Natural River Research Articles

Комплексная оценка экологического состояния участка трансграничной реки Нарвы

A comprehensive study of the ecological state on the transboundary Narva River (Russia –Estonia border) downstream of Ivangorod, Kingisepp District, Leningrad Region, was carried out. It included the determination of chemical indicators of pollution of the natural river water, with an emphasis on some heavy metals content in water, sediments and soft tissues of local filter-feeding mollusks; the latter were used as bioindicators of river pollution in this location. Zn was determined in maximum concentrations in bottom sediments, then Cu, to a lesser extent Pb, Cd and Ni. The coefficient of metal bioconcentration calculated in soft tissues of mollusks showed maximum for Zn and Cu, to a lesser extent for Pb, Cd and minimum for Ni. However, such values have no negative effect on local biota as shown by physiological testing of bivalves from study site. The functional assessment of bivalves by non-invasive cardiac rhythm recording made it possible to characterize their functional status and adaptive capabilities as good. The relative abundance of hydrocarbon-oxidizing bacteria in the digestive tract of mollusks from the Narva River has been determined as an indicator of environmental pollution by oil products. It was noted that the number of hydrocarbon-oxidizing bacteria does not exceed the values characteristic of the reference waters of the Eastern Gulf of Finland. Based on the totality of the studies, it was concluded that the river waters in the studied location can be characterized as slightly polluted and its ecological status as of a good quality.

Constructed wetland-based wastewater treatment of a coffee-washing plant and its impacts: a case study of Kege processing plant, Ethiopia

Abstract Coffee washing units in Sidama Regional State, Southern Ethiopia pose a serious threat to surface water sources and coffee growing soils. A comparison field performance test was conducted on the capacity of natural and constructed wetland on wastewater treatment. The study indicates that treating the effluent from the processing plants using constructed wetland supported by sedimentation pond, filtration, and gravity aeration can reduce pollutant levels significantly. The treated waste water can be safely disposed-of to the natural river. It was observed that BOD5 was reduced by 90 and 96% in natural and constructed wetlands, respectively. TDS was reduced substantially whereas nitrate was reduced by 63%. Soil quality parameters from the wetland are within the permissible limits of Ethiopian Environmental Protection Agency. Heavy metals from coffee beans produced from the surrounding farms using water from Gidabo River discharged with treated effluent from the processing units are within the permissible limits. It is evidenced that wetlands in general and constructed wetlands in particular are promising technologies to treat the effluent water and recommended to be adopted by all the processing units in the region.

Optimizing reservoir operation incorporating ecological demand and stability requirement

AbstractThe construction and operation of large‐scale reservoirs alter the natural river flow regimes and destroy the original suitable reproduction and living conditions of aquatic organisms. Especially during the spawning season, the fluctuating water flow process is particularly important for fish reproduction. A bi‐objective optimization model considering both the environmental flow demand for fish spawning and the stability of hydropower output is established in this study and solved by one of the most widely used algorithms for multi‐objective problems, the Non‐Dominated Sorted Genetic Algorithm‐II. The derived Pareto Front can be used to guide reservoir operation during fish spawning seasons. The model proposed in this paper is applied to the Yangqu Reservoir, currently under construction on the upper Yellow River in China. The results show that the stability of hydropower output can still be improved by 4.80% to 10.56%, even if the environmental flow demand is taken into account. Even for the scheme preferring ecological recovery, the stability of hydropower output can be increased by more than 0.85%.

Comparison of fresh and hardened properties of concrete made with various artificial and natural sands

Concretes made with different fine aggregates (natural river sand, artificial crushed sand, artificial gravel sand and a hybrid sand (50% river sand and 50% gravel sand by mass)) were experimentally investigated. The concretes were found to have little differences in compressive strength and elastic modulus (less than 6.4%). Compared with the concrete made with natural river sand, the concretes made with artificial sands demonstrated higher flexural strengths (3.5–11.1%) and splitting strengths (4.4–12.8%) at 28 days, but lower workability (21.4–35.7%), water permeation resistance (34.3–53.7%) and abrasion resistance (2.9–20.6%). The compressive, flexural and splitting tensile strengths of the concretes increased with time under water curing. Under sulfate attack, the compressive strengths of the concretes were enhanced at 56 days but reduced at 90 days. In comparison with water curing, the reductions in compressive strength due to sulfate attack were 2.6–7.7% at 56 days and 11.8–24.4% at 90 days. Weibull distribution analysis was conducted to explore the sensitivity of concrete strength to the type of fine aggregate.

