مجله هیدرولیک
سال شانزدهم شماره 1 (بهار 1400)

The roughness parameter in hydraulic modelling of natural river and channel flows is not measurable easily and accurate point determination of roughness coefficient, its spatial and temporal variations includes several uncertainties that acts as the main source of error and uncertainties in hydraulic modeling. These drawbacks restricts the applicability of hydraulic modelling in river engineering projects, flood control and management, re-habitation and river restoration. Due to the uncertainties in rating curve, stage, top water width, stream power, shear stress, Froude number and velocity. Because of these drawbacks, in the current study uncertainty analysis by Monte-Carlo Simulation (MCS) combined with HEC-RAS model.

In the current study uncertainty analysis, by Monte-Carlo Simulation (MCS) combined with HEC-RAS model is used to study a 105 km reach of Karoon River from Mollasani to Farsiat as shown in Fig. 1. The model is calibrated and verified using two year daily data of river flow and stage levels in Ahvaz station at the middle of the river reach. A computational control module is developed and combined with computational core of HEC-RAS to perform MCS automatically and the flowchart of modeling strategy and uncertainty analysis is presented in Fig.2. The MCS approach is coupled with computational core of HEC-RAS model by developing a subprogram that create and modifies the input files of HEC-RAS, run it automatically based on random samples of n Manning, and extracting the results of HEC-RAS model in each execution for further analysis in an automatic procedure. By using probability distribution of Manning roughness, 3000 simulations performed and graphical and quantitative indices used to evaluate the uncertainties of model results. In order to refine proper MCS from non-proper ones, the NSE>0.75 index is used to objectively sample n Manning from uncertainty analysis. The uncertainty analysis of proper MCS evaluated by 5 and 95% uncertainty bounds. The uncertainty analysis of model results are evaluated based on the six parameters of water surface elevation, top width of water, flow velocity, Froude number, stream power and shear stress in 3000 runs of peak flow and mean flow discharges respectively and quantified by two indices of 95PPU and d-factor.

The calibration and verification results of the HEC-RAS model in Figs 3-4 shows that in the calibration data set the R2 and RMSE of model in discharge are 0.94 and 21 (m3/s); and 093 and 0.6 (m) for water stage respectively. These values in the verification stage were 0.94 and 25.2(m3/s) for discharge; and 0.91 and 0.1(m) for water stage respectively. The results in 105 km length of Karoon River reveals high level of uncertainties with d-factor greater than 1 up to 11 in peak discharge of 3000 and mean daily discharge of 457 m3/s. These results revealed that using conditional evaluations based on NSE>0.75 reduced the uncertainty of d-factor in results of rating curve, stage, top water width, stream power, shear stress, Froude number and velocity. The d-factor of water stage reduced from 2 to 0.07 in peak discharge, and from 0.96 to 0.02 in average flows. These uncertainty reductions in top width of water were 2.5 to 0.19 in peak discharge and 1.3 to 0.078 in average flows of Karoon river. The highest uncertainty of HEC-RAS model results observed in water velocity and Froude number with di-factor 10.85 and 7.44 in peak discharge respectively. This trend of uncertainty reduction observed for water velocity, Froude number, stream power and shear stress along the river, as provided in Tables 1-3. The spectral responses of hydraulic parameters in model result that presented in Figs 5-10, indicate that although the HEC-RAS model produced high uncertainty values, especially in the complex domain of Karoon river, but these uncertainties dos not deviates the hydraulic patters of river flow in the study reach. The peak and maximum values and the zones of high vales of parameters, show high level of uncertainty than the small or moderate hydraulic situations. These indicates the inherent uncertainty in model results that causes high extents of spectral responses for model simulations. The provided findings necessitates the accurate determination of roughness coefficient according to its spatial and dynamic variations along the river reach.

The uncertainty results revealed high level of latent uncertainties in HEC-RAS model results and probabilistic analysis of models results is required for river re-habitation and management practices of large rivers such as Karoon River to provide certain and reliable results. The presented methodology and framer in the current study that uses automatic control and automation of HEC-RAS runs, strengths the modeling capability of one dimensional river flows for probabilistic analysis and automatic calibration of this mode.

