Vertical mass transfer in open channel flow

Jobson, Harvey E.

doi:10.3133/ofr68146

Cited by 41 publications

(55 citation statements)

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“…Other net deposition experiments have been performed by Jobson & Sayre (1970). In this work, the particles were released near the water surface, so that the initial concentration profiles exhibit a peak close to z = H , as shown in figure 8(a).…”

Section: Net Deposition Experimentsmentioning

confidence: 99%

“…The suspended sediment response is expected to have a strong effect on the dynamics of the erodible bottom, especially on the formation of dunes or bars or, at a larger scale, on the development of meanders (Seminara 2006). Specific relaxation problems have been investigated by numerical integration of the Reynolds-averaged Navier-Stokes equations, using mixing length or k-ε turbulence models (Apmann & Rumer 1970;Hjelmfelt & Lenau 1970;Jobson & Sayre 1970;van Rijn 1986b;Celik & Rodi 1988;Ouillon & Le Guennec 1996). In the case in which bed load is dominant, or in the aeolian situation (saltation), it has been shown that the evolution of the sediment flux q can be accounted for by a relaxation equation of the form…”

Section: Introductionmentioning

confidence: 99%

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Transport relaxation time and length scales in turbulent suspensions

2011

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We show that in a turbulent flow transporting suspended sediment, the unsaturated sediment flux $q(x,t)$ can be described by a first-order relaxation equation. From a mode analysis of the advection-diffusion equation for the particle concentration, the relaxation length and time scales of the dominant mode are shown to be the deposition length $H U/V_{\rm fall}$ and deposition time $H/V_{\rm fall}$, where $H$ is the flow depth, $U$ the mean flow velocity and $V_{\rm fall}$ the sediment settling velocity. This result is expected to be particularly relevant for the case of sediment transport in slowly varying flows, where the flux is never far from saturation. Predictions are shown to be in quantitative agreement with flume experiments, for both net erosion and net deposition situations.Comment: 16 pages, 8 figures, accepted for publication in J. Fluid Mec

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Section: Net Deposition Experimentsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Transport relaxation time and length scales in turbulent suspensions

2011

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“…According to the Reynolds' analogy for the equivalence of mass and momentum transfer in turbulent shear flow (Jobson and Sayre 1970), negative Reynolds stresses indicate diffusivity of momentum from the water column to the sediment that could be similar to the diffusivity of a scalar and depending on the gradient, be a rough proxy for transport of mass. Comparing profiles in control (sand) situations with vegetated treatments indicates the flow reduction and increased transport of particles attributed to hydrodynamic changes due to the presence of the canopy.…”

Section: Leaf Area Indexmentioning

confidence: 99%

Effects of seagrasses and algae of the Caulerpa family on hydrodynamics and particle-trapping rates

et al. 2009

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The widespread decline of seagrass beds within the Mediterranean often results in the replacement of seagrasses by opportunistic green algae of the Caulerpa family. Because Caulerpa beds have a different height, stiffness and density compared to seagrasses, these changes in habitat type modify the interaction of the seafloor with hydrodynamics, influencing key processes such as sediment resuspension and particle trapping. Here, we compare the effects on hydrodynamics and particle trapping of Caulerpa taxifolia, C. racemosa, and C. prolifera with the Mediterranean seagrasses Cymodocea nodosa and Posidonia oceanica. All macrophyte canopies reduced near-bed volumetric flow rates compared to bare sediment, vertical profiles of turbulent kinetic energy revealed peak values around the top of the canopies, and maximum values of Reynolds stress increased by a factor of between 1.4 (C. nodosa) and 324

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“…), which may be considered as vertical transfer (w') of longitudinal momentum fluctuations (u') and an indicator of vertical mixing activity (Velasco et al 2001). Negative Reynolds stresses indicate fluxes from the water column to the sediment which, according to the Reynolds' analogy for the equivalence of mass and momentum transfer in turbulent shear flow (Jobson & Sayre 1970), can be also interpreted as transport of mass. Comparing profiles in control (sand) conditions and vegetated treatments gives an indication of increased transport of particles attributable to hydrodynamic changes caused by presence of the canopy.…”

mentioning

confidence: 99%

Experimental assessment and modeling evaluation of the effects of the seagrass Posidonia oceanica on flow and particle trapping

Hendriks¹,

Sintes²,

Bouma³

et al. 2008

Mar. Ecol. Prog. Ser.

256

187

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Retention of particles in seagrass canopies is usually attributed to only the indirect, attenuating effects canopies have on flow, turbulence and wave action, promoting sedimentation and reducing resuspension within seagrass meadows. Yet recent evidence suggests that seagrasses are also able to affect particle flux directly through loss of momentum and increased path length derived from collisions with leaves and binding of particles. We evaluated the role of Posidonia oceanica on flow and associated particle trapping through flume experiments. Our results confirm the existence of 2 dynamically different environments, viz. (1) the below-canopy habitat, with low shear stress and reduced turbulence, and (2) the canopy-water interface region, characterized by high shear stress and turbulence intensity, where vertical transport of momentum is enhanced. At relatively low free stream velocities (i.e. 0.05 and 0.10 m s -1 ) sediment concentration decreased much faster when a Posidonia meadow was present in the flume, indicating major particle trapping in the seagrass canopy. Fluxes to the sediment (as shown by large negative peaks in Reynolds stress inside the Posidonia meadow) indicated 2 to 6 times more sediment transport to the bottom when a meadow is present. However, calculations based on the experimental results point to loss rates an order of magnitude larger in a Posidonia meadow. We hypothesize that direct effects of particle collisions with leaves are responsible for this discrepancy, and we explore possible interactions with a simple model. Using only collisions as a loss factor, the model predicts that the probability a particle is lost from the flow upon a collision is 2 to 3%. Previously observed leaf density and flow velocity effects on particle loss rates were explained by the model. Fitting the model to our experimentally obtained particle disappearance rates in vegetation indicated that around 27% of particle momentum is lost upon each collision with a leaf. We hypothesize that physical filtration by sediment collisions with plant structures plays a role in particle removal in aquatic systems. KEY WORDS: Particle trapping · Physical filtering · SeagrassResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 356: 163-173, 2008 Explaining the full mechanism by which seagrasses enhance particle retention is less clear. Usually, it is assumed that the role of seagrasses is solely indirect, driven by their effects on flow (Fonseca et al. 1983, Ackerman & Okubo 1993, Lopez & Garcia 1998, Granata et al. 2001). Yet recent evidence suggests that once particles are brought near the canopy (through the effects on flow listed above), seagrasses also affect particle flux directly, leading to potentially high particle removal rates (Agawin & Duarte 2002). Possible mechanisms for such direct effects on particle trapping include (1) the loss of momentum and increased path length derived from collisions of the particles with leaves within the dense seagrass canopie...

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Vertical mass transfer in open channel flow

Cited by 41 publications

References 0 publications

Transport relaxation time and length scales in turbulent suspensions

Transport relaxation time and length scales in turbulent suspensions

Effects of seagrasses and algae of the Caulerpa family on hydrodynamics and particle-trapping rates

Experimental assessment and modeling evaluation of the effects of the seagrass Posidonia oceanica on flow and particle trapping

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