Erosion mechanism of point bar retreat under the protection of a flexible mattress

Channel retreat can be responsible for the significant loss of banks, farmland, and wetlands, leading to drastic changes in fluvial sediment and local river regimes. Although current studies focus on the erosion process of natural river channels, the mechanism by which revetments, such as the flexible mattress, influence bank evolution is still unclear. Hence, by conducting a generalized model experiment, this study investigates the point bar failure process under the mattress protection, i.e., the episodic event when the soil reaches the static equilibrium state. Specifically, a scour hole develops at the junction between the soft and hard materials, causing mattress suspension on the bank toe’s side wall and resulting in a reduction coefficient for transverse scouring rate ranging from 0.08 to 0.15. Based on the theories of soil mechanics and river dynamics, the critical conditions for point bar instability were deduced, and a mechanical model describing its erosion process under the mattress protection was established. Furthermore, our model calculated the bank morphology and total erosion volume at different periods in the flume experiment, demonstrating a good agreement with the measured data. Additionally, variations in stability coefficient and forces exerted on soil (including shear strength, gravity, and fluid pressure) of the typical sections during point bar retreat were analyzed. Sensitivity analysis of the bank toe stability emphasized the controlling effect of soil mechanical properties and the negative feedback of mattress weight. The results reveal the interaction mechanism between the mattress protection and point bar failure, theoretically guiding the bank erosion strengthening and river management planning.

Heterogeneity and influencing factors of algal blooms in the reservoir-impacted tributary: Evidence from remote sensing and physical-based model

The construction of reservoirs often caused algae blooms but there is controversy over pattern and attribution of blooms in reservoir-impacted tributaries due to lacking of detailed records. Herein, the remote sensing data with resolution of 10-meter pixels and physics-based model were combined to explore spatiotemporal distribution of algae blooms in key tributaries of Three Gorges Reservoir, China. Self-organizing maps were used for identifying the drivers of blooms. The results revealed that algae blooms were more serious during dry years and there was little difference between dry or wet years for multi-stage gate river with coverage remaining at 42.90%. The open waters with slow currents were identified as high-risk zones. Hotspots of algae blooms changed during the year, which were close to estuary in spring, upstream in autumn, and dispersed in backwater region in summer. Water level was major measure for managing algae blooms, while meteorological conditions were identified as the key factor (40.25%) for river with multi-stage dams and characteristics of shallow lakes. For the nutrients-rich river, blooms were driven by both water level (13.98%) and climate factors (23.34%). The blooms of natural backwater river were stimulated by combination of water level, climate and waterhed nutrients. These results highlighted necessity of region water quality management in reservoir-impacted regions.

Flexural fatigue properties of concrete based on different replacement percentage of natural sand with manufactured sand

Compression and flexural fatigue tests were conducted on concrete specimens with varying strengths under different replacement percentages of natural river sand (R-sand) with manufactured sand (M-sand) at 0%, 30%, 70%, and 100%. The objective was to compare and analyse differences in fatigue damage of concrete across different replacement percentages. Findings were made that when the replacement percentage of M-sand was 30% and 70%, the concrete exhibited higher fatigue life, irrespective of any variations in compressive strength. The fatigue life of concrete with a compressive strength of 30 MPa could be increased by an average of 20%–30% compared to concrete with a compressive strength of 50 MPa, and this increasing trend reached its peak when the replacement percentage of M-sand was in the range of 30%–70%. In order to enhance the prediction of fatigue life for concrete subjected to varying replacement percentages of natural sand with M-sand and to elucidate the evolution process of fatigue damage in M-sand concrete, dynamic strain gauges and DIC methods were employed. This enabled determination of the cycle-varying maximum strain and residual strain during the fatigue process across different replacement percentages. The findings indicate that the addition of M-sand can significantly delay the degradation of dynamic elastic modulus (E), with this effect being enhanced by over 15%. The fatigue damage was defined based on fatigue deformation modulus; therefore, a new fatigue damage model was established. It was observed that this new model improved accuracy by up to 6.1% compared to the traditional fitted model in this test. Finally, the effects of different replacement percentages of natural sand with M-sand on the brittle transition of concrete were analysed at the microscopic level. This analysis involved a combination of microhardness testing and examination of the interfacial transition zone and hydride map of concrete under a scanning electron microscope across different replacement percentages of natural sand with M-sand.