Due to the simplicity of construction, vertical drops are widely used to reduce the steep slope of the canal and the volume of earthworks in irrigation and drainage canals. The upstream regime of flow in structures can be subcritical or supercritical. Stilling basins are usually used to dissipate the energy and prevent the bed erosion. Due to the fact that concrete materials are used in the construction of the stilling basin, Hydraulic engineers are always looking for a way to minimize the construction cost of downstream stilling basin and increase downstream energy loss of these structures. The dimensions of the downstream stilling basin depend on the geometry and hydraulic parameters of vertical drop. So In the present study, the effect of serrated drop edge on energy dissipation is investigated numerically using Flow3D software. Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses computers to analyze and simulate complex fluid problems. Flow-3D software is one of the most widely used software in the field of computational fluid dynamics.One of the prominent features of this software is the ability to simulate free-surface flow by VOF method. The governing equations of fluid flow are continuity and momentum equations. In Flow 3D software, several turbulence models are implemented. In the present study, k-ε and RNG turbulence models were used to perform the simulation. An experimental vertical drop set up with a height of 25 cm, width of 46 cm and a relative critical depth ranging from 0.2 to 0.35 was used for simulation. Total relative energy loss was used to validate the numerical results. Afterwards, different arrangements of dented (serrated) edge were used to simulate the flow on a vertical drop. The squared shapes in plan were used. The dimensions of dented edges which distributed symmetrically along the width were 6.9 and 4.6 cm (0.15 and 0.1 times the width of the flume) and their thicknesses were 2 cm. So, the number of dented edges was 3 and 4, respectively. The total number of meshes was considered to be 1237500. According to the dimensional analysis, the relative energy loss can be expressed as equation (1): ΔE/Eu=f(yc/h, n, α) (1) where, yc/h is the relative critical depth, n is the number of serrated edge and α is the relative dimensions of the serrated edge. The RNG turbulence model showed better agreement with laboratory values compared to the k-ε turbulence model. The results showed that the use of dented vertical drop increases the energy loss for the same relative depth in downstream, length of falling jet and the turbulence intensity compared to the simple vertical drop. In the dented model, irregularities in the streamlines of downstream increased significantly. Increasing in dimensions of the dented edges and decreasing their number caused more irregularity in streamline and augmentation of the turbulence. So, the model with 3 dented edges (relative dimension of 0.15) performed the most turbulence and irregularity in the downstream streamlines. Energy losses in vertical drop with 3 and 4 dented edges and ordinary vertical drop are compared. The average energy losses were 26, 38, 15 and 25 percent, respectively. Although the use of dented edges increases the length of falling jet, but the stilling basin length for energy loss in models with dented edges is less than the ordinary model. According to the results of the present study, the vertical drop with 3 dented edges and relative dimension of 0.15 performs the highest energy loss as compared to the ordinary vertical drop and other models of the present study. In this study, the Froude number ranged from 3.7 to 4.5 in the ordinary vertical drop to 2.7 to 2.9. Since a stilling basin is usually constructed at downstream of the vertical drop to dissipate the destructive kinetic energy of the flow and the dimensions of the stilling basin depends on the Froude number, so the use of dented edges in the vertical drop has such advantages as reduction in basin dimensions, augmentation in energy loss and lower depth for tail-water to form the hydraulic jump. Therefore, considering the hydraulic and economic conditions of the design, it is possible to use dented edges in practice to reduce the dimensions of the stilling basins and increasing the energy loss of flow in downstream of vertical drops. Some other features and conditions are not considered in this study. So, it is suggested that the effect of angle of dented edges on energy loss and other hydraulic parameters could be investigated.

