Convex Bank Research Articles

Modeling Bedload Transport Trajectories along a Sine-Generated Channel

This study explores the influences of flow discharge and particle size on bedload transport trajectory by applying a depth-averaged two-dimensional model to a 110° sine-generated laboratory flume with wide-and-shallow sections. Calculated results exhibit two erosion regions in a bend: Zone-1—oreside of the point bar near the convex bank and Zone-2—near the apex of the concave bank. Sediments eroded from Zone-1 are mainly transported along the same-side convex bank rather than crossing the channel centerline, indicating the crucial role of longitudinal flow in shaping point bars. Most particles from Zone-2, however, behave more complicated by changing their trajectories with the developing bar-pool topography. Besides, sensitivity analyses indicate that, the shifting of bedload trajectory in the curved channel is not susceptible to particle size while considerably varies with flow discharge. Moving particles in a meandering channel are ultimately constrained within the belt of “concave bank‒crossing bar‒concave bank” after the bend topography is fully developed and the bed deformation reaches a dynamic equilibrium.

Numerical Analysis on Hydraulic Characteristics of U-shaped Channel of Various Trapezoidal Cross-Sections

Curved channel with trapezoidal cross-section is approximate to the common form in nature fluvial networks and its hydraulic characteristics are considerably complex and variable. Combined with volume of fluid (VOF) method, renormalization group (RNG) k-ε turbulence model was employed to numerically investigate the flow properties in the U-shaped channel with various trapezoidal cross-sections. Analyses were performed from the aspects of the water surface transverse slope in bend apex (WTS-BA), longitudinal velocity, secondary flow, shear stress and turbulent kinetic energy (TKE) under several scenarios, specifically, four types of radius-to-width ratio and seven types of slope coefficient with a constant aspect ratio. The calculated results suggested that the maximums of shear stress and TKE in the bend were observed in the convex bank and the maximal intensities of secondary flow were observed within the range of 60 to 75 degrees for various varieties. As the radius-to-width ratio increased, the maximums of shear stress, TKE and WTS-BA decreased; but increased with increasing slope coefficients. The intensity of secondary flow decreased as slope coefficients increased and the angle of maximum intensity of secondary flow moved to the upstream for the increasing radius-to-width ratios. In addition, a new equation concerning the vertical distribution of longitudinal velocity in trapezoidal cross-sectioned channel was presented.

Diagnosis of Erodible Locations in River Bends Using a Combined Method (GIS, RS and the CCHE2D Model) (Case Study: The Karkheh River in Iran)

Abstract In this research, a combined method was developed to determine the erodibility of bends in the Karkheh River. For this purpose, a 40 km reach of the Karkheh River downstream of the Karkheh Dam was considered. The value of the shear stress was the calculated using the CCHE2D model. The results from the model show that in 1996 (before construction of the Karkheh dam), the length of the erodible reach was 1314 m; in 2011 (after construction of the Karkheh dam), this length was reduced to 840 m. Furthermore, the model illustrates that the location of the maximum shear stress is a function of the relative curvature (R/W) in the bends. For small values of the R/W (less than 1.5), the maximum shear stress occurs on the convex bank of a river bend. By increasing the R/W, the location of the maximum shear stress transfers to the concave bank of the river bend. Also, this location is displaced towards downstream by increasing the R/W.

Experimental Research on Boundary Shear Stress in Typical Meandering Channel

A novel instrument named Micro-Electro-Mechanical System (MEMS) flexible hot-film shear stress sensor was used to study the boundary shear stress distribution in the generalized natural meandering open channel, and the mean sidewall shear stress distribution along the meandering channel, and the lateral boundary shear stress distribution in the typical cross-section of the meandering channel was analysed. Based on the measurement of the boundary shear stress, a semi-empirical semi-theoretical computing approach of the boundary shear stress was derived including the effects of the secondary flow, sidewall roughness factor, eddy viscosity and the additional Reynolds stress, and more importantly, for the first time, it combined the effects of the cross-section central angle and the Reynolds number into the expressions. Afterwards, a comparison between the previous research and this study was developed. Following the result, we found that the semi-empirical semi-theoretical boundary shear stress distribution algorithm can predict the boundary shear stress distribution precisely. Finally, a single factor analysis was conducted on the relationship between the average sidewall shear stress on the convex and concave bank and the flow rate, water depth, slope ratio, or the cross-section central angle of the open channel bend. The functional relationship with each of the above factors was established, and then the distance from the location of the extreme sidewall shear stress to the bottom of the open channel was deduced based on the statistical theory.

