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In spite of its practical relevance, little is known about the turbulence characteristics in sharp open-channel bends, which may largely be attributed to a lack of accurate experimental data. This paper reports an experimental investigation of the turbulence structure in one cross-section of an open-channel bend. The flow pattern in this section is...
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... vectorial representation of the normalized cross-stream motion, (v n , v z )/U , in the section investigated is shown in figure 3. In the centre region, it shows a circulation cell -termed centre-region cell -with outward velocities near the water surface and inward velocities near the bed. ...
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Citations
... Point bar deposits are the products of lateral migration of meandering channels (Bagnold, 1960;Leopold & Wolman, 1960;Dietrich et al., 1979;Blanckaert & De Vriend, 2005;Seminara, 2006;Ghinassi et al., 2014;Bhattacharyya et al., 2015). The sinuous shape of a channel promotes helical flow characterized by the highest energy at the channel base at the cutbank, and the waning of flow magnitude, velocity and stream capacity towards the channel top at the point bar (Bagnold, 1960;Allen, 1970;Hey & Thorne, 1975;Dietrich & Smith, 1983;Dietrich, 1987;Miall, 1996;Bridge, 2003;Seminara, 2006). ...
Stratigraphic positions and the nature of some breccias and mudstones observed in the Lower Cretaceous McMurray Formation type section (Alberta, Canada) challenge the existing fluvial point bar facies models. The currently applied large‐scale, tidally influenced, sandy bedload facies model suggests that point bars fine upward, where breccias and cross‐strata occur at the base of the succession and mud content increases upward. Instead, this study documents that: (i) mudstone clast breccias are not exclusively associated with basal channel‐lag deposits and can occur throughout the point bar; and (ii) stratified mudstones are not limited to the top of the fining‐upward succession and can occur at or near the channel base. This outcrop data, including lithology, sedimentary structures, trace fossils, bed thickness and boundaries, and the nature of stratal packages, further suggests dynamic interplay between fluvial and tidal processes. This article discusses the potential role of high‐magnitude river floods in temporarily elevated suspended sediment concentrations and highlights that fluid mud processes played a crucial role in mud deposition in both tide‐influenced and fluvial parts of the system. Fluid mud processes allowed mud deposition along the channel base and across the point bar accretion surfaces. The breccias predominantly consist of broken pieces of inclined heterolithic strata and mostly occur on accretion sets of point bars, suggesting erosion and collapse of the point bar rather than of the cutbank. This work introduces a novel perspective to the existing models of point bar sedimentation, offering a new end member and concepts for the interpretation of subsurface data. The authors hope that this research encourages further investigations into similar phenomena elsewhere in the world.
... However, reproducing laterally migrating, single-thread meandering rivers with repeated neck cutoffs in laboratory experiments remains a challenge (Van Dijk et al., 2012;Kyuka et al., 2021;Parker, 1998;Smith, 1998;Song et al., 2016), leading researchers to predominantly rely on increasingly complex numerical models informed by field-based studies to model long-term meander morphodynamics (e.g., Dubon et al., 2019;Eke et al., 2014bEke et al., , 2014aParker et al., 2011;Posner and Duan, 2012). However, laboratory experiments with fixed banks are still widely employed to study local flow structure as a function of bend morphology (e.g., Blanckaert, 2009Blanckaert, , 2010Blanckaert, , 2011Blanckaert and de Vriend, 2005;Ottevanger et al., 2012;Termini, 2009Termini, , 2013Termini, , 2016Termini and Piraino, 2011) and the effects of changes in vegetation and rate of sediment supply (e.g., Azarisamani et al., 2020;Keshavarzi et al., 2016;Modalavalasa et al., 2023;Zhao et al., 2022). Kozarek et al. (2024, this volume) provide insights into the relation between turbulent flow and bank erosion in meandering rivers using the unique Outdoor StreamLab (St. ...