In analyzing of hydraulic phenomena and turbulent flow problems, the most appropriate solution is to use physical and laboratory models. Due to the effects of precise measurement in the experimental works, in the turbulent flow fields, accurate measurement of velocity components helps to a better understanding of flow dynamics. Acoustic Doppler velocimetry (ADV) is a velocity measurement instrument and is among the most widely used methods in various hydraulic engineering applications both in the laboratory and field. The ADV instrument measures both of the mean and fluctuating characteristics of all three components of the velocity field. The most important reasons for using the ADV as a priority to other velocity measurement techniques are the portability of the device, the ability to measure turbid current, and the three-dimensional velocity measurement. Due to the presence of noise and spikes in ADV's velocity measurements, statistical parameters of velocity may be affected. For this reason, the post-processing of velocity measurements of acoustic Doppler velocimetry is essential in the hydraulic based research. Most of the studies in this field have been carried out within the channel, where the turbulence intensity is relatively low. So, it is necessary to evaluate the efficiency of the proposed methods in the flows, especially with high turbulence intensity. The experiments were carried out at the hydraulic laboratory of the civil engineering department of Amirkabir University. To minimize wall effects, experiments were carried out in a 1 × 1.7 × 0.54 m3 upstream basin connected to a 6-m-long flume filled with water. An axisymmetric turbulent jet with a circular cross-section with 1cm diameter was emitted into the upstream basin with quiescent water. The jet was fed from a constant-head tank. A Georg Fischer d32 (Schaffhausen, Switzerland) DN 25 flowmeter with a measurement accuracy of 1% (of full-scale value) maintained to adjust the jet flow with a Reynolds number of 10000. The temperature of the water in the jet and that of the water in the basin were the same because the jet was fed from the water of the flume. The velocity field was measured using Nortek Vectrino Plus ADV at two sampling frequencies of 25 and 200 Hz in the self-similarity region. The instrument’s probe consisted of four ceramic receivers and one ceramic transmitter connected to the electronics housing by a stem. To validate the ADV measurements, experiments were carried out in the self-similar zone of an axisymmetric turbulent jet issued into quiescent water. Different types of spike removal and noise reduction filters, such as Goring and Nikora (spike removal), Hurther and Lemmin, and Khorsandi et al. (noise reduction), as well as their combination, have been used to improve the velocity statistics measured at different sampling frequencies. Results were compared with the measurements conducted using other techniques in past research such as Panchapakesan and Lumley (1993), Darisse et al. (2015), and Hussein et al. (1994). ADV measurement verification shows that the amounts of spreading rate (S) and decay rate (B) of the jet are in a good agreement with previous studies that used different types of velocity measurement tools. Results highlight that changing sampling frequency does not significantly affect the amounts of decay rate and spreading rate. It is observed that the spike effect on the mean axial velocity is negligible. Evaluation of the velocity variance for the different ADV sampling frequencies reveals that noise causes a difference in the velocity measurement by different sampling frequencies. Applying different filters to velocity measurement data shows that Khorsandi et al. filter has the best agreement with the previous study. Results also show that Khorsandi et al. filter less dependent on the sampling frequency changing. In the near field of the jet nozzle, a combination of spike and noise post-processing filters has more efficiency on 200Hz sampling frequency data. This can be attributed to the presence of more spikes and noise in the area with more turbulence intensity. Applying combinations of both noise and spike post-processing filters improves the accuracy of the results. Results revealed that the velocity variances measured at the higher sampling frequency were overestimated when compared to those measured at the lower sampling frequency. Post-processing of the data resulted in a better agreement of the statistics measured at different sampling frequencies. The application of combinations of both noise and spike filters are more effective than just using one filter. Finally, for the post-processing of velocity field measurements or near boundary flow measurements with low-quality data, the application of both the noise and spike filters is recommended.

Simulating and understanding of abnormal conditions is one of the most important applications of hydraulic models of water distribution networks. Hence, existence of calibrated models is essential to network behavior realization. This process requires field data collection to improve model's performance by comparing predicted and actual data. Sampling from network has different constraints. Therefore the sampling design process is performed in order to optimize it, which includes different aspects of sampling, such as location, number and frequency. This paper focuses on pressure sampling nodes for hydraulic model calibration. To implement sampling design, first by sensitivity analysis, uncertainty of each nodal pressure is divided between model inputs.

In this paper, a global sensitivity analysis method, Sobol, is used which divides the variance of model into model inputs and their interactions. Then, two criteria for selecting sampling points are defined. The first criterion maximizes the entropy and magnitude of sensitivity values of each parameter for the set of sampling design points. The second criterion, by replacing number of points with sampling costs, follows minimization of sampling costs. To solve the integer multi-objective optimization problem, the multi-objective integer genetic algorithm called MI-NSGA-II is employed.