Experimental and Numerical Investigation on Vanes’ Effects on the Flow Characteristics in Sharp 60° Bends

The aim of this paper is to study the effect of middle vanes on the flow structure in open-channels with sharp 60° bends. For this purpose, two cases are considered, i.e. bends with (one-vane case), and without (no-vane case) middle vanes. To minimize the systematic errors, the models are also numerically simulated using the Standard and RNG based k-ε turbulence models. The validation process shows satisfactory agreement between the measured and predicted results with average RMSEs of 0.18 and 4.82, for the water depth and velocity values, respectively. The comparison of the obtained results for the no-vane and one-vane cases is presented. It is found that middle vanes are efficient to minimize the intensity of secondary currents, super-elevation, and the tendency to flow separation in the convex bank of the bend. According, non-submerged vanes can help to reduce erosion and sedimentation the bend.

Experimental Analysis of the Source and Sink Relationship in a Meandering Bend

The source-sink-relationships in natural rivers are compound results of various influencing factors, and correlated fitted equations are kind of qualitative description. Experiment analyses was conducted in a laboratory flume to reach a quantitative description, whose center line follows a sine-generated curve with deflection angle of 30 degree. Three-dimensional velocities, bed deformation and composition of the pool-bar complexes in the meander bend are analyzed. The comparison of deformed bed and the depth-averaged velocities founds that, the maximum depth-averaged velocities of sections around the transition reach occurred at the edge of pool with smaller transverse slope, and the maximum depth-averaged velocities for sections around the apex section occurred at the edge of pool with larger transverse slope. Quantitative analysis of the composition of pool-bar complexes revealed the source-sink relation of moved particles. It shows that, moved particles sourced from the concave bank are mainly come from area down the apex section, and moved particles sourced from the convex bank are mainly come from area upper the apex section. Besides, there exit an area with strong sediment exchange, namely the bar around the convex-side of the apex section.

Impacts of bridge piers on water level during ice jammed period in bend channel–An experimental study

Experiments are carried out in an “S-shaped” flume in the laboratory under both open flow and ice-jammed conditions to study the impacts of bridge piers in a bend channel on the variation of the water level. The variations of the water level under the ice jammed condition with bridge piers are compared to those without bridge piers in an 180° bend channel. Results indicate that the bridge piers in the S-shaped channel have obvious impacts on the ice accumulation and the water level. The increment of the water level with the presence of the bridge piers is less than that without the bridge piers in the channel. Different arrangements of the bridge piers result in different increments of the water level. When one bridge pier is installed in the straight section of the channel (between 2 bends) and another one at the bend apex (for a convex bank), the increment of the water level during the equilibrium ice jammed period is between that with a single bridge pier located in the straight section of the bend channel and that with a single bridge pier located at the bend apex. It is also shown that the increment of the water level during the equilibrium ice jammed period increases with the increase of the average thickness of the ice jams.

THREE DIMENSIONAL FLOW STRUCTURES AND SCOURING FEATURE IN A RIVER WITH CONVEX BANK AND BACKWARD STEP

THREE DIMENSIONAL FLOW STRUCTURES AND SCOURING FEATURE IN A RIVER WITH CONVEX BANK AND BACKWARD STEP

Erosion of collapsed riverbank and interaction with channel

After failure of a riverbank, a cohesive block of soil can temporarily lie at the toe of the bank until it is eroded away. To investigate the erosion of this collapsed block and its interaction with the river channel, a series of experiments was conducted under similar hydraulic conditions within a U-bend flume. The flow structure and channel topography were measured and analysed in detail. The presence of the collapsed block was found to cause a 1·1–1·6 times greater downstream bank retreat, while the counter-rotating circulation cell and smaller near-bank velocity protect the bank against erosion upstream of the block end. The scour percentage of the block head or end was about 1·5–2 times larger compared with that at the middle. Cohesive soil from the block fills the gap of non-cohesive riverbed material in the downstream convex bank and can result in a maximum bed deposition thickness increase of more than 300%. The results also provide a valuable experimental dataset and a theoretical basis for practical application.

