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).

Water level determination during a flood is always a challenging task for river engineers. During the flood, river channel becomes Compound consisting of the main river channel which carries low flows and floodplains that carry overbank flows.  Flow velocity and structures are affected by vegetation, the degree to which depends on vegetation density, flexibility, type, and whether it is in a submerged or emergent condition. Water surface modeling help for the study of flood waves, water level calculation during flood, stage discharge relation, design of water work structures. This work develops a model which can be used to simulate water surface profile in Compound channel with vegetated floodplains with various vegetation covers. To predict the water surface, experiments have been conducted in the laboratory for different hydraulic conditions. It can be seen from the results that the trend of stage-discharge relationships is found to be an exponential function giving a high value of R2. A multivariable regression model (MRM) has been developed to predict the water surface profile for such channels. The dependency of water surface profiles on four different non-dimensional parameters such as canopy arrangement, canopy density, relative depth and relative distance are analyzed. Using the relevant experimental data, non-linear regression has been performed. The results obtained from the present water surface profile model shows good agreement with the observed data.

Introduction A Compound channel usually consists of a main channel in the middle and one or two floodplains around it. The flow velocity in the main channel is higher than the floodplains, due to its greater depth and smaller roughness. This difference causes the formation of a shear layer at the interface between the main channel and floodplains (as shown by Sellin, 1964); Shiono and Knight, 1991; Tominaga and Nezu, 1991; Bousmar, 2002; and Rezaei, 2006). Tominaga and Nezu (1991), Rezaei (2006), and Sum (2007) investigated the velocity distribution in prismatic Compound channels. Their observations showed that the highest flow velocity is below the free surface. In prismatic Compound channels, as the relative depth increases, the difference between the velocity in the main channel and floodplains decreases. At high relative depths, the effect of the shear layer formed between the main channel and floodplains can be almost ignored (Knight et al., 2018). The maximum interactions between the main channel and floodplains have been observed in relative depths between 0.1 and 0.3 (Shiono and Knight, 1991; Rezaei, 2006). Investigations reviled that, so far, the effect of the floodplain's side slope in prismatic Compound channels has not been investigated. The main objective of this research is the experimental study of the flow field in the prismatic Compound channel with inclined floodplains.Methodology This research was carried out on the flume located in the hydraulic research laboratory of Bu-Ali Sina University. The flume is 18 meters long, 1.2 meters wide, and 0.6 meters deep with a bed slope of 1.63×10-3. Figure 1 shows an overview of the research channel used in this research. In this flume, smooth and rigid boundaries were constructed using PVC material. As seen in Figure 2(b), the flume has a Compound cross-section with a 0.4 m main channel wide, 0.05 m deep, and also two 0.4 m wide inclined floodplains (lateral slope of 0.075). The downstream end of the flume has an adjustable tailgate which is used to control the water surface profile and make uniform flow along the flume. In all experiments, the water surface profiles were measured using a pointer gauge with an accuracy of 0.1 mm. Velocity distributions were measured using an Acoustic Doppler Velocimeter (ADV) every 20 mm