Concave Bank Research Articles

4D GIS FOR MONITORING RIVER BANK EROSION AT MEANDER BEND SCALE: CASE OF MOSELLE RIVER

Abstract. The "Wild Moselle" regional nature reserve extends over 13 km at the western foothills of the Vosges Mountains (France). The hydrological regime of the river is characterized by high flow in winter and spring and low flow in summer. Its average slope is 0.12 % and its average bankfull width is 60 m. The coarse sediment load comes mainly from bank erosion. Although this sector is relatively less affected by past or present human activities, the propagation of morphodynamic adjustments initiated by actions carried out both upstream and downstream of this sector impacts the current functioning of the river. These erosion waves converge today towards the central part of the reserve, which led to the collapse of the central pier of the Bainville-aux-Miroirs bridge during a 2-year flood in 2011, and could induce potential risks of defluviation which may destabilize infrastructures. In this context, the study carried out aims to characterize and anticipate the morphodynamic evolutions of the Moselle to be able to propose scenarios of management and restoration of the lateral mobility of the river. For this purpose, a 2D hydro-sedimentary model is being built over the entire reserve, combined with a detailed morpho-sedimentary monitoring. In order to improve the understanding of the lateral migration of the Moselle River, a photogrammetric monitoring was carried out along the concave bank of the most active meander of the studied sector. To follow this morphological evolution more closely, it was decided to establish a 4D GIS. The objective of this monitoring is to compare the rate of bank retreat with hydrodynamic parameters in order to estimate the geotechnical properties of the bank. Comparison of the observed and modelled bank retreat must thus allow these different parameters to be calibrated in the hydro-sedimentary model. As part of this work, this paper aims to highlight the use of 4D GIS to monitor bank retreat at the scale of a meander bend and is divided into three different parts: (i) a state of art to situate the study into the current knowledge and technologies, (ii) a presentation of the study area and the measurements carried out and (iii) a description of the different 3D or 4D data produced and the consequent spatial analyses.

BASIC LAWS AND LOCAL FEATURES OF DEVELOPMENT OF SMALL AND MEDIUM-SIZED RIVERBEDS (results of monitoring observations)

The article describes the results of monitoring of stream-bank erosion on small and medium-sized rivers of Central Russia - the wide-bottomed Kerzhents and Sherna and the narrow-bottomed embedded Tarusa. It was found that horizontal channel deformations generally occur according to the expected patterns: erosion of concave banks of bends, alluvium of their convex banks, curvature and then straightening. However, both on medium and small rivers, local non-fluvial factors interfere with this process, disrupting the interaction of the flow and the valley floor, bringing elements of unpredictability to typical channel deformations. This is especially noticeable on embedded rivers that lack a wide floodplain - a natural buffer that protects the channel from the effects of slope and other processes occurring on the slopes and in the near-valley parts of the watersheds.

Neural Network Prediction of Maximum Scour in Bends of Sand-Bed Rivers

Scoured streambeds can undermine revetments built on concave banks in bends of sand-bed rivers formed in alluvium. For this reason, protection work along the banks needs to extend to at least the lowest level reached along the toe of the underwater slope. In this investigation, 202 onsite measurements of bend scour assembled from several previous studies of streams with beds composed primarily of sand-sized sediment are evaluated. The data are used to fit coefficients of an artificial neural network model that offers a robust and easy to apply relation for rapidly predicting the maximum depth of bend scour in sand-bed rivers. A calculation of model variance by a similar neural network provides a means of generating the upper prediction limit of bend-scour depth. Consequently, one can determine the safety margin needed for a design scour depth that is statistically well-reasoned and not disproportionately large.

Operation damless intake of the Amudarya river (Central Asia)