Catastrophic flood disturbance and a community's response to plant resilience in the heart of the Texas Hill Country

The Blanco River, which flows through the limestone Balcones Canyonlands of central Texas (USA), experienced catastrophic flooding in May 2015 that resulted in significant biogeomorphic disturbance to its riparian corridor. High-resolution aerial and satellite imagery from pre- and post-flooding for a 55-km reach of river were used to map and categorize patterns of disturbance by degree of severity ranging from complete floodplain stripping to no disturbance. The most severe disturbance occurred within the floodway near the channel and decreased with lateral distance into the 100- and 500-year floodplains. Disturbance patterns previously identified in the literature including meander scour, parallel chute scour, convex bank erosion, and macroturbulent scour were all present following this event, as well as substantial disturbance proximal to tributary confluences. In the aftermath of this event, TreeFolks, a local nonprofit organization, engaged with the community to actively replant and restore the riparian corridor of the Blanco River on public and private lands. These reforestation efforts supplement the natural passive recovery of the riparian corridor, enabling the system to recover more quickly and be resilient to future flood events.

Exploring the role of trees in the evolution of meander bends: The Tagliamento River, Italy

AbstractTo date, the role of riparian trees in the formation of scroll bars, ridges, and swales during the evolution of meandering channels has been inferred largely from field observations with support from air photographs. In situ field observations are usually limited to relatively short periods of time, whereas the evolution of these morphological features may take decades. By combining field observations of inner bank morphology and overlying riparian woodland structure with a detailed historical analysis of airborne LiDAR data, panchromatic, and color images, we reconstruct the spatial and temporal evolution of the morphology and vegetation across four meander bends of the Tagliamento River, Italy. Specifically we reveal (i) the appearance of deposited trees and elongated vegetated patches on the inner bank of meander bends following flood events; (ii) temporal progression from deposited trees, through small to larger elongated vegetated patches (pioneer islands), to their coalescence into long, linear vegetated features that eventually become absorbed into the continuous vegetation cover of the riparian forest; and (iii) a spatial correspondence between the resulting scrolls and ridge and swale topography, and tree cover development and persistence. We provide a conceptual model of the mechanisms by which vegetation can contribute to the formation of sequence of ridges and swales on the convex bank of meander bends. We discuss how these insights into the biomorphological processes that control meander bends advance can inform modeling activities that aim to describe the lateral and vertical accretion of the floodplain during the evolution of vegetated river meanders.

Modeling meander morphodynamics over self‐formed heterogeneous floodplains

AbstractThis work addresses the signatures embedded in the planform geometry of meandering rivers consequent to the formation of floodplain heterogeneities as the river bends migrate. Two geomorphic features are specifically considered: scroll bars produced by lateral accretion of point bars at convex banks and oxbow lake fills consequent to neck cutoffs. The sedimentary architecture of these geomorphic units depends on the type and amount of sediment, and controls bank erodibility as the river impinges on them, favoring or contrasting the river migration. The geometry of numerically generated planforms obtained for different scenarios of floodplain heterogeneity is compared to that of natural meandering paths. Half meander metrics and spatial distribution of channel curvatures are used to disclose the complexity embedded in meandering geometry. Fourier Analysis, Principal Component Analysis, Singular Spectrum Analysis and Multivariate Singular Spectrum Analysis are used to emphasize the subtle but crucial differences which may emerge between apparently similar configurations. A closer similarity between observed and simulated planforms is attained when fully coupling flow and sediment dynamics (fully‐coupled models) and when considering self‐formed heterogeneities that are less erodible than the surrounding floodplain.

