عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The river, as the main artery and natural drainage, plays a significant role in the transmission of floods. During floods, the water crosses the main section of the river and enters the floodplains. In this case, the river crossing becomes a compound cross section. Compound channels consist two hydraulic sections: main channel and floodplain. Natural rivers are rarely in direct flow because of the regulating energy grade-line, and usually have a curved path to which it is referred to as "meandering channels". In meandering rivers, the floodplain width is constantly changing according to the topography of the region. The change of the floodplain width has a significant role on the hydraulic conditions of the flow during the floods. Therefore, the aim of this study is to investigate the effect of changing the width of the floodplain and the relative depth of the channel on the flow velocity and shear stress of the bed in the meandering compound channels, using numerical modeling.
In this research, using FLOW3D software, the flow velocity and bed shear stress in meandering compound channel under the influence of floodplain width change and relative depth are evaluated. For this purpose, four sections of meandering compound channels with floodplain widths of 3.3, 4.31, 5.32 and 6.33 m, and relative depths of 0.26, 0.35 and 0.45 m have been used. According to studies performed on compound channels, the RNG turbulence model has better adaptation to laboratory data, therefore it has been employed used in the current study. In this study, the boundary conditions applied to the numerical model are for the channel’s upstream boundary, the volume flow rate and for the downstream boundary of the Outflow. The boundary condition of the wall was used to apply the boundary conditions in the floor and side walls of the channel. The upper surface of the flow field was also defined as symmetry boundary conditions in the model. The laboratory data of Liu et al. (2014) were used to validate and control the results extracted in the present study. In the laboratory study of Liu et al., the flow conditions in channels FA1, FA2 and FA3 (according to Table 1) were investigated. In the present study, the data of the depth averaged velocity in CS1 section, stage-discharge curve and the ratio of transverse velocity to longitudinal velocity in CS1.5 sections were used for validation. In order to evaluate the accuracy of the numerical model results, after examining the different meshes according to Table 3, the Root Mean Square Error (RMSE), according to Equation 5, has been used. The root error value of depth averaged velocity in CS1 section is 0.081 and 0.052 m/s for the relative depths of 0.45 and 0.35, respectively, and 0.025 m3/s for the stage-discharge curve, which indicates the appropriate accuracy of the flow simulation by the software.
The results of the numerical simulation show that by increasing the floodplain width, the depth averaged velocity and bed shear stress are reduced. Therefore the maximum flow velocity changes by increasing the floodplain width from 3.3 to 6.33 m (92% increase), from 0.51 to 0.39 m/s (24% decrease) and the depth averaged velocity at relative depths of 0.45 and 0.26 are reduced by 17% and 21%, respectively; and the rate of depth averaged velocity change is more noticeable due to a change in the floodplain width for low relative depths. The effect of changing the floodplain width on the bed shear stress has the highest value in the mid-section between two apexes (CS3 section); therefore, an increase of 92% in the floodplain width in the mid-section (CS3), resulted to a 35% decrease in bed shear stress. The shear stress of the inner arch wall of the main channel is higher than the shear stress of the outer arch wall. In each channel, the maximum shear stress of the wall occurs near the bankfull level of the main channel (h = 0.14) and by increasing the relative depth, the wall shear stress increases. By increasing the relative depth and flow depth in the main channel and floodplain, the average depth velocity and shear stress in the bed increases.