Concentrations in oscillatory sheet flow for well sorted and graded sands
TL;DR: In this article, a new empirical equation is presented which characterises the time-dependent concentration profile in the sheet flow layer, and the dependence of both parameters on flow and bed conditions is analysed.
About: This article is published in Coastal Engineering.The article was published on 2004-01-01. It has received 172 citations till now.
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TL;DR: In this paper, U-tube measurements of instantaneous velocities, concentrations, and fluxes for a well-sorted, medium-sized sand in oscillatory sheet flow are analyzed.
Abstract: [1] U‐tube measurements of instantaneous velocities, concentrations, and fluxes for a well‐sorted, medium‐sized sand in oscillatory sheet flow are analyzed. The experiments involved two velocity‐asymmetric flows, the same two flows with an opposing current of 0.4 m/s, and a mixed skewed‐asymmetric flow, all with a velocity amplitude of 1.2 m/s and flow period of 7 s. We find that the net positive transport rate beneath velocity‐ asymmetric oscillatory flow results from large, but opposing sand fluxes during the positive and negative flow phase. With an increase in velocity asymmetry and, in particular, velocity skewness, the difference in the magnitude of the fluxes in the two half cycles increases, leading to larger net transport rates. This trend is consistent with the observed increase in skewness of the oscillatory bed shear stress. Phase‐lag effects, whereby sand stirred during the negative flow phase has not settled by the time of the negative‐to‐positive flow reversal and is subsequently transported during the positive flow phase, are notable but of minor importance to the net transport rate compared to earlier experiments with finer sands. In the vertical, the oscillatory flux is positive above the no‐ flow bed. Within the sheet flow pick‐up layer, the oscillatory flux is negative and similar in magnitude to the positive flux induced by the residual flow. The 0.4 m/s opposing current causes more sand to be picked up during the negative than during the positive flow phase. Above the no‐flow bed the resulting negative oscillatory flux is comparable in magnitude to the current‐related flux.
160 citations
Cites background or methods from "Concentrations in oscillatory sheet..."
...6 of 13 [20] The instantaneous concentration data within the sheet flow layer show the expected [e.g.,O’Donoghue and Wright, 2004a; Hassan and Ribberink, 2005; Van der A et al., 2009] difference in behavior above and below z ≈ 0 mm for all five flows....
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...z = 0, as made in the flux computations of O’Donoghue and Wright [2004b] and Van der A et al....
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...…focused on the predominantly onshore sand transport rates induced by velocity‐skewed flows [e.g., Ribberink and Al‐Salem, 1994; Davies and Li, 1997; O’Donoghue and Wright, 2004b; Hassan and Ribberink, 2005], as velocity‐asymmetric flows were assumed to result in zero transport because of their…...
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TL;DR: In this paper, velocities were measured using an ultrasonic velocity profiler (UVP) capable of measuring deep within the sheet flow layer and the results showed that the near-bed velocity leads the main flow velocity by approximately 21° and a small offshore-directed current is generated near the bed.
158 citations
TL;DR: In this paper, the authors carried out measurements of the detailed time-dependent velocity and suspended sand concentration field around vortex ripples for regular oscillatory flow conditions and made velocity measurements for 14 different flows and concentration measurements for three of these flows.
Abstract: The knowledge and modeling of wave-induced sand transport over rippled beds still has significant shortcomings, which is partly related to a lack of measurements of the detailed processes from controlled laboratory experiments. We have carried out new measurements of the detailed time-dependent velocity and suspended sand concentration field around vortex ripples for regular oscillatory flow conditions. The fact that the ripples were mobile and the flow conditions were full-scale makes these measurements unique. We made velocity measurements for 14 different flows and concentration measurements for three of these flows. The velocity and concentration field above ripples are dominated by the generation and ejection of vortices on the ripple flanks around the time of flow reversal. Vortex formation results in near-ripple flow reversals ahead of free-stream reversals and velocity maxima near the ripple crest that are much higher than the free-stream maxima. Asymmetry in the free stream produces steady circulation cells with dominant offshore mean flow up the ripple lee slope, balanced by weaker onshore streaming up the ripple stoss slope as well as higher up in the flow. The time- and bed-averaged horizontal velocity profile comprises an offshore streaming near the bed and an onshore drift higher up in the flow. The vortices are responsible for three main concentration peaks: one just after on-offshore flow reversal associated with the passage of a sand-laden vortex followed by two smaller peaks due to advected suspension clouds generated by vortex action at the neighboring onshore ripples. The sand flux field measured for one typical asymmetric flow condition is dominated by an offshore flux associated with the suspended sand cloud generated by vortex shedding from the ripple's lee slope around the time of on-offshore flow reversal. The net (time-averaged) current-related and wave-related horizontal sand fluxes are generally offshore directed and mostly contained within 1.5 ripple heights above the ripple crest. The wave-related suspended transport component is larger, but the contribution of the current-related suspended sand transport cannot be neglected. In addition to the measured offshore net transport of suspended sand, there is an onshore-directed transport very close to the ripple surface. The total net transport is in the offshore direction for this specific asymmetric flow condition.
127 citations
Cites result from "Concentrations in oscillatory sheet..."
...[82] The observed streaming profile looks similar to the tunnel measurements by Ribberink and Al-Salem [1995] and O’Donoghue and Wright [2004] for regular asymmetric flows and sheet flow (flat bed) sand transport conditions....
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TL;DR: In this article, the wave-related sand transport is still very difficult to predict due to the complexity of its underlying processes, which mainly take place in a thin layer near the sea bed in the wave boundary layer.