Effects of sediment characteristics on the erosion of Pelton turbine

As Pelton turbines are increasingly used in high mountain regions because of their unique advantage of high-water head adaptability, the hydraulic characteristics and multi-phase flow behaviour in Pelton turbines have attracted the attention of many scholars. Most notably, the presence of solid particles such as sand exists in natural river systems, which end up in the turbines, causing erosion. This has become an international academic issue which has not been fully developed yet. In this study, three-phase simulations based on Eulerian–Lagrangian methodology are conducted to investigate the erosion phenomenon, especially in the bucket region. Sediment particle characteristics such as size and concentration in both the distribution system and buckets region are analysed. The innovation of this study is that, unlike other erosion investigations, it focuses on the impact angle of the jet flow as this is a unique characteristic of Pelton turbines. Furthermore, the sediment particle's flow characteristics at different bucket rotating angles and how they influence the erosion mechanism are also investigated. Results reveal that larger particles cause greater erosion on the bucket. In small particle and standard diameter conditions, the impact angle is mostly below 5 ∘ . Finally, this study presents the first successful attempt in using rotating angle to describe the erosion in a Pelton turbine. A corrected erosion model adapted to the Pelton turbine is given through regression and proves effective in predicting erosion.

Neonicotinoid insecticides in waters of the northern Jiangsu segment of the Beijing-Hangzhou Grand Canal: Environmental and health implications

Neonicotinoid (NEO) insecticides have been frequently detected in natural aquatic environments. Nevertheless, the distribution of NEOs in artificial environments is not clear. The Beijing-Hangzhou Grand Canal is the longest canal in the world. The northern Jiangsu segment of the Grand Canal was selected to study the spatiotemporal variation and source of eight NEOs in the canal water and assess their ecological and health risks. The total NEO concentration in the canal water was 12–289 ng L−1 in the dry season and 18–373 ng L−1 in the wet season, which were within the concentration range in other 11 natural rivers worldwide. The average total NEO concentrations were not statistically different between the seasons; only the concentrations of imidaclothiz, thiacloprid (THI), acetamiprid, and dinotefuran were different. At city scale, the total NEO concentration in the dry season showed a decreasing trend along the water flow from Xuzhou City to Yangzhou City. The total NEO concentrations were found to be positively correlated with the sown area of farm crops and the rural labour force, indicating the agricultural influence on the spatial distribution of NEO concentrations. In the wet season, relatively high NEO concentrations were distributed in downstream sites under the influence of artificial regulation. The primary contributor to the NEO inputs into the canal was the nonpoint source in the dry and wet seasons, with a relative contribution of 68 %. THI, imidacloprid, clothianidin and thiamethoxan would produce chronic ecological risks in both seasons. Further consideration needs to be given to the above four NEOs and NEO mixtures. The human health risks that NEOs posed by drinking water were assessed based on the chronic daily intake (CDI). The maximum CDI for adults and children was lower than the reference doses. This suggested public health would not be at risk from canal water consumption.

Effect of equilibrium and non-equilibrium sediment transport flows on the shear velocity in an open channel.

