A PIV-based method to measure spatial gradients in bedload transport over a dune

Traditional sediment transport equations calculate sediment flux from bed shear stress and the equations predict that transport increases nonlinearly with an increase in flow velocity. In a dune field, the dune geometry affects the flow velocity causing accelerating flow over the dune crest and de- and reattachment of the flow downstream of the dune crest. Sediment flux predicted from the reach-averaged bed shear stress gives fairly good results for dune fields, though their simplification is discordant for the complexity of the processes involved. Measurements of the displacement of sand particles over the dune bed were derived from highfrequency image capturing. The two main methods to measure particle velocities from images are particle tracking velocimetry (PTV) and particle image velocimetry (PIV). We compare individual particle tracking with a PIV-based correlation method. The PIV-based method promises to be a more efficient and effective approach to track particle motion. It is more suitable for the conditions of high bedload transport, as present in our experiments. The PIV-based method is based on using images of difference (IoD) and is fully automated and identifies spatial gradients at a support scale in the order of centimetres. Findings align with our general knowledge of accelerating flow over the dune crest. The mean streamwise particle velocity and activity over a dune stoss slope increase. At the scale of 0.026 m the observed particle velocity variability can be explained in the context of general onset and cessation of sediment transport, the effect of the reattachment zone and observed sweep/burst events. By decreasing the streamwise distance between cross-sections, the variations in mean particle velocity induced by superimposed bed defects are distinguished as well. The maximum particle velocity and activity occurred at the same location and consequently the location of the maximum transport over the dune crest was identified. The measurements bridge the gap between individual particle motion studies and (non-local) sediment transport flux measurements.

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Particle velocity and sediment transport at the limit of deposition in sewers

Water Science & Technology ◽

10.2166/wst.2013.646 ◽

2013 ◽

Vol 67 (5) ◽

pp. 959-967 ◽

Cited By ~ 31

Author(s):

J. J. Ota ◽

G. S. Perrusquía

Keyword(s):

Shear Stress ◽

Sediment Transport ◽

Particle Velocity ◽

Water Velocity ◽

Angle Of Repose ◽

Bed Load ◽

Bed Shear Stress ◽

Transport Capacity ◽

Sediment Particle ◽

Velocity Measurements

This paper focuses on the sediment particle while it is transported at the limit of deposition in storm sewers, i.e. as bed load at the limit of concentration that leads to sediment deposition. Although many empirical sediment transport equations are known in the literature, there is only limited knowledge concerning particle velocity. Sediment particle and sphere velocity measurements were carried out in two pipe channels and these results led to the development of a semi-theoretical equation for sediment transport at the limit of deposition in sewers. Even in the transport process without deposition, sediment movement is slower than water velocity and depends on the angle of repose of sediment with a diameter d on the roughness k of the pipe channel. Instead of classical dimensionless bed shear stress ψ, a modified dimensionless bed shear stress ψ (d/k)2/3 was suggested, based on the angle of repose and this parameter was proved to be significant for quantifying the transport capacity. The main purpose of this article is to emphasize the importance of careful observation of experiments. Not only number of tests, but physical understanding are essential for better empirical equations.

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Numerical Model for Bank Erosion in the Brahmaputra River

Journal of Disaster Research ◽

10.20965/jdr.2016.p1073 ◽

2016 ◽

Vol 11 (6) ◽

pp. 1073-1081 ◽

Cited By ~ 1

Author(s):

Robin K. Biswas ◽

◽

Atsuhiro Yorozuya ◽

Shinji Egashira ◽

◽

...

Keyword(s):

Shear Stress ◽

Sediment Transport ◽

Suspended Sediment ◽

Deposition Rate ◽

Bank Erosion ◽

Bedload Transport ◽

Bed Shear Stress ◽

Brahmaputra River ◽

Erosion And Deposition ◽

Sand Bar

A method is proposed to predict bank erosion and sand bar migration in river reaches where suspended sediment transport is dominant. The method focuses on the influence of the lateral bed slope on the erosion and deposition rate of suspended sediment, as well as on the profile of lateral bedload transport, assuming that geometric similarity holds in the bank region. In the proposed model, the erosion and deposition rate can be evaluated using either the bed shear stress at a reference location or the average bed shear stress in the bank region. In order to simulate bank erosion and associated bank shifting with a depth-integrated-base treatment, stretchable grids were added to the conventional coarse grid system near the bank. The proposed method, including the bank erosion model, is applied to the lower reach of the Brahmaputra River, which is ∼90 km long and ∼12.50 km wide. The computed results on bank shifting, sand bar migration, and sediment transport rates are compared with data obtained from field investigations and remote sensing. These results suggest that the proposed method is applicable for predicting sediment issues in river reaches dominated by suspended sediment.

