Concave Bank Research Articles

Research on sedimentation characteristics of the end bend water extraction project of cascade reservoir backwater

Abstract The operation mode of upstream and downstream cascade power stations significantly impacts the river bed in the water intake section, leading to complex sediment deposition characteristics in the curved river section at the reservoir’s end. In this research, a standard physical model for bed load and suspended load at a scale of λl=λh=100 was conducted, combined with the operation mode of upstream and downstream power stations, to investigate the sediment deposition patterns in the intake section and to study the sediment deposition characteristics in the intake area. The experimental results reveal that bed load sediment predominantly accumulates on the right bank (convex bank) of the river, with no bed load sediment accumulation at the mushroom head layout of the water intake project on the left bank (concave bank). When the operation of upstream and downstream power stations is uncoordinated, suspended sediment settles in the water intake section, with no sediment accumulation in the local area of the mushroom head of the intake due to the disturbance effect. Safety sensitivity analysis indicates that the safety of the water intake project can be ensured.

Influences of channel bed morphology on flow structures in continuous curved channels

Introduction: The formation of bars and pools, characterized by concave and convex bed morphology, is a typical feature of curved rivers. The channel bed morphology has a significant influence on the flow structures in curved channels.Methods: Based on data from physical model experiments, this study employs the RNG k-ε model and the VOF (Volume of Fluid) method to perform three-dimensional numerical simulations of flow in continuous curved channels.Results: By comparing the variations in flow structures between channels with a flat bed and channels with bars and pools, the results show that the presence of bars and pools leads to an increase in longitudinal flow velocity on the convex bank side near the entrance of the upstream bend, while in the downstream bend it is opposite. The high-velocity region shifts slower towards the concave bank along the bend. The presence of point bars weakens the circulation near the convex bank in the upstream bend, resulting in a smaller circulation intensity. The decrease in circulation intensity is the largest (−23.91%) at the apex of the bend. In the downstream bend, the remaining circulation from the upstream bend attenuates slower in the pool and has a greater impact distance, increasing the circulation intensity in the downstream bend. The section near the bend entrance shows the largest increase in circulation intensity, with a rate of 128.18%. The unevenness of the bed topography increases the unevenness of the bed shear stress in the downstream bend.Discussion: The findings of this study contribute to a deeper understanding of the complex flow structures and evolution trends in natural curved rivers, providing scientific basis for the management of curved river channels.

The deployment of spur dike group improves physical habitats in urban meandering rivers: Insight from a 2D habitat suitability model

Although channel meandering is currently a popular approach for river ecological restoration, the spiral flow formed in bends may lead to fragmentation of aquatic habitats due to the concave bank erosion and convex bank sedimentation. The aforementioned issue motivated this study, which aims to mitigate the adverse effects of curved flow on aquatic habitats within bends. We proposed a placement strategy of dike group within the curved channel to adjust the water–sediment structure of meandering rivers, thereby safeguarding against bank erosion and the fragmentation of river habitats. The effectiveness of this strategy has been validated by the development of numerical models based on 2D depth-averaged water–sediment equations, coupled with the IFIM method. These models investigate the mechanism of aquatic habitat enhancement achieved by deploying groups of submerged and non-submerged dikes on the convex and concave banks of the Fuhe River in Chengdu, China. The findings indicate that the implementation of dike groups resulted in an enhancement of approximately 30% in the weighted usable area (WUA) of the specific river. This improvement was accompanied by a 25% decrease in the ecological flow threshold (WUAmax). Furthermore, subsequent comprehensive comparative analyses under various conditions revealed that the fluctuations in enhancement effects were mitigated by flow increases. The degree of submergence parameter exhibits heightened sensitivity to enhancement effects, especially when the flow remains below the ecological threshold. The findings of this study can serve as a reference for the ecological restoration of river systems.