Equilibrium and non-equilibrium sediment transport flow in natural rivers and artificial channels can result in degradation or aggradation of the river bed, which will affect many hydraulic parameters, one of which is shear velocity. This study evaluates the influence of sediment transport on shear velocity in open channels. In this study, laboratory experiments were carried out using a sediment-recirculating flume, and measurements of the velocity and Reynolds shear stress distributions were measured using a Sontek-acoustic Doppler velocimeter (ADV). This experimental study measured 120 velocity distributions, and Reynolds stress profiles, consisting of 12 different sets of measurements involving flow with sediment transport and movable sand bed. The results showed an average shear velocity, u*, difference between sediment feeding and without sediment feeding by 9.41%. For the integration constant Br value, the average difference between sediment feeding and without sediment feeding was 5.58%.

Closed-form fracture model for evaluating crack resistance of 3D printed fiber-reinforced alkali-activated slag/fly ash recycled sand concrete

To reduce resource consumption and greenhouse gas emissions, 3D printed alkali-activated slag/fly ash recycled sand (RS) concrete (3DP AARSC) was developed in this paper. The reasonable evaluation of its crack resistance is crucial to structural safety and stability since steel bars are still difficult to embed in 3DP samples automatically so far. The realistic fracture parameters can be responsible for the reasonable evaluation and are extremely difficult to predict accurately based on traditional methods unless the specimen sizes are large enough. Therefore, the purpose of this study is to determine the size-independent tensile strength (ft) and fracture toughness (KIC) through a predictive fracture model incorporating the heterogeneity and discontinuity of 3DP AARSC. First, fracture tests of small-sized specimens were conducted to clarify the fracture mechanisms of 3DP AARSC in different loading directions. Subsequently, based on the boundary effect model, the size-independent ft and KIC were explicitly correlated to the maximum fracture load (Fmax) in different loading directions and directly obtained if Fmax was measured from the tests. The results displayed that the predicted ft and KIC of 3DP AARSC loaded perpendicular to the printing direction were significantly higher than the counterparts of concrete loaded parallel to the printing direction by 108.7% at most. The greatest cracking resistance was demonstrated in the specimens with the RS replacement rate of 60% for natural river sand (NS), resulting in the maximum increase of 19.9% in ft and KIC compared with the 3DP concrete with 100% NS. The fiber addition and partial RS replacement increased the difference in the crack resistance along various loading directions. Moreover, the fiber incorporation had no significant improvement in the crack resistance of 3DP AARSC loaded parallel to the printing direction and apparently increased its ft and KIC along other loading directions.

REA-FM: automated generation of natural-looking flow maps through river extraction algorithm

ABSTRACT A flow map is a type of thematic visualization that depicts the movement of objects across a geographical space using a tree layout resembling a natural river system. In this paper, we introduce an innovative and automated approach called REA-FM, which leverages the power of the maze-solving algorithm to extract rivers from digital elevation models (DEMs). This enables the creation of flow maps that originate from a single source and extend to multiple destinations. Initially, REA-FM represents the mapping space of a flow map using a DEM. Subsequently, a maze-solving algorithm is adapted to extract flow paths from the destinations to the origin within the DEM data, with constraints on search directions, direction weights, and search ranges based on quality criteria specific to flow maps. To obtain comprehensive flow maps, the maze-solving algorithm is employed iteratively, considering the importance of each flow path, as determined by their respective lengths. These obtained paths are finally rendered smoothly with varying widths using Bézier curves, thereby enhancing the visual aesthetics of the flow map. A comparative evaluation with existing approaches demonstrates that REA-FM can generate natural-looking flow maps with reduced total length and improved node distribution, eliminating node overlaps and edge crossings. Furthermore, the effectiveness of REA-FM is validated through three extension experiments involving heterogeneous mapping spaces and areas with obstacles. Parameter analysis confirms that REA-FM offers intuitive control over the layout of flow maps. Project website: https://github.com/TrentonWei/FlowMap

Responses of annual streamflow variability to annual precipitation, extreme climate events and large-scale climate phenomena in the Qinghai-Tibet Plateau