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Efficiency prediction for storage chambers using computational fluid dynamics

Water Science & Technology ◽

10.2166/wst.1996.0202 ◽

1996 ◽

Vol 33 (9) ◽

pp. 163-170 ◽

Cited By ~ 4

Author(s):

Virginia R. Stovin ◽

Adrian J. Saul

Keyword(s):

Fluid Dynamics ◽

Computational Fluid Dynamics ◽

Shear Stress ◽

Particle Tracking ◽

Critical Value ◽

Complex Geometries ◽

Bed Shear Stress ◽

Breadth Ratio ◽

Computational Fluid Dynamics Cfd ◽

Simulated Flow

Research was undertaken in order to identify possible methodologies for the prediction of sedimentation in storage chambers based on computational fluid dynamics (CFD). The Fluent CFD software was used to establish a numerical model of the flow field, on which further analysis was undertaken. Sedimentation was estimated from the simulated flow fields by two different methods. The first approach used the simulation to predict the bed shear stress distribution, with deposition being assumed for areas where the bed shear stress fell below a critical value (τcd). The value of τcd had previously been determined in the laboratory. Efficiency was then calculated as a function of the proportion of the chamber bed for which deposition had been predicted. The second method used the particle tracking facility in Fluent and efficiency was calculated from the proportion of particles that remained within the chamber. The results from the two techniques for efficiency are compared to data collected in a laboratory chamber. Three further simulations were then undertaken in order to investigate the influence of length to breadth ratio on chamber performance. The methodology presented here could be applied to complex geometries and full scale installations.

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Bed Shear Stress Influence on Local Scour Geometry Properties in Various Flume Development Conditions

Water ◽

10.3390/w11112346 ◽

2019 ◽

Vol 11 (11) ◽

pp. 2346 ◽

Cited By ~ 3

Author(s):

Kiraga ◽

Popek

Keyword(s):

Shear Stress ◽

Sediment Transport ◽

Mutual Influence ◽

Three Dimensional ◽

Critical Shear Stress ◽

Water Structure ◽

Correlation Coefficients ◽

Bed Shear Stress ◽

Hole Dimensions ◽

Bed Material

Numerous approaches in sediment mobility studies highlighted the key meaning of channel roughness, which results not only from bed material granulation but also from various bed forms presence, caused by continuous sediment transport. Those forms are strictly connected with the intensity of particle transport, and they eventuate from bed shear stress. The present paper comprised of local scours geometric dimensions research in three variants of lengthwise development of laboratory flume in various hydraulic properties, both in “clear-water” and “live-bed” conditions of sediment movement. Lots of measurements of the bed conformation were executed using the LiDAR device, marked by a very precise three-dimensional shape description. The influence of the bed shear stress downstream model on scours hole dimensions of water structure was investigated as one of the key factors that impact the sediment transport intensity. A significant database of 39 experimental series, lasting averagely 8 hours, was a foundation for delineating functional correlations between bed shear stress-and-critical shear stress ratio and geometry properties of local scours in various flume development cases. In the scope of mutual influence of bed shear stress and water depth, high correlation coefficients were attained, indicating very good and good functional correlations. Also, the influence of bed shear stress and the total length of the scour demonstrated a high correlation coefficient.

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Self-organisation of morphology and sediment transport in alluvial rivers

10.5194/egusphere-egu2020-2758 ◽

2020 ◽

Author(s):

Eric Lajeunesse ◽

Anais Abramian ◽

Olivier Devauchelle

Keyword(s):