Abstract: Shoaling short gravity waves at sea approaching the shore become asymmetric and are able to generate a net resulting sand transport in cross-shore direction (on-shore-offshore transport). The wave-related sand transport is still very difficult to predict due to the complexity of its underlying processes, which mainly take place in a thin layer near the sea bed in the wave boundary layer (thickness of order centimeters). The development of models for cross-shore sand transport heavily relies on experimental lab research, especially as taking place in large oscillating water tunnels (see, e.g., Nielsen, 1992). In oscillating water tunnels the near-bed horizontal orbital velocity, as induced by short gravity waves, can be simulated above fixed or mobile sandy beds (for a detailed description, see, e.g., Ribberink and Al-Salem, 1994). It should be realized that the vertical orbital flow and relatively small wave-induced residual flows as streaming and drift are not reproduced in flow tunnels. Research aimed at their contribution to the net sediment motion under surface waves is still ongoing (see Ribberink et al., 2000).
120 citations
Cites background or methods from "Concentrations in oscillatory sheet..."
...McLean et al. [18] successfully cross-correlated the concentration signals of two CCM probes, spaced 20 mm apart in the streamwise direction, to estimate particle velocities inside the sheetflow layer with a spatial resolution of approximately 1 mm. Hassan and Ribberink [29] improved this technique by reducing the distance between the probes to 11 mm. O’Donoghue and Wright [22, 23] used UVP to measure velocity profiles in the complete wave boundary layer including the upper sheet-flow layer with a spatial resolution of approximately 0.5 mm....
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...New detailed analysis of O’Donoghue and Wright’s [23] sheet-flow measurements has revealed (i) the dominance of the current-related flux in the sheet-flow layer and (ii) the different character of the current-related flux for fine and medium sand....
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...O’Donoghue and Wright [22, 23] obtained further insight into the influence of concentration phase lag on the horizontal sand fluxes inside the sheet-flow layer....
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...Here, we present new analysis of the sheet-flow measurements of O’Donoghue and Wright [23] in order to obtain new insights into the importance of the flow asymmetry-induced streaming for the total mean sediment flux....
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...The pick-up function boundary condition results in highly dynamic concentration at the reference level (z = a), but actual concentration measurements show that concentration varies little with time at the same elevation [22]....
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TL;DR: In this article, the effect of flow irregularity and ripple three-dimensionality on ripple dimensions is investigated. But the authors focus on ripple length and not on ripple width. But they do not consider ripple height and length.
120 citations
References
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TL;DR: In this paper, a simple mathematical model for sediment transport in straight alluvial channels is presented, which is based on physical ideas related to those introduced by Bagnold (1954), was originally developed in two steps, the first describing the bed load transport and the second accounting for the suspended load.
Abstract: The paper presents a simple mathematical model for sediment transport in straight alluvial channels. The model, which is based on physical ideas related to those introduced by Bagnold (1954), was originally developed in two steps, the first describing the bed load transport (Engelund 1975) and the second accounting for the suspended load (Fredsoe and Engelund 1976). The model is assumed to have two advantages as compared with empirical models, first it is based on a description of physical processes, secondly it gives some information about the quantity and size of the sand particles in suspension and the bed particles.
657 citations
TL;DR: In this article, an empirical relation for the bed concentration of suspended sediment cb defined at a few grain diameters above the bed is developed based on the experimental data sets of Guy et al. in 1966.
Abstract: An empirical relation for the bed concentration of suspended sediment cb defined at a few grain diameters above the bed is developed. Its derivation is based on the experimental data sets of Guy et al. in 1966. The values of the near‐bed concentration were obtained by first isolating the suspended load transport from the observed rates of total load transport by use of a bed‐load formula. The suspended load was then equated to Einstein's formulation, which allowed the determination of cb. The influence of large concentrations of suspended sediment and of the reference level for cb on the calculated values of bed concentration has been analyzed. The proposed relation compares satisfactorily to values of near‐bed concentration obtained from independent data sets. The procedure used to derive cb provides a simple method for the calculation of the total transport rate of noncohesive sediment.
290 citations
TL;DR: In this article, time-dependent measurements of flow velocities and sediment concentrations were conducted in a large oscillating water tunnel, aimed at the flow and sediment dynamics in and above an oscillatory boundary layer in plane bed and sheet-flow conditions.
257 citations
DOI•
29 Jan 1982TL;DR: In this paper, a series of laboratory experiments in an oscillatory tank was carried out to investigate the sheet flow of sediment, and the results showed that the criteria for the inception of sheet flow given by Manohar C1955 and by Komar and Miller (1974) are both applicable to materials composed of spheroidal particles, and that the average rate of sediment transport for sheet flow is well described by an empirical relationship given by Madsen and Grant (1976) for the bed load transport rate on a plane bed in oscillatory flow.
Abstract: A series of laboratory experiments in an oscillatory tank was carried out to investigate the sheet flow of sediment. Objectives of the study were to determine the criterion for inception of sheet flow, and to evaluate the sediment transport rate under the sheet flow condition. In order to proceed with the investigation, it was necessary to develop devices appropriate for tracing the sediment particle movement, and for measuring the extremely dense sediment concentration in the moving layer of sheet flow. The chief results are: 1) the criteria for the inception of sheet flow given by Manohar C1955) and by Komar and Miller (1974) are both applicable to materials composed of spheroidal particles, and 2) the average rate of sediment transport for sheet flow is well described by an empirical relationship given by Madsen and Grant (1976) for the bed load transport rate on a plane bed in oscillatory flow.
155 citations
TL;DR: In this article, a semi-unsteady model was developed that includes the effects of phase lags on the net transport rate in oscillatory sheet flow, which showed that phase lag becomes important for fine sand, high velocities and short wave periods.
142 citations