The article describes the nature of the ongoing channel processes occurring in large damless water intakes from the Amudarya-Karakum, Karshi, and Amu – Bukhara channels located in its middle course. The characteristic features of the ongoing channel processes in the area of water intake are given. Based on the analysis of the current state of damless water intake, a method for improving its operation is recommended. To solve the problems of ensuring high-quality water intake and water supply, experimental studies were carried out to determine the angle of the bottom threshold in the inlet section of the head structure in the ABMCh. The analysis of the studies showed that for the threshold angles to the shore β = 30°, 45° 60°, an increase in the angle β from 30° to 60° increases the intensity of the artificial transverse circulation formed in the flow. It is proved that this circumstance is also true for a constant angle of threshold β with an increase in the relative threshold height H rth . The bottom threshold ensures the entry of bottom sediments into the head structure of the damless water intake. The threshold height is set to 1.44 m, which is determined in comparison with the height of the beds of sediments. To prevent the entry of a large amount of bottom sediment into the head structure of a damless intake, it is recommended that the device be used especially in the inlet section of the channel. It is shown that a Karakum channel is intensively exposed to deigish. Moreover, in this process a certain sequence is traced. First, one deigish is formed, the erosion products of which create favorable conditions for the occurrence of the next deigish. So the chain of deigish “walks” along the shore. The distance between deigish, also a “step,” varies widely: from 3-7 to 15-40 m or more. Then, the remaining protrusions between the deigish are washed out, which leads to a general expansion of the channel. It has been established that deigish appears and develops in both straight and curved sections of the channel. In the latter case, they are formed mainly on the concave bank, although there are cases of their location on the convex. This means that there are corresponding conditions for its formation in both straight and curved sections of the channel. It is substantiated that the formation and intensive development of deigish are associated with the intense channel-forming activity of the flow, which manifests itself both in the process of developing channel forms and in active sediment transport. It is recommended that in order to prevent the deigish process, it is necessary to carry out water management measures aimed at strengthening the channel of the channel. The implementation of concrete cladding using modern materials to reduce losses on filtering the channel in addition to preventing deigish channel and also helps to increase the efficiency of the irrigation channel.

Model Test of the Effect of River Sinuosity on Nitrogen Purification Efficiency

River pollution is a significant problem within the urbanization process in China. Nitrogen is one of the most important pollutants in rivers, and the nitrogen purification capacity of rivers can be affected by their sinuous morphology. In this study, a set of sandy circulating water test models was constructed, consisting of four river channel simulation models with sinuosities of 1.0, 1.4, 1.8, and 2.2. Each model was then infused with the same concentration of nitrogen-polluted water, which circulated for 52 h. The nitrogen reduction processes of rivers with different sinuosities were studied through water quality monitoring. The positive correlation between river sinuosity and nitrogen purification capacity was verified in physical laboratory test models. The effect of sinuosity on the spatiotemporal distribution of total nitrogen in pore water was confirmed. Additionally, the near-shore substrate was more involved in the process of river self-purification than the far-shore substrate. The concave bank of the sinuous rivers was more prone to pollutant accumulation and had a higher purification capacity than the convex bank. After the polluted water entered the sinuous channel systems, pollutant concentration differed within the convex bank between the more polluted upstream section and the less polluted downstream section. This study lays a foundation for studying the mechanism by which river sinuosity influences self-purification capacity.

Recognizing the product of concave‐bank sedimentary processes in fluvial meander‐belt strata

AbstractDownstream migration of point bars is an important process in meander belts. Inherent to downstream migration is sediment accumulation in concave channel banks, immediately adjacent to and downstream of convex point bars. Despite this, associated concave bank processes are often overlooked, with depositional products sparsely identified in the stratigraphic record. Counter‐point‐bar deposits are a type of concave‐bank deposit that have been positively identified in subsurface three‐dimensional seismic datasets, yet outcrop examples are not well‐constrained. This study characterizes and establishes recognition criteria of counter‐point‐bar deposits in outcrop using extensive exposure of Late‐Cretaceous meander‐belt deposits in eastern Alberta, Canada. Using a combination of traditional field‐based sedimentological analyses and three‐dimensional outcrop mapping with an Uncrewed Aerial Vehicle and Structure‐from‐Motion photogrammetry, point bar, counter‐point bar, and associated abandoned‐channel deposits, as well as adjacent floodplain deposits are identified. Bed‐scale characteristics of counter‐point‐bar deposits include interlaminated and interbedded siltstone and fine‐grained sandstone, abundant organic detritus, and evidence of deformation and slumping. At the bend scale, accretion packages bounded by internal erosion surfaces are composed of dipping siltstone and minor sandstone beds that extend from the top to the base of the meander belt. At the belt scale, positive identification relies on concave accretion surface mapping, their orientation relative to the meander‐belt edge (i.e. dipping away), and consideration of meander‐bend evolution. These results have implications for recognition of counter‐point‐bar deposits in analogous, less‐constrained data sets, which provides a foundation for more complete palaeoenvironmental interpretations.