Investigating different scenarios demonstrates effect of parameter type on the position of selected points. In the meantime, similarity between the results of combinatorial and individual scenarios decreases from cases including roughness to cases involving demand. This indicates effective role of roughness in selecting points in combinatorial scenarios. Also, analysis of combinatorial scenarios suggests that parameter interactions are effective in selecting points.

The results showed that the developed approach offers good performance in selecting sampling points with different scenarios. The MI-NSGA-II algorithm has a good ability to find the solutions of the integer multi-objective optimization problem. The use of pressure driven simulation method is effective on the results of sensitivity analysis and sampling design.

Due to the increasing need water resources, analysis, design and construction of dams is one of the most widely used fields in engineering sciences. In general, dams with their special characteristics have been able use to another of types of the hydraulic structures.One of the most popular numerical methods proposed for the analysis of hydraulic problems is the meshless methods. In the meshless method, the amplitude and boundaries of the structure are created by nodes. The shape function is used to communicate between nodes.In this study first the meshless local Petrov-Galerkin (MLPG) method by Radial Basis Function (RBF) has been explained entirely. In the following, MLPG method is verified by exact solution in a numerical example. The Results show that MLPG method presented high accuracy and capability for solving the governing equation of differential equations problem in meshless methods. Finaly, using RBF (MatLab code was adopted) in the fluid flow in dam breaking problem.

Several numerical methods, such as the finite element (FE) and meshless methods, have been developed in the last few decades for solving governing partial differential equations of engineering problems. Approximation in geometry and imposition of boundary conditions in meshless approaches can be mentioned as the drawbacks of the methods. Furthermore, in some engineering problems such as those which are solved in a Lagrangian framework, geometry and boundaries change and, therefore, discretization of the domain should be modified in case of using the FE method, which is quite costlyIn the meshless methods, the calculation of the integration is based on the Gaussian integration method in the general and the local forms. In the general method, in order to integrate, it is necessary to create meshes in the background of the problem domain; therefore, this method is not a true meshless method. But meshless methods based on the local integration method, such as the meshless local Petrov-Galerkin (MLPG) method, have been proposed. In this way, the governing fluid flow in dam breaking problem is expanded using MLPG method. Radial Basis Functions (RBF) is used to communicate between nodes. In order to discretize the derived equations in time domains, Zienkiewicz and Codina (1995) scheme with suitable time step is used. The Mass and momentum conservation laws are governing equations of flow, which are solved by pressure correction in Lagrangian approach. Then these results are compared with another method results. The results showed high accuracy and good conformity compared to available another solutions and the ability of the proposed method in solution of moving fluid with moving boundaries.

In order to demonstrate the accuracy of the present method for dam breaking, at the first a problem verifying with analytical solution. Table 2 shows the analytical, numerical and error values obtained. The comparison shows the high capacity and accuracy of the present method.After verification, the dam breaking problem is investigated. The geometry of the dam breaking problem is shown in Figure 4 and then the present method compare with isogeometric and the least squares method. By comparing the free surface profile between the three methods, it can be easily understood that the meshless local Petrov-Galerkin (MLPG) method based on Radial Basis Functions (RBF) has the high accuracy. On the other hand, the close nature of the meshless local Petrov-Galerkin (MLPG) method with the least squares method, it is quite clear that the results are in good agreement. The following results are shown in Figures 7 to 9. the water flow velocity resulting from the present method results with the base function of the radial function in 0.15 seconds in the problem compared to the least squares method (Shobeyri and Afshar 2010) and isogeometric (Amini, Maghsoodi et al. 2016). Finally, the pressure obtained from the MLPG method in 0.15 seconds compared to the least squares method (Shobeyri and Afshar 2010) and isogeometric (Amini, Maghsoodi et al. 2016) in Figures 10 to 12 are showed.