Two new types of sandbars in channels of the modern Ganjiang Delta, Poyang Lake, China: Depositional characteristics and origin

Two new types of sandbars are found in distributary channels of the modern Ganjiang Delta, Poyang Lake, China, respectively named as branching-point bar (BPB) and merging-point bar (MPB). BPB is defined as sandbar formed at the branching point of a channel where one channel branches into two. MPB is defined as sandbar formed at the merging point of two channels.BPB results from current velocity decrease at the branching point. BPB, arrowhead-shaped and growing towards upstream through upstream accretion, can be divided into bar head, middle and tail. Bar head refers to the lower part of the bar below average low water level, submerges under water whether in rainy seasons or dry seasons, and no plants grow on it. Bar middle refers to the middle part of the bar between average low and average high water levels, and sparse plants grow on it. Bar tail refers to the upper part of the bar above average high water level, and plants grow densely on it. From bar head to bar tail, sediment becomes finer and sorting degree becomes poorer; plant fossils increase in number, and silting-mud layers increase both in number and thickness. Vertically BPB shows a fining-upward sequence. BPB can be classified into 3 types: baby BPB with only bar head, child BPB with bar head and middle, and adult BPB with bar head, middle and tail. Generally from the lower delta plain to upper delta plain, BPB becomes more and more mature. If a channel is suddenly abandoned, BPB at different stages may be preserved. MPB is very similar to BPB in depositional characteristics, and main differences lie in its origin and growth direction.Discovery of BPB and MPB has important implications in finding remaining oil in oilfields. Previously, geologists considered point bars on convex bank of meandering channels or longitudinal bars and transverse bars in the middle of braided channels as favorable locations for finding remaining oil. Our study shows such locations as branching points and merging points of channels are also significant because BPB and MPB develop at these locations. Remaining oil has been found at branching points of channels in Pubei Oilfield, Songliao Basin, Northeast China.

Three-dimensional water-quality simulation for river based on VOF method

In the present study, considering the transport and transformation processes of variables, a three-dimensional water quality model for the river system was established, which coupled the volume of fluid (VOF) method with the k-e turbulence mathematical model. Then, the water hydrodynamic characteristics and transport processes for BOD5, NH3-N and TP were analyzed. The results showed that the water surface of convex bank was a little lower than that of concave bank due to the centrifugal force near the bend, and most concentrations were inferior to the type V standard indexes of surface water environmental quality. The model validation indicated that the errors between the simulated and monitored values were comparatively small, satisfying the application demands and providing scientific basis and decision support for the restoration and protection of water quality.

Effect of bank curvatures on hyporheic water exchange at meter scale

The micro-topography feature of a riverine system is a controlling attribute to induce the change of patterns and magnitudes of hyporheic water exchange. The study aims to determine how hyporheic water exchange is affected by the bank curvatures of test points at meter scale. A one-dimensional heat steady-state transport model was applied to determine patterns and magnitudes of vertical hyporheic water exchange in January and July 2015. The bank curvatures were calculated based on the curvature formula. The results demonstrate that vertical water exchange patterns of all test points were upwards during the two test periods, and the higher vertical fluxes mostly occurred in January 2015. Large curvatures for either sides of convex banks in the two periods resulted in higher vertical water exchange fluxes, and the significantly higher vertical fluxes occurred near the apex of bends. Additionally, a flow pattern from river bank discharging into stream was derived during the campaign in July 2015, and significantly higher fluxes were obtained along the straight bank where more riparian vegetation was adjacent to the bank/water interface. It can be suggested that the bank curvatures and riparian vegetation are considered the crucial attributes influencing hyporheic water exchange.

Meandering channel sandstone architecture characterization based on architecture seismic forward simulation

