In this research an attempt has been made to model flow field in compound channel with skewed floodplains for two skew angles of 5. 1o and 9. 2o and relative depths of 0. 15, 0. 24, 0. 41 and 0. 50. For numerical modelling the k- turbulence model and the ANSYS-CFX software were used. The results of numerical modelling of the velocity and boundary shear stress distributions at two selected sections were then compared to the Flood Channel Facility Experimental Data. The study shows that more or less there are good between the experimental data and the results of numerical modelling. Using the momentum equations for the control volumes on the floodplains, the interaction between flow in the main channel and on the floodplains for different skew angles and relative depths has been investigated. The study indicates that in general by increasing the skew angle and water depth due to mass exchange between the subsections, the apparent shear forces at the vertical interface between the main channel and floodplains increases.

During a flood, water flows out of the main channel and enters the floodplains. In nature, due to the influence of local topographic conditions, rivers have often irregular and non-prismatic shape. In non-prismatic compound channels, because of the effect of changing in cross-sectional area along the channel, the flow structure is more complex than prismatic compound channels. Therefore, it is important to study the flow characteristics in such channels and it can help researcher to better understand the flow behavior in rivers, especially during floods. In the present study, the flow characteristics in a compound channel with inclined and skew floodplains has been investigated experimentally. The experiments were performed at three relative depths of 0.2, 0.3 and 0.4 and for two skew angles of 11.31° and 3.81°. The water surface profile, lateral distributions of velocity and boundary shear stress along the skew part of the flume have been measured and reported. The results of experiments indicate that the lateral distributions of velocity and boundary shear stress in skewed compound channels with inclined floodplains are non-uniform and asymmetric. Also, the flow velocity and boundary shear stress in diverging floodplain are always higher than its value in converging floodplain. By increasing the skew angle of the floodplains from 3.81 to 11.31 degrees, the boundary shear stress and flow velocity in the main channel and on the floodplains increase.

Compound channels form the cross section of many rivers, especially in adjacent areas of residential and agricultural areas Investigation of hydraulic behavior of compound channels is of great importance in floodplain control and river management plans. In this study, by modeling a concrete compound channel with Ansys Fluent software using k-ε,turbulence model and VOF method, shear stress and average flow velocity in compound channel with divergent floodplains were determined for different divergence angles. Also, the boundary conditions for flow in the studied channel are defined as input mass flow rate, output limit as pressure outlet, solid boundaries as non-slip and rough wall and upper channel boundary as non-slip and rough wall. The modeling and comparison between the laboratory data and the results of the present study showed that the maximum velocity in the channel occurs with a divergence angle of 3 degrees and with increasing the divergence angle, the flow velocity also decreases. In addition to, maximum of shear stress in the compound channel occurs with a lower divergence angle,In other words, as the divergence angle of the floodplain increases, the amount of shear stress of the fluid also decreases, and the higher the divergence angle, the lower the velocity. Also, the maximum shear stress in the channel with a divergence angle of three degrees and the minimum shear stress in the divergence angle is 15 degrees.

Differences in the flow properties in the main channel and flood plains causes, mass and momentum tensions between the both sections. The non-prismatic compound open channel cross section intensifies the mass and momentum transferring between the main channel and floodplains and has significant effect on the flow properties through the compound open channel.In this study the flow properties in the heterogeneous Roughness Non-Prismatic Compound Open Channel was assessed using the numerical and physical modeling. The physical modeling was conducted in the hydraulic laboratory center of Tehran University and numerical modeling was carried out using the Flow-3D as famous computation fluid dynamic tool (CFD).The results indicates that the Flow-3D is an effective tool for modeling the flow in the heterogeneous roughness non-prismatic compound open channel. During the CFD modeling it was found that the RNG turbulence model is more precise for simulation and modeling the flow properties. The results show that the heterogeneous roughness has significant effect on the flow characteristics such as velocity distribution and share stress gradient.