By considering the dam breaking problem, it was found, the Meshless Local Petrov-Galerkin (MLPG) method is useful in modeling problems with variable boundary conditions, because only by producing nodes at each stage of analysis can define a new boundary conditions and then in the shortest possible time modeling is done. It is clear the modeling this problem with the other methods such as finite element method is complex, because by changing the boundary conditions, produced the new elements becomes a time-consuming and complex matter. The Meshless Local Petrov-Galerkin (MLPG) Method is an intelligent design for solving problems of variable geometric conditions.

Masonry check dams are one of the common structures which used to regulate slope and control erosion in the watersheds. Due to the hydraulic head difference of water flows upstream and downstream of these structures, the flow has a lot of kinetic energy after passing through the structure that should be dissipated in the settling basin and before entering the river in downstream. Experience has shown that this phenomenon can make continuously erode the substructure and destroy it. One of the methods to control scour of downstream of the structures is using the stilling basin with appropriate length. The stilling basin is usually designed in such a way that the hydraulic jump is completely formed inside the stilling basin. However, due to the high energy of jet passing over the crest of check dam, scouring may still occur downstream. Looking to design of check dam shows that the there are two rows of pipes which can act during low flow condition in order of connectivity of upstream and downstream flow in rivers. During high flow condition the flow passes from the series of pipes and over the crest of check dam structures. Due to the fact that flow over the crest can interact with flow through pipes during high flow condition, therefore the flow pattern is complex and causes the scouring downstream of stilling basin.

A physical hydraulic model of Tul Beneh with scale of 1:20 was used for simulating of scouring during different hydraulic conditions. The check dam height in prototype is 6 m and the check dam width is 20m. There are two rows of pipes with diameter of 0.6 m inside the body of checkdam structure. The experiments were conducted in a recirculating channel flume 9 m length, 1m width and 1m height. The side wall is made with glass to facilitate observing the scouring and flow pattern from side view. the flow depth was adjusted by tail gate. The sediment sample was taken from surface and subsurface of the river bed and size distribution of sediment was obtained with sieve analysis. The experiments were design in such condition that the flow can passes the series of pipes installed in two rows and over the weir structure. The flow discharge was measured by ultrasonic flow meter. The water surface profile and scouring pattern were also measured by 3D bed profiler instrument. The experimental results were also compare with different empirical formulas such as Borman and Julien (1991),Scurlock et al. et al. (2012), Fahlbusch(1994), Catakli et al et al. (1973), Novak(1961), Farhoudi and Smith (1982, 1985) and Dargahi (2003).

For assessing the time of experiments, a test was done for 48 hours and the temporal evolution of scour depth was measured. The results showed that 75 and 94 percent of maximum scour depth was occurred during 1 and 8 hours from the starting of experiments, respectively. Comparison of results with available empirical formulas showed that the Farhoudi and Smith (1985,1982) and Dargahi (2003) give more close results with the present results. The results also showed that the maximum scour depth occurred when the flow pass over the check dam structure. The comparison of results showed that when the flow pass through only the pipes the hydraulic jump formed in stilling basin and the energy of flow can be dissipated along the stilling basin. By increasing the flow, the flow pass through the pipes and over the crest of check dam structure. The sedimentation upstream of check dam causes the pipes filled with sediment, so the flow passes over the check dam and causes major scouring downstream of stilling basin. The results showed that there is sedimentation bar downstream of scouring region which can extend more when the flow passes both through the pipes and over the crest of check dam structure.

Complex flow pattern was observed when the flow passes through the pipes and over the crest of check dam structure. The scouring pattern showed that the scouring expanded both vertically and laterally when the flow pass over the crest of check dam. The ratio of maximum scour depth to downstream flow depth is 2.2 when the flow passes through the pipes and crest of check dam. This ratio is 0.66 when the flow only passed through the pipes. The maximum scour depth in prototype was 0.86 m which is 66 percent of downstream flow depth in river and shows that the flow passes through the pipes during the operation of check dam structure due to sand mining upstream of check dam structure.