Influence of bottom protection structures group on the hydraulic characteristics in sharp bends

Bottom protection (BP) is a commonly used engineering measure in river training. A 3D hydrodynamic numerical model is established to study the influence of the overall arrangements of BP structures on the hydraulic characteristics of the sharp bends. Under the action of the BP structures group, the maximum longitudinal flow velocity is closer to the water surface vertically and closer to the concave bank laterally compared with no BP. The high shear stress zones on the riverbed at the exit straight section of the bend do not occur. The so-called primary circulation after the apex of the bend gradually weakens and disappears. If the coverage rate of BP is increased to 1.5 times that of the sequential BP, the average longitudinal flow velocity near the bottom of BP decreases by 10% and the average bed shear stress at the outlet section of the bend decreases by 35%. In the staggered BP condition, the average bed shear stress at the outlet section of the bend is reduced by 23% compared with the sequential BP. Moreover, the results show that the protection effects of the dense BP and the staggered BP are better than that of sequential BP.

Analysis of bed changes in the segment of the Tien river flowing through Tan Chau

The segment of the Tien River flowing through Tan Chau town is a meandering section with complex flow patterns, influenced by the flow dynamics and the lack of upstream sediment supply. This has resulted in the formation of deep erosional channels that affect the bank stability. This study focuses on evaluating the bed change of the Tien River within Tan Chau town through the analysis of measurement data and some scenarios from numerical modeling. The findings reveal that the Tan Chau - Hong Ngu curved section experiences severe channel erosion, particularly concentrated in the topographic channel and inclined towards the concave bank of Tan Chau (the inner bank). An analysis of causative factors indicates that reduced sediment supply due to dam construction and sand mining activities have led to changes in the riverbed, deeper erosion, and bank erosion near the sand mining area. The results quantifies the sediment deficit in the area at 0.86%, while the impact of sand mining in this area is 0.26%. The findings from this research provide a database to support local planning for bank protection projects and disaster mitigation measures due to bank erosion.

Geological Hazard Susceptibility Analysis and Developmental Characteristics Based on Slope Unit, Using the Xinxian County, Henan Province as an Example.

Geological hazards in Xinxian County, Xinyang City, Henan Province, are characterized by their small scale, wide distribution, and significant influence from regional tectonics. This study focuses on collapses and landslide hazards within the area, selecting twelve evaluation factors: aspect, slope shape, normalized difference vegetation index (NDVI), topographic relief, distance from geological structure, slope, distance from roads, land use cover type, area of land change (2012-2022), average annual rainfall (2012-2022), and river network density. Utilizing data from historical disaster sites across the region, the information quantity method and hierarchical analysis method are employed to ascertain the information quantity and weight of each factor. Subsequently, a random forest model is applied to perform susceptibility zoning of geological hazards in Xinxian County and to examine the characteristics of these geological disasters. The results show that in the study area, the primary factors influencing the development of geohazards are the distance from roads, rock groups, and distance from geological structure areas. A comparison of the susceptibility results obtained through two methods, the analytic hierarchy process information quantity method and the random forests model, reveals that the former exhibits a higher accuracy. This model categorizes the geohazard susceptibility in the study area into four levels: low, medium, high, and very high. Notably, the areas of very high and high susceptibility together cover 559.17 km2, constituting 35.99% of the study area's total area, and encompass 57 disaster sites, which represent 72.15% of all disaster sites. Geological hazards in Xinxian County frequently manifest on steep canyon inclines, along the curved and concave banks of mountain rivers, within watershed regions, on gully inclines, atop steep cliffs, and on artificially created slopes, among other sites. Areas with very high and high vulnerability to these hazards are mainly concentrated near the county's geological formations. The gneiss formations are widely exposed in Xinxian County, and the gneisses' strength is significantly changed under weathering, which makes the properties of the different degrees of weathering of the rock and soil bodies play a decisive role in the stability of the slopes. This paper provides a basis for evaluating and preventing geologic hazards in the Dabie mountainous area of the South Henan Province, and the spatial planning of the national territory.

