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Meandering channels are common in natural rivers, especially in lowland alluvial plains. In this research, a meandering channel was designed in order to explore the effect of sediment supply on the flow characteristics. The water level, three-dimensional (3D) instantaneous velocity and riverbed elevation were measured using a digital wave altimeter, a 3D acoustic Doppler velocimeter and a 3D laser scanner, respectively, and the flow characteristics with and without sediment supply were compared. Under the same discharge, the transverse slope of the water surface in the case with sediment was larger than that without sediment and the change was steep; the change in the longitudinal slope of the water surface was small in the upper half bend and large in the lower half bend. By increasing the water discharge, the number of secondary flow vortices at the apex of the meandering channel increased. The trends of the mainstream lines with and without sediment supply were roughly opposite and the difference between them decreased with increasing discharge. The lateral and vertical turbulence intensities of the flow near the outer bank were larger in both cases (with and without sediment). In the case with sediment, the turbulent kinetic energy of the cross-section was larger near the bed and the water surface. [ABSTRACT FROM AUTHOR]

In this study, flow behavior in a meandering channel was investigated experimentally and numerically. The experiments were carried out for nine different cases on a channel consisting of 180° and 120° angle bends following successively. Measurements were conducted employing Acoustic Doppler Velocimetry (ADV) at 13 different points in the inner, middle, and outer bends of the sections located at significant bends along the channel. Depth-averaged velocity, velocity profiles, bed shear stress, and turbulence kinetic energy parameters were considered to understand the flow behavior in the meandering channel. A 1:1 scale numerical model of the experimental setup was generated using the Computational Fluid Dynamics (CFD) method through the verified FLOW-3D software (HYDRO 2022R1). It was found to be successful in estimating all parameters and was capable of investigating the flow behavior in the meandering channel. Additionally, a mesh independence study was performed, and four different turbulence models were compared. As a result, as the flow encountered the first meander in the channel, secondary flow occurred, and lateral momentum transfer took place. Therefore, velocity increased by approximately 30% from the first meander of 180° angles to the second meander of 120° angles. Therefore, the most critical zone was the inner bend of the 120-angle meander. [ABSTRACT FROM AUTHOR]

Contaminant trapping by recirculation zones occurring at the apex of natural meandering channels induces a long tail in the contaminant cloud, thereby complicating the prediction of mixing behaviors. Thus, the understanding of the interaction between solute trapping and recirculating flow is important for responding to and mitigating water pollution accidents. In this research, the EFDC model was employed to reproduce three-dimensional flow structures of recirculating flow at the channel apex and investigate the influence on contaminant mixing. To investigate the contaminant transport characteristics from the storage zone in meandering channels, simulations were conducted using various discharge values to assess the impact of storage zone development on the concentration–time curves. The analysis of the relationship between the storage zone size and mixing behaviors indicates that an increase in discharge could result in a shorter tail and larger longitudinal dispersion even with the larger storage zone size. On the other hand, the enlarged recirculation zone size contributes to reducing transverse dispersion, evidenced by flatter dosage curves under lower flow rate conditions. These findings suggest that the increase in longitudinal dispersion with a larger flow rate is primarily caused by the reduction in transverse dispersion resulting from the formation of the recirculation zone. [ABSTRACT FROM AUTHOR]

A river's planform pattern changes due to erosion of banks and the bed near the outer bend. The primary cause of these planform changes is the formation of helical flow patterns in response to centrifugal forces. Uncontrolled bed scouring can have a negative impact on the river's geometry, aquatic habitat, and floodplains. To alleviate this scouring, various structures, such as spur dikes, can be placed at any accessible location along the bend. The current research was accomplished by installing two meandering models with different sinuosities of 1.3 and 1.5, in a flume. For both sinuosities, the maximum bed scour was observed at an approximate angle of 45° relative to the bend apex. Thus, the main objective was to control this maximum scour by installing spur dikes with varying porosities, ranging from 0% to 75%, at five locations along the outer bend. The spur dikes were found to divert the helical flow regime away from the outer bend and protect the riverbed from severe scouring. However, the results show that the effectiveness of spur dikes decreases as sinuosity increases. Furthermore, for both meandering models, a 50% permeable spur dike installed at the +30° location yielded the best performance. Finally, a regression-based predictive equation is proposed to determine the proportion of scouring around a spur dike in a meandering channel. [ABSTRACT FROM AUTHOR]