laterally and every 10 mm vertically (see Figure 3). The lateral distributions of boundary shear stress were also measured using a Preston tube (outer diameter 4 mm). The velocity distributions and boundary shear stress were measured for five discharges of 27, 30, 35, 40, and 45 lit/s.Results and Discussion The velocity distributions for different discharges are shown in Figure 4. From the figure, it is clear that in the vicinity of the junction edge, the isovel lines bulge upward, and the velocity is decelerated, probably due to low mass and momentum exchanges in this region. Near the main channel walls (0.1 m < y < 0.18 m), the isovel lines bulge downward, and the velocity is accelerated. Also, near the middle of the main channel (0 < y < 0.1 m), the isovel lines bulge upward, and flow is decelerated due to flowing away from the main channel bed. As seen in Figure 3, the flume cross-sectional area was divided into subareas. The point velocity distributions were integrated numerically over the local water depth at each subarea to get the streamwise depth-averaged velocity. The results of depth-averaged velocity calculations for different relative depths are shown in Figure 5. In order to investigate the effect of the floodplain's side slope on the velocity distribution, the depth-averaged velocity has been normalized (Ud/Um) and compared to Rezaei’s Data (see Figure 7). The normalized depth-averaged velocities in the main channel, are almost equal to Rezaei’s data (with some fluctuations). While on the floodplains, those velocities are less than Rezaei’s Data (velocity in Compound channels with flat floodplains).Boundary shear stress is used in river engineering and in studies related to riverbed protection and sediment transport. The boundary shear stress distributions for different discharges are measured and shown in Figure 8. As seen in Figure 8, the bed shear stress distribution follows the same pattern as the depth-averaged velocity.The apparent shear forces at the vertical interface between the main channel and floodplains can be calculated using the momentum equation for a control volume in the main channel (see Figure 12). The results of the apparent shear force calculation show that by increasing discharge or relative depth, the apparent shear force increases and reaches its peak value at a relative depth of 0.363 (see Table 3). The apparent shear forces expressed as a percentage of the total channel shear force on the vertical interface are shown in Figure 13. From Figure 13, it can be seen that the percentage of apparent shear forces at the interface between the main channel and floodplains are always smaller than those values in the Compound channel with flat floodplains.Conclusion In this research, the flow field, including the velocity and bed shear stress distribution, in a prismatic Compound channel with inclined floodplains (side slope 0.075) has been studied, experimentally. The results of experiments have been also compared with Rezaei’s data. The most important results obtained from this research are as follows:The depth-averaged velocity and boundary sear stress distributions follow the same pattern. Both of them show almost uniform flow in the main channel with some fluctuations. While in the flood plains, they are non-uniform with an extreme decreasing trend. In the main channel, the normalized depth-averaged velocity and normalized boundary shear stress are almost similar to Rezaei’s Data. While on floodplains, the normalized velocity and shear stress are non-uniform and less than Rezaei’ data. The study also shows that the apparent shear force at the interface between the main channel and floodplains increases with the increasing relative depth and reaches a peak value at the relative depth of 0.3. The same observations were made by Shiono and Knight (1991), and Rezaei (2006).