The dynamics of marsh-channel slump blocks: an observational study using repeated drone imagery

Abstract. Slump blocks are widely distributed features along marsh shorelines that can disturb marsh edge habitats and affect marsh geomorphology and sediment dynamics. However, little is known about their spatial distribution patterns or their longevity and movement. We employed an unoccupied aerial vehicle (UAV) to track slump blocks in 11 monthly images (March 2020–March 2021) of Dean Creek, a tidal creek surrounded by salt marsh located on Sapelo Island (GA, USA). Slump blocks were observed along both convex and concave banks of the creek in all images, with sizes between 0.03 and 72.51 m2. Although the majority of blocks were categorized as persistent, there were also new blocks in each image. Most blocks were lost through submergence, and both decreased in area and moved towards the center of the channel over time. However, some blocks reconnected to the marsh platform, which has not been previously observed. These blocks were initially larger and located closer to the marsh edge than those that submerged, and they increased in area over time. Only 13 out of a cohort of 61 newly created blocks observed in May 2020 remained after 5 months, suggesting that most blocks persist for only a short time. When taken together, the total area of new slump blocks was 886 m2, and that of reconnected blocks was 652 m2. This resulted in a net expansion of the channel by 234 m2 over the study period, accounting for about 66 % of the overall increase in the channel area of Dean Creek, and this suggests that slump block processes play an important role in tidal creek channel widening. This study illustrates the power of repeated UAV surveys to monitor short-term geomorphological processes, such as slump block formation and loss, to provide new insights into marsh eco-geomorphological processes.

Devising a procedure for integrated modeling of riverbed shape in the area of bridge crossing in order to avoid dangerous washing erosion

The object of research is a riverbed in the area of a bridge crossing, which is subject to the action of high water during the passage of a flood. The selection of parameters and contours of the scheme of the river section was carried out using a mathematical model of the flow and deformation of the bed of the riverbed before and after the change of its shape. It was established that the assignment of parameters of the operational slot should be performed on the basis of the principle of creating natural analogs of riverbed shapes, riverbed and floodplain terraces, with the use of mathematical modeling. To reduce erosion in the area of bridge crossings, it is proposed to regulate the riverbed by constructing a canalized riverbed. Modeling according to this option showed that the erosion in the span of a road bridge decreased and did not exceed 1.5 m. In addition, it was established that a significant role in ensuring the stability of the river is played by the vegetation cover in floodplain areas. Using the method of mathematical modeling, it was established that the area near the convex bank with a width of 150 m is subject to the strongest erosion in the design of the supports. Siltation of the existing left-bank bottom basin is also observed. It is recommended to make a decision to carry out restorative cutting of riverbed alignment annually after receiving a flood forecast from the local hydrometeorological station. In the case of a high flood warning, work should be carried out to clear the sleeves on the approach to the construction of the bridge deck. It was established that the most stable shape of riverbeds is in the form of curvilinear incomplete meandering when the radii of curvature R and the width of the riverbed B satisfy the ratio R=(4...7) B at riverbed-forming flow rate. At the same time, radii of curvature R<3.5 B should not be allowed, as this leads to separation of the flow from the convex bank and excessive erosion of the concave bank

Occurrence and influencing factors of high groundwater manganese in the oxbow lakes of the middle reaches of Yangtze River