Natural debris floods travel in straight and meandering courses. The flow behaviour greatly depends on the volume fractions of solid and fluid, as well as on their dynamic interactions with the channel geometry. For the quasi three-dimensional simulations of flow dynamics and mass transport of these floods through meandering and straight channels, we employ a two-phase debris flow model to carry out simulations for debris floods within straight and sine-generated meandering channels of different amplitudes. The results for different sinuous meandering paths are compared with that in the straight one in terms of phase velocity, downslope advection and dispersion, depths of the maxima, deposition of mass, position of front and rear parts of the solid and fluid phases, and also the flow dynamics out of the conduits. The results reveal the slowing of the flow and increase of momentary deposition of the mixture mass in the vicinity of the bends along with the increasing sinuosity. The numerical experiments are useful to better understand the dynamics of debris floods down meandering channels as seen in the natural paths of the rivers as well as already existing channels like episodic rivers in hilly regions. The results can be extended to propose some appropriate mitigation strategies.

Natural debris floods travel in straight and meandering courses. The flow behaviour greatly depends on the volume fractions of solid and fluid, as well as on their dynamic interactions with the channel geometry. For the quasi threedimensional simulations of flow dynamics and mass transport of these floods through meandering and straight channels, we employ a two-phase debris flow model to carry out simulations for debris floods within straight and sinegenerated meandering channels of different amplitudes. The results for different sinuous meandering paths are compared with that in the straight one in terms of phase velocity, downslope advection and dispersion, depths of the maxima, deposition of mass, position of front and rear parts of the solid and fluid phases, and also the flow dynamics out of the conduits. The results reveal the slowing of the flow and increase of momentary deposition of the mixture mass in the vicinity of the bends along with the increasing sinuosity. The numerical experiments are useful to better understand the dynamics of debris floods down meandering channels as seen in the natural paths of the rivers as well as already existing channels like episodic rivers in hilly regions. The results can be extended to propose some appropriate mitigation strategies. [ABSTRACT FROM AUTHOR]

The study investigates the role of cocolog, an eco-friendly material, as groyne, in protecting the river banks in meandering channels. Laboratory experiments were conducted to compare the flow pattern and sediment dynamics in a mobile-bed meandering channel for cocolog (permeable) and impermeable groynes. It is observed that as the permeability increases, the vertical and transverse velocities are seen to be reduced in the groyne fields. The results also indicate that places of maximum scour depth display amplified velocity, turbulent intensity and turbulent kinetic energy. However, considerable reduction in these characteristics is being observed in the case of permeable groynes that have densities varying between 140 and 160 kg/m3. The eroded volume is seen to be reduced by 60% for permeable groynes of medium density compared to the tests done without groynes. Also impermeable groynes exhibit a wider distribution of the maximum values for scour depth, turbulence intensity and turbulent kinetic energy than that of permeable groynes. It is found that the cocolog groynes with suitable density (or permeability) can perform best in meandering channels by dampening the velocity and turbulence of the groyne fields. [ABSTRACT FROM AUTHOR]

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.;

In the present study, a boundary treatment scheme named the partially saturated method (PSM) is incorporated into the discrete Boltzmann model (DBM) for shallow water flows to deal with complex curved boundaries. In this PSM-DBM, the two-dimensional 16 velocity levels scheme is adopted and the finite difference method is used to solve the governing equation. Then, the PSM-DBM has been applied to simulate four cases, i.e. such as steady flow in a bending channel, the flow past a stationary cylinder, a jet-forced flow in a circular basin and flow in a meandering channel with 90° consecutive bends. The simulated results have been compared with the experiments and the simulation by traditional numerical simulation. The study shows that the agreement between simulation and experiments is good. It is demonstrated that the PSM-DBM is stable and accurate to deal with the curved boundaries. Moreover, the implementation of the PSM is relatively straightforward in treating stationary curved geometries and is easy to be incorporated into the DBM for shallow water flows. In conclusion, the proposed PSM-DBM can be used widely for flows with curved boundaries. [ABSTRACT FROM AUTHOR]