One of the important aspects in describing river behavior is its curvature. The production of successive bends in natural rivers is an inevitable component of the process through which the river changes. Understanding the flow behavior in the interaction between the main channel and the floodplain, especially during floods, is necessary to protect soil and water structures. This study was conducted to better understand the hydrodynamics of the middle and turbulent flow in Compound meandering channels. According to the natural conditions of most rivers, due to variation of discharge and depth ratios (flow depth in floodplain to flow depth in the main channel), in this study, a rectangular meander laboratory channel with a constant sinuosity 1. 3 for different depth ratios of 0. 35 and 0. 55 was investigated. The results showed that the size of velocity components (u, v, w) at 0. 35 relative depth was greater than 0. 55, which indicating higher vortex strength and interaction intensity between the main channel and floodplain at lower relative depth. Also, by investigating the secondary flows, the existence of clockwise and counterclockwise rotational eddy currents in the main channel and floodplains and the location of these currents were determined.

Natural rivers are rarely in direct flow because of regulating energy grade-line, and usually have a curved path to which it is referred to as "meandering channels". After the appearance of meandering rivers, with the passage of time and lateral movement of the meanders, the external bending progression and the sinusoidal or curvature is increased. In meandering channels, the curvature of the meandering sections with a dimensionless number can be defined as the sinusoidal which is the ratio of meander length of main channel to the floodplain length. By increasing sinusoidal slope number, flow velocity and river discharge capacity decrease. As a result, the risk of flood has increased significantly and during floods the water level exceeds to the bankfull and then enters to the floodplains. In this case, due to the interaction between higher velocities in the main channel and the lower velocities in the floodplains and the momentum transfer between these two regions, the flow profile is constantly changing. In this research, the hydraulic characteristics of flow including the depth-averaged velocity, the water surface profile, longitudinal velocity distributions, ratios of transverse to longitudinal velocities in the central axis of the main channel and the mean velocity and flow rate of the main channel along the meandering Compound channel have been investigated numerically, regarding the change 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 used. So that, the turbulence model with experimental data have a better compliance. for this purpose, two RNG and k-ε turbulence models were then used and the performance of these two models were investigated to simulate the important hydraulic characteristics of the flow, such as the flow velocity, and it was determined that the RNG turbulence model has a better accuracy than the k-ε turbulence model. In the following, this model was introduced as a final turbulence model for numerical simulations. Numerical simulation results show that by increasing the sinusoidal ratio of channel from 1 to 1. 641, the mean velocity of the main channel section is decreased by 54% on average and the flow rate of the main channel decreases by the average of 38%. Also, by increasing the sinusoidal ratio, the maximum depth-averaged velocity, Ud, decreases from 0. 55 m/s to 0. 38 m/s, and the maximum free surface height of the water rises from 0. 305 m to 0. 332 m in the outer bend of the CS1 cross section. By increasing the sinusoidal ratio causes the ratio of the transverse velocity to be increased longitudinally in the central axis of the main channel, so that its value in the most critical state reaches from zero to 0. 4. As the sinusoidal ratio increases, the maximum length velocity moves towards the right side floodplain (internal bend) and decreases its value. So that by increasing the sinusoidal ratio from 1 to 1. 641, the maximum longitudinal velocity 0. 55 m/s to 0. 42 m/s and its position moves from the centerline of the main channel to the inner bend over the depth of the main channel overflow.

Floodplain vegetation can alter the flow characteristics of a river through the application of redundant drag forces. In this study, turbulence characteristics and flow structure were investigated under the influence of partially double-layered vegetation in a Compound channel. To investigate the phenomenon, a three-dimensional numerical model was used to solve the Navier-Stokes equations and track the evolution of the free surface. To ensure the performance of the model, the numerical results were validated using data from previous experimental studies. The validation results showed that this model captured the flow dynamics with high accuracy. In the next step, the model was used to predict the free surface fluctuations and velocity field of the steady flow in the layered vegetated floodplains. The modeling results showed that the formation of a velocity gradient at the interface between the main channel and the floodplain can lead to the development of secondary flows and the mass and momentum exchange at this interface. In addition, turbulent kinetic energy and turbulent dissipation of the flow through vegetation in floodplains was observed in the numerical results. It was concluded that the layered vegetation can increase the flow turbulence and the dissipation rate of the flow energy. The maximum values of turbulence kinetic energy and turbulence intensity were observed at the interface between the floodplain and the main channel. Therefore, the flow disturbance at the interface between the floodplain and the main channel may increase the mass and momentum exchange in this region.