Reducing of Bend Scour by Setting Spurs in a Curved Channel

In Taiwan, the rivers not only are fast-flowing with high discharge, but they badly erode their beds during the typhoon seasons. In addition, erosion on the concave bank in a meandering channel is the primary cause of levee break. Therefore, the study conducted a down-scale experiment on erosion induced by oblique flow in a laboratory. It was similar to number 27–34 cross sections of the Fengshan river of Hsinchu County, Taiwan. The region was chosen because there are some special attacking angles of water flow and historical precedents of levee break. The study adopted the discharges of return periods of 10 and 20 years and measured the flow field by laser doppler velocimetry (LDV). Then the protective effects with different spur types were examined. The results indicate that increasing velocity induces side erosion when the flow impacts with the adjacent angle on the concave bank. However, the decreasing of velocity causes deposition of sediment on the concave bank. Furthermore, based on the vertical velocity profile of water flow, a higher flow rate is measured in the downstream on the concave bank. After the spurs are installed, the velocity at the spurs in the downstream is reduced, and the cross section with the larger velocity is moved to upstream. In addition, after setting the spurs, the reduction rates in volume of scour are 7.97% of a 10 year return period and 4.65% of a 20 year return period, respectively. That demonstrates the scour is effectively reduced as long as the spurs are set. Although the erosion mitigation rate and protection effect are decreased when the velocity is high, there is still a good protection effect at the bank. The setting of spurs has the following effects: First, the maximum scour depth generates in the front spur, while the maximum scour position keeps away from the bank. Then, the overall flow rate can be reduced to approximately 35%–40% comparing with the original flow field. Lastly, the spur on the slope of 1/30 degrees demonstrated the best function of stretching the distance from the embankment.

Scour at River Bend: A Parametric Study

Scour at river bend is a complex process involving various parameters. Experiments were conducted in two laboratory flumes having 90° and 180° bends to study the influence of different parameters on scour at bends. Uniform non-cohesive sediments having median diameters varying from 0.28 to 1.18 mm were used as bed materials under different flow conditions. Three different cross-sectional shapes of the bed, namely flat, trapezoidal and parabolic, were used to study the effect of shape on scour. From the experiments, it is found that, at 90° bend, maximum scour and deposition occurred on concave and convex bank, respectively, between 50° and 60° azimuthal sections, whereas for 180° bend, the location was between 130° and 140° sections. Magnitude of scour was maximum for parabolic bed on the concave bank under flooded condition and deposition was maximum for flat bed on convex side. The deposition pattern on trapezoidal and parabolic bed was also different than that of flat bed. Again, for the same shear velocity ratio, the bigger size bed sediment produced larger scour depth but, for the same bed particle size, scour was more for greater approach shear velocity ratio. Further, measured three-dimensional time averaged velocity components before and after the scour on the 180° bend having initial parabolic bed shows the dominance of a spiral flow at the first half of the bend, and cross-flow from concave to convex at lower layer of the second half of the bend. The combined effect of vortex, cross-flow and accelerated flow is responsible for the erosion on the concave bank and deposition on the convex bank.

Evolution process of different abnormal river bends in the braided reach of the Lower Yellow River