Fluid flows occur due to internal or external forces such as wind, gravity, pressure gradients, side-wall motion, MHD, and free convection. This study examines how meanders impact heat transfer by studying the behavior of viscous fluid flow with streamwise vortices in a sinusoidal wavy meandering channel of non-uniform radius. The study simplifies the motion and energy equations governing the fluid flow using novel transformations and a regular perturbation method. By plotting graphs for different parameter values, such as Pr, Re, and Ec, it reveals that decreasing the wavelength leads to flow separation near the channel surface. However, the stream moves forward with a sudden meander disturbance, causing the flow to become rectilinear and independent of vertex-generating centrifugal forces. The study identifies a stream function using standard and established relations. The fluid flow patterns and temperature distribution behavior are shown in various plots, highlighting the significant impact of meanders on fluid flow.

In single-bend channels or meandering channels, groynes are widely used as a river-training structure. Geometric factor is an important principle of groyne design. In this paper, the numerical method based on the Renormalization Group (RNG) k-ε turbulence model is used to study the effects of groynes with different lengths and orientations on the mean flow pattern and turbulence characteristics in a mildly meandering channel. The analysis shows that compared with equal-length groynes, the groynes arranged in descending order perform well in improving the flow velocity of the main channel, and the maximum longitudinal velocity at the channel center can be increased by 1.57 times the average velocity. However, at the same time, they bring higher Normalized Turbulent Kinetic Energy (NTKE) and Normalized Bed Shear Stress (NBSS) values. The attracting groynes arranged in descending order help to reduce scouring. The groynes arranged in ascending order reduce the velocity of the riverbank in some downstream areas and are suitable for bank protection. It effectively decreases the NTKE and NBSS around the groyne fields and can reduce the inner bank scouring of the mildly meandering channel. The findings are helpful for the management of meandering rivers. [ABSTRACT FROM AUTHOR]

In this paper, the flow characteristics around T-shape spur dyke situated in the reverse meandering channel with a rigid bed is simulated using renormalization group (RNG) turbulence model with ANSYS 2018 Fluent software. To solve the model in 3D ways we used Navier-Stroke's equation based on the principle of conservation of mass and momentum within a moving fluid. For studying the flow characteristics, computational fluid dynamics were applied with all geometric parameters and the turbulence was simulated using (RNG) equations of model. In this simulation, the structured meshes were used with different diameters and diameters of meshes are high at the exit channel for obtaining accuracy in the result. In this study, we mainly focus on the effect of Froude number on flow pattern and several other characteristics like velocity distribution, flow separation, bed-shear-stress distribution. The final result of this research work is compared with the condition when no structure is present in the channel. HIGHLIGHTS A numerical investigation is carried out to study the flow field characteristics around a T-shaped spur dike located in a reverse meandering channel with rigid bed.; RNG k – ε turbulence model is implemented using ANSYS fluent software.; Study the effect of Froude number on flow pattern, several characteristics.; The results are then compared with the condition when no structure is placed.;

Effect of bed roughness on flow field and turbulence characteristics due to spur dyke in rigid bed meandering channel is presented in this study. This is an extension of authors' earlier work. In the experiments, cement mortar, sand particles on cement mortar and small aggregates on the cement mortar surfaces are used to generate different bed roughness. 3-D velocity measurements are obtained with Acoustic Doppler Velocimeter (ADV). In a rigid bed meandering channel without spur dyke, flow concentrates near the convex bank up to inflection point in the downstream of Apex. Inflection point moves towards the upstream with increase in the roughness of the channel. The water surface slope along the center line increases with increase in bed roughness. In presence of spur dyke, the maximum velocity in the channel decreases with increase in bed roughness. When the spur dyke is located on the convex bank of Apex, the length and width of separation zone decrease with increase in bed roughness. The magnitude of turbulence intensities decreases with increase in roughness. [ABSTRACT FROM AUTHOR]