Solar activity and large-scale climate phenomena have significant effects on extreme climate events and streamflow variability. As the roof of the world and the water tower of Asia, the Qinghai-Tibet Plateau (QTP) is highly sensitive to climate change. Therefore, it is of great significance to study the relationship between extreme hydrometeorological events of the QTP and climate change for global hydroclimate research. In this study, we analyzed the spatiotemporal variation and significant oscillation period of several hydrometeorological variables such as extreme precipitation indices (EPIs), extreme temperature indices (ETIs) and annual streamflow variability based on the observation data of hydrometeorological stations in the QTP during 1962–2019 using Sen’s slope estimator, Mann-Kendall test and continuous wavelet analysis (CWT). And the teleconnection patterns and the lead-lag relationship between annual precipitation, extreme climate events, multiple nonstationary climate mode and annual streamflow were evaluated using wavelet coherence (WTC). The results show that the climate of the QTP has been wetter and warmer in the past 58 years. The annual streamflow of several natural rivers in the QTP mainly show an increasing trend over the last 60 years. The annual streamflow of natural river (except Yellow River) increases significantly in three-rivers headwater region, and the rivers in the northeast and northwest also show a significant increase trend, but there is no significant change in the natural river streamflow in the south of QTP. In addition, the annual streamflow of natural rivers had significant periodicities in the bands of 2–4 years, 4–7 years and 7–11 years. The significant coherence of WTC spectra between annual precipitation and streamflow indicate that precipitation is the main reason for annual streamflow variation in the QTP. The variation of extreme climate events significantly regulates the regional annual streamflow. All EPIs except consecutive dry days show an in-phase correlation with annual streamflow of most headwater. Annual streamflow of Naijin River is positively and negatively correlated with extreme warm and extreme cold, respectively, and has a clearer lead-lag relationship than that with EPIs. Annual streamflow of headwater has higher correlations with EI Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) than with sunspot number (SSN). Solar activity may first affect ENSO and PDO, and then this effect is transferred sequentially to the hydrological cycle of the QTP by SSN-modulated PDO and PDO-SSN co-modulated ENSO. It is of great significance to investigate the response of streamflow variation to annual precipitation, extreme climate events and large-scale climate phenomena for water resources management, flood control, ecosystem restoration, and the well-being of surrounding residents and sustainable development in the QTP.

Behavior of large-scale LRS FRP-confined steel-reinforced recycled aggregate concrete square columns under axial compression

The combination of environmentally friendly large rupture strain fiber-reinforced polymers (LRS FRPs) and recycled fine and coarse aggregates presents a promising alternative to overcome the scarcity of natural river sand and natural gravels. Recycled aggregate concrete (RAC) can obtain sufficient ductility and strength by the confinement of LRS FRPs, and these two green materials are particularly suited to sustainable construction. However, existing research on LRS FRP-confined RAC is very limited and primarily focused on small-scale specimens. The objective of this paper is to investigate the axial compression performance of large-scale steel-reinforced RAC square columns wrapped with LRS FRP and promote the practical application of this sustainable structural system. The experimental variables include the effective confinement stiffness ratio (ρk,eff) and the type of LRS FRP (i.e., polyethylene naphthalate (PEN) FRP and polyethylene terephthalate (PET) FRP). Test results showed that the application of RAC had a negligible effect on the axial compression behavior of large-scale LRS FRP-confined square columns. The behavior of the test columns was primarily influenced by ρk,eff. Finally, evaluations were conducted on the existing models of LRS FRP-confined concrete with square cross-sections, based on the compilation of available test results from large-scale specimens.

Effect of sand–precursor ratio on mechanical properties and durability of geopolymer mortar with manufactured sand