Oxbow lake, acting as an efficient factory, plays an important role in the attenuation and accumulation of groundwater contamination. However, there are few concerns on the enrichment of manganese in the oxbow lakes. Based on 113 groundwater samples collected from Wuhan Geological Survey Center, China Geological Survey in 2019, the distribution and influencing factors of high groundwater Mn were studied in the oxbow lakes of the middle reaches of Yangtze River. The results showed that the elevated groundwater Mn were mainly distributed in the concave bank with 75 % of Mn concentration exceeded 0.10 mg/L, and up to 3.79 mg/L. Moreover, groundwater Mn concentrations were related to the depth and the well types. According to principal component analysis, Mn and HCO3–, Mg2+, Fe, NH4-N, Ca2+, As, Eh and NO3– dominated in PC1 with significantly correlations which may result from natural attenuation processes. The sedimentary environments including geological properties and reducing conditions in the studied area were responsible for the enrichment. Not only the dissolution of rich Mn-bearing minerals in the studied area driven by redox conditions provided the source of groundwater Mn but also the unique sedimentary architecture characteristics and the slow flow limited the transfer of dissolved Mn in groundwater, resulting in the enrichment of Mn. This study provides us understand the occurrence and influencing factors of high groundwater manganese in the oxbow lakes/wetlands.

Research on the Protective Effect of Twin-groyne Arrangement on Riverbank

A curved channel with intersecting streams can be easily scoured by incoming flow, and the concave bank is badly damaged. This research showed that the twin-groyne could effectively adjust and optimize the flow velocity distribution, change the shape of the free water surface of the bend, prevent erosion, and promote silting on the concave bank, and it could provide a scouring and silting effect on the convex bank. When the spacing of twin-groyne was increased to more than four times the body length of the single-groyne (spur dike), the protective effect on the concave bank was weakened, and the scouring and silting effect of the convex bank was reduced. Excessive spacing of the twin-groyne could cause local erosion damage to the concave bank. When the distance exceeded the theoretical optimum, it was equivalent to the effect of single-groyne. With the increase in the submergence degree, the velocity of the concave bank decreased first and then increased, while the velocity of convex bank decreased continuously. The protective effect of a non-submerged twin-groyne with a dam spacing of four times the body length of the single-groyne was better than that of other conditions, and it is recommended to be used in practice.

Riverbank Following Planner (RBFP) for USVs Based on Point Cloud Data

Autonomous path planning along riverbanks is crucial for unmanned surface vehicles (USVs) to execute specific tasks such as levee safety detection and underwater pipe inspections, which are vital for riverbank safety and water environment protection. Given the intricate shapes of riverbanks, the dynamic nature of tidal influences, and constraints in real-time cartographic updates, there is a heightened susceptibility to inaccuracies during manual waypoint designation. These factors collectively impact the efficiency of USVs in following riverbank paths. We introduce a riverbank following planner (RBFP) for USVs to tackle this challenge. This planner, utilizing 2D LiDAR, autonomously selects the following point to follow riverbank shapes. Additionally, a PID controller is integrated to compensate for position and yaw errors. Our proposed method reduces the deviation between the USV’s planned path and the actual riverbank shape. We simulated straight, convex, and concave riverbanks in the Virtual RobotX (VRX) simulator while considering the impacts of wind, waves, and USV dynamics. The experimental result indicates the following performance of 96.92%, 67.30%, and 61.15% for straight, convex, and concave banks, respectively. The proposed RBFP can support a novel autonomous navigation scenario for autonomous paths following along the riverbank without any preplanned paths or destinations.

Local Scour around Side-by-Side Double Piers in Channel Bends under Ice-Covered Conditions—An Experimental Study

The pier scour process is normally intensified in the presence of an ice cover, which poses risks to the longevity and safety of bridges. In the present study, the impact of the densimetric Froude number, locations, and pier spacing of side-by-side piers on the local scour depth under ice-covered flow conditions were investigated based on clear water scour experiments in an S-shaped laboratory flume. The results demonstrated that the local scour at piers along the convex bank was more substantial than that along the concave bank when other factors stayed identical. The densimetric Froude number clearly has more impact on local scour at piers along the convex bank than that along the concave bank. Different from the mechanism of the pier scour in a straight channel, the scour depth around a pier along the convex bank in the S-shaped flume increases as the distance between two piers (or pier spacing) increases, while it decreases around the piers along the concave bank. Similar scour patterns were observed when the side-by-side piers were installed at different bend apex cross-sections. The maximum local scour depths at piers along the convex bank measured at different bend apex cross-sections were relatively unchanged when other influencing factors were held constant. However, the maximum scour depth around piers along the concave bank decreased as the bends increased toward downstream.