Use of spur dikes by modifying the flow pattern is among the common methods for control of erosion at the river banks and their protection. In this research by construction of a laboratory meandering channel and use of five spur dikes, the effects of permeability and length of a series of spur dikes on the erosion control was investigated. The experiments were performed using three effective lengths, four permeability values and three Froude numbers in the non-submerged state. The results showed that the highest amount of bank retreat was achieved in impermeable spur dikes with a length of 9 cm and the lowest amount of bank retreat in permeable spur dike was 54% and a length of 15 cm. In this research use has been made of two criteria R2 and RMSE for comparing the data obtained from the predicted relationship and the observed data and showed a good compatibility.

Neck cutoff as an extreme geomorphic process, occurred in flood season of July 2018 in the lower Black River in the Yellow River Source region. Neck cutoff strongly adjusting flow structure is of great importance to understand the hydrodynamic mechanism of meander cutoff. Acoustic Doppler Current Profiler (ADCP) was used to conduct field measurements on a total of 45 cross-sections in May 2019 and August 2020 in order to compare and analyze three- dimensional flow structure in different cross sections. Results demonstrated that due to the influence of channel shape, boundary conditions, hydrological conditions and other factors, the flow structure in the upstream straight reach is less affected by the neck cutoff, while the flow structure of the bend section is obviously adjusted after neck cutoff. The specific performance is that the neck cutoff has little effect on the spatial distribution of velocity and circulation intensity in the straight reach. It will change the transverse distribution of the maximum velocity at the top of the bend and the section distribution of circulation structure, and affect the scale and distribution position of the separation area. The results also deepen the understanding of the impact of flow structure adjustment after neck cutoff on deposition of oxbow lake, scouring and depositing of riverbed in separation area and scouring of new channel. [ABSTRACT FROM AUTHOR]

Floodplains of meandering rivers are colonized with various plant species that differ in how they stabilize streambanks, modulating sinuosity evolution. Here, we compile observations of meander migration from North and South America, categorizing channels based on the riparian vegetation as cropland, forest, grassland, and rainforest. Our analysis reveals that the most stable meanders are those developed in rainforests and the reason is likely related to their established root systems and clay‐rich soils. The most unstable meanders were found in cropland areas, which is explained by the lack of vegetation cover and the frequent land disturbances associated with cultivation. Rivers in grassland and North American forest environments have intermediate migration rates. We found that meanders in grasslands have lower migration rates than those in North American forests. The reason for this may lie in the fact that grassland meanders in general have higher sinuosity, lower gradients, and layered banks. Plain Language Summary: Riparian vegetation growing in meadering river floodplains have differing abilities to stabilize streambanks depending on the vegeation type and density, which influences meander bend migration. Here, we categorize meandering channels from North and South America based on the type of riparian vegetation, that is, cropland, forest, grassland, and rainforest. Our analysis reveals that the most stable meanders are those developed in rainforests and the most unstable were found in cropland areas. Deep and strong root systems and clay‐rich banks are likely the reasons for the low migration rates of rainforest meanders. The high migration rate in cropland is explained by the lack of vegetation cover and the frequent land disturbance associated with cultivations. Rivers in grassland and North American forest environments have intermediate migration rates. More stable meanders occur in grasslands than in croplands partially due to their layered banks, higher sinuosity, and lower gradients. The influence of specific types of riparian vegetation on channel migration vegetation type is complex and remains an open question in geomorphology. Key Points: The most stable meanders are those developed in rainforest regions and the most unstable meanders were found in cropland areasMeanders in rainforest are more stable than those in North American forests likely due to their stronger root systems and clay‐rich banksMore stable meanders occur in grasslands than in croplands partially due to their layered banks, higher sinuosity, and lower gradients [ABSTRACT FROM AUTHOR]