Abstract The geopolymer mortar (GPM) prepared from industrial by-products and alkali activation solution (AAS) is one of the hot spots of current building materials. As a feasible alternative to natural river sand, manufactured sand (MS) alleviates the global ecological pressure. In this study, MS was used for fine aggregate. Sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solution were used as AAS. Metakaolin (MK) and fly ash (FA) were used as the precursor to prepare MK-FA-based GPM with MS (MS-GPM), which was of great significance for saving non-renewable resources, mitigating the greenhouse effect, and recycling waste. Numerous studies were conducted to explore the effect of sand–precursor ratio (r sp) on mechanical and durability characteristics of MS-GPM. Relationships between compressive strength and tensile or flexural strength were established by linear fitting equation. Finally, analysis of variance (ANOVA) was used to systematically calculate the effect of r sp on performance. The results indicated that the mechanical strength and impermeability of MS-GPM decreased and crack resistance increased with r sp from 1 to 5. The strength of MS-GPM was the best when r sp was 1. With the increase of r sp, the proportion of MS in MS-GPM increases, and the relative cementitious material decreases, which has an adverse impact on mechanical properties and impermeability. Linear fitting revealed that the compressive strength of MS-GPM was closely related to tensile strength and flexural strength. ANOVA results indicated that r sp in the range of 1–5 had great effects on the performance of MS-GPM. The aim of this article is to further promote the possibility of applying MS-GPM in practical engineering by designing reasonable r sp.

Recycling of municipal solid waste incineration bottom ash (MSWIBA) particles into natural fine sands for sustainable engineering cementitious composites

The manufacture of engineered cementitious composites (ECCs) consumes large amounts of natural fine aggregate. In this study, replacement of the natural fine aggregates in ECCs with municipal solid waste incineration bottom ash (MSWIBA) was found to reduce the consumption of natural river sand and the negative environmental impact of MSWIBA disposal. Previously, scholars have shown the potential of using MSWIBA particles as fine aggregates in concrete, but few scholars have evaluated ECCs with high percentages of MSWIBA powder in place of the natural fine aggregates. This study was conducted to design green ECCs by incorporating MSWIBA (BA–ECCs) to completely replace the natural silica sand with various fibers (PVA, PP and PE) and different fiber volume blends (0%, 0.5%, 1%, 1.5% and 2%) and to evaluate the BA–ECCs based on the fluidities, mechanical properties, microscopic morphologies, pore size distributions and heavy metal leaching capacities. The results showed that the addition of MSWIBA increased the porosities of the BA–ECCs. Compared with those of the control group, the maximum flexural strength of the BA–ECCs with 1.5% fiber doping were approximately 27.75% higher. There were relationships between the compressive strength and flexural strength and between the fiber volume content and strength. Moreover, microstructural observations revealed that the MSWIBA powder interacted strongly with both the matrix and the fibers, and the performance of the MSWIBA powder in the matrix was similar to that of a lightweight aggregate during internal curing. Additionally, fiber agglomeration (winding) at a 2% fiber content reduced the compressive and flexural strengths. The BA-ECCs showed lower heavy metal leaching contents than the MSWIBA, and the concentrations of various heavy metals were below the Chinese standards. Overall, this study could support research on the use of MSWIBA particles to prepare sustainable ECCs and recycle MSWIBA particles.

River video flow measurement algorithm with space-time image fusion of object tracking and statistical characteristics

River flow monitoring serves as a crucial data source in hydrological stations, employing the widely adopted space-time image velocimetry (STIV) method both domestically and internationally. However, current methods yield space-time (ST) image trajectories with limited usable information, particularly due to the interference of complex flow characteristics and harsh outdoor environments in natural rivers. This interference leads to errors in detecting the main orientation of texture, consequently reducing measurement accuracy. To overcome these challenges, this paper proposes an river video flow measurement algorithm with ST image fusion of object tracking and statistical characteristics. The Lucas–Kanade tracking algorithm is employed to track velocity points and generate tracking trajectories. These trajectories’ coordinates are then fitted into straight lines to automatically arrange velocity-measuring lines in natural rivers. The algorithm leverages multi-scale detail boosting and Gaussian directional stretch filtering to enhance the quality and texture trajectories of ST images. Furthermore, background noise filtering and stripe noise filtering are applied to denoise the ST images, effectively mitigating the impact of noise caused by the natural river environment and camera equipment. The proposed algorithm incorporates projection statistical characteristics algorithms to accurately detect the dominant texture direction, enabling precise calculation of mean velocity and cross-sectional discharge of the river. Experimental validation was conducted in both natural and regular river channels, demonstrating the superior performance of the proposed solution compared to widely used large-scale particle image velocimetry, STIV, MobileNet-STIV and FD-DIS-G algorithms. The algorithm exhibited higher accuracy, stability, and applicability, with errors at each velocity-measuring point below 10%, and relative errors of the calculated mean velocity and cross-sectional discharge below 3%. This affirms the algorithm’s superior accuracy, stability, and applicability.