Morphology Dynamics of Ice Cover in a River Bend Revealed by the UAV-GPR and Sentinel-2

After the formation of the bend ice cover, the ice thickness of the bend is not uniformly distributed, and an open-water area is usually formed downstream of the bend. The spatial and temporal variation of the ice thickness in seven cross sections was determined via Unmanned Aerial Vehicle Ground Penetrating Radar (UAV-GPR) technology and traditional borehole measurements. The plane morphology change of the open water was observed by Sentinel-2. The results show that the average dielectric permittivity of GPR was 3.231, 3.249, and 3.317 on three surveys (5 January 2022, 16 February 2022, and 25 February 2022) of the Yellow River ice growing period, respectively. The average ice thickness of the three surveys was 0.402 m, 0.509 m, and 0.633 m, respectively. The ice thickness of the concave bank was larger than that of the convex bank. The plane morphology of the open water first shrinks rapidly longitudinally and then shrinks slowly transversely. The vertical boundary of the open water was composed of two arcs, in which the slope of Arc I (close to the water surface) was steeper than that of Arc II, and the hazardous distance of the open-water boundary was 10.3 m. The increased flow mostly affected the slope change of Arc I. Finally, we discuss the variation of hummocky ice and flat ice in GPR images and the physical factors affecting GPR detection accuracy, as well as the ice-thickness variation of concave and convex banks in relation to channel curvature.

Effect of a Circular Cylinder on Hydrodynamic Characteristics over a Strongly Curved Channel

Curved channels are one of the most fundamental units of natural or artificial channels, in which there are different kinds of obstacles; these include vegetation patches, bridge piles, electrical tower foundations, etc., which are all present over a channel bend, and can significantly alter the hydrodynamic characteristics of a channel when compared to a bare bed. In this study, laboratory experiments and numerical simulations were combined to investigate the effect of a circular cylinder on the flow characteristics of a 180-degree U-shaped curved channel. Experimental data, including on water depth and three-dimensional velocity, which was obtained by utilizing acoustic Doppler velocimetry (ADV), were used to calibrate and verify the simulation results of the Reynolds-Averaged Navier–Stokes (RANS) model in the FLOW-3D software. Numerical results show that a larger cylinder diameter leads to an overall greater depth-averaged velocity at the section, a greater shear stress acting on the banks on which the cylinder is placed, and a greater increase in the depth-averaged velocity along the concave bank compared to that along the convex bank. When the diameter of the cylinder placed at the 90° section increases, two weaker circulations with the same direction are found near the water surface; for the submerged one, the two weaker circulations appear at the further downstream section, unlike the emergent one. The degree of variation degree in the shear stress acting on the banks is larger than that of the flowrate. As the flowrate increases or the radius of curvature decreases, the secondary flow intensity correspondingly elevates. However, the curvature radius of the curved channel plays a more important role in the secondary flow intensity than the flowrate does. For both the emergent and submergent cylinders, the large cylinder produces a greater secondary flow strength, but the emergent one has a greater secondary flow strength than the submergent one. In summary, the present study provides valuable knowledge on the hydrodynamics of flow around emergent and submergent structures over a curved channel, which could improve the future design of these structures.

