Bottom‐slope‐induced net sheet‐flow sediment transport rate under sinusoidal oscillatory flows

Yuan, Jinliang; Li, Zhiwei; Madsen, Ole Secher

doi:10.1002/2016jc011996

Cited by 12 publications

(15 citation statements)

References 38 publications

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“…Thus, the facility can provide full‐scale approximation of wave‐driven near‐bed flows. A 9‐m‐long and 20‐cm‐deep trough is attached to the bottom of the test channel for holding sediments, so this facility can be used for studying sediment transport under oscillatory flows (e.g., Wang & Yuan, ; Yuan et al, ).…”

Section: Methodsmentioning

confidence: 99%

An Experimental Investigation of Acceleration‐Skewed Oscillatory Flow Over Vortex Ripples

Yuan

Wang

2019

JGR Oceans

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A full-scale experimental study of acceleration-skewed oscillatory flow over vortex ripples, which is an approximation of wave-driven flow over a rippled seabed, was conducted in an oscillatory water tunnel. Three acceleration-skewed-flow tests and a reference sinusoidal-flow tests were involved in this study. In each test, two-dimensional vortex ripples were generated from a coarse-sand movable bed, and the flow field was measured with a particle image velocimetry. The dominant feature of phase-averaged flow is the generation of onshore (offshore) coherent vortices during the onshore (offshore) half-cycle. The acceleration skewness expedites the offshore-onshore flow reversal and therefore speeds up the ejection of offshore vortex, while the opposite occurs for the onshore vortex. The ripple-averaged flow is similar to the acceleration-skewed turbulent oscillatory flow over a flat bed in terms of leading Fourier harmonics, boundary layer thickness, and cycle-averaged flow, suggesting a possible analogy between the two. From the aspect of flow turbulence, acceleration skewness elongates the offshore accelerating quarter, so the turbulence embedded in the offshore vortex can be better developed. This, together with a quick ejection process, allows the offshore vortex to carry a significant amount of residual turbulence to the onshore-side ripple flank, which may even enhance the production of turbulence within the onshore half-cycle. Some results on pressure field and form drag are also presented in this paper. Plain Language SummaryIn shallow coastal regions, shoaling waves produce oscillatory bottom flows, which create sand ripples on a sandy seabed. The acceleration skewness of the oscillatory flow is a result of the nonlinear feature of the overlying water waves, and it can differentiate the two half-cycles of the oscillatory flow and therefore leading to a net (cycle-averaged) sediment transport. In this study, we use an oscillatory water tunnel and particle image velocimetry to investigate the oscillatory boundary layer flow over vortex ripples with a focus on the effect of acceleration skewness. Our measurements provide detailed descriptions of the formation-ejection process of coherent vortices, which is produced by the flow separation behind the ripple crest. Some useful observations on flow turbulence are also provided. The results showed that acceleration skewness can substantially differentiate the two coherent vortices developed within the two half-cycles. The offshore-side vortex can carry more turbulence over the ripple crest than the onshore-side vortex, which possibly leads to some net onshore sediment transport rate. We also showed that the ripple-averaged flow is fairly close to that over a flat bed, so flat-bed boundary layer model can be possibly used for rippled bed with some parameterizations.

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Section: Methodsmentioning

confidence: 99%

An Experimental Investigation of Acceleration‐Skewed Oscillatory Flow Over Vortex Ripples

Yuan

Wang

2019

JGR Oceans

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“…The main possible sources of this mismatch are the compaction of sand bed and some three‐dimensionality, which cannot be well captured by the LBP system. In a previous sheet flow study, that is, Yuan, Li, and Madsen (), this mismatch is mainly due to bed compaction. However, for our rippled‐bed tests, bed compaction should not be the main reason because the sand bed had been compacted for O (1,000) periods when the ripples were first generated on a horizontal bottom.…”

Section: Net Sediment Transport Ratementioning

confidence: 89%

“…

(〈〉, q_{net})

is determined based on the principle of volume conservation. Considering the Exner equation,

\frac{\partial q_{net}}{∂x} = - false(1 - ϵ false) \frac{∂z}{∂t},

where ϵ = 0.482 is the sand's porosity reported by Yuan, Li, and Madsen (; we used the same coarse sand from their study). Note that q net in equation varies with spatial x and time t because of ripple migration.…”

Section: Net Sediment Transport Ratementioning

confidence: 99%

“…Alternatively, starting the integral from the upslope end of the test section x = x L gives another estimate

q_{su} false(x false) = \frac{V_{L}}{normalΔ Tb} + \frac{1 - ϵ}{normalΔ T} \int_{x}^{x_{L}} normalΔ z_{b} normald x,

where V L is the volume of sand collected outside the upslope end ( x > x L ), which can be obtained following the same method as determining V 0 . Theoretically speaking, q su should be identical to q sd , but a mismatch δ ( q ) is usually observed, which can be expressed as (see Yuan, Li, & Madsen, )