Adaptive river flow measurement method based on spatiotemporal image velocimetry and optical flow.

This paper proposes an adaptive river discharge measurement method based on spatiotemporal image velocimetry (STIV) and optical flow to solve the problem of blurred texture features and limited measurement accuracy under complex natural environmental conditions. Optical flow tracking generates spatiotemporal images by following the flow mainstream direction of rivers with both regular and irregular natural banks. A texture similarity function filtering method effectively enhances spatiotemporal texture features. The proposed method is applied to a natural river, with measurement results from a propeller-type current meter used as truth values. It is evaluated and compared with three other methods regarding measurement accuracy, error, and other evaluation indices. The results demonstrate that the method significantly improves spatiotemporal image quality. Its estimation outcomes perform better across all evaluation metrics, enhancing the adaptability and accuracy of the flow measurement method.

Timescale of the Morphodynamic Feedback Between Planform Geometry and Lateral Migration of Meandering Rivers

AbstractAcross varied environments, meandering channels evolve through a common morphodynamic feedback: the sinuous channel shape causes spatial variations in boundary shear stress, which cause lateral migration rates to vary along a meander bend and change the shape of the channel. This feedback is embedded in all conceptual models of meandering channel migration, and in numerical models, it occurs over an explicit timescale (i.e., the model time step). However, the sensitivity of modeled channel trajectory to the time step is unknown. In numerical experiments using a curvature‐driven model of channel migration, we find that channel trajectories are consistent over time if the channel migrates ≤10% of the channel width over the feedback timescale. In contrast, channel trajectories diverge if the time step causes migration to exceed this threshold, due to the instability in the co‐evolution of channel curvature and migration rate. The divergence of channel trajectories accumulates with the total run time. Application to hindcasting of channel migration for 10 natural rivers from the continental US and the Amazon River basin shows that the sensitivity of modeled channel trajectories to the time step is greatest at low (near‐unity) channel sinuosity. A time step exceeding the criterion causes over‐prediction of the width of the channel belt developed over millennial timescales. These findings establish a geometric constraint for predicting channel migration in landscape evolution models for lowland alluvial rivers, upland channels coupled to hillslopes and submarine channels shaped by turbidity currents, over timescales from years to millennia.

Effect of the transition section on the flow structure of consecutive river bends with point bars

The transition section (TS), linking the front bend and the back bend in consecutive river bends, plays a key role in the flow hydraulic characteristics, sediment transport, and riverbed evolution in meandering rivers, which is crucial for flood control, navigation, and river stability. Most existing studies on the hydraulic effect of the TS focus on flat-bed scenarios, rendering an inadequate understanding of the TS in consecutive bends with point bars, often found in natural rivers. Based on three-dimensional numerical simulations, the effects of the length and the topography of the TS are investigated in detail on the flow structure in consecutive bends with point bars, respectively. The results reveal that without an adequate TS length, the residual circulation of the front bend (RC) inhibits the development of the main circulation in the back bend (MC), causing the positions of the maximum strength of the MC and the maximum value of cross-section-averaged turbulent kinetic energy (<tke˜>) in the back bend moving upstream. As the length of the TS increases, the above effect of the front bend gradually decreases. The critical length of the TS to fully decay the RC is 4.8 times the bend width (B) in consecutive bends with point bars, longer than that (4B) in the flat-bed bend. Furthermore, the central bar in the TS is found to facilitate the generation of the outer bank cell and accelerate the decay of the RC. These effects of the TS can profoundly change the flow hydraulics, sedimentation, and stream stability of consecutive bends. The results of the study will help to further understand the linkage role of the TS in consecutive bends and may serve as a rational reference for managing natural meandering rivers with multiple hydrological, geomorphological, and/or ecological goals.