1980s中期以来，黄河下游游荡段经常出现畸形河湾，分析其演变过程及特点对游荡段治理具有重要意义.本研究采用黄河下游游荡段的遥感影像和实测断面地形资料，描述了不同畸形河湾的演变过程并分析其河湾形态参数和断面形态的变化.研究表明，黄河下游游荡段的畸形河湾具有演变周期短、扭曲程度较大和易发生自然裁弯等特点.游荡段不同位置畸形河湾的演变特点不同：游荡段上段的畸形河湾演变缓慢，裁弯历时长；中段的畸形河湾演变速率较高，河湾扭曲程度大；下段的畸形河湾演变速率高，容易发生自然裁弯.河湾形态参数可反映畸形河湾的演变过程，其变化特点与畸形河湾的形成和裁弯过程相对应.在畸形河湾形成过程中，曲率半径和河湾间距减小，弯曲度、水流夹角和河湾振幅增大.游荡段3个畸形河湾弯曲度的最大值分别为1.20、2.10和1.61，反映了不同畸形河湾的扭曲程度.“Ω”形畸形河湾裁弯后的曲率半径、水流夹角、河湾振幅和河湾间距约为其演变过程中最值的605%、59%、27%和133%.“M”形畸形河湾裁弯后水流夹角、河湾振幅和河湾间距约为其最值的37%、83%和152%.在畸形河湾形成时期，伊洛河口断面位于畸形河湾凹岸侧的滩地以94 m/a的速率崩塌.裁弯后，河槽冲刷，两岸滩地崩退，河槽展宽速率为148 m/a.河床底部的冲刷从深泓点逐渐向两边发展.;Since the mid-1980s, the braided channel reach of the Lower Yellow River has formed some abnormal river bends. Analysis of the evolution processes and characteristics of abnormal river bends are of great significance for the regulation of the braided reach. Based on the remote sensing imagery and the cross-sectional topography, this study describes the evolution processes of abnormal river bends and variation of morphological characteristic parameters. The result shows that the abnormal river bends in braided channel reach of the Lower Yellow River have the characteristics of short evolution duration, high distortion level and prone to natural cutoff and the bends in different location have different evolution characteristics. The first bend in the upper braided reach evolves slowly and the cut-off period is long. The second bend in the middle braided reach evolves rapidly and highly distorted. The third bend in the lower braided reach evolves rapidly and is prone to generate natural cutoff. Variation of river bend morphology reflects the evolution process of three abnormal river bends, which are corresponding to the bend formation and cutoff processes. The formation of the abnormal river bend is a process in which the curvature radius and river bend spacing decreases, and the bend curvature radius, angle of water flow and bend amplitude increase. The maximum curvature radius of the three bends in the braided reach are 1.20, 2.10 and 1.61, respectively, reflecting the distortion degree of different bends. After cut-off, the curvature radius, angle of water flow, bend amplitude and river bend spacing are about 605%, 59%, 27%, and 133% of the extreme value during the evolution of the “Ω” shape river bend. For “M” shape river bend, angle of water flow, bend amplitude and river bend spacing after cutting are about 37%, 83%, and 152% of their extreme values. The cross-sectional profile at the Yiluo River outlet reflected that the beach on the concave bank of the river bend continuously collapsed at a rate of about 94 m/a during the formation period of abnormal river banks. The cut-off has a great influence on the cross-sectional profile. After cut-off, the beaches on both bank collapsed, and the river channel widen at a rate of 148 m/a. The erosion at the riverbed developed from the deep point to its two sides.

Research on atypical evolution characteristics of convex bank of typical bend in lower Jingjiang river under changing environment

Since Three Gorges Reservoir was put into operation, the water and sediment conditions have changed significantly, and the lower reach of the project have been adjusted to adapt to this change. The continuous bend of the Jingjiang section of the lower reach of the reservoir (Tiaoguan-Laijiapu) is selected as the research reach. The research methods of prototype data observation and physical model test were carried out, and the continuous erosion evolution and channel shape change characteristics of the reach are analyzed. The results show that the convex bank at the upper of the bend is eroded, the concave bank is deposited, the deep groove is moved to the right, widened and cut down, the deep groove at the lower of the bend is widened and brushed deep, the convex bank at the upper of the Laijiapu bend is scoured, the concave bank gradually deposited, and the deep groove of the whole bend is widened and deep scoured after Three Gorges Reservoir was built and used. The change of runoff conditions after the construction of the reservoir provide dynamic conditions for the continuous scouring of the convex bank. The main factors for the continuous erosion of the convex bank are the substantial reduction of sediment inflow downstream of the dam caused by reservoir sediment interception, sand mining and soil and water conservation. This study is universal and can provide reference for the research on evolution characteristics of the downstream bend reach of other large water conservancy projects.

