a 13 km reach of the River Waal, Netherlands, in a time series of bi-weekly multibeam echo sounder surveys over the last 12 years. Parameterization includes dune height, wavelength, leeside angle and leeside shape, to assess dune kinematics and hysteresis during different flood hydrographs.Rivers, Ports, Waterways and Dredging Engineerin
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
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