The study of meandering patterns created by geophysical flows is important for a number of fundamental and applied research topics, including stream and wetland restoration, land management, infrastructure design, oil exploration and production, carbon sequestration, flood-hazard mitigation, and planetary paleoenvironmental reconstructions. This volume, Meandering Streamflows: Patterns and Processes across Landscapes and Scales , contains 13 papers that present field, laboratory, and numerical investigations of meandering channels found in distinct environmental and geological contexts, and focus on how the interactions of different autogenic and allogenic processes, both in the horizontal and the vertical dimension, affect meander kinematics and the resulting morphology, sedimentology, and stratigraphic architecture. In this introductory chapter, we offer an overview of the evolution of scientific research on meandering streams over time, aiming to review and discuss meandering patterns in both fluvial and non-fluvial settings. Additionally, we present a new compilation of data on meander morphological features, drawn from both existing literature and novel sources, encompassing over 8000 meander bends discovered across a diverse array of environments.
... Many researchers (Blanckaert 2002;Blanckaert & De Vriend 2005) concluded that a meandering channel has a different RSS and turbulence intensity than a straight channel. An experiment carried out at a 200°deflection angle of a compound bend channel by Abad & Garcia (2009) concluded that TKE depends on normal shear stress. ...
This research aims to investigate the near-bed turbulent flow characteristics in a meandering channel with both mobile bed and immobile bed conditions. Experiments were performed in a prismatic rectangular meandering channel with a non-uniform sand bed of size d50 = 0.523mm. The three-dimensional instantaneous fluid velocity was collected using the Acoustic Doppler velocimeter which will provide important results related to the flow turbulence such as mean flow velocity, turbulence intensity, Reynolds shear stress, turbulent kinetic energy, skewness, kurtosis and turbulent anisotropy. The secondary current flow and the exchange of momentum in the form of turbulence kinetic energy, Reynolds shear stress and turbulent intensity at the inner layer of the flow are identified more in a mobile bed condition as compared to an immobile condition, which causes sediment transport. For the inner layer of the flow, turbulence intensity and turbulent kinetic energy are decreased in magnitude and gradually increase in the outer layer of flow for both the bed conditions. Higher turbulence anisotropy is noticed in the mobile bed condition than in the immobile bed condition, which shows more nonuniformities near the bed level for the mobile bed condition. This study may help in understanding the effect of sediment transport due to a turbulent flow structure in a sinuous alluvial channel.
HIGHLIGHTS
Comparison of turbulence between mobile and immobile bed conditions.;
The near-bed longitudinal velocity profile increases with an increase in distance from the bed surface.;
Negative values of Reynolds shear stress due to a narrow channel.;
At the bend portion, the mobile bed has higher TKE than the immobile bed and vice versa at the cross-over section.;
Turbulent anisotropy shows more nonuniformities of flow in the case of mobile beds.;
... This study applies the same principle to bend flow, with an added challenge of distinguishing overlapping structures rotating in the same direction. There is already an indication that secondary flow of the first kind manifests as coherent structures in instantaneous data: [6] found coherent fluctuations, termed "bulk oscillation", in velocity data across a channel bend. ...
A method is introduced to cluster instantaneous vortices using a density-based spatial clustering technique to better distinguish overlapping secondary circulation from different mechanisms. Applying the method to large eddy simulation results of a tight open channel bend, two secondary circulation sub-cells are distinguished: the inner bank cell and the center cell. The identification of these structures using instantaneous vortices shows a connection between channel bend mean secondary flow and instantaneous coherent structures, which is further solidified by the agreement of mean bend circulation and circulation calculated using instantaneous vortices. The inner bank sub-cell exhibits high maximum circulation early in the bend followed by a rapid decline, a pattern which is characteristic of tight bends. The center sub-cell exhibits slower development and retains its circulation longer, which is characteristic of milder bends. The locations of the sub-cells within the channel cross section lead to different opportunities for vorticity generation in each sub-cell, which explains their different development patterns.
Article Highlights
A method is introduced to identify secondary circulation structures using clusters of instantaneous vortices.
Distinct sub-cells are identified in the secondary circulation of a tight open channel bend: one at the inner bank and one in the center.
The inner bank sub-cell’s development behavior resembles that of tight bends, while the center sub-cell’s resembles mild bends.
... Ejection bursting events, according to Sumer and Deigaard (1981), are principally responsible for the saltation motion of sediment particles in the inner region of the flow. In the presence of arrays of submerged vanes, such as shown in Figure 5, information on the coherent turbulent structures in the wake of the vane can be used to determine the dominant sediment transport mechanisms controlling bed topography evolution (Jackson 1976;Raupach 1981;Bogard and Tiederman 1986;Gad-el-Hak and Bandyopadhyay 1994;Patel and Yoon 1995;Ganapathisubramani et al. 2003;Blanckaert and De Vriend 2005;Keshavarzi and Gheisi 2007). Numerical models of flow in open-channels frequently use extensions of turbulence closures that were developed for two-dimensional boundary-layer flows. ...
Diverting or deflecting sediments is an essential part of river training. Under the effect of an induced helical motion, the submerged vanes are best known for deflecting sediments. This study presents current state-of-the-art information on employing submerged vanes for sediment deflection, as well as concluding notes outlining the current knowledge’s limitations and suggestions for future research on sediment deflection using submerged vanes. The analytical background for basic physics of submerged vanes’ operation is first described. The paper then moves on to a thorough examination of vane design parameters based on analytical relationships. A brief overview of the applications of submerged vanes is also included in the paper. A review of recent studies aims at designing a layout of submerged vanes to assist in river training tasks, such as river bends protection, sediment exclusion at water intakes, improving safety at bridges on waterways, improving inland navigation is provided to this end. The study also presented a future program of research on submerged vanes for improving inland navigation in braided rivers. The major lag in literature is a lack of information on macro turbulence and the associated sediment motion induced by the wake of the submerged vane.
... Water flow in alluvial river meanders usually undergoes centrifugal acceleration which induces transverse velocity components and water-surface slopes (Fong, et al., 2009;Blanckaert and de Vriend, 2005). Although these curved sections look stable, there is a general tendency of bank failure and bed scour at the outer bend accompanied by corresponding sediment deposition at the inner one. ...
The current research aims to study the suitability of Damietta branch meanders for safe first-class cargo transportation. This branch is the River Nile eastern waterway bifurcated at the beginning of Delta region which is about 22.0 km downstream El-Roda gauge in Cairo. It extends for 244 km to the Mediterranean Sea. The branch plan form was examined for meanders. Fifty one meanders could be identified throughout. Their characteristics were analyzed. The results revealed the existence of eight sharp meanders and other seven critical looping ones that would slow down and jeopardize the first-class navigation traffic. Also, it was found that about 68 % of the looping meander lengths could be saved if cutoffs are executed. Accordingly, examples of straightening the river at four meanders were proposed for clarification. It was found that their lengths could be reduced by 51.10 %. Finally, the study concluded that in order for Damietta branch to be qualified and entitled to safe first-class cargo transportation, the sharp meanders and critical loops have to be subject to training works and re-channelized.
... The ADV mount consisted of a movable carriage upon which the ADV was aligned parallel to the stream flow (Fig. 4a). According to Blanckaert and de Vriend (2005) and Engel and Rhoads (2017), the structure and pattern of turbulence parameters would be different in meandering channels than in straight channels. Thus in the present field study, a curved channel can be described by an intrinsic coordinate system with streamwise ū, transverse v, and vertical w (Fig. 4a) velocities similar to Engel and Rhoads (2017). ...
The shapes of a meandering river at the different stages of its evolution forming meander loops result from the interaction of the turbulent flow structures and channel planform. Based on field investigations and analysis of Google Earth (aerial) images, series of self-similar lobed-shaped patterns of different scales evolving in both space and time were observed at the outer curved bank of large rivers such as Hooghly River (India), Yamuna River (India), Madhumati River (Bangladesh) and in the White Nile (Africa). The present study attempts to develop insight into the turbulence characteristics of the flow leading to the formation, growth and migration of these self-similar lobed-shaped patterns. Measurements of turbulent flow characteristics were carried out at five different locations within a distinct dynamic lobe of an outer bend of the River Hooghly. Results show that the outer bank of a curved channel is under the action of strong secondary currents, turbulent burst-sweep cycles, flow separation and strong intermittent turbulent shear stress at different scales leading to enhanced bank erosion activity. The results from the present study form the basis for a conceptual representation of the bankline retreatment process through the formation, growth and evolution of dynamic lobed-shaped erosive patterns.
... The velocity profile is not perfectly logarithmic in nature as that can be seen in straight rivers (Anwar, 1986;Blanckaert and Graf, 2002;Booij, 2003;Sukhodolov, 2010). The Reynolds stresses of a meandering river also got variations from straight rivers (Blanckaert, 2002;Blanckaert and Vriend, 2005). Early works from researchers have confirmed the presence of outer bank circulation which acts as counteracting consequence on outer banks of sinuous or meander bend (Blanckaert and Graf, 2001). ...