δ false(q false) = q_{sd} false(x false) - q_{su} false(x false) = \frac{1}{normalΔ Tb} false[V_{LBP} - false(V_{0} + V_{L} false) false] = \frac{normalΔ V}{normalΔ Tb},

where

V_{LBP} = - false(1 - ϵ false) b \int_{x_{0}}^{x_{L}} normalΔ z_{b} normald x

is the volume loss of sands measured by the LBP system and ( V 0 + V L ) is the total sand volume collected outside the test section. Equation indicates that the mismatch between q su and q sd is due to the mismatch of sand volume Δ V = V LBP −( V 0 + V L ).…”

Section: Net Sediment Transport Ratementioning

confidence: 99%

“…In a recent study by Yuan, Li, and Madsen (), sheet‐flow‐regime experimental study was conducted in an inclinable OWT. In their tests, sinusoidal oscillatory flows were produced over movable bed with a bottom slope up to 2.6°.…”

Section: Introductionmentioning

confidence: 99%

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Bottom‐Slope‐Induced Net Sediment Transport Rate Under Oscillatory Flows in the Rippled‐Bed Regime

Wang

Yuan

2018

JGR Oceans

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Full-scale experiments in the rippled-bed regime were conducted in an oscillatory water tunnel to study the net sediment transport rate due to oscillatory flows on sloping bottoms (up to 2.6 ∘ ). The tests cover a variety of sinusoidal flows, under which uniform two-dimensional ripples are generated from a coarse sand movable bed. A laser-based bottom profiler was developed to measure ripple geometry and net sediment transport rate. Under the same flow condition, a small bottom slope can slightly change the shape of the equilibrium ripples; for example, ripples become upslope leaning, but the ripple dimensions remain almost unchanged. Ripples are also found to migrate in the downslope direction with uniform speed and permanent shape, which is due to the spatial variation of net bedload transport rate. For a given flow condition, the slope-induced net sediment transport rates are in the downslope direction and increase linearly with the bottom slope. A conceptual model, which is an extension of a sheet flow model (Yuan, Li, & Madsen, 2017, https://doi.org/10.1002/2016JC011996), was also developed. This calibration-free model can reasonably predict the net transport rate, despite that a moderate underestimate (a factor slightly less than 2) is observed. Both our experiments and model suggest that the presence of ripples significantly enhances sediment mixing and leads to a large net suspended-load transport rate, which will not occur under sheet flow conditions. Therefore, net transport rate in the ripple-bed regime can be comparable to that in the sheet flow regime, although the flow condition is much weaker.Plain Language Summary Sediment transport over seabed covered by wave-generated sand ripples (or vortex ripple) is an important coastal process related to many coastal engineering problems (e.g., coastal erosion). Understanding the mechanisms of net (time-averaged) sediment transport is of the primary interest to coastal modelers. The bottom-slope effect, namely, a bottom-parallel gravity force acting on sediment grains, is believed to be a key mechanism, but very few quantitative experimental results or modeling work on this effect are available in the literature. In this study, we conduct a full-scale experiment, in which sinusoidal oscillatory flows over vortex ripples are generated in an inclinable oscillatory water tunnel. Measurements of ripple shape and net transport rate are obtained, and a conceptual model is developed for interpreting experimental results and governing physics. It is found that ripples become slightly upslope leaning on sloping bed, and they migrate in the downslope direction. A net downslope transport rate, which includes a strong suspended-load component, is observed in all tests, and it increases quite linearly with bottom slope. The conceptual model can reasonably reproduce measured net transport rates.

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Field methods of a near-bed suspended sediment experiment in the Yangtze River, China

Shu

Tan

et al. 2020

Arab J Geosci

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Using experimental data of near-bed suspended sediment concentrations at five typical hydrometric stations of the Three Gorges Reservoir at the early reserving stage, the differences were investigated between the common method and improved method during flood seasons and non-flood seasons. The impact of taking measurements below 0.2 times the water depth on the results was discussed. The results show that the average discharges and velocities at each station calculated by the common method were slightly larger than those calculated by the improved method. Regarding the suspended sediment concentration at each station, the errors in the reservoir and downstream channels in dynamic equilibrium state were small, and the largest errors occurred where the river bed was strongly scoured in the downstream reach below the large dam. There was no significant relationship between water discharge and flow velocity, and the missed measurement phenomenon also occurred. The sediment discharge error was affected by the suspended sediment concentration, implying that errors usually occurred in channels with serious erosion during flood seasons. The correction coefficients (R2) of sediment discharge at each station were given during the experiment, which showed that the sediment discharges at the hydrometric stations where a large amount of sediment transport occurred near the river bottom, needed to be modified. Furthermore, the test methods proposed in this study were applied to calculate the sediment discharges of three rivers, and the results indicate that this method can narrow the gap between bathymetric comparisons and sediment load measurements.

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Bottom‐slope‐induced net sheet‐flow sediment transport rate under sinusoidal oscillatory flows

Cited by 12 publications

References 38 publications

An Experimental Investigation of Acceleration‐Skewed Oscillatory Flow Over Vortex Ripples

An Experimental Investigation of Acceleration‐Skewed Oscillatory Flow Over Vortex Ripples

Bottom‐Slope‐Induced Net Sediment Transport Rate Under Oscillatory Flows in the Rippled‐Bed Regime

Field methods of a near-bed suspended sediment experiment in the Yangtze River, China

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