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Compound channels are a common hydraulic section of many alluvial and fluvial systems, serving as conduits for the flow of water and sediment. The proper design of these structures requires an understanding of the entrainment and mixing mechanisms in them. This paper presents a three-dimensional numerical study of lock-exchange flow in a compound channel covering the Reynolds numbers (Re) range of 17 000–39 000. This study investigates flow structures and mixing patterns for various geometries and flow characteristics. The results show that the entrainment coefficient decreases as the dense flow progresses, and the amplitude of instantaneous mixing fluctuations decreases with an increase in the width of the floodplain and reduced gravity. The averaged flux entrainment profiles show a minimum at Re = 2 7 0 0 0 at different times in the front area. In the head area also, a downward concavity can be seen in the profiles around Re = 2 7 0 0 0. This study also reveals that changes in floodplain height do not affect the establishment of lobe-and-cleft instability. The wider the floodplain channel, the larger the lobe-and-cleft instability created. Small-scale mixing dominates at high Reynolds numbers, leading to higher fluid mixing. [ABSTRACT FROM AUTHOR]

In the study of natural waterways, the use of formulas such as Manning's equation is prevalent for analyzing flow structure characteristics. Typically, floodplains exhibit greater roughness compared to the main river channel, which results in higher flow velocities within the main channel. This difference in velocity can lead to increased sedimentation potential within the floodplains. Therefore, accurately determining Manning's roughness coefficient for compound channels, particularly during flood events, is of significant interest to researchers. This study aims to model the Manning roughness coefficient in compound channels with both converging and diverging floodplains using advanced soft computing techniques. These techniques include a multi-layer artificial neural network (MLPNN), Group Method of Data Handling (GMDH), and the Neuro-Fuzzy Group Method of Data Handling (NF-GMDH). For the analysis, a dataset from 196 laboratory experiments was used, which was divided into training and testing subsets. Input variables included parameters such as longitudinal slope (So), relative hydraulic radius (Rr), relative depth (Dr), relative dimension of flow aspects (δ*), and the convergent or divergent angle (θ) of the floodplain. The relative Manning roughness coefficient (nr) was the output variable of interest. The results of the study showed that all the models performed well, with the MLPNN model achieving the highest accuracy, characterized by R² = 0.99, RMSE = 0.001, SI = 0.0015, and DDR = 0.0233 during the testing phase. Further analysis of the soft computing models indicated that the most critical parameters influencing the results were Sr, Rr,Dr*, and θ. These findings highlight the effectiveness of soft computing techniques in accurately modeling the Manning roughness coefficient in complex channel conditions and provide valuable insights for future research and practical applications in the management of flood events and waterway analysis. [ABSTRACT FROM AUTHOR]

This paper studies the phenomenon of hydraulic jump in compound channels using a numerical model and provides remarkable results. Several values of the opening parameter, h1, are utilized to generate the hydraulic jumps. A recirculation zone is detected by studying the RNG-K-ε turbulent Volume Of Fluid (VOF) model, which is distinguished by the modified directions of the velocity vectors. When compared with the experimental values, the numerical simulation demonstrated very good accuracy, with an error of no more than 9.4%. The results underline the reliability and usefulness of the VOF turbulence model for understanding and simulating the hydraulic processes in compound channels. [ABSTRACT FROM AUTHOR]

In the study of natural waterways, the use of formulas such as Manning's equation is prevalent for analyzing flow structure characteristics. Typically, floodplains exhibit greater roughness compared to the main river channel, which results in higher flow velocities within the main channel. This difference in velocity can lead to increased sedimentation potential within the floodplains. Therefore, accurately determining Manning's roughness coefficient for compound channels, particularly during flood events, is of significant interest to researchers. This study aims to model the Manning roughness coefficient in compound channels with both converging and diverging floodplains using advanced soft computing techniques. These techniques include a multi-layer artificial neural network (MLPNN), Group Method of Data Handling (GMDH), and the Neuro-Fuzzy Group Method of Data Handling (NF-GMDH). For the analysis, a dataset from 196 laboratory experiments was used, which was divided into training and testing subsets. Input variables included parameters such as longitudinal slope (S_o), relative hydraulic radius (R_r), relative depth (D_r), relative dimension of flow aspects (δ^*), and the convergent or divergent angle (θ) of the floodplain. The relative Manning roughness coefficient (n_r) was the output variable of interest. The results of the study showed that all the models performed well, with the MLPNN model achieving the highest accuracy, characterized by R² = 0.99, RMSE = 0.001, SI = 0.0015, and DDR = 0.0233 during the testing phase. Further analysis of the soft computing models indicated that the most critical parameters influencing the results were S_o, R_r, D_r, δ^*, and θ.

Compound channels consisting of obvious riverbank beaches and deep grooves are widely found in the upper reaches of the Yangtze River, and the special and complex flow structure of compound channels has an important impact on fish habitats. Currently, there are insufficient studies on how the flow characteristics of compound channels affect the suitability of spawning habitats for fish. In this study, two different spawning types of fishes, namely, the Four major Chinese carps and the Schizothorax prenanti, were taken as representatives, and two different hydrodynamic effects, namely, the characteristic flow and the typical flow pulse process, were considered. Using the 3D hydrodynamic model and physical habitat suitability model, we investigated the spatio-temporal distribution characteristics of the habitat suitability indicator for representative fishes in a compound channel, and analyzed the impact of hydraulic characteristics of the compound channel on the suitability of spawning habitats for representative fish species. The results show that the suitable areas for spawning and habitat of the Four major Chinese carps are concentrated in the middle and upper layers of the flow field, with the distribution characteristics of locating in deep grooves under low flow and locating in riverbank beaches under high flow; the spawning and habitat suitability area for the Schizothorax prenanti is located mainly in the near-bottom flow field of the riverbank beach of the compound channel, and with the increase of the flow, the size of the suitable area in the riverbank beach decreases and migrates to the bank gradually. Under the effect of a high-flow pulse process in a certain flow interval, the ideal weighted usable area (IWUA) of habitat suitability indicator greater than 0.8 (HSI ≥ 0.8) for the Four major Chinese carps and Schizothorax prenanti will increase with the increase of flow, and the connectivity of ideal habitat patches will also be enhanced. The results of this study are of great significance for the conservation of river fish resources and ecological engineering construction, etc.