Introduction: Sediment transport is one of the most basic and important characteristics in river hydraulics and bed morphology. The prediction of sediment transport path in rivers and also cannels is absolutely complicated, and mostly conducted with semi-empirical methods. In such cases, the Lagrangian method is essential for exploring the physics of individual sediment particles. The investigation of the flow pattern in the compound open-channel originated in 1960s and followed by the exploration of turbulence structures of overbank flows. However, studies on the characteristics and processes of sediment transport in the compound channels are rarely conducted. For completion this gap, in this experimental study, the rolling and sliding motions of individual bed particle in the floodplain of a rectangular compound open-channel have been experimentally investigated. Specifically, the mechanical parameters of particle motions such as velocity and acceleration are investigated. In this regard, different statistical distributions, especially Gaussian or normal distribution, are employed to introduce the properties of bed sediment motions in the floodplain. Methodology: The experiments were conducted in the hydraulic laboratory of Tarbiat Modarres University in a straight open channel with length of 10 m, width of 1 m and height of 0. 7 m (Fig. 1). The laboratory flume is a wide rectangular channel with a compound section (Fig. 2), where the side wall and bottom of the channel are made of glass. The main channel is 0. 4 m wide and the floodplain is 0. 6 m wide. To control the water depth, an adjustable weir was used at the end of channel. The discharge at the inlet of the channel was controlled using a regulating valve downstream of pump and measured by an electromagnetic flow-meter. The hydraulic conditions of the experiments are summarized in Table 1. According to the calculations, the Reynolds and Froude numbers are respectively 28000 and 0. 34. Therefore, the flow in the compound channel of the present study is turbulent and subcritical. The flow depths in the floodplain and main channel are 5 and 20 cm, respectively. To capture high quality images from bed particle motions in short intervals, a camera with the speed of 24 frames per second and FullHD resolution was used (Fig. 3). To improve the quality of the images, the floodplain and main channel bottoms were coated with black color in the measurement zone. Moreover, for detection of the particle trajectories, the measurement zone was regularly meshed by the perpendicular lines with the distance of 10 cm. Several projectors were applied at different angles for illumination of the measuring plane. The spherical bed particle characteristics of the present study are mentioned in Table 2. Particle tracking were conducted at the distances of 5, 20, 40, and 50 cm from the floodplain side wall (Fig. 4), and repeated about 20 times for each one. Results and discussion: Chi-Squared test were used to determine the appropriate distribution to describe the longitudinal and transversal velocity and acceleration of individual particles (Fig. 5). Also, skewness and kurtosis of the data are employed to investigate the fitness of velocity and acceleration data to the normal distribution (Eqs. 2 and 3). In the case of sediment release at 20 cm from the floodplain side wall, the skewness values for the particle longitudinal and transversal velocities are always close to zero and their kurtosis values are close to 3, . This indicates that the particle longitudinal and transversal velocities follow the normal distribution. However, kurtosis of longitudinal acceleration diverges from 3, and consequently, it does not follow normal distribution (Table 3). The averaged longitudinal and transversal velocities of the sediment particles increase, approaching to the interaction zone (Fig. 6). Also, the standard deviation of longitudinal and transversal velocity and acceleration values increase with the increase of distance from the floodplain side wall (Fig. 7 and 8). Kurtosis of streamwise and spanwise velocity and acceleration of sediment particles increase far from floodplain side wall (Fig. 9), duo to the uniformity of particle motions in the interaction zone. The linear relationship between the average particle velocity and flow shear velocity indicates that there is a good agreement between the results of the present study and previous researches. Conclusion: The results of this study show that the sreamwise and lateral velocity and spanwise acceleration histograms of spherical particles in the floodplain far from the interaction zone, could be fitted to the normal distribution. While the kurtosis of histograms increases considerably, approaching to the junction. The histogram of streamwise acceleration does not fitted by the normal distribution. The histogram kurtosis of velocity and acceleration is enhanced approaching the interaction zone.