Introduction: Vegetation in Compound channels by increasing factors such as roughness in the floodplains rather than main channel, velocity difference and momentum exchange between the sub sections, makes the transverse velocity gradient and apparent shear stress increase of the channel interface. In natural rivers, changing the cross section makes the uniform flow convertion to non-uniform. In prismatic channels, shear stress in the interface between main channel and floodplains, influences the transfer capacity and velocity distribution pattern significantly. This effect in non-prismatic channels, due to the extra momentum exchange between the sub-sections is more intense. In such conditions, identifying the flow hydraulic is too complex. Although forming the vegetated and non-prismatic floodplains at the same time in natural rivers is highly probable, there are no specialized studies to investigate the hydraulic of flow in such the conditions. Therefore, in the present study, experimental measurements were conducted in a Compound channel with nonprismatic and vegetated floodplains simultaneously and flow behavior is investigated on it. Methodology: The experiments were conducted in a 10 m long, 2 m wide Compound channel located at the National Laboratory for Civil Engineering (LNEC) in Lisbon, Portugal. The channel cross-section consists of two equal rectangular fl oodplains (fl oodplain width Bfp=0. 7 m) and one trapezoidal main channel (bank full height, hb=0. 1 m, bottom width bmc = 0. 4 m, bank full width Bmc = 0. 6 m and side slope of 45° , sy= 1). The channel bed is made of polished concrete and its longitudinal slope is S0= 0. 0011 m/m. The vegetated floodplains were obtained by covering their bottoms with a 5 mm hight synthetic grass. For the polished concrete, the roughness coefficients i. e. n= 0. 0092 m-1/3s and ks=0. 15 mm are considered and for the synthetic grass, n=0. 0172 m-1/3s and ks=6. 8 mm are used. Measurements were performed for relative depths of 0. 21 and 0. 31. The experiments in the non-prismatic channel performed at two convergent angles of floodplains (θ =7. 25° and θ =11. 3° ). In this cases, the mentioned relative depths were set up in the middle sections of convergences by changing the downstream tailgate. The velocity measurements performed for Entrance, Middle and End sections of convergent angles. Results and discussion: High velocity distribution pattern gradients are observed in nonprismatic channel rather than prismatic channel for a given relative depths. Comparisons between similar sections indicates that by increasing the relative depth, the interaction intensity through the main channel and floodplain decreases. As presence of vegetation on the floodplain leads to the channel transfer capacity decrease and in high values of relative depth, this effect decreases. Except the areas close to wall and interface, the flow on floodplain is twodimensional, while in the main channel and especially in low relative depths is threedimensional. This issue has also been affected by different convergence angles. The maximum velocity generally occurs near the outer wall of the main channel. But, by increasing the relative depth, position of the maximum velocity moves to the floodplain. In the lower relative depth, in vicinity of interface, a bulge is visible in isovel lines that is already reported in previous works. Due to the mass transfer from the floodplain towards the main channel, this bulge occurred more intensively in the middle sections in both the convergent angels. In nonprismatic channel, for all the flow cases, at the interface, the intensity of the secondary flows is more apparent and in the down part main channel flow, a vortex is formed that by increasing the relative depth from 0. 21 to 0. 31 and convergent angels from 7. 25° to 11. 3° , moves from the outer wall of the main channel towards the floodplain. Also at the beginning of the floodplain, one vortex is formed that becomes more apparent by increasing the convergence angles. Due to converging floodplains, in the upper layers, a transverse current is directed from the floodplains to the main channel. This transverse current enters the main channel from both sides and, due to symmetry of flow, plunges to the channel bed and as a result, two helical secondary flows are generated in the main channel, rotating in the channel length which is very important in terms of sediment and pollutant transport. By increasing relative depth, velocity gradient between the main channel and floodplains decreases. Conclusions: In present study, using an experimental model, flow behavior in a prismatic and non-prismatic Compound channel is investigated. Non-prismatic channel consists of two convergence angles; 7. 25° and 11. 3° . All the experiments are conducted at two relative depths of 0. 21 and 0. 31. In order to investigate vegetation cover effects, floodplains are covered with synthetic grass and a vecterino (ADV) is used to measure the fluctuations of instantaneous flow velocity. Variations in the streamwise velocity distribution, secondary currents and Reynolds stresses based on proportions of vegetation and non-prismaticity in the flow hydraulic are investigated. Results show for high relative depth, by increasing convergent angles, the floodplains are less involved in discharge carrying and transferring. The maximum velocity values which occur at the main channel center, by increseang the relative depth and extending secondary currents, towards to the floodplains. By increasing the convergent angle, the roughness values in the main channel and floodplains increases. Distribution of flow mean kinetic energy shows that by increasing relative depth, its values in the middle section decreases for both the convergent angles.

Prediction of flow through the Compound open channel is one of the main problems in the field of hydraulic engineering. One of the main parameter related to the flow properties in the Compound open channel is Shear Stress. The shear stress is because of difference of velocities between the main channel and floodplains. The Shear Stress causes of turbulence and vortex creation on the border of main channel and floodplains. The difference between the roughness of main channel and floodplains intensities the shear stress in the border zone and also decreases total discharge. In this paper, the discharge of flow in Compound open channels was predicted using group method of data handling technique. To this end, related dataset were collected from literature. Involved parameters in modeling are relative hydraulic depth ( ), relative hydraulic radius ( ), and relative roughness ( ) and relative area ( ). To compare the performance of GMDH with other types of soft computing methods, the MLPNN as most well-known soft computing technique is developed as well. Results indicate that the GMDH model with coefficient of determination 0. 91 and root means square error 0. 057 is more accurate than the MLPNN. Reviewing the structure of developed GMDH model shows that and are the most effective parameters on prediction of discharge in Compound open channels.

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.

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.