Little is known about the evolution and gradual abandonment of the fluvial channel and associated sedimentary architecture preserved in the ancient sedimentary rock record. This research aims to analyze the changes and evolution of a gradually abandoned fluvial valley and associated within‐channel deposits and architecture from the rock record. This is achieved through detailed photogrammetry, outcrop sedimentology and quantification of palaeochannel parameters for the Oligocene palaeovalley fill of the north Western Desert basin of Egypt. The three‐dimensional vertical Oligocene outcrops and trenches of the study area provide the opportunity to analyze sedimentary facies, ichnology, architectural elements and evolution of the gradually abandoned fluvial channel belt. This paper presents a detailed model for the gradual abandonment and evolution of the fluvial palaeovalley through the stepwise transition from braided to meandering channel systems. This includes the transition from a bedload‐dominated, low sinuosity, braided channel that prominently features downstream‐accreted, mid‐channel and compound bars, to a small, mixed and suspended load‐dominated intermediate sinuosity (underfit) channel that is characterized by heterolithic‐dominated point bars, floodplain and mud‐plugs via a large, sand‐dominated, intermediate sinuosity channel with laterally accreted bars of transitional stage. The gradual abandonment of the fluvial palaeovalley is also associated with a successive decrease of the channel width and depth from >800 to 166 m and from 12 to 5 m, respectively. The successive abandonment is also associated with a gradual increase in the diversity and abundance of Scoyenia–Termitichnus ichnofacies groups, and vice versa in the Skolithos ichnofacies group. The successive decrease in channel width with gradual abandonment of the Oligocene palaeovalley is probably attributed to autogenic gradual avulsion, rather than allogenic. Comparison between the Oligocene palaeovalley fill and the modern analogues indicate the similarities in channel pattern changes and channel dimension through filling stages and differ from most modern counterparts because it is aggradational. This research enhances the understanding of the sedimentological characteristics and stratigraphic architecture of the gradually abandoned fluvial channels in the rock record and introduces insights for prediction of within‐channel facies and architectural heterogeneities of the fluvial channel reservoirs. [ABSTRACT FROM AUTHOR]

This study investigates the three-dimensional turbulence flow properties in a meandering channel under subcritical flow conditions, specifically focusing on a few critical parameters such as three-dimensional velocity behaviour, Turbulent kinetic energy, skewness, and kurtosis. A model study has been attempted by considering the same sinuosity, Froude number, and bed material from an Indian peninsular river to understand the basic hydrodynamics of a meandering channel. Understanding these critical parameters is crucial for comprehending the complex flow behaviour and its implications in meandering channels. Experimental measurements were conducted to collect data on Velocity, Turbulent Kinetic energy, skewness, and kurtosis in all three directions. The objective was to analyse their variations and behaviour along the meandering path, providing insights into the flow dynamics and turbulence characteristics. Advanced measurement techniques, such as Acoustic Doppler Velocimeters (ADV), were utilized to obtain accurate and detailed data. The study revealed distinct patterns and trends in the velocity, Turbulent Kinetic energy, skewness, and kurtosis along the flow path of a sand bed meandering channel. These parameters were found to vary significantly in different sections, highlighting the influence of channel geometry and flow conditions. Graphical representations of the flow profiles were employed to visualize the spatial distribution and understand the variations in these turbulence properties. The findings of this study enhance our understanding of the three-dimensional turbulence flow properties in sand bed meandering channels under subcritical flow conditions. The results offer valuable information for designing hydraulic structures, flood control measures, and river restoration projects. • This study aims to understand the behaviour of three-dimensional flow parameters in a meandering channel which are responsible for the morphological variations. • The Froude number data of the Mahanadi River is collected and same Froude number, sinuosity and bed material were maintained in the laboratory setup to perform the experiment. • The graphical variations of the Velocity, TKE, Skewness and Kurtosis in three directions were presented which describes the changes in the morphology and river bank erosion. [ABSTRACT FROM AUTHOR]

In the present study, a physical model of a meandering river with the sloping bank was used to assess the effects of rigid vegetation on flow velocity distribution and bed scouring at the toe and bank slope of a meandering river. The experimental tests were carried out under non-vegetated condition as a reference and vegetated conditions with six different patterns. The results revealed that the flow velocity distribution and bed topography at each cross-section were considerably affected by vegetation density and planting patterns. For the vegetated condition, the longitudinal flow velocity component increased in the main channel, while it decreased in the bank zone when compared to the non-vegetated conditions. Additionally, in the presence of vegetation, the core of maximum velocity diverts toward the centerline of the flume, which reduces erosion risk. Furthermore, a comparison of the longitudinal velocity profiles under vegetated and non-vegetated conditions showed that the area occupied by the core of maximum velocity at the bend apex decreases up to 26%, notably for the conditions with double rows and hybrid patterns. The results of this study showed that the use of vegetation can be recommended to stabilize rivers' bed and banks a green and cost-effective alternative to hard-engineering methods. [ABSTRACT FROM AUTHOR]

The selection of turbulence model and wall function is very important to solve the flow and heat transfer. By studying the flow and heat transfer characteristics in the winding path of turbine blades by different turbulence models, the mechanisms of the effect of different turbulence models and wall functions on the calculation were revealed. The research results show that the turbulence model has a great influence on the calculation results of flow and heat transfer under the same boundary conditions, but the turbulence model has little effect on the static pressure coefficient. In CFX, the heat transfer results calculated by the Scalable Wall Functions are higher than the results from the Automatic Wall Functions. Compared with the experimental results, the meandering channel heat transfer results from the Realizable k - ε turbulence model and Scalable Wall Functions are ideal, while the SST turbulence model is better than other turbulence models. Choosing the reasonable dissipative equation and estimating the accurate turbulent kinetic energy near the wall can affect the calculation results of the turbulence model. [ABSTRACT FROM AUTHOR]

Erosion-deposition changes of meandering channels are a dynamic equilibrium process in the long-term time scale. The part of sediment load transport is roughly equal the difference between bank erosion in concave bank and point bar deposition in convex bank. In summer 2018,the Unmanned Aerial Vehicle (UAV) aerial survey was used to obtain high-resolution imagery data in four meandering rivers of Yellow River source(i.e.,Maiqu,Haqu,Gequ,and Lanmucuo Rivers),and the high-resolution topographic dataset were produced by post-processing of the Structure-for-Motion technology. Channel topography of cross sections of single bend and subsequent bends are extracted using ArcGIS. The differences of the cross-sectional area between convex and concave bank zone are calculated in single section and adjacent sections of meandering channels. Results demonstrate that sediment loss amount generated by bank collapse on concave bank and sediment deposition amount on convex bank are imbalanced,i.e.,a net loss of sediment amount existing. For a single bend,the sediment loss per unit channel length in Lanmucuoqu River is about 0.191 m3,and about 0.045 m3 on average in Maiqu,Haqu,and Gequ Rivers. Nonetheless,the sediment loss in the reach scale of meandering channel is non-uniformity that indirectly reflects the difference in lateral migration rates among different bends. [ABSTRACT FROM AUTHOR]

A new method is proposed to calculate flow in a compound meandering channel by considering the momentum transfer between different zones. Vertical momentum transfer, at the horizontal interface of the lower main channel and the meander belt; and the horizontal momentum transfer, at the vertical interfaces between the meander belt and the adjoining outer floodplains. Modelling of this analytical method for the bend apex section is carried out, for a control volume of unit length. Calibration has been carried out for different laboratory channels and a natural river, demonstrating that the present model is capable of providing an adequate prediction of discharge in experimental as well as in field study. A constant value of 0.01 as the momentum transfer coefficient is verified to act at the interacting interfaces for the meandering channels. The zonal velocity for each subsection is determined, which facilitates prediction of discharge distribution. [ABSTRACT FROM AUTHOR]

Abstract Neck cutoff is an essential process limiting evolution of meandering rivers, in particular, the highly sinuous ones. Yet this process is extremely difficult to replicate in laboratory flumes. Here we reproduced this process in a laboratory flume by reducing at the 1/2500 scale the current planform of the Qigongling Bend (centerline length 13 km, channel width 1.2 km, and neck width 0.55 km) in the middle Yangtze River with geometric similarity. In five runs with different constant input discharges, hydraulic parameters (water depth, surface velocity, and slope), bank line changes, and riverbed topography were measured by flow meter and point gauges; and bank line migration and a neck cutoff process were captured by six overhead cameras mounted atop the flume. By analyzing the neck cutoff process, development of the cutoff channel, and adjustment of the old channel to cutoff, we found that (i) bank erosion around the upstream and downstream channel segments of the neck reduced its distance, subsequently increased water head gradient on both sides of the neck inducing the occurrence of neck cutoff in a short time period; (ii) the width of the new cutoff channel increased quickly after neck cutoff because the increased local slope generated a higher unit stream power in the cutoff channel; and (iii) neck cutoff significantly increased bank erosion and channel widening in upstream and downstream channels as it is a gradual process compared with chute cutoff. These results suggest that bank revetment around the Qigongling Bend might be necessary to prevent the abrupt occurrence of natural neck cutoff in the future. Highlights • Neck cutoffs are first successfully achieved in flume experiments. • Neck cutoff is controlled by bank erosion mainly caused by moderate flows. • Neck cutoff enhances bank erosion and channel widening in upstream and downstream channels. [ABSTRACT FROM AUTHOR]

A novel workflow is presented for building static models of fluvial reservoirs composed of large point-bar architectural elements, based on the application of a specialized forward stratigraphic model, named ‘Point-Bar Sedimentary Architecture Numerical Deduction’ (PB-SAND). The approach uses interpreted horizontal slices from 3D seismic datasets to reconstruct the planform evolution of meander loops, on which basis the geometry of point-bar deposits and associated accretion units can be simulated deterministically. The resulting meander-belt geometry is then populated with different types of facies, through a rule-based algorithm that generates facies architectures that reflect geologic understanding, enabling users to establish linkages between styles of meander evolution (e.g., meander growth via expansion, translation, rotation) and facies distributions. Input parameters define the proportions, geometries and distributions of types of point-bar deposits, as captured from subsurface data and/or from geologic analogs. Multiple stochastic realizations of facies architecture can be generated. To demonstrate the application of this modeling approach, the workflow has been applied to a meander-belt reservoir where large point-bar and channel-fill elements are imaged in seismic. A detailed example is used to illustrate workflows that can be used to build high-resolution sector models in pre-drill contexts, suitable for guiding development plans. An additional example is used to show how to achieve well match for densely drilled sectors, by means of a hybrid approach that combines the new algorithm with traditional geostatistical techniques. It is shown how the workflow allows consideration of point-bar growth styles, as inferred from seismic data, on distribution and geometry of heterogeneities, and how this facilitates the reproduction of geologic features that are important controls on the static connectivity of point-bar reservoirs (e.g., distribution and characteristics of bar-front mud drapes, and of mud-prone packages arising from progressive meander-bend tightening or from downstream fining of deposits beyond the apex of a meander bend). A comparison with traditional variogram-based methods is undertaken to compare metrics that describe intra-point-bar static connectivity and that represent proxies for the degree of compartmentalization of upper-bar sands by mud drapes. [ABSTRACT FROM AUTHOR]

Factors affecting roughness coefficient in a meandering compound channel are investigated and used to predict Manning’s roughness by dimensional analysis. Factors affecting Manning’s roughness, are considered to be geometric as well as hydraulic, namely relative depth of flow, width ratio, bed slope and sinuosity for a meandering compound channel. An experimental investigation was carried out for the highly meandering channel of sinuosity 4.11. Observations of various researchers, on the laboratory study of meandering channels have also been used in the investigation. The proposed equation for predicting composite Manning’s roughness is used to estimate the conveyance of a channel with floodplain flow. The proposed dimensional analysis model is observed to predict the conveyance capacity better than other existing methods of Manning’s n prediction. [ABSTRACT FROM AUTHOR]

Accurate prediction of Manning's roughness coefficient is essential for the computation of conveyance capacity in open channels. There are various factors affecting the roughness coefficient in a meandering compound channel and not just the bed material. The factors, geometric as well as hydraulic, are investigated and incorporated in the prediction of Manning's n. In this study, a new and accurate technique, gene expression programming (GEP) is used to estimate Manning's n. The estimated value of Manning's n is used in the evaluation of the conveyance capacity of meandering compound channels. Existing methods on conveyance estimation are assessed to carry out a comparison between them and the proposed GEP model. Results show that the discharge capacity computed by the new model provides far better results than the traditional models. The developed GEP model is validated with three individual sections of a natural river, signifying that the model can be applied to field study of rivers, within the stated range of parameters. [ABSTRACT FROM AUTHOR]

Gas-liquid mass transfer around Taylor bubbles moving in a meandering millimetric square channel was locally visualized and characterized in the present study. For that, the colorimetric technique proposed by Dietrich et al. (2013) was implemented. With this technique, the evolution of equivalent oxygen concentration fields in the liquid slugs passing through one and several bends was firstly described. In particular, it was observed how the flow structure (recirculation zones) inside the liquid slugs were twisted and split by the periodic bends (centrifugal effect), until reaching, after several bends, a uniform O 2 concentration inside the liquid slugs. The influence of the “turning point”, joining two “straight” sections of meandering channel was also highlighted: a slowing down of the gas-liquid mass transfer was clearly shown. Volumetric mass transfer coefficients were determined at last by fitting the experimental axial profiles of averaged oxygen concentrations in the liquid slugs (before the turning point) with the ones predicted by a classical plug-flow model. [ABSTRACT FROM AUTHOR]

The sediment discharge has been reduced sharply since the operation of Three Gorges Project in 2003, leading to the severe erosion of Lower Jingjiang Reach (LJR), a representative meandering channel located in the middle of Yangtze River. Some sharp bends in LJR are unusually undergoing convex bank erosion and concave bank deposition resulting in submerged shoal, which are contrary to the basic fundamentals of curved riverbed evolution. These abnormal riverbed deformations induce changes in flood control, waterway condition and the relationship between Yangtze River and Dongting Lake, and indeed the corresponding mechanism needs to be investigated. In the paper, based on a number of field data, the characteristics of riverbed evolution and channel morphology adjustments are fully demonstrated by quantitative statistics of erosion-deposition distribution and channel morphology for selected curved reaches. The results indicate that the annual redistribution of flow, the reduction of sediment discharge and mutation of sediment gradation are the dominant causes. [ABSTRACT FROM AUTHOR]

The predictive capability of Reynolds-averaged numerical simulation (RANS) models is investigated by simulating the flow in meandering open channel flumes and comparing the obtained results with the measured data. The flow structures of the two experiments are much different in order to get better insights. Two eddy viscosity turbulence models and different wall treatment methods are tested. Comparisons show that no essential difference exists among the predictions. The difference of turbulence models has a limited effect, and the near wall refinement improves the predictions slightly. Results show that, while the longitudinal velocities are generally well predicted, the predictive capability of the secondary flow is largely determined by the complexity of the flow structure. In Case 1 of a simple flow structure, the secondary flow velocity is reasonably predicted. In Case 2, consisting of sharp curved consecutive reverse bends, the flow structure becomes complex after the first bend, and the complex flow structure leads to the poor prediction of the secondary flow. The analysis shows that the high level of turbulence anisotropy is related with the boundary layer separation, but not with the flow structure complexity in the central area which definitely causes the poor prediction of RANS models. The turbulence model modifications and the wall treatment methods barely improve the predictive capability of RANS models in simulating complex flow structures. [ABSTRACT FROM AUTHOR]