Effect of Rigid Vegetation Arrangement on the Mixed Layer of Curved Channel Flow

Curved channels and aquatic vegetation are commonly present in the riverine environment. In this study, the effects of vegetation density and distribution on the hydrodynamic characteristics of a mixed layer developed over a 180-degree curved channel were investigated through flume experiments. Wooden sticks were used to simulate rigid vegetation distributed along the half side of the channel, and a 200 Hz acoustic Doppler velocimeter (ADV) was employed to measure the three-dimensional instantaneous velocity at five selected cross sections along the curved channel. Experimental results show that the vegetation covering the half of the channel significantly affects the hydrodynamic structure of the curved channel flow, and the unequal vegetation resistance induces the K-H instability at the vegetation and non-vegetation interface, resulting in a standard hyperbolic tangent function of streamwise velocity distribution along the lateral direction. The influence of curve position on turbulence kinetic energy is far greater than that of vegetation density and vegetation distribution. The peak value of turbulent kinetic energy is comprehensively affected by vegetation density and distribution, and the peak position of turbulent kinetic energy at the interface is changed by different vegetation distribution. The combined effect of the curve and the partly covered vegetation increases the mixing between the water bodies, enhancing turbulent kinetic energy, and vegetation along the concave bank plays a more significant role. For turbulent bursting, the inward and outward interactions are mainly bursting events in the vegetation area, while ejections and sweeps are dominant in the non-vegetation area. However, the critical vegetation condition to initiate large-scale coherent structure (LSS) in the mixed layer and the influence of flexible vegetation need to be further studied in the future.

Flow Structure and Short-Term Riverbed Evolution in Curved Flumes

River bending is the major effect responsible for bed topography and bank changes. In this study, fluid velocity (measured by a three-dimensional Doppler advanced point current meter) and bed topographical data have been collected in 40 sections of an experimental model. The whole flume was composed of an organic glass bend, upstream and downstream water tanks, two transition straight sections, a circulation pump, and a connection pipeline. Each section has been found to be characterized by a primary circulation and a small reverse circulation, with some sections even presenting three more or more circulation structures. The minimum circulation intensity has been detected in proximity to the top of the curved channel, while a region with small longitudinal velocity has been observed near the concave bank of each bend, corresponding to the flat bed formed after a short period of scouring. The maximum sediment deposition and scour depth in the presence of a uniform distribution of living flexible vegetation within 10 cm of the flume wall have been found to be smaller than those observed in the tests conducted without vegetation.

Numerical Investigation of Hydrodynamics in a U-Shaped Open Channel Confluence Flow with Partially Emergent Rigid Vegetation

The effects of partially emergent rigid vegetation on the hydrodynamics of a curved open-channel confluence flow were simulated using OpenFOAM. The numerical model using the Volume of Fluid method and the RNG k-ε turbulence model in the Reynolds-averaged Navier–Stokes equations was first validated by existing experimental data with good agreement. Then the characteristics of hydrodynamics were analyzed in aspects of separation zone, water level, streamwise velocities, secondary flows, bed shear stress and flow resistance. Some main conclusions can be drawn from the results. Compared to the non-vegetated cases, the separation zones in vegetated cases are smaller in both length and width. With higher vegetation Solid Volume Fraction (SVF), the separation zone is divided into two parts, a smaller one right after the confluence point and a larger one on the second half of the curved reach after the confluence. The main circulation cell shrinks and the circulation near the concave bank moves towards the channel midline. The differences in velocities and bed shear stress between the convex and concave banks become larger with a higher SVF. Under the same SVF, a larger vegetation density has more disturbance on the tributary than a larger stem diameter.

Numerical study of flow characteristics in compound meandering channels with vegetated floodplains

Large eddy simulations were conducted to simulate the flow in compound meandering channels whose main channel sinuosity was 1.381. Then, the floodplain vegetation was generalized using the momentum equation coupled with the drag force formula. The mean flow pattern, secondary flow, coherent structure, turbulence characteristics, and lateral mass and momentum transport with and without floodplain vegetation with relative depths (Dr) of 0.3–0.5 were studied. Results showed that the floodplain vegetation enabled the flow of the main channel to be more concentrated. The maximum average velocity in the cross section of the main channel increased by 100% and 30% when the relative depth was 0.3 and 0.5. Under the influence of floodplain vegetation, the secondary flow cell transformed greatly with the change in relative depth. When Dr < 0.3, the vegetation caused the vortex center of the secondary flow to move closer to the concave bank side, and the secondary flow distribution presents a flow pattern not flooding the floodplain. When Dr > 0.3, the spatial change in the secondary flow was not obvious. In addition, the floodplain vegetation did not change the large-scale vortex that was separated from the boundary layer of the convex bank side. Meanwhile, the floodplain vegetation increased the overall turbulence intensity, turbulent kinetic energy, and Reynolds stress of the main channel, and it increased the range of lateral mass exchange of the inbank flow and the mean and turbulent transport flux of each cross section.

