Lower‐stage plane bed topography is an outcome of rarefied, intermittent sediment transport

Lateral intakes are hydraulic structures used for domestic, agricultural and industrial water conveyance, characterized by a very complex three-dimensional morphodynamic behavior: since streamlines near the lateral intake are deflected, some vortices form, pressure gradient, shear and centrifugal forces at the intake generate flow separation and a secondary movement, responsible for local scour and sediment deposition. On the other side, the modeling of flows, besides the sediment transport, in curved channels implies some more complications in comparison with straight channels. In this research, this complex process has been investigated experimentally and numerically, with the mechanism of sediment transport, bed topography evolution, flow pattern and their interactions. Experiments were performed in the Laboratory of Tarbiat Modares University, Iran, where a U-shaped channel with a lateral intake was installed and dry sediment was injected at constant rate into a steady flow. Due to the spiral flow, the bed topography changes significantly and the bed forms in turn affect the sediment entering the intake. Different from the previous works on this topic which were mainly based on laboratory experiments, here, Computational Fluid Dynamics (CFD) numerical simulations with FLUENT software were also performed, specifically with the two-phase Eulerian Model (EM) and Discrete Phase Model (DPM), at the aim of evaluating their performance in reproducing the observed physical processes. This software is used for a large variety of CFD problems, but not much for simulating sediment transport phenomena and bed topography evolution. The comparison of the results obtained through the two models against the laboratory experimental data proved a good performance of both the models in reproducing the main features of the flow, for example, the longitudinal and vertical streamlines and the mechanism of particles movement. However, the EM reveals a better performance than DPM in the prediction of the secondary flows and, consequently, of the bed topography evolution, whereas the DPM well depicts the particles pattern, predicts the location of trapped particles and determines the percentage of sediment entering the intake. The numerical models so calibrated and validated were applied to other cases with different positions of the intake in the bend. The results show that mechanism of sediment entrance into the intake varies in different position. If the intake is installed in the second half of the bend, the sediment accumulates along the inner bank of the bend and enters the intake from downstream edge of intake; on the other side, if it is placed in the first half of the bend, the sediment accumulates along both the inner and the outer bends and, therefore, more sediment enters the intake. Also the results of the simulations performed with the DPM model for different positions of the lateral intake show that for all discharge ratios, the position of 120∘ is the one which guarantees the minimum ratio of sediment diverted to the intake (Gr).

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Bar dynamics and sediment transport pulses in gravel-bed channels

E3S Web of Conferences ◽

10.1051/e3sconf/20184005047 ◽

2018 ◽

Vol 40 ◽

pp. 05047

Author(s):

Blaise Dhont ◽

Christophe Ancey ◽

Patricio Bohorquez

Keyword(s):

Sediment Transport ◽

Laser Sheet ◽

Water Discharge ◽

Bedload Transport ◽

Transport Rate ◽

Experimental Conditions ◽

Gravel Bed ◽

Bed Topography ◽

Sediment Transport Rate ◽

Alternate Bars

Mountain rivers exhibit sediment transport rate fluctuations that often cover more than two orders of magnitude. Bedform migration is often cited as the key process that causes giant fluctuations in the sediment transport rate. To quantify the effect of bedform migration on transport rate, we ran laboratory experiments in a 19-m long 60-cm wide flume with well-sorted gravel bed. At the flume inlet, the water discharge and the particle flux were kept constant. Experiments were conducted over long times (typically > 500 h). Sediment transport rate was monitored at the flume outlet using accelerometers. Bed topography was scanned at high spatial resolution using a laser sheet. Water depth was measured using ultrasonic probes mounted on an automated rolling carriage. We observed that, under steady state experimental conditions, bed morphology played a key part in the generation of bedload transport fluctuations. The bars migrated downstream intermittently, producing the most important pulses. When the bar position was stable for a few hours, additional pulses resulted from sediment transfer from pool to pool, in the form of sediment waves (bedload sheets). Thus, in our experiments, alternate bars formed a two-entity system (bar + pool) with two distinctive functions: the bars contributed to fix and stabilize the bed whereas the pools were the preferential zones of short-term storage and transfer of sediment.

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Numerical simulation of sediment transport in a U-shaped channel with lateral intake: Effects of intake position and diversion angle

International Journal of Modern Physics C ◽

10.1142/s0129183119500712 ◽

2019 ◽

Vol 30 (09) ◽

pp. 1950071 ◽

Cited By ~ 1

Author(s):

Keivan Tavakoli ◽

Hossien Montaseri ◽

Pourya Omidvar ◽

Stefania Evangelista

Keyword(s):