Research on the water level in a bending channel using a guide wall

AbstractIn order to study the water level at the convex and concave banks after installing a guide wall in a spillway chute bend, with the original condition that the Fr at the entrance of the channel bend is larger than 1.0 (supercritical flow) when there is no the guide wall, systematic experiments with the guide wall were conducted for three radii (2.4B, 3.2B and 4B; B is the width of the channel), bottom slopes (0.01, 0.005 and 0.02), and discharges (50, 100 and150 m3 h−1). Results show that, firstly, after installing a guide wall, the Fr becomes smaller and even lower than 1.0, which means the flow status changes from supercritical to subcritical in some conditions with the help of the guide wall. Secondly, the water depth at the convex bank decreases with the increase of the relative axial radius while this presents to be adverse at the concave bank. Thirdly, for water surface differences in cross-sections, the maximum value decreases with the increase of the relative axial radius, and increases with the increase of the discharge per unit width or the bottom slope. Additionally, a novel formula for calculating the maximum water surface difference was obtained in this article.

Downstream geomorphic impact of the Three Gorges Dam: With special reference to the channel bars in the Middle Yangtze River

AbstractThe Three Gorges Dam (TGD) has altered downstream flow–sediment regimes and led to significant changes in the morphodynamic processes in the Middle Yangtze River (MYR). However, due to the complexity of this large river, the driving forces and implication of the morphodynamic processes remain insufficiently understood. This study selected two typical meandering and bar‐braided reaches, the Zhicheng (ZC) and Shashi (SS) reach, to examine their responses to the TGD operation. The results showed that in the post‐dam period significant channel erosion occurred with a higher erosion rate in the ZC reach (closer to the TGD) compared with the SS reach. The area of the Guanzhou mid‐channel bar (ZC reach) and the Sanba mid‐channel bar (SS reach) shrank by 30 and 90% from 2003 to 2015, respectively. The increased fluvial erosion intensity due to the reduction in suspended sediment concentration (SSC) drove the shrinkage of the mid‐channel bars, as demonstrated by empirical relationships between bar geometry and fluvial erosion intensity. An increase of 22 days per year in the frequency of post‐dam medium‐to‐high discharges (10 000–25 000 m3 s−1), and associated with the reduction in SSC, jointly led to the greater erosion at the convex (inner) banks than the concave (outer) banks, which has negatively affected the designed navigation channels at the concave banks by decreasing their discharge partitioning ratios. The post‐dam water level at a given high discharge (>25 000 m3 s−1) showed no evident change, but the water level at a given low discharge (<10 000 m3 s−1) decreased. The reduction in water levels at low flows can affect water supply and riverine ecosystems in the MYR. © 2019 John Wiley & Sons, Ltd.

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.

Experimental research on the maximum backwater height in front of a permeable spur dike in the bend of a spillway chute

Abstract Hydraulic experiments on installing a permeable spur dike at three positions (1/4, 1/2, and 3/4) on the concave bank of the bend of a spillway chute with three angles (45°, 60° and 75°) were carried out for studying the backwater condition in front of the permeable spur dike. Results show that the maximum backwater height occurs at the cross-section where the permeable spur dike meets the concave bank of the bend. A formula for the maximum backwater height was derived by the employment of the principle of momentum conservation, and the formula indicates that the height is influenced by the geometric parameters of the permeable spur dike and the bend, the layout of the spur dike in the bend, and the inflow discharge. Based on experimental data, a regression analysis was implemented on the water depth coefficient in the formula. Furthermore, the maximum backwater height can be obtained through the water depth at the concave bank of the end of bend when the parameters and installing pattern of the permeable spur dike are determined.

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.

Case Study on Application of the Step with Non-Uniform Heights at the Bottom Using a Numerical and Experimental Model

Steps effectively dissipate the energy of water along a path and reduce the size of the stilling basin but are rarely used in curved spillways. The shore spillway of a reservoir, which is restricted by topography, must be arranged in a curved shape. At high flow velocity and low water depth, some areas of the base plate of the curved spillway were not covered by the water. The water flow into the stilling basin did not form a submerged hydraulic jump. It was proposed that a step with bottom non-uniform heights be placed in the smooth base plate of the curved spillway to improve these undesirable hydraulic phenomena. A physical model experiment with a length scale of 1:40 verified the feasibility of the curved stepped spillway in engineering. Based on the k-ε model and volume-of-fluid (VOF) method, a three-dimensional numerical model was established, and the reliability of the numerical model was verified by measured data. The main flow region, velocity field, cavitation on a step, and the energy loss rate of steps were discussed. The comparison between a curved spillway with and without steps shows that the steps balance the partial centrifugal force in the curved section, making the water depth of the cross-section evenly distributed, and the base plate was no longer covered by water. The flow pattern on the steps was skimming flow, and the velocity of the flow into the stilling basin was greatly reduced. The elevation of the concave bank of the base plate was raised, resulting in the formation of transverse flow, which in turn constituted a three-dimensional energy dissipation pattern with the longitudinal flow. The energy loss was significantly higher than that of the smooth curved spillway. However, the triangular region near to the concave bank on the base plate experienced negative pressure, and an aeration device in front of the steps was needed.