Climate change can have a profound impact on river flooding, leading to increased frequency and severity of floods. To mitigate these effects, it is crucial to focus on enhancing early warning systems and bolstering infrastructure resilience through improved forecasting. This proactive approach enables communities to better plan for and respond to flood events, thereby minimizing the adverse consequences of climate change on river floods. During river flooding, the channels often take on a compound nature, with varying geometries along the flow length. This complexity arises from construction and agricultural activities along the floodplains, resulting in converging, diverging, or skewed compound channels. Modelling the flow in these channels requires consideration of additional momentum transfer factors. In this study, machine learning techniques, including Gene Expression Programming (GEP), Artificial Neural Networks (ANN), and Support Vector Machines (SVM), were employed. The focus was on a compound channel with converging floodplains, predicting the shear force carried by the floodplains in terms of non-dimensional flow and hydraulic parameters. The findings indicate that the proposed ANN model outperformed GEP, SVM, and other established approaches in accurately predicting floodplain shear force. This research underscores the efficacy of utilizing machine learning techniques in the examination of river hydraulics.

River discharge estimation is vital for effective flood management and infrastructure planning. River systems consist of a main channel and floodplains, collectively forming a compound channel, posing challenges in discharge calculation, particularly when floodplains converge or diverge. In the present study, ML algorithms such as XGBoost, CatBoost, and LightGBM were developed to predict discharge in a compound channel. PSO algorithm is applied for optimization of hyperparameters of gradient boosting models, denoted as PSO-XGBoost, PSO-LightGBM, and PSO-CatBoost. ML model discharge predictions were validated with existing empirical models and feature importance was explored using SHAP and sensitivity analysis. Results show that all three gradient-boosting algorithms effectively predict discharge in compound channels and are further enhanced by application of PSO algorithm. The R2 values for XGBoost, PSO-XGBoost, CatBoost, and PSO-CatBoost exceed 0.95, whereas they are above 0.85 for LightBoost and PSO-LightBoost. PSO-CatBoost performance is better than other models based on findings of statistical performance parameters, uncertainty analysis, reliability index, and resilience index for prediction of discharge in a compound channel with converging and diverging flood plains. The findings of this study validate the suitability of the proposed models especially optimized with PSO is recommended for predicting discharge in a compound channel. HIGHLIGHTS A diverse range of datasets is used for discharge prediction in non-prismatic compound channels.; The SHAP and sensitivity analysis is employed to find out the effect of influencing input parameters that affect discharge prediction.; Six ML models have been proposed for the prediction of discharge.; Recommendation of the different models for range-wise variation of width ratio and relative flow depth.;

A compound channel's discharge capacity and boundary shear force can be predicted as a sum of the discharge capacity of different sub-regions once the apparent shear stress of the dividing line is reasonably quantified. The apparent shear stress was usually expressed as a coefficient multiplied by the difference between two squared velocities of two adjacent regions. This study investigated the range of the coefficient values and their influencing factors. Firstly, the optimal values of the coefficient were obtained based on experimental data. Then, comparisons between the optimal values and several parameters used in quantifying the apparent shear stress were conducted. The results show that the coefficient is mainly related to a morphological parameter of the floodplain and the ratio of resistance coefficients between the floodplain and the main channel. An empirical formula to calculate the coefficient was developed and introduced to calculate the flow discharge and boundary shear stress. Experimental data, including 142 sets of test data of symmetric-floodplain cases and 104 sets of one-floodplain cases, have been used to examine the prediction accuracy of discharges and boundary shear stress. For all these tests, the ranges of water depth of the main channel and the total width of the compound cross-section are about 0.05~0.30 m and 0.3~10 m, respectively; the Q range and the range of Froude numbers of the main channel flow are about 0.0033~1.11 m3/s and 0.3~2.3, respectively. Comparison with other methods and experimental data from both rigid and erodible compound channels indicated that the proposed method not only provided acceptable accuracy for the computation of discharge capacity and boundary shear stress of compound channels in labs but also gave insights for calculating discharge capacity in natural compound channels. [ABSTRACT FROM AUTHOR]

As floodplains are inundated during floods in a compound channel, solutes in the surface water column reach the hyporheic zone and react with solutes upwelled from the groundwater. These biogeochemical reactive processes, such as aerobic respiration, nitrification, and denitrification, need more clarification. In this study, a 3D hydrodynamic model combined with a 2D groundwater and biogeochemical model was used to examine the influence of bank slope angle and ambient groundwater discharge on these processes. A denitrification zone was found under the interface between the main channel and the floodplain when the bank slope angle was 90°, while lower angles extended that zone horizontally. In addition, a lower bank angle decreased N entry into the streambed and enhanced nitrogen removal. A decrease in ambient groundwater had a negative impact on both aerobic respiration and denitrification. When the ambient groundwater discharge reached below −0.9 m/d, nitrification was dominant in the model domain, and the hyporheic zone turned into a NO3− source. The greatest removal efficiency, equal to 0.8, was attained at a discharge rate of −0.5 m/d for ambient groundwater and a bank slope angle of 30°. The hyporheic zone should lose its ability to remove N when ambient groundwater discharges exceed 0.25 m/d and removal efficiency fluctuates by 0. In conclusion, our findings indicate that bank slope angle and ambient groundwater discharge have a substantial impact on solute transport and biogeochemical activities in the hyporheic zone of a compound channel. [ABSTRACT FROM AUTHOR]

This paper presents the results of bed scouring near an abutment that spans the entire floodplain width and terminates at the edge of the main channel, termed here as the bankline abutment. The cross section can be divided into a scoured section and an un-scoured section. The scoured bed profile can be approximated using a power function. An analytical method has been suggested to predict the maximum scour depth at bankline abutment. This method is valid irrespective of whether the original bed is at or below the threshold condition of sediment motion. The proposed method is consistent with the experiments from this study and previous studies. [ABSTRACT FROM AUTHOR]