Sediment Transport ◽

Equilibrium Shape ◽

Boltzmann Distribution ◽

Physical Review ◽

Bedload Transport ◽

Sediment Flux ◽

Sediment Discharge ◽

Alluvial Rivers ◽

Braided Rivers ◽

Streamwise Streaks

<div> <div> <div> <p>The coupling of sediment transport with the flow that drives it shapes the bed of alluvial rivers. The channel steers the flow, which in turns deforms the bed through erosion and sedimentation. To investigate this process, we produce a small river in a laboratory experiment by pouring a viscous fluid on a layer of plastic sediment. This laminar river gradually reaches its equilibrium shape. In the absence of sediment transport, the combination of gravity and flow-induced stress maintains the bed surface at the threshold of motion (Seizilles et al., 2013). If we impose a sediment discharge, the river widens and shallows to accommodate this input. Particle tracking reveals that the grains entrained by the flow behave as random walkers. Accordingly, they diffuse towards the less active areas of the bed (Seizilles et al., 2014). The river then adjusts its shape to maintain the balance between this diffusive flux, which pushes the grains towards the banks, and gravity, which pulls them towards the center of the channel. This dynamical equilibrium results in a peculiar Boltzmann distribution, in which the local sediment flux decreases exponentially with the elevation of the bed (Abramian et al., 2019). As the sediment discharge increases, the channel gets wider and shallower. Eventually, it destabilizes into multiple channels. A linear stability analysis suggests that it is diffusion that causes this instability, which could explain the formation of braided rivers (Abramian, Devauchelle, and Lajeunesse, 2019).</p> </div> </div> </div><p>&#160;</p><p>References:</p><ul><li>Abramian, A., Devauchelle, O., and Lajeunesse, E., &#8220;Streamwise streaks induced by bedload diffusion,&#8221; Journal of Fluid Mechanics 863, 601&#8211;619 (2019).</li> <li>Abramian, A., Devauchelle, O., Seizilles, G., and Lajeunesse, E., &#8220;Boltzmann distribution of sediment transport,&#8221; Physical review letters 123, 014501 (2019).</li> <li>Seizilles, G., Devauchelle, O., Lajeunesse, E., and M &#769;etivier, F., &#8220;Width of laminar laboratory rivers,&#8221; Phys. Rev. E. 87, 052204 (2013).</li> <li> <p>Seizilles, G., Lajeunesse, E., Devauchelle, O., and Bak, M., &#8220;Cross-stream diffusion in bedload transport,&#8221; Phys. of Fluids 26, 013302 (2014).</p> </li> </ul>

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A new vectorial bedload formulation and its application to the time evolution of straight river channels

Journal of Fluid Mechanics ◽

10.1017/s002211209400114x ◽

1994 ◽

Vol 267 ◽

pp. 153-183 ◽

Cited By ~ 130

Author(s):

Agnes Kovacs ◽

Gary Parker

Keyword(s):

Shear Stress ◽

Time Evolution ◽

Fluid Shear Stress ◽

Constant Width ◽

Sediment Flux ◽

Force Balance ◽

Angle Of Repose ◽

Bed Shear Stress ◽

River Channels ◽

Fully Nonlinear

The derivation of a new vectorial bedload formulation for the transport of coarse sediment by fluid flow is presented in the first part of the paper. This relation has been developed for slopes up to the angle of repose both in the streamwise and transverse directions. The pressure distribution is assumed to be hydrostatic. The bed shear stress for the onset of particle motion and mean particle velocity are obtained from the mean force balance on a particle. A new generalized Bagnold hypothesis is introduced to calculate the sediment content of the bedload layer. The new formulation possesses two innovative features. It is fully nonlinear and vectorial in nature, in addition, it behaves smoothly up to the angle of repose.A mathematical model of the time evolution of straight river channels is presented in the second half of the paper. This study focuses on the evolution process due to bank erosion in the presence of bedload only. The bed and bank material is taken to be coarse, non-cohesive and uniform in size. The sediment continuity and the fluid momentum conservation equations describe the time evolution of the bed topography and flow field. These equations are coupled through the fluid shear stress acting on the bed. This bed shear stress distribution is predicted with the aid of a simple algebraic turbulent closure model. As regards the computation of the sediment flux, the new fully nonlinear vectorial formulation is found to perform well and renders the evolution model fully mechanistic.The formation of an erosional front in the time development of straight river channels has been so far obscured in physical experiments. Herein, with the help of the new bedload formulation, the existence and migration speed of the front of erosion are inferred from the analysis of the sediment continuity equation.The model successfully describes the time relaxation of an initially trapezoidal channel toward an equilibrium cross-sectional shape, as evidenced by comparison with experimental data. This equilibrium is characterized by a constant width, vanishing sediment transport in the transverse direction, and a small but non-vanishing streamwise transport rate of bed sediment.