One of the most important topics in river engineering is the study of the morphological condition of the river, which deals with the expression of the geometric shape, bed form, longitudinal profile of the canal, cross sections, deformation and displacement of the river over time. In fact, rivers and streams are a completely dynamic system, and their position, shape, and other morphological features are constantly changing over time (Rangzan et al., 2008). So far, several studies have been conducted to evaluate the effects of river engineering and water resources engineering projects on the hydrodynamic status of the river (e.g., Yamani et al., 2007; Arshad et al., 2008; Vaezipour et al., 2020; Asghari Saraskanroud, 2013; etc.). The aim of this study is to evaluate these changes according to the implementation of various projects in recent years in the Karaj River that have imposed significant morphological changes on the river system and will change its behavior and response in the future. In this study, possible morphological changes in the Karaj riverbed due to Karaj river canalization projects, construction of Alborz recreational lake and artificial recharge have been investigated. The models used in this research include one and two dimensional HEC-RAS flow models (4.1 & 5), HEC-RAS sediment model and RVR Meander model for morphological simulation. In these modellings, three hydraulic scenarios were performed to investigate the flow status in the initial state (without designs), within the channelization design and within the artificial recharge project, and modeling of bed changes (sediment) and morphology were performed separately. Scenario 1- According to the results obtained from the implementation of the two-dimensional model, the areas located on the right bank of the river, especially the area located in Chamran Park, as well as residential areas located at the beginning of the study reach due to being located in the river area are significantly flooded. One of the main reasons for this is the low altitude of the areas located on the right bank of the river. Scenario 2 - In this scenario, the hydraulic flow is evaluated during the implementation of the Karaj River channelization and Alborz Lake projects. The results of two-dimensional modeling in the area of the main outlet indicate that the velocity of the current passing through this area is extremely high. According to calculations, the values of velocity along the longitudinal section of the channel vary between 2.5 to 4 meters per second, which is a significant number. Such velocity values can lead to erosion of the riverbed and destruction of the riverbed at the outlet of the waterway. Scenario 3- In this scenario, the effect of constructing lakes of artificial recharge plan of Shahriar plain on the flow pattern and sedimentation process of Karaj river has been evaluated. According to the results, the proposed structures increase the water level, reduce the velocity and provide suitable conditions for water infiltration in the riverbed. Turkey nest (dam in the earth) reduce the velocity of the flow and penetrate more and more into the ground, but it is necessary to mention that due to the significant sedimentation of the river and the presence of sand factories upstream, a lot of fine sediments (caused by washing materials in the mines) will enter the lake of each of these structures. The most important results can be summarized as follows: • Backflow upstream of Alborz Lake due to insufficient dimensions of the lake intake and the constriction caused by the implementation of the channelization project, will cause significant energy losses at the beginning of the project and this will cause water return and flood spread upstream. • The occurrence of turbulence and rotational flows due to improper angle of connection between the channel and the freeway bridges in the future will have a significant impact on the hydraulic flow and sediment of the river, especially in the area of the bridges. • Turkey nest structures reduce the velocity of the flow and penetrate the ground as much as possible, but it is necessary to mention that due to the significant sedimentation of the river and the existence of sand mines in the upstream areas, a lot of fine sediments enter the lakes and it will reduce their permeability and their performance. • Construction of wide structures with only one outflow weir will cause the formation of passive areas in the reservoirs. The existence of such areas, in addition to providing suitable conditions for sedimentation, will also create a wetland state. • Due to the high velocity of the outflow from the lake, the riverbed has suffered significant erosion and this erosion is due to the concentration of flow in the bed and the left bank of the river and will greatly increase the possibility of bank failures. • Sediment modeling showed that in a relatively short period of time, relatively large sediments occurred upstream of the Turkey nests. The results show well that the reservoirs are almost full of sediment to overflow crest height, and in practice the discussion of aquifer recharging will face a major challenge. • Based on the morphological model outputs, the main axis of the river, especially downstream of the Karaj-Tehran freeway bridge, has undergone many changes over time, and this issue poses a major threat to the left bank of the river. • Based on the future morphological simulation of the Karak River, the presence of the Karaj-Tehran metro bridge in the outer arch of the river will cause the river to flow to the right bank in the long run and this will cause the destruction of the right bank wall and this area will need to be protected.

The velocity difference in the main channel with higher velocity and floodplain with lower velocity creates a strong shear layer in their junction, causing the production of additional turbulence structures, especially large-scale vertical vortices in this interface. In addition, because of turbulence anisotropy in the bottom and wall of the channel, secondary currents occur around the longitudinal axis and in a spiral shape. On the other hand, in most cases, due to the existence of vegetation on floodplains, investigation of the flow mechanism is far more complicated. There are usually three methods to explain the flow field and shear stress with the existence of vegetation on floodplains: 1) field measurements, 2) hydraulic models, and 3) analytical and numerical models. In natural rivers, since the flow cross-section changes along the river and the cross-section shape changes from prismatic to non-prismatic, with these conditions causing more mass and momentum exchange from the floodplain to the main channel and vice versa. this study has explored the effects of divergence angle and vegetation density on the flow structures in a non-prismatic compound channel. The experiments of this study were performed in an asymmetric compound channel made of Plexiglas with a length of 12 m, width of 0.6 m with a bed slope (So) of 0.8810-3. In order to model the vegetation on the floodplain, rigid cylindrical plastic rods with a diameter of (D) 10 mm were used. The spacing ratio (Sr = ly / D) for the three vegetation densities will be equal to 5, 7.5, and 10. Three divergence angles () equal to 3.8, 5.7, and 11.3 ͦ were created on the floodplain. Due to the formation of non-uniform flow in non-prismatic sections, the relative depths (Dr=yf/H) of 0.15, 0.25, 0.35, and 0.45 were set in the middle of the divergence region for all experiments. The longitudinal, transverse, and vertical components of the instantaneous flow velocity were measured by a 3D Vectrino profiler velocimeter at three sections: entrance, middle, and end of the divergence region. Using the transverse distribution of depth-averaged velocity, contribution of each section to the conveyance capacity was calculated. Due to the interaction between the rods, the flow structures are very different from the behavior of a single rod; thus, this should be considered in calculating the drag coefficient of an element set on the floodplain. For determine f, the Keulegan (1938) equation for smooth surfaces was modified. Jafari et al. (2011) proposed the an equation to calculate Strouhal number in a row arrangement. Because of outbreak the Kelvin–Helmholtz instability due to the existence of vegetation on the floodplain, in the interface between the main channel and the floodplain, coherent vortices and intense momentum exchange were formed from the main channel to the floodplain. Since the flow momentum prepared a shear layer around the vegetation stems, which causes inflection points in the velocity profile, which is consistent with Sanjou et al. (2010), Mulahasan et al. (2017), and Ahmad et al. (2020) results. At Sr = 7.5, the distance of the elements well forms Von Kàrmàn vortex streets and increases the flow resistance. At all relative depths, increasing vegetation density has reduced the Ufp / Umc ratio. The discharge rate through floodplain with vegetation has reduced by an average of 58.6 and 69.3% compared to non-prismatic channel without vegetation in the middle and end of the divergence reach, respectively. The results indicate that with increasing Dr, zonal roughness coefficient in the floodplain has increased nonlinearly and is linear in the main channel. This result is consistent with the Musleh and Cruise (2006) research. Drag coefficient has decreased nonlinearly with increasing the rod Reynolds number. In addition, it can be found that the drag coefficient caused by floodplain vegetation is directly related to the vegetation density. The results show that with increasing the vegetation density from Sr = 10 to Sr = 5 on the floodplain in the middle and end of the divergence, the bed shear stress has decreased by 44.2 and 54.6%, respectively. The vortex frequency is a linear function of Rerod and the increasing rate of vortex frequency versus Rerod in the middle of the divergence is higher than the end. In the zone close to the vertical interface between the main channel and the floodplain, the secondary currents have suddenly reached their maximum and minimum values. The results showed that with emergent vegetation, Kelvin-Helmholtz instability caused the generation of primitive Von Kàrmàn vortex streets in downstream of the elements. The existence of vegetation in the floodplain caused a sharp reduction in the bed shear stress in this region and increased it in the main channel. As the vegetation density increased, so did the drag coefficient and flow friction factor significantly. The flow passing through the vegetation was controlled by coherent vortices whose maximum size was in the interface between the main channel and the floodplain.