The bed degradation of a compound channel induced by a base level fall can significantly impact the flood conveyance capability and stability of riverine structures. This study found that severe compound channel degradation is due to the coupled effects of flood discharge and base level fall. Laboratory experiments examined the impact of base level fall on the grain size distribution at the bed surface, flow discharge in the main channel and floodplains, and longitudinal profile of the bed elevation. Two riverbed degradation patterns of a compound channel after base level fall were clarified. Relative to the original bed slope, degradation pattern (I) had a higher bed slope, but degradation pattern (II) had a lower one. A calculation method was proposed to determine the critical conditions for identifying these two riverbed degradation types. Both experimental and published data confirmed that the proposed method is valid for identifying these two compound channel degradation patterns. [ABSTRACT FROM AUTHOR]

Rivers as a natural drainage of a watershed have always been of interest in draining the runoff resulting from precipitation, especially during floods. Due to the formation of most cities and villages on the banks of rivers and the construction of structures in their floodplains, during floods, the pattern of river flow always undergoes changes, so it is necessary to study the interaction of flood flows of meandering compound channel in these areas. In this research, by using Flow3D software, which is a powerful software in the field of computational fluid dynamics, the flow structure and bed shear stress in meandering compound channel under the effect of structural density of floodplain during floods have been investigated. For this purpose, six types of structural density of 0, 8, 16, 11, 25 and 44% have been used on the floodplain using non-submerged blocks with distances of 7, 14, 21, 28 and 35 cm. The results of the numerical simulation showed that with the increase of structural density in the floodplain from zero to 44%, the average velocity of the main channel, the maximum water surface elevation and the amount of flow passing through the main channel increased by 51%, 25% and 84% respectively. Also, with the increase in structural density on the floodplain, the amount of bed shear stress has increased so that the maximum bed shear stress has increased from 1.32 to 4.61 pascal (250 percent) and moves towards the center of the main channel. [ABSTRACT FROM AUTHOR]

Investigation of communication over a wiretap channel is one of the problems of information–theoretic security. The aim in the general wiretap channel model is to maximize the rate of the reliable communication from the source to the legitimate receiver, while keeping the confidential information as secret as possible from the wiretapper (eavesdropper). Here we consider the compound wiretap channel model, when the channels to the legitimate receiver and to the wiretapper depends on the number of possible states. We investigate the -capacity–equivocation region which is the closure of the set of all achievable rate-reliability and equivocation pairs, where the rate-reliability function represents the optimal dependence of rate on the error probability exponent (reliability). Here the outer bound of this region is constructed in the case, when the states are not known to all terminals. [ABSTRACT FROM AUTHOR]

Riparian vegetation grows excessively in floodplain areas of natural rivers and canals which affects the conveyance capacity of the channel and alters the flow velocity. Therefore, it is necessary to understand the impact of floodplain vegetation on flow dynamics in a trapezoidal compound channel. Therefore, the main focus of this study is to utilize a three-dimensional (3-D) numerical model to investigate the flow dynamics in a floodplain vegetated compound channel. ANSYS FLUENT software was employed to conduct the numerical simulations, and Reynold's Stress Model (RSM) was chosen to accurately estimate turbulence parameters during simulations. The numerical simulation data were validated using previously published data. The cross-sectional flow characteristics revealed a significant reduction in flow velocity within the floodplain region due to the high resistance offered by the vegetation, whereas velocity in the main channel increased by around 70% comparatively. Additionally, turbulence intensity was also reduced in the vegetation region. As the flow velocity and turbulence decrease within the floodplain-vegetated region of the trapezoidal compound channel, favorable conditions for sedimentation may be created in such regions of low flow velocity which are suitable for flood mitigation and the growth of aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Estimating the water surface elevation of river systems is one of the most complicated tasks in formulating hydraulic models for flood control and floodplain management. Consequently, utilizing simulation models to calibrate and validate the experimental data is crucial. HEC-RAS is used to calibrate and verify the water surface profiles for various converging compound channels in this investigation. Based on experimental data for converging channels (θ = 5°, 9°, and 12.38°), two distinct flow regimes were evaluated for validation. The predicted water surface profiles for two relative depths (β = 0.25 and 0.30) follow the same variational pattern as the experimental findings and are slightly lower than the observed values. The MAPE for the simulated and experimental results is less than 3%, indicating the predicted HEC-RAS value performance and accuracy. Therefore, our findings imply that in the case of non-prismatic rivers, the proposed HEC-RAS models are reliable for predicting water surface profiles with a high generalization capacity and do not exhibit overtraining. However, the results demonstrated that numerous variables impacting the water surface profile should be carefully considered since this would increase the disparities between HEC-RAS and experimental data. HIGHLIGHTS In this article, research was conducted for the non-prismatic compound channel with converging floodplains, utilizing the HEC-RAS software.; The findings depict the HEC-RAS models are accurate for forecasting the water surface profile of non-prismatic rivers, have a high capacity for generalization, and do not display any signs of overexertion.; The usefulness of HEC-RAS tool for the design of flood control and diversion structures in the non-prismatic rivers.;

Floods have become one of the most dangerous and frequent natural disasters. Most rivers are characterised by compound cross-sections that are usually contain vegetation. The ability to simulate water surface profiles (WSPs) in vegetated rivers quickly and accurately is crucial in flood forecasting operations. The aim of this study was to develop a low-cost and practical tool for predicting the WSP in compound channels with vegetated floodplains. In particular, artificial neural network (ANN) and support vector machine (SVM) techniques were used to devise a model for the prediction of the WSP in an experimental channel. Two approaches were employed: the first was based on the use of non-dimensional data and the second used dimensional data. The performance of the prediction methods was determined using a ten-fold cross-validation approach. Comparative results revealed that the SVM algorithm outperformed the ANN and regression models. The performance of the SVM model using dimensional data (correlation coefficient of 0.99 ± 0.005 and mean absolute error of 0.0019 ± 0.0002) was shown to be marginally better than the model using dimensionless data. Sensitivity analysis also indicated that the relative discharge and relative depth played the most important roles in estimating the WSP. [ABSTRACT FROM AUTHOR]

Abstract The local scouring around the pier, is one of major cause of the bridges failure worldwide. Compared to the flow hydraulics in single channels, the flow pattern in compound channels is completely different with flood plains covered with vegetation and this issue can affect the amount of scouring in the area of bridge piers. However, the combined effect of these factors has not been systematically investigated. Therefore, due to the complex nature of the local scouring phenomenon and the absence one of an accurate prediction model, in this research, an experimental study on the hydraulic characteristics of the flow approaching the bridge deck in compound channels with floodplain vegetation in pressurized flow conditions, has been done. It was found that increasing vegetation density on average will reduce scour depth by 15% for the same floodplain width and relative depth. In free-flow conditions, increasing the vegetation density on average will increase the scour depth. The aim is to develop an optimized model to estimate the bridge pier scour in consideration of the combined effect. The newly developed relationship shows a good correlation coefficient of more than 92% with the experimental data and yielded better results than the previous equations. The finding of this study will have potential applications for the prediction of the bridge pier scour in clear water conditions.

The influence of different vegetations widely habituating on the floodplain of natural rivers on the hydrodynamic characteristics of compound channel has always been concerned. This study has established a numerical model of riparian vegetation flow of compound channel based on Delft 3D software, and adopted the diameter, height and density of vegetation as control parameters, to investigate the influence of these parameters on the longitudinal water surface slope, lateral profile of depth-averaged longitudinal velocity, and the ratio of difference of velocities in main channel and flood plain to the average velocity of whole cross section. Research indicate that the increase in diameter, depth and density of vegetation changes the lateral profile of longitudinal velocity significantly. The ratio of difference velocity and the water surface slope are correlated with the diameter of vegetation quadratic, with the height of vegetation linearly, with the density of vegetation logarithmically. The difference between water surface slopes of main channel and flood plain increase as long as the increase in diameter, height and density of vegetation; and the maximum velocity in the main channel is shifted due to the imbalance distribution of vegetation. [ABSTRACT FROM AUTHOR]

The essential information required by hydraulic engineers for estimating the stage and discharge characteristics of open channels is a comprehension of velocity distribution. The current work employed time-averaged normalized velocity as a random variable in Tsallis and Renyi entropy-based model to construct velocity estimations in compound channels with varying bank slopes. According to the approach, both entropies should be maximized using the maximum entropy principle, with certain restrictions. Two Lagrange multipliers for Renyi entropy and dimensionless parameter G for Tsallis entropy must be derived for the model, and these quantities can be determined using average and maximum experimental velocity data. Laboratory measurements were used by considering 45 ° and 90 ° bank angle compound channels and to evaluate the velocity distributions, which were then contrasted with predicted velocity distributions by Renyi and Tsallis entropy methods. The resulting velocity profile matches the experimental data with great accuracy, mainly in the floodplain and main channel domains. However, the accuracy is less in and around slopes because of the nonuniform cross-section and involvement of secondary currents, ultimately affecting the velocity profile. This entropy-based velocity estimation in compound channels should open the way for the future inclusion of vegetation for a better understanding of flow in actual field circumstances. [ABSTRACT FROM AUTHOR]

Some hydraulic structures and phenomena, including compound channels, must be studied in relation to open channel flow. Despite the fact that the primary channel and watersheds share a similar degree of roughness, estimating discharge in composite channels with mainstreams and flood plains has proved tricky. The flow discharge for a compound channel with different roughness in the primary and flood plain channels has been studied, and the results computed experimentally using horizontal division level have been compared with those predicted using dimensional analysis and an adaptive neuro-fuzzy inference system. The results show good agreement between experimental and numerical for discharge calculation according to root-mean-square error, MARE, R 2, SI, and Nash–Sutcliffe efficiency, with a percentage error not exceeding ±5%.

This paper investigates the effect of different geometries of approach embankments and guides banks on the flow pattern and bed shear stress values in the skewed bridges in the compound channel using three-dimensional numerical modelling. First, the numerical model was evaluated based on the results of existing laboratory studies. After ensuring its proper performance, the elliptical guide bank and the three types of abutments: vertical-wall, spill-through, and wing-wall, at different skew angles are examined. Investigation of the values of maximum velocity and bed shear stress at the flow-conducting embankment and the flow-splitting embankment showed that in the flow-conducting embankment, the best performance is assigned to the elliptical guide bank. In contrast, the performance of various abutments is different for the flow-splitting embankment depending on the skew angle of the bridge. Then, different patterns based on streamlines for the geometry plan of the guide bank were proposed and studied. The results show that the most suitable pattern for the guide bank reduces the maximum flow velocity by up to 15% and reduces the maximum bed shear stress by up to 80% around the flow-splitting embankments.

The experimental study of hydraulic jump characteristics is always a difficult task, particularly regarding the instantaneous velocity, which is difficult to measure experimentally because of its fluctuation and the presence of air bubbles, mainly at the level of the hydraulic jump. However, it is possible to experimentally measure velocities in two-phase flows (air-water) using an intrusive measuring instrument such as a Microreel. This work will experimentally examine two different hydraulic jumps with two flow rates (26.78 l/s and 17.82 l/s) controlled by two thin sills (HS = 14 cm and HS = 15 cm) in a channel composed of a trapezoidal shape with a rectangular base. The results of the measurements of the velocities (relative velocity V*exp = V/V0) as a function of the positions (relative positions X* = X/Ls). The relationship V*exp = f(X*) was analyzed by multiple regression and given a second-degree polynomial function. The coefficients of determination of this function are very high (R2 approximately 0.9), which describes a strong relationship between speeds and distances. These results allow us to reflect on the application of learning techniques (ML) to predict relative velocities at any position of the hydraulic jump. Three learning techniques were tested: RBFNN, MPLNN, and GRNN. However, the results prove that the RBFNN has a very accurate speed productivity and records the lowest deviations from the experimental results. Its performance indicators are RMSE = 0.0163 and MAE = 0.0085 for level 1, RMSE = 0.0138 and MAE = 0.0089 for level 2, and RMSE = 0.0166 and MAE = 0.0099 for level 3. However, the MLPNN model gives competitive results to the RBFNN precisely at level 4, where it registers the smallest deviation (RMSE = 0.0457, MAE = 0.0308). [ABSTRACT FROM AUTHOR]

The study of vegetation evolution is essential for further understanding of biogeographic feedback and ecological restoration. In this paper, a vegetation evolution model based on velocity threshold (scaled by the average flow velocity of the bare bed) was established to simulate the vegetation evolution process in compound channels. In this model, the effect of vegetation on water flow was generalized as equivalent Manning coefficient, and the velocity field was obtained by solving two-dimensional shallow water equations. The model defined that new vegetation was added in areas where the velocity was less than the velocity threshold, and conversely, vegetation was destroyed in areas where the flow velocity exceeded the velocity threshold. The model was used to explore the effect of velocity threshold, initial vegetation coverage, and relative water depth (the ratio of the flow depth in the floodplain to that over main channel) on final vegetation coverage and longitudinal dispersion coefficient (Ke) in compound channels, and compare the difference of vegetation evolution between rectangular channels and compound channels. Results showed that the velocity threshold played a decisive role in vegetation evolution, and the effect of relative water depth and cross section type on vegetation evolution was only reflected when the velocity threshold was small. The longitudinal dispersion coefficient gradually increased with the expansion of vegetation, and tended to a constant value (Kf) when a stable vegetation landscape was reached. As the relative water depth decreased, the longitudinal dispersion coefficient presented an increasing trend. Regular distribution of initial vegetation patches can produce larger longitudinal dispersion coefficient compared to the cases of random distribution in compound and rectangular channels, and the increasing effect was more significant in compound channels. [ABSTRACT FROM AUTHOR]

The planting of the vegetation on the floodplain helps the ecological restoration, which is the main form of the river's ecological corridor. Therefore, the current research of the river dynamics focuses on the water movement under a compound channel with the vegetated floodplains. Two simulated vegetation species are selected in this paper for the flume simulation experiments of the floodplain vegetation, and the compound channel is divided into three subregions in the transverse direction. The Navier-Stokes equation and the eddy viscosity theory are applied to obtain the transverse distribution of the depth-averaged velocity and the results agree well with the experimental data. This paper proposes a new method based on the analytical solution of the flow velocity distribution to calculate the average flow velocity in each section. Calculation results can effectively simulate the average flow velocity of the measured sections. The description of the pollutant transport processes in a moving stream requires a refined determination of the dispersion coefficients in the compound channel. The process of the pollutant concentrations in each zone and the reasons for their occurrence are elucidated on the basis of the experimental results. Simultaneously, the measured values of the longitudinal dispersion coefficients are obtained by the "routing procedure," and a two-zone model of the pollutant dispersion is constructed on the basis of the hydrodynamic study. The prediction method for the longitudinal dispersion coefficients is also presented. Applying the predicted and measured section average flow velocities to the two-zone model to predict the longitudinal dispersion coefficient, and the average relative errors are only 4.17%, 7.15%, respectively. This result indicates that the two-zone model can effectively predict the longitudinal dispersion coefficients. The calculation methods for the longitudinal dispersion coefficients from—various studies are compared. The results reveal that the predicted values of these calculation methods are all larger than the measured values, indicating that the vegetation has a considerable influence on the dispersion process. This study comprehensively shows the dispersion features of the pollutants and provides a theoretical basis for the planning and the design of the vegetated ecological corridors. [ABSTRACT FROM AUTHOR]

This technical note presents results from flume experiments conducted in a straight prismatic asymmetric compound channel with two different floodplain roughnesses and rigid emergent vegetation along the floodplain edge in combination with a mobile and fixed main channel bed, respectively. It is found that the contribution of floodplain roughness and vegetation along the floodplain edge to the bulk friction factor is considerably reduced when studied in combination with a mobile main channel bed featuring bedforms. This in turn means that the effect of vegetation along the floodplain edge on the conveyance capacity and bulk friction factor may be overestimated if it is studied solely with fixed bed conditions. The presented results reveal the need to improve our understanding of the complex interplay between mobile beds and vegetation along the floodplain edge and its combined effect on compound channel conveyance capacity and hydraulics. [ABSTRACT FROM AUTHOR]

The large confluence between the Yangtze River and the outflow channel of Poyang Lake is receiving attention due to its importance in flood control and ecological protection in the Yangtze River basin. There is a large floodplain along the outflow channel of Poyang Lake, which is submerged during high flow and dry during low flow. The effects of the submergence of this floodplain on sediment and morphological characteristics at this large confluence have not been known. Hence, a field investigation was done in March 2019 (relatively high flow, Survey 3) to complement the previous field studies done in August (high flow, Survey 1) and December 2018 (low flow, Survey 2) to identify the temporal variations of sediment and morphological characteristics considering the submergence of this large floodplain. The predominant sediment transport modes were wash load for Poyang Lake and confluence particles and mixed bedload/suspended load for the Yangtze River particles. The sediment transport processes were largely affected by both the secondary flows and the water density contrast between the tributaries with a lock-exchange sediment rich, denser flow moving across the inclined mixing interface in Surveys 1 and 2. The sediment flux across the mixing interface was weakened in Survey 3 when the density contrast was very small. The stagnation zone near the confluence apex had a low sediment concentration and played a role in preventing the sediment flux exchange between the two flows, and its size, and, thus, its importance as a barrier to sediment mixing were related to the submergence of the floodplain. The bed morphology with the local scour holes at the confluence was largely affected by the large-size helical cells, and this kind of effect was weakened as the secondary flows got restricted in Survey 3. The current results expand the database and knowledge on the sediment transport and morphological features of large river confluences. [ABSTRACT FROM AUTHOR]

Bridge abutment-induced sediment scour in compound river channels is time-sensitive during floods, and, thus, estimating the scour rate has significance in design. In the current study, large-scale laboratory experiments were done to examine the influence of multiple factors on scour evolution, including channel geometry, bed mobility (clear-water or live-bed), and lateral (embankment intrusion) and vertical (deck submergence) flow contractions. A new method is applied for scour measurement and determination of characteristic scour trends. Results show that the non-dimensional time scale is related to flow intensity and may peak before reaching the clear-water scour threshold; the corresponding equations also are derived. It is found that the use of scour countermeasures has a significant impact on the scour rate, and the existing equations for unprotected abutments have limitations. Under clear-water conditions, the scour rate is closely related to the overall flow contraction, which can be better reflected by the ratio of the approaching unit discharge to that at the bridge section. Specifically, the effect of vertical flow contraction on scour rate is more dominant than other factors. Two scour rate curves are derived to suit different structural and flow contraction types for clear-water scour. In contrast, the live-bed scour rate data are generally consistent and fall in a narrow band, showing less dependence on other factors. Finally, a new design approach is established to estimate the time scale and scour rate using the new equations proposed in this paper. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of different geometries of approach embankments and guides banks on the flow pattern and bed shear stress values in the skewed bridges in the compound channel using three-dimensional numerical modelling. First, the numerical model was evaluated based on the results of existing laboratory studies. After ensuring its proper performance, the elliptical guide bank and the three types of abutments: vertical-wall, spillthrough, and wing-wall, at different skew angles are examined. Investigation of the values of maximum velocity and bed shear stress at the flow-conducting embankment and the flow-splitting embankment showed that in the flowconducting embankment, the best performance is assigned to the elliptical guide bank. In contrast, the performance of various abutments is different for the flow-splitting embankment depending on the skew angle of the bridge. Then, different patterns based on streamlines for the geometry plan of the guide bank were proposed and studied. The results show that the most suitable pattern for the guide bank reduces the maximum flow velocity by up to 15% and reduces the maximum bed shear stress by up to 80% around the flow-splitting embankments. [ABSTRACT FROM AUTHOR]

A compound open channel is composed of the main channel and flood plain. In experiments with compound open channel conducted to ensure that the flow is uniform and fully developed, it is necessary to study the distribution of discharge in the main channel and flood plains. The purpose of the present study is to investigate the effects of channel inlet condition on flow uniformity by considering distribution of discharge in channels with length to flood plain width ratio lower than 35 (needed for fully developed flow condition) by analyzing the flow field and turbulence parameters. For this purpose, particle image velocimetry method has been used in a rectangular compound open channel. To provide correct measurement of secondary velocities, using a non-intrusive method such as particle image velocimetry is completely essential. The results of this study show that in short compound channels with the same screen installed in the main channel and flood plain, there is significant mass transfer from the flood plain to the main channel until the end of the channel length. It was found that in this case, a considerable downfall occurs for the maximum velocity position in the main channel. However, with a supplementary screen installed in the flood plain, in addition to the typical screen, expected conditions are established similar to the fully developed compound channels. In this condition, the time-averaged streamwise velocities vary considerably in the flood plain along the spanwise direction. On the other hand, in short compound channels with the same screen installed in main channel and flood plain, the streamwise velocities do not change significantly along the flood plain width duo to the imperfect interaction of main channel and flood plain. These observations express that to provide correct distribution of discharge, a supplementary screen should be installed in the flood plain of the compound channel.

The use of apparent shear stress at the interface between the adjacent subregions of a cross section to represent the effect of momentum transfer is a common method for the one-dimensional calculation of compound open-channel flow. The apparent shear stress of the dividing lines is affected by the compound geometry and boundary conditions. The expressions of the apparent shear stress established with previous studies are very different. Empirical treatment was proposed in this study to simplify the expressions based on a momentum transfer–equivalent states assumption, in which (1) the apparent shear stress was assumed to be the difference of the momentum transfer from two sides of a dividing line, and (2) two assumed equivalent states were employed to define the equivalent value of the momentum transfer. The apparent shear stress could be calculated based on the deviation of the momentum transfer from its equivalent value. The new expressions were used to calculate the discharges and the boundary shear force. Comparisons between the calculated results based on different methods and the measured data showed that the proposed method improved the calculation of the subregion boundary shear force and the discharges in the compound cross sections. [ABSTRACT FROM AUTHOR]

Transverse hyporheic transport and biogeochemical reactions in a compound channel remain poorly understood. As floodplains are submerged, solutes in the surface water enter the hyporheic zone of the compound channel and react with the solutes upwelling below the floodplain. Aerobic respiration (AR) and denitrification (DN) processes in the hyporheic zone of the compound channel still need to be clarified. In this paper, a 3D hydrodynamic model coupled to a 2D groundwater flow and biogeochemical model was applied to investigate such processes. The model results were verified using laboratory experimental measurements. The effects of bank slope angle, ambient groundwater flow, and ambient groundwater solute concentration on aerobic and anaerobic biogeochemical processes were studied. The denitrification zone was found below the interface between the main channel and the floodplain. The lower bank slope angle favours nitrogen removal. Small losing groundwater discharges promote aerobic respiration and denitrification reactions. As the ambient groundwater oxygen concentration rises, O2 consumption increases while N removal decreases. As the NO3 concentration in ambient groundwater increases, NO3 consumption increases and proportionally NO3 consumption decreases. These findings not only provide a better understanding of biogeochemical reactions in a compound channel but can also be applied to river restoration to efficiently remove nitrate by modifying bank slope angles. [ABSTRACT FROM AUTHOR]

Abstract Overbank flows occur in alluvial valleys during flood events when the conveyance of main channel of rivers is exceeded. Once floodplains are inundated and the so-called compound channel flow is observed, the faster flow in the main channel interacts with the slower flow in the floodplain featuring a much more pronounced 3D flow structure compared to single channel flow. These flow mechanisms comprise a shear layer near the interface, lateral momentum transfer and strong secondary currents due to the non-isotropic turbulence. This paper starts by giving an overview of the main flow mechanisms in compound channels pointing out the importance of taking into account the apparent shear stress generated between the main channel and the floodplain flows due to the interaction of these flows. A new simple model was developed to include the apparent shear stress concept as a correction of the Manning roughness coefficient of main channel and floodplains. The proposed method for predicting stage–discharge relationships was calibrated and validated by experimental data from several compound channel facilities. A significant improvement in prediction of the compound channel conveyance in comparison with the traditional methods was achieved.

The study of floods has always been important for researchers due to the great loss of life and property. Investigation of flood bed can provide appropriate solutions to reduce this phenomenon to managers and researchers. In this research, the compound channel (with flood plain on one side of the main channel) Been paid, Therefore, two experimental models of compound channel in laboratory flume were examined by considering dimensional analysis. With the goal Investigation of lateral slope of flood wall in laboratory model In the first model, transverse slope 0 And in the second model, a value equal to 50% Was considered. Also in order to investigate the effect of longitudinal slope of river bed sediments Longitudinal slope in three steps 0.00 2, 0.004 and 0.006 Was changed. Examining the ADV speedometer data, the results showed that with increasing the longitudinal and transverse slope (slope of the flood wall) of the channel, the maximum longitudinal velocity changes to the floor of the channel. In order to investigate the effect of average sediment diameter on the scouring process during experiments Mm was used. The results showed that increasing the longitudinal and transverse slope had a great effect on increasing the volume of washed sediments 3 and 0.9 of sandy sediments with a diameter Along the canal and with the increase of these longitudinal and transverse slopes in the channel, more sediment transport volume occurs. In the following, using Investigation of dimensionless numbers obtained from dimensional analysis, dimensionless weight landing number was introduced to evaluate this value value of other hydraulic parameters and Was introduced. A relationship based on nonlinear regression with correlation coefficient Acceptable was introduced at around 0.88.

Flow in compound sections, due to the differences in velocity between the main channel and floodplains, is structurally quite different from that in simple channel sections. In the present research work, FLOW3D software was applied in a rough rectangular compound channel with single floodplain. The model geometry was selected from the experimental channel by Bousmar (2002), and the numerical solution results were validated by comparing the depth-averaged velocity parameter. The purpose of this study was to investigate the effects of roughness, depth and relative width on flow velocity distribution, bed shear stress variation, and location of maximum stress in compound channel sections and simulated flow for 3 relative roughness values of 1, 2 and 2.9. Results showed that flow rate difference between the main channel and floodplain as the generator of shear stress and secondary flows in those sections increased by 3 times. Relative roughness for the Model A, 33.5% and for Model B, increased by 15.12%. While the relative depth parameter increased from 65% (from 0.2 to 0.33), the difference for the two models A and B decreased to 36.32% and 37.85%, respectively. So, it seems that using Model B is more appropriate for high relative roughness. Also, by increasing the relative depth from 0.2 to 0.33 in all the relative roughness values studied, the location of maximum shear stress for both models changed from the main channel to the floodplain.

Natural levees are wedge-shaped morphological features developing along the boundaries of mass flows. When they form in fluvial landscapes, they can have multiple implications for river management of trained inland rivers. This paper summarizes the present knowledge in regard to the formation and evolution of so-called fluvial levees of trained inland river sections and provides novel hypotheses in regard to the significance of bedforms and vegetation strips along the floodplain on levee formation, evolution, and characteristics. The hypotheses that (i) bedforms contribute to levee formation by altering the interface hydraulics between the main channel and the floodplain and enhancing entrainment of sediment into suspension and (ii) vegetation stripes along the floodplain additionally affect the interface hydraulics resulting in a changed levee geometry are supported by combining existing knowledge on bedform dynamics and flow-vegetation-sediment interaction with results reported in recent flume studies. Article Highlights: Levee formation is associated with both turbulence induced and advective lateral sediment transport processes. Flume experiments indicate that main channel bedform dynamics is an important factor enhancing levee formation. Riparian floodplain edge vegetation enhances levee formation and alters its typical geometry. [ABSTRACT FROM AUTHOR]

We derive universal classical-quantum superposition coding and universal classical-quantum multiple access channel coding by using generalized packing lemmas for the type method. Using our classical-quantum universal superposition code, we establish the capacity region of a classical-quantum compound broadcast channel with degraded message sets. Our universal classical-quantum multiple access channel codes have two types of codes. One is a code with joint decoding and the other is a code with separate decoding. It is not so easy to construct a former code that universally achieves general points of the capacity region beyond corner points. First, we construct the latter code that universally achieves general points of the capacity region. Then, converting the latter code to the former coder, we construct the above desired code with the former type. [ABSTRACT FROM AUTHOR]

This study focuses on scour depth development around 45° wing‐wall setback abutments in a compound channel. The results show that the scour development curves almost have similar shape and they can be described using exponential function. It is generally observed that the scour hole achieved 80% of the equilibrium scour depth at a scour time equals to 25% of the equilibrium scour time. Additionally, an increase in the relative abutment length and the contraction ratio causes an increase in the equilibrium time. However, this time decrease with increasing flow depth ratio. Finally, new empirical equations are proposed to predict the development of scour depth and the time to reach the equilibrium state. These equations will be useful to predict these characteristics of scour hole especially for setback abutments in compound channel and predict the temporal development scour depth for previous studies that have available data related to maximum scour depth. [ABSTRACT FROM AUTHOR]

Compound channels are consisting of two hydraulic sections namely main channel and floodplain. In meandering rivers, with the passage of time and lateral movement of the meanders, the external bending progression and the sinusoidal or curvature is increased. The curvature of meandering sections can be defined by a dimensionless parameter as the sinusoidal number which is the ratio of meandering length of main channel to the floodplain length. In this research work, the hydraulic characteristics of flow including the velocity magnitude, boundary shear stress, turbulence intensity and turbulence energy of the main channel along the meandering compound channel have been investigated numerically, regarding changes in the sinusoidal ratio for six types of channels with different sinusoidal ratios. In order to investigate the effect of sinusoidal ratio in meandering compound channels on the hydraulic characteristics of the flow, the FLOW3D software is applied. Numerical simulation results show that by increasing the channel sinusoidal number from 1 to 1.641, the velocity and bed shear stress decrease and the turbulence intensity and energy increases. So that the maximum value of the above parameters occurs in the inner arc.

Abstract The roughness in the floodplains in a compound canal and its impact on hydraulic parameters such as the shear stress and their estimation are one of the problems that have attracted the attention of engineers. In this research, the aim is to investigate the effect of artificial on the floodplain of the compound channel on hydraulic parameters. In experiments, slope of channel bed was 0.0015 and three different discharges have been used. The four types of rigid roughness were used to investigate the effect of these parameters. These roughness elements were arranged with zigzag state with two distances of 4 k and 8 k (k is the height of roughness) in the floodplain. A Preston tube with an external diameter of 3 mm that equipped with dynamic pressure sensors was used to compute the shear stress. The Patel calibration curve was used in order to convert the difference between the static and dynamic pressure measured by the Preston tube to the shear stress values. The results showed that for the zigzag arrangement with the density of 4 k, the shear stress is reduced due to the high roughness density and the greater roughened area. In a rough bed, the shear stress in floodplain was significantly higher than smooth bed, and the stress distribution is such that it has descending trend from the main channel toward the wall of the floodplain. The shear stress increase for roughness with a spacing of 8 k is 22–36% higher than the similar hydraulic condition in a smooth bed and the shear stress for condition with the presence of a cylinder with D = 3 cm and roughness spacing of 8 k was 14–18% higher than the shear stress of bed without a cylinder and the same roughness density. The shear stress for condition of the presence of a cylinder with D = 6 cm and roughness spacing of 4 k is 24–30% more than the roughened plain with distances of 4 k.

Aim of the study: The aim of the study was to determine the parameters of the stream transport capacity using turbulent diffusion coefficients using the imaging technique and to compare them with the coefficients obtained as a result of the measurement with the ADV (acoustic Doppler velocimeter) probe in a compound channel. Material and methods: Model tests were carried out on a physical concrete model of a straight open channel with symmetrically complex trapezoidal cross section. On the model, the study of particle motion was carried out using the digital image recording technique with the analysis of the motion of the dosed indicator (solid particles), (PIV) (Particle Image Velocimetry), allowing to determine the basic parameters characterizing the process of their transport in the subsurface layer of water in the channel. Results and conclusions: Primary analysis of test results obtained in laboratory conditions using the PIV (Particle Image Velocimetry) technique based on particle trajectory (in Lagrange’s approach), in comparison with the results of calculations of turbulence characteristics based on measurements with the ADV probe (in Euler’s approach), show significant differences between the results obtained. Turbulence intensity, vortex length, and values of turbulent diffusion coefficients, calculated on the basis of Lagrange’s autocorrelation function, are much higher than the values calculated according to Euler’s function.

Aiming at the bottleneck problem of channel capacity in the development of artificial ports, a solution of compound channel was proposed. In view of the water area of Tianjin Port, the key issues such as cross-section design, traffic organization mode, and navigation rules of compound channel were studied. It is proved by practice that the scheme can improve the channel capacity and efficiency, and can provide reference for similar projects. [ABSTRACT FROM AUTHOR]

Location of maximum scour depth is a key feature for the accurate design of scour countermeasures. Experimental results show that the maximum scour depth around wide setback abutments migrates to a location further downstream than that observed conventionally at the tip for narrower abutment. The wide abutment caused two scour holes to form, with a larger primary scour hole and a smaller secondary scour hole downstream from the first, and this has important implications for scour protection at the structure's foundation. For this study, 55 experiments were conducted with seven abutment aspect ratios and the migration of the maximum scour locations were measured. The results reveal that Rx (the normal distance from abutment face to the location of maximum scour depth) is affected by the flow intensity, flow depth, and abutment aspect ratio. The value of Rx is larger as flow intensity decreases for the same abutment aspect ratio. However, Ry (the distance from the centerline of abutment face to the location of maximum scour depth parallel to flow direction) is only affected by flow depth and abutment aspect ratio. Empirical equations are proposed to predict the locations of the maximum scour depth for wide abutments in compound channels. [ABSTRACT FROM AUTHOR]

We consider the fundamental limits of reliable communication over a two-state compound channel when the state of the channel needs to be masked. Our model is closely related to an area of study known as covert communication, a setting in which the transmitter wishes to communicate to legitimate receiver(s) while ensuring that the communication is not detected by an adversary. Our main contribution is the establishment of upper and lower bounds on the throughput-key length region when the constraint that quantifies how much the states are masked is defined to be the total variation distance between the channel output distributions of the two states. When length of the key is sufficiently large, we provide sufficient conditions for the bounds to coincide. Our results follow the so-called square-root law and hence are reminiscent of results in covert communications. Numerical examples, including that of a Gaussian channel, are provided to illustrate our results. [ABSTRACT FROM AUTHOR]

The study establishes different analytical model for velocity distribution, boundary shear stress, stage–discharge curves and their application to the trapezoidal compound channel. The analytical solution to the depth-integrated Navier–Stokes equations is used for the same, and their results are validated using experimental data and numerical solution. The parameters used to predict the three-dimensional characteristics of the flow field over compound channel in the analytical solution require modelling of friction factor, eddy viscosity coefficient and secondary current term. The Shiono and Knight model (SKM), which is an analytical solution for RANS equation through empiricism, is compared with two similar analytical-based models, viz. K-method and extended SKM, and with one numerical method, i.e. K–ε model through simulation based modelling. The results obtained in the analyses show that the secondary flow parameter plays a vital role and its significance proliferates near side slope where momentum transfers takes place from the main channel to the flood plain. The contrast in the results is shown with the help of variation in percentage error over the cross section, root mean square error and Nash–Sutcliffe efficiency (E). [ABSTRACT FROM AUTHOR]

Motivated in part by the problem of secure multicast distributed storage, we analyze secrecy rates for a channel in which two transmitters simultaneously multicast to two receivers in the presence of an eavesdropper. Achievable rates are calculated via extensions of a technique due to Chia and El Gamal and the method of output statistics of random binning. Outer bounds are derived for both the degraded and non-degraded versions of the channel, and examples are provided in which the inner and outer bounds meet. The inner bounds recover known results for the multiple-access wiretap channel, broadcast channel with confidential messages, and the compound MAC channel. An auxiliary result is also produced that derives an inner bound on the minimal randomness necessary to achieve secrecy in multiple-access wiretap channels. [ABSTRACT FROM AUTHOR]