Introduction: A compound channel consists of one main channel with a deeper flow in the middle and one or two floodplains around the main channel with lower flow depth. The difference between velocity in the main channel and on the floodplains in compound channels creates a strong shear layer at the interface between the main channel and floodplains. Also, because of the three-dimensional (3D) structure of flow, the investigation of flow characteristics in compound channels is completely complicated. In non-prismatic compound channels, due to the mass exchange between subsections, the study of flow is more complex. Therefore, the prediction of flow behavior in the non-prismatic compound channel is an important subject for river and hydraulic engineers. The skewed compound channel is one kind of non-prismatic compound channels. In compound channel with skewed floodplains, one of the floodplains is divergent and the other is convergent. The flow patterns in skewed compound channels have been studied experimentally by many researchers (James and Brown, 1977; Jasem, 1990; Elliott, 1990; Ervine and Jasem, 1995; Chlebek, 2009; Bousmar et al., 2012). However, numerical studies on flow characteristics in skewed compound channels were rarely performed. In this research, the velocity, boundary shear stress distributions, secondary current circulation, and water surface profile in a compound channel with skewed floodplains have been numerically investigated using the Computational Fluid Dynamics (CFD) and two turbulence models of the RNG and LES. Methodology: In the present research, modeled compound channel is similar to the experimental channel used by Chlebek (2009) at the hydraulic laboratory of Birmingham University, Department of Civil Engineering. The experimental studies were performed in a straight flume of 17 m long, 1. 198 m wide, 0. 4 m deep, and with an average bed slope of 2. 003×10-3. The PVC material was used to make compound cross-section. A rectangular main channel of 0. 398 m wide and 0. 05 m deep in the middle, and two floodplains with 0. 4 m wide around the main channel (Fig. 2). The skewed compound channel was made by isolated floodplains using L-shaped aluminum profiles. Experiments were conducted at the skew angle of 3. 81° and four relative depths of 0. 205, 0. 313, 0. 415, and 0. 514. The lateral distributions of depth-averaged velocity and boundary shear stress were measured at six sections along the skewed part of the flume (see Fig. 3), using a Novar Nixon miniature propeller current meter and Preston tube of 4. 77 mm diameter, respectively. For numerical simulations of the flow field in the skewed compound channel, the FLOW-3D computational software was used. Also, the renormalization group (RNG) and Large Eddy Simulation (LES) turbulence models were selected as turbulence closure. Two mesh blocks were utilized for gridding, mesh block 1 by coarser mesh size at the upstream of the skewed portion of the channel, and mesh block 2 by smaller mesh size for skewed part (Fig. 5). The flow field is numerically simulated by three computational meshes (fine, medium, and coarse mesh size). Details of gridding for different computational meshes are summarized in Table 2. Finally, the medium mesh by 1653498 cells was selected. For boundary conditions, using volume flow rate condition for inlet, outflow condition for the outlet, symmetry condition for water surface area and the interface of two mesh blocks, and wall condition for lateral boundaries and floor (see Fig. 8 and Table 3). Results and Discussion: The results of the numerical simulations show that the RNG turbulence model, can predict the depth-averaged velocity and boundary shear stress distributions in the skewed compound channel fairly well (Figs. 9 and 10). In addition, in the skewed compound channel, the mean velocity and boundary shear stress on the diverging floodplain is more than converging floodplain at the same section. The longitudinal discharge distribution on floodplains of the skewed compound channel is linear, and the numerical modeling can compute those values very well (Figs. 11 and 12). By moving along the skewed part of the flume, the regions with higher velocity move toward the diverging floodplain. Also, the position of the maximum velocity, instead of the main channel centerline, move to the interface between the main channel and diverging floodplain (see Figs. 13 and 14). The lateral flow that leaves the converging floodplain, plunging into the main channel flow, creates a secondary flow circulation in the main channel and near the converging floodplain. Also, as moving along the flume and get close to the end of the skewed portion, this secondary flow becomes stronger (Figs. 15 and 16). Regarding the water surface profile in the skewed compound channel, two turbulence models can predict the water depth along the channel fairly well, especially the RNG turbulence model (Fig. 17). In addition, the error analysis by using experimental data and numerical results are investigated. For error analysis, Mean Absolute Error (MAE), Mean Absolute Percentage Error (MAPE), Root Mean Square Error (RMSE), and the coefficient of determination (R 2 ) were calculated by using the equations of (12) to (15), respectively. The computational errors between the results of numerical simulation and experimental data are presented in Table 5 and are showed in Figs. 18 and 19. Conclusion: In this research, the flow field in a compound channel with skewed floodplains has been numerically simulated. The FLOW-3D software and two turbulence models of the RNG and the LES were used to model the depth-averaged velocity, boundary shear stress distributions, and discharge distribution at different sections along the skewed compound channel. The results of simulations indicated that compared to the LES turbulence model, the RNG turbulence model are able to predict the velocity and bed shear stress distributions quite well especially at the first half of the skewed portion. Also, by increasing the flow depth, the accuracy of numerical modeling for prediction of the velocity and bed shear stress increase, while for the water surface profile decreases (see Fig. 18).

In non-prismatic compound channels due to the mass and momentum exchange between the main channel and floodplains, the flow interaction between channel sub-sections is higher than prismatic compound channels. As a result, in this form of channels, the study of apparent shear forces create at the vertical interface between the main channel and floodplains are much more complex. In the present research, the velocity and boundary shear stress distributions were measured at different sections along the skew part of the flume. Also using the experimental data and the momentum equations the flow interaction between the main channel and floodplains have been investigated. Measurements have been performed for two skew angles of 11.31 and 3.81 degrees and three relative depths of 0.2, 0.3 and 0.4. Investigations indicate that the flow interaction between the main channel and the converging floodplain is generally greater than the flow interaction between the main channel and the diverging floodplain. Also, the side slope of the floodplains, has increased the amount of apparent shear force at the interface between the main channel and floodplains.

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.