A sinuous river system has sinuosity between 1.1-1.5. It’s the bridge between straight river (sinuosity<1.1) and meandering river (sinuosity>1.5). The continuous change of the sinuous river course and direction to the meandering river makes the study of flow characteristics and morphological behaviour in sinuous river system important. The turbulence study helps us to follow the mechanism of continuous erosion of sediments in outer of a sinuous or meandering river. The study of the velocity profile, Reynolds shear stress (RSS), turbulent intensities, turbulent kinetic energy(TKE), morphology etc. are important to understand its effects on the bank stability. Secondary flow also plays a vital role to determine the behaviour of the flow. The present study is concentrated to understand the turbulent flow characteristics and how it affects the morphology of the sinuous bend by performing experiments in a flume. It also addresses how the flow behaviour is different from that of a straight river. Turbulent intensity profiles show higher turbulence intensity in the middle and near-surface of the flow depth as compared to that of the straight river where maximum intensity can be found in the lower portion of the flow depth. Bursting events analysis suggests that the outer bend of the channel is subjected to erosion. This study of turbulent flow characteristics and morphological changes in the sinuous river system helps us to understand the formation of a meandering river in a better way.
... In general, the velocity profile is not perfectly logarithmic as observed in straight streams (Anwar 1986;Graf and Blanckaert 2002;Booij 2003;Sukhodolov and Kaschtschejewa 2010). The Reynolds shear stress and turbulence intensity of a sinuous channel are also different from that in a straight channel (Blanckaert 2002;Blanckaert and Vriend 2005). Some of the previous work has shown the existence of counterrotating circulation cell at the outer bank that causes erosion and deformation of banks. ...
An alluvial river system mostly comprises of series of turns, loops or bends in its course. This study presents the experimental investigation of the turbulence structure at the centre of a sinuous sand bed channel with rigid banks. In order to examine the turbulent flow, distribution of instantaneous velocities, Reynolds stresses and bursting events were analysed at the centre of bend. In this regard, Reynolds shear stresses (RSS) distribution exhibits the existence of reversal of flow in a bend. Close to the channel bed, RSS presents a triangular positive distribution as has been observed in straight flow, and away from the bed it attains negative value. Furthermore, an assessment of bursting events shows the ejection and sweep events prevail whereas away from the bed outward and inward events are prominent. In addition to this, analyses of these events reveal that contributions from sweep events (inrush of fluid moving towards the bed) are maximum nearer to the channel bed. Turbulence scale such as Taylor micro scale and Eulerian scale were also determined to measure the eddy size and its turn over time. The Eulerian length scale and Eulerian time scale are observed greater at the centre of the flow depth.
... The plane curvature and cross-sectional asymmetry topography are shown to produce significant secondary currents and transverse water-surface slopes [2,3]. The secondary currents derive from the centrifugal acceleration acting on the stratification between the upper and lower flow layers. ...
Meandering is a common feature in natural alluvial streams. This study deals with alluvial behaviors of a meander reach subjected to both freshwater flow and strong tides from the coast. Field measurements are carried out to obtain flow and sediment data. Approximately 95% of the sediment in the river is suspended load of silt and clay. The results indicate that, due to the tidal currents, the flow velocity and sediment concentration are always out of phase with each other. The cross-sectional asymmetry and bi-directional flow result in higher sediment concentration along inner banks than along outer banks of the main stream. For a given location, the near-bed concentration is 2−5 times the surface value. Based on Froude number, a sediment carrying capacity formula is derived for the flood and ebb tides. The tidal flow stirs the sediment and modifies its concentration and transport. A 3D hydrodynamic model of flow and suspended sediment transport is established to compute the flow patterns and morphology changes. Cross-sectional currents, bed shear stress and erosion-deposition patterns are discussed. The flow in cross-section exhibits significant stratification and even an opposite flow direction during the tidal rise and fall; the vertical velocity profile deviates from the logarithmic distribution. During the flow reversal between flood and ebb tides, sediment deposits, which is affected by slack-water durations. The bed deformation is dependent on the meander asymmetry and the interaction between the fresh water flow and tides. The flood tides are attributable to the deposition, while the ebb tides, together with runoffs , lead to slight erosion. The flood tides play a key role in the morphodynamic changes of the meander reach.