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.

Immediately downstream effects of Three Gorges Dam on channel sandbars morphodynamics between Yichang-Chenglingji Reach of the Changjiang River, China

Sandbars are of vital ecological and environmental significance, which however, have been intensively influenced by human activities. Morphodynamic processes of sandbars along the Yichang-Chenglingji Reach of the Changjiang River, the channel immediately downstream of the Three Gorges Dam (TGD), are assessed based on remote sensing images between 2000 and 2016. It can be found that the entire area of sandbars reduces drastically by 19.23% from 149.04 km2 in 2003 to 120.38 km2 in 2016, accompanied with an increase in water surface width. Owing to differences in sediment grain size and anti-erosion capacity, sandbar area in the upstream sandy gravel reach (Yichang-Dabujie) and downstream sandy reach (Dabujie-Chenglingji) respectively decreases by 45.94% (from 20.79 km2 to 11.24 km2) and 14.93% (from 128.30 km2 to 109.14 km2). Furtherly, morphological evolutions of sandbars are affected by channel type: in straight-microbend channel, mid-channel sandbars exhibit downstream moving while maintaining the basic profile; in meandering channel, point sandbars show erosion and deposition in convex and concave bank respectively, with mid-channel sandbars distributing sporadically; in bending-branching channel, point sandbars experience erosion and move downstream while mid-channel sandbars show erosion in the head part along with retreating outline. We document that the primary mechanism of sandbars shrinkages along the Yichang-Chenglingji Reach can be attributed to TGD induced suspended sediment concentration decreasing and increasing in unsaturation of sediment carrying capacity. Additionally, channel type can affect the morphological evolution of sandbars. Along the Yichang-Chenglingji Reach, sandbars in straight-microbend channel are more affected by water flow than that in bending-branching channel.

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.

The mechanism of barrier river reaches in the middle and lower Yangtze River

Alluvial channel has always adjusted itself to the equilibrium state of sediment transport after it was artificially or naturally disturbed. How to maintain the equilibrium state of sediment transport and keep the river regime stable has always been the concerns of fluvial geomorphologists. The channel in the middle and lower reaches of the Yangtze River is characterized by the staggered distribution of the bifurcated river and the single-thread river. The change of river regime is more violently in the bifurcated river than in the single-thread river. Whether the adjustment of the river regime in the bifurcated river can pass through the single-thread river and propagate to the downstream reaches affects the stabilities of the overall river regime. Studies show that the barrier river reach can block the upstream channel adjustment from propagating to the downstream reaches; therefore, it plays a key role in stabilizing the river regime. This study investigates 34 single-thread river reaches in the middle and lower reaches of the Yangtze River. On the basis of the systematic summarization of the fluvial process of the middle and lower reaches of the Yangtze River, the control factors of barrier river reach are summarized and extracted: the planar morphology of single-thread and meandering; with no flow deflecting node distributed in the upper or middle part of the river reach; the hydraulic geometric coefficient is less than 4; the longitudinal gradient is greater than 12‰, the clay content of the concave bank is greater than 9.5%, and the median diameter of the bed sediment is greater than 0.158 mm. From the Navier-Stokes equation, the calculation formula of the bending radius of flow dynamic axis is deduced, and then the roles of these control factors on restricting the migration of the flow dynamic axis and the formation of the barrier river reach are analyzed. The barrier river reach is considered as such when the ratio of the migration force of the flow dynamic axis to the constraint force of the channel boundary is less than 1 under different flow levels. The mechanism of the barrier river reach is such that even when the upstream river regime adjusts, the channel boundary of this reach can always constrain the migration amplitude of the flow dynamic axis and centralize the planar position of the main stream line under different upstream river regime conditions, providing a relatively stable incoming flow conditions for the downstream reaches, thereby blocking the upstream river regime adjustment from propagating to the downstream reaches.