To improve the accuracy of the meandering channel sandstone reservoir characterization in offshore oilfield, this article completes the meandering channel sandstone reservoir architecture characterization of the NmII-3 layer of Low Minghuazhen Group in the Qinhuangdao 32-6 oilfield by the architecture-guided seismic forward simulation based on the meandering river depositional mechanism. By the research mentioned above, the seismic forward simulation identification criteria of composited meandering channels boundaries are summarized that when the mud layer thickness is greater than or equal to 1 m and the channel sandstone thickness is between 8 and 9 m, the mud layer between composite meandering channels can be identified. The seismic forward simulation identification criteria of single meandering channels boundaries are summarized that the elevation difference which is greater than or equal to 5 m can be identified while the channel sandstone is greater than or equal to 7 m. And the seismic forward simulation waveform features of elevation difference of channels, small sandstone among channels, and abandoned channel are summarized. The seismic waveform of elevation difference has two side-by-side troughs. The seismic crest center position of large channel sandstone is lower than the seismic crest center position of the small sandstone. The seismic crest center falls slowly and rises quickly from convex bank to concave bank in abandoned channel laterally. Finally, the architecture-guided seismic forward simulation method is established and the applying criteria are summarized in this paper. And the accuracy of the meandering channel sandstone architecture characterization with well logs and seismic data is improved.

陕西省县级行政中心城区与河流关系考察

城市的形成、发展和演变与河流水系有着密不可分的关系。河流是影响和制约城市空间结构的重要因素。通过对陕西省的河流与县级行政中心城区资料的考察后发现：河流左岸的城区多于右岸，干支交汇处也易形成城市。这一结论与地理学中河流的流水作用——凸岸堆积，凹岸侵蚀——密切相关。 The formation, development and evolution of the city have a close relationship with the river sys-tem. River is an important factor to affect and restrict the urban spatial structure. Through the study of data that rivers and county administrative center in Shaanxi Province we can find: The city in left bank is more than the right bank, also interchange of main rivers and branch rivers are easy to form city. Those conclusions are bound up with fluvial action of river in geography— convex bank accumulation and concave bank accumulation.

Experimental study on improvement effect of guide wall to water flow in bend of spillway chute.

In order to improve water flow in a bend of a spillway chute using a guide wall, modeling experiments with or without a guide wall under conditions of three different bend axial radii, three chute bottom slopes and three flow rates were carried out in this study. Two indexes were calculated, which are the improved water surface uniformity and the reduced rate of water surface difference in concave and convex banks of the cross-section. The results show that: (1) setting a guide wall in a bend can improve water flow in the bend because it increased the water surface uniformity of the cross-section and reduced the water surface difference in the concave and convex banks; (2) the smaller the bend axial radius, the better the water surface improvement effect will be using a guide wall; (3) the steeper the bottom slope, the more cross-sections with less water surface difference; and (4) flow rates have a great influence on water surface improvement in the bend, and the guide wall can improve water flow obviously when the water depth in the starting section of the bend is lower than the height of the guide wall. This study has important implications in engineering design of guide walls.

Flow separation at convex banks in open channels

Laboratory experiments in an open channel bend provide insight into the physics of convex bank flow separation occurring in a variety of channel configurations, including confluences and bifurcations. The edge of the zone of flow separation is characterized by a shear layer, enhanced velocity gradients, tke, turbulent shear stresses and reversal of the streamwise vorticity and vertical velocity. The latter result from turbulence-induced secondary flow near the convex bank. When bankline curvature abruptly increases, flow tends to move away from the convex bank along a straight path, as represented by the inertial forces – including the centrifugal force – in the transverse momentum equation written in curvilinear coordinates. Mass accumulation at the opposite bank leads to a transverse tilting of the water surface, and a pressure gradient towards the convex bank that causes the flow to change direction. The pressure gradient force lags spatially behind the inertial forces, which promotes flow separation. Flow separation typically occurs downstream of the location of maximum change in the bankline curvature, because an abrupt increase in bankline curvature also leads to water surface gradients that cause local flow redistribution towards the convex bank that opposes flow separation. The zone of convex bank flow separation is shaped by the secondary flow induced by streamline curvature and turbulence. The latter is conditioned by the production rate of tke, which crucially depends on the accurate description of the Reynolds stresses. Hydrodynamic, geometric and sedimentologic control parameters of convex bank flow separation are identified and discussed.

Hydraulic and geomorphic processes in an overbank flood along a meandering, gravel‐bed river: implications for chute formation

AbstractHydraulic interactions between rivers and floodplains produce off‐channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel‐bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off‐channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer‐grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation. Copyright © 2015 John Wiley & Sons, Ltd.