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Effect of vegetation on flows and sediment transport

E3S Web of Conferences ◽

10.1051/e3sconf/20184002017 ◽

2018 ◽

Vol 40 ◽

pp. 02017

Author(s):

Hela Romdhane ◽

Amel Soualmia ◽

Ludovic Cassan ◽

Gilles Belaud

Keyword(s):

Shear Stress ◽

Sediment Transport ◽

Open Channel ◽

Sediment Accumulation ◽

Bedload Transport ◽

Reasonable Accuracy ◽

Physical Processes ◽

Flow Measurements ◽

Flow And Sediment Transport ◽

Movable Bed

Vegetation is a common feature in natural coastal and riverine waters, interacting with both water flow and sediment transport. However, the physical processes governing these interactions are still poorly understood, which makes it difficult to predict sediment transport and associated morphodynamics in a vegetated environment. In this context, an experimental study was conducted in laboratory with a movable bed trapped in artificial vegetation. The experimental flume is a rectangular open channel 5.75 m long and 0.29 m wide. For flow measurements, the channel is equipped with a fast camera and ADV probe. This work focuses on identifying the vegetation effects on flows and sediment transport. In fact, it was shown that the vegetation presence in a watercourse promotes deposition and sediment accumulation. This is explained by a reduction of the bed shear stress, since the friction occurs mainly by the drag force effect exerted by the vegetation. It was shown too that the vegetation reduced the bedload transport. Thanks to the partitioning of shear stress, it was possible to predict the bedload transport using standard formulas with a reasonable accuracy.

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Laboratory studies on bedload transport under unsteady flow conditions

Journal of Hydrology and Hydromechanics ◽

10.1515/johh-2017-0032 ◽

2018 ◽

Vol 66 (1) ◽

pp. 23-31 ◽

Cited By ~ 5

Author(s):

Magdalena M. Mrokowska ◽

Paweł M. Rowiński ◽

Leszek Książek ◽

Andrzej Strużyński ◽

Maciej Wyrębek ◽

...

Keyword(s):

Shear Stress ◽

Sediment Yield ◽

Water Surface ◽

Bedload Transport ◽

Bed Shear Stress ◽

Surface Slope ◽

Stream Power ◽

Transport Dynamics ◽

Water Surface Slope ◽

Total Sediment

Abstract Two sets of triangular hydrographs were generated in a 12-m-long laboratory flume for two sets of initial bed conditions: intact and water-worked gravel bed. Flowrate ranging from 0.0013 m3 s-1 to 0.0456 m3 s-1, water level ranging from 0.02 m to 0.11 m, and cumulative mass of transported sediment ranging from 4.5 kg to 14.2 kg were measured. Then, bedload transport rate, water surface slope, bed shear stress, and stream power were evaluated. The results indicated the impact of initial bed conditions and flow unsteadiness on bedload transport rate and total sediment yield. Difference in ratio between the amount of supplied sediment and total sediment yield for tests with different initial conditions was observed. Bedload rate, bed shear stress, and stream power demonstrated clock-wise hysteretic relation with flowrate. The study revealed practical aspects of experimental design, performance, and data analysis. Water surface slope evaluation based on spatial water depth data was discussed. It was shown that for certain conditions stream power was more adequate for the analysis of sediment transport dynamics than the bed shear stress. The relations between bedload transport dynamics, and flow and sediment parameters obtained by dimensional and multiple regression analysis were presented.

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Efficient non-hydrostatic modelling of flow and bed shear stress in a pier scour hole

Canadian Journal of Civil Engineering ◽

10.1139/cjce-2013-0160 ◽

2014 ◽

Vol 41 (5) ◽

pp. 450-460 ◽

Cited By ~ 2

Author(s):

S. Pournazeri ◽

S.S. Li ◽

F. Haghighat

Keyword(s):

Experimental Data ◽

Shear Stress ◽

Local Maximum ◽

Bedload Transport ◽

Bed Shear Stress ◽

Pier Scour ◽

Law Of The Wall ◽

Scour Hole ◽

The Law ◽

Wall Method

Predicting 3-D flow in a pier scour hole and the associated bed shear stress τb is important for the safe and economical design of bridge piers. This paper combines layered, hydrostatic hydrodynamic computations with non-hydrostatic pressure corrections, exploring a new modelling approach for efficient and reliable predictions of 3-D flow velocity. The law of the wall method is used for estimating τb. Its suitability for incorporation into layered models for bedload transport and pier scour simulations is also discussed. The predicted flow shows realistic features: strong downward flow adjacent to the upstream nose of a circular pier, vortex motions in the vertical and horizontal direction, and meandering flow wakes. The velocity results compare well with available experimental data. In the approach region, τb is uniform. It attains a local maximum immediately before flow enters the scour hole and then drops non-linearly in the scour-hole region toward the pier. In the wake region, τb has very low values. The τb predictions are consistent with the experimental data. In multi-layer models, when applying the law of wall method, one should use near-bed velocities as opposed to bottom-layer velocities to obtain more reliable τb estimates and avoid noisy results, which can cause a numerical instability problem in bedload transport simulations.

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