Introduction
Most of the rivers in the world have a sinuosity shape, and the migration of the meandering river over time causes the transfer and expansion of the main channel towards the flood plains and the change of the flood plains. The momentum transfer between the main channel and the floodplains affects the velocity distribution in the cross section of the channel and emphasizes the importance of studying this complex flow phenomenon in meandering and compound meandering channels (Julian et al., 2012; Morvan et al., 2005). Reynolds shear stress, turbulent kinetic energy and Froude number. Studies have shown that in meandering channels with moving beds, there is a higher momentum exchange leading to an increase in turbulent kinetic energy and Reynolds shear stress, especially near the bed surface (Sukhodolov & Uijttewaal., 2010; Mohanty, 2013; Pradhan et al, 2018; Pradhan et al. 2024). By studying past research, it can be said that despite the complexities of flow in meandering and compound meandering channels, it is very necessary to investigate the hydrodynamic behavior of the flow in the main channel and floodplain. The importance of understanding and knowing the flow hydraulics in the meandering compound channel to investigate the changes of the river and banks during the flood event and the overbank flow conditions will greatly help the better performance of engineers and researchers.
Methodology
The present study was conducted in order to investigate the hydrodynamics of the flow in the main channel of a compound meandering channel in the laboratory of physical and hydraulic models of Ferdowsi University of Mashhad. The experiments were carried out in a flume equipped with a water circulation system, with a length of 10 meters, a width of 0.78 meters and a depth of 0.5 meters. The wall of the channel was made of transparent glass. The floor of the main channel and floodplain was made of concrete. Doppler Velocimetry (ADV) was used to measure the velocity components in (x, y, and z) directions. The flow rate passing through the main channel (inbank flow) was measured as 10.4 liters per second, Froude number as 0.3 and Reynolds number as 21306. Flow rate in the overbank flow with relative depth (0.46) is equal to 31.64 L/s and Froude number and Reynolds number are 0.21 and 23000 respectively. The vertical distances of the measuring points of the velocity components for 8 points close to the bed were 0.5 mm and the next 8 points up to the water level were considered to be one centimeter in the main channel. In the flow of the inbank, the components of the flow velocity in three directions were taken in eight sections of the main channel and 208 points were taken in each section, which in total and considering the eight sections, 1664 points were taken. The spacing of transverse points varies between 5 and 50 mