1,721,012 research outputs found
Representation of Vegetation in Two-Dimensional Hydrodynamic Models
No full textHerein we present the work product from an ASCE Task Committee charged with providing a succinct review of the available approaches for determining flow resistance coefficients for vegetated floodplains for use in two-dimensional hydrodynamic model simulations, with comparison of the most promising approaches. In the future, with the restoration and conservation of ecosystems of growing importance, engineers will increasingly design and manipulate floodplain vegetation. Conceivably, the appropriate placement of plants along stream corridors may direct overbank flows, improve flood storage, reduce scour and erosion, facilitate sediment transport, and alleviate other flood risk factors, while also providing critical habitat. Previous approaches to predicting water surface elevations along vegetated channels and floodplains consisted of running computer models that typically allowed little temporal variation in flow resistance coefficients during the course of a flow event. More recently, the sophistication of flow models has improved to allow simulation of interactions among hydraulic variables and vegetation properties. The literature dealing with the effects of vegetation on open channel flow is vast and growing. Several workers have reviewed the current state of the science (Montakhab et al. 2012; Curran and Hession 2013; Hession and Curran 2013; Dombroski 2014, Zahidi et al. 2014), and the paper by Aberle and Jarvela (2013) summarizes the state-ofthe-art
in research on the flow resistance of emergent rigid and flexible floodplain vegetation. This paper compiles and interprets this recent literature, with an focus on the estimation of vegetated floodplain resistance for two-dimensional numerical modeling
Wake structure and sediment deposition behind models of submerged vegetation with and without flexible leaves
No full textThis laboratory study explored the flow structure and pattern of deposition directly downstream of submerged patches of vegetation, focusing on whether, or not, the presence of flexible trailing leaves enhanced deposition. Both leaves of different length and patches of different geometry (circular and channel-spanning) were considered. The study defined the length of wake within which the velocity was diminished and net deposition was enhanced. The model sediment represented an organic or mineral solid smaller than fine sand. For a channel-spanning, submerged patch, recirculation or turbulent diffusion in the x-z (streamwise-vertical) plane set the wake length within which velocity was diminished and deposition was enhanced. This length was greater for patches with lower stem densities, because greater flow through the patch displaced the recirculating eddy farther downstream. In addition, for a channel-spanning patch, the presence of flexible trailing leaves extended the wake length, which in turn increased the length of the deposition region. In contrast, for a circular patch, the wake contained oscillations in the x-y (streamwise – lateral) plane. The onset of the wake unsteadiness set the length of the deposition region. Because the presence of flexible trailing leaves on a circular patch did not affect the formation distance for the wake oscillation, the length of the deposition region was unchanged with the addition of the flexible trailing leaves. For both circular and channel-spanning patches, a longer deposition region was associated with a larger deposition mass. Keywords: Aquatic vegetation; Wake structure; Sediment deposition; EcohydraulicsNational Science Foundation (Grant EAR-1414499
The incipient motion of sediment in a channel with model emergent vegetation
No full textIn a bare channel (without vegetation), the incipient velocity for sediment motion, U[subscript crit], has historically been related to the mean bed shear stress ([bar over τ]) o or friction velocity (U[subscript ∗] = √[bar over τ]/ρ). More recent studies, however, suggest turbulence also plays a role. This paper examines whether the onset of sediment motion in a vegetated channel is correlated with U[subscript ∗], or turbulence (k[subscript τ). Images collected with a digital camera were interrogated with a particle-tracking code to measure sediment transport for different vegetation density and channel velocity. The trend in sediment transport with channel velocity was used to identify U[subscript crit] for each stem density. The values of k[subscript τ and U[subscript ∗] were estimated at Ucrit. However, none of these parameters produced a constant threshold across all stem density and bare bed. We construct a new metric representing the peak turbulent velocities impinging on the bed that produces a constant threshold value for all cases
Numerical modeling study to compare the nutrient removal potential of different floating treatment island configurations in a stormwater pond
No full textConstructed wetlands, which are commonly used in wastewater treatment, are difficult to use for stormwater treatment, because the water level variation in a stormwater pond makes it difficult for rooted vegetation to survive. As an alternative, vegetation can be grown on floating mats, called floating treatment islands (FTI), with roots extending downward into the water. Nutrient removal is achieved through uptake and trapping by the matrix of roots and bio-film. The overall treatment provided by a pond with FTIs depends on the fraction of flow exposed to the root zone. This study used numerical modeling to study the flow through the root zone, with the goal of determining which configuration of FTI achieved the greatest overall treatment. Six different configurations were considered, all with root zone volume equal to 11% of the pond volume. The permeability of the root zone was estimated using velocity measurements within real floating vegetation. A first-order removal rate within the root zone (k[subscript r]) was estimated from removal rates reported in the literature. Preliminary studies considered the similarity in flow and removal between systems of different physical scale. Geometric similarity of the root zone guaranteed flow similarity within the root zone. To achieve performance similarity (same mass reduction), systems need to have the same non-dimensional removal rate (k[subscript r]t[subscript n], with t[subscript n] the nominal residence time of the pond). The consideration of different FTI configurations showed that wakes generated by upstream FTIs lowered the mass removal of downstream FTIs, so that segmenting a single large FTI into multiple smaller FTIs in series did not improve overall nutrient removal. However, segmenting a single FTI into a pair of parallel FTIs did improve the nutrient removal, and this configuration provided the best pond-scale removal. Keywords: Floating treatment islands; Stormwater pond; Mass removal; Numerical modelin
Turbulent Kinetic Energy in Submerged Model Canopies Under Oscillatory Flow
No full textLaboratory experiments measured the velocity inside a model meadow of submerged, flexible vegetation under 1 and 2 s period waves. The model plant consisted of a rigid stem and strap‐like blades, similar to the seagrass Zostera marina and the freshwater eelgrass Vallisneria Americana. The ratio of wave excursion (A[subscript w]) to stem spacing (S) determined whether, or not, plant‐generated turbulence enhanced the turbulence level within the meadow, compared to bare bed. Specifically, near‐bed turbulence was enhanced for conditions with A[subscript w]/S > 0.5, and for these conditions the turbulence (TKE) normalized by the RMS wave velocity squared, TKE/U[subscript w], RMS², increased monotonically with the plant solid volume fraction, ϕ. The plant‐generated turbulence was greater in the stem region than in the blade region, and this was attributed to the greater relative motion between the waves and rigid stem, compared to the flexible blades. A model previously developed to predict TKE in unidirectional flow through a rigid emergent canopy was modified by replacing the time‐mean current with the RMS wave velocity. With a fitted scale coefficient, the modified model predicts TKE as a function of RMS wave velocity in the meadow, stem and blade geometry, and solid volume fraction. Wave decay was also measured and shown to have a linear correlation with the measured TKE within the canopy, providing a second method to predict meadow TKE in the field.National Science Foundation (U.S.) (Grant EAR-1659923
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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