Meandering development of tidal creeks splits bacterial communities and functional characteristics: Evidence from a field study in Jiangsu Coast, China

Tidal creeks are common and important geomorphic features in intertidal zones, which provide a broad range of ecosystem services. The meandering development of tidal creeks can cause a discrepancy in sediment properties between the two banks in tidal creek bends, which affects a few geomorphic and biological processes and seems to be another incentive for the ecological effects of tidal creeks. However, how this discrepancy affects the distribution of bacterial communities in tidal creek bends remains unclear. To this end, we collected sediment samples from a typical tidal creek bend in Jiangsu Coast, China. The bacterial communities were characterized by 16S rRNA gene sequencing and the sediment properties were determined. Results showed that bacterial communities and their functions exhibited significant differences between the convex bank (VB) and the concave bank (CB). Bacterial diversity was higher in VB than in CB, mainly attributed to higher nutrients (TOC and TN), moisture, and finer sediment in VB. The oligotrophic (e.g., Chloroflexi), drought-resistant (e.g., Actinobacteria), and mobile (e.g. Nitrospirae) bacteria had higher abundances in CB. In contrast, bacteria with opposite niche preferences (e.g., Proteobacteria and Acidobacteria) tended to inhabit VB. Network analysis revealed that CB bacterial communities had a higher efficiency of nutrient cycling and carbon uptake but were more vulnerable to environmental disturbances. FAPROTAX results showed that bacterial functions associated with carbon cycles were more abundant in CB, whereas those associated with nitrogen and sulfur cycles were more abundant in VB. Overall, these results demonstrate that the meandering development of tidal creeks splits bacterial communities and functional characteristics in tidal creeks bends. These findings are expected to provide useful information for further understanding and predicting the ecological effects of tidal creeks.

Morphological evolution mechanism of Gravel-bed braided river by numerical simulation on Da-jia River

• “central bar formation” and “channel avulsion” are the main river evolution mechanism in the simulated gravel-bed reach. • As the river width expands downstream, the longitudinal profile gets stable, while the transverse oscillation becomes wild. • If the longitudinal structure (bank revetment) on the concave bank of a gravel-bed river is made by concrete with relatively smooth surface, it is very likely to cause further damage on the longitudinal structure. Due to a steep slope, rapid flow, and large amounts of sediment, rivers in Taiwan are braided rivers that are full of bars and bifurcated flows. The channel shifting behavior and bar migration within natural braided rivers results in difficult river engineering. In this study, the Nays2DH model was used to simulate the braiding processes and morphological changes downstream of Shigang Dam in the Da-jia River from 2012 to 2013. Furthermore, the characteristics of riverbed erosion, the stability of the channel in the longitudinal and transverse directions and the impact of river structures on the morphology were simulated. The numerical model can present morphological and evolutionary processes similar to an actual river, such as: central bar formation, chute cutoff, and channel avulsion. The scour pool mostly occurred at the apex of the river bend and downstream of the bar (confluence) in a braided river, which means that bar migration is the main factor that alters scour position. Under the assumption of equilibrium sediment transportation across the upstream inflow section, the width of the channel determines whether the river demonstrates longitudinal erosion or transverse changing. Results from this research can be used as a substantial reference for future research concerning river morphology evolution, river engineering, and river space planning.