Sediment Transport ◽

Numerical Model ◽

Transport Mechanism ◽

Discrete Phase Model ◽

Reference Time ◽

Bed Topography ◽

Discharge Ratio ◽

Discrete Phase ◽

Sediment Particles ◽

Good Agreement

In this work, the mechanism of sediment transport in a U-shaped channel with a lateral intake is investigated experimentally and numerically, together with the processes of sediment entry into the intake itself and formation of bed topography. Dry sediment is injected into a steady flow in a rigid channel with a bend and sediment particles are traced in time. In order to validate the numerical model, the three components of the flow velocity, as well as the sediment path in time and the diverted sediment ratios, are measured experimentally. A numerical Discrete Phase Model (DPM) is then applied to study the effect of the intake position and diversion angle on the sediment transport mechanism in the bend. The DPM has, in fact, the capability of specifying for each particle its position relative to a reference time and space and, thereby, it is used in this study to analyze the phenomenon evolution and determine the sediment particles diverted into the intake. The comparison between the experimental data and the DPM numerical results shows a good agreement. In order to investigate the mechanism of sediment transport and to evaluate the percentage of the diverted sediments, a parametric study is then conducted through the numerical model, with different positions of the outer bend of the channel, diversion angles of the lateral intake and diversion discharge ratios. The results show that the mechanism of sediment entry into the lateral intake is affected by the diversion discharge ratio. For low discharge ratios, the mechanism of sediment entry to the lateral intake only consists of continuous entrance from the upstream edge of the intake. With the increase of the discharge ratio, it consists of a continuous entrance from the downstream edge and a periodic entrance from the upstream edge of the intake. The DPM results show that, for all diversion discharge ratios, the minimum percentage of sediment entered into the lateral intake corresponds to the position of 120∘ and diversion angle equal to 50∘.

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A depth-averaged two-dimensional model for flow, sediment transport, and bed topography in curved channels with riparian vegetation

Water Resources Research ◽

10.1029/2004wr003730 ◽

2005 ◽

Vol 41 (3) ◽

Cited By ~ 71

Author(s):

Weiming Wu ◽

F. Douglas Shields ◽

Sean J. Bennett ◽

Sam S. Y. Wang

Keyword(s):

Sediment Transport ◽

Riparian Vegetation ◽

Two Dimensional ◽

Dimensional Model ◽

Bed Topography ◽

Curved Channels ◽

Two Dimensional Model

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A coupled 1-D/2-D model for simulating river sediment transport and bed evolution

Journal of Hydroinformatics ◽

10.2166/hydro.2020.020 ◽

2020 ◽

Vol 22 (5) ◽

pp. 1122-1137

Author(s):

Mezbache Salheddine ◽

Paquier André ◽

Hasbaia Mahmoud

Keyword(s):

Sediment Transport ◽

Flow Direction ◽

Coupled Model ◽

Computational Time ◽

Bed Topography ◽

Sedimentary Model ◽

Lateral Coupling ◽

Morphological Processes ◽

Bed Evolution ◽

D Model

Abstract The paper details the method to couple a 1-D hydro-sedimentary model to a 2-D hydro-sedimentary model in order to represent the hydrodynamics and morphological processes during a flood event along a river. Tested on two field cases, the coupled model is stable even in the case of generalized overflow over the riverbanks or of levee breaching. For lateral coupling, the coupled model allows saving computational time compared to a full 2-D model and to provide valuable results concerning the flooding features as well as the evolution of the bed topography. However, despite a similar simplified representation of the sediment features in the 1-D and 2-D models, some discrepancies appear in the case of upstream/downstream coupling along a cross section perpendicular to the flow direction because the assumption of homogeneous velocity and concentration is not valid for estimating sediment transport. Further research is necessary to be able to define a suitable distribution of the sediments on the 1-D side of the boundary between the two models.

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Flow resistance in alluvial channels

Progress in Physical Geography Earth and Environment ◽

10.1177/0309133311414604 ◽

2011 ◽

Vol 35 (6) ◽

pp. 765-781 ◽

Cited By ~ 7

Author(s):

André Robert

Keyword(s):

Sediment Transport ◽

Flow Resistance ◽

Large River ◽

Flood Hazards ◽

Alluvial Channels ◽

River Systems ◽

Spatial And Temporal Scales ◽

Bed Topography ◽

Technological Means

There have been numerous fluvial studies of flow resistance in alluvial channels during the last few decades. Significant progress has been made towards predicting flow resistance (and therefore velocity) for a given discharge. These past applications rely heavily on the characterization of particle sizes and the effects of changing relative submergence on flow resistance estimates. Different types of equations have been shown to provide reasonably good estimates in specific environments. Major difficulties arise from characterizing mobile beds, bed topography and its evolution and how these factors control rates of change of average velocity as discharge rises along a given river reach. Different issues can be recognized as a function of the spatial and temporal scales of investigation. A case can made that more emphasis should be placed upon reach-scale investigations. Detailed studies of bed topography, its maintenance, its evolution (at the reach scale) and its interactions with macroturbulence structure and sediment transport would ultimately provide valuable information and improved knowledge on both flow resistance processes and applications (predictions). Moreover, technological means now allow detailed characterization of bed topography and flow fields of large river systems. Such promising avenues should be further pursued with the goal of providing not only a better understanding of flow-bed-sediment transport interactions in large river systems but also a better understanding of flow stage variations, flood hazards, flow resistance estimates and therefore partitioning of depth and velocity as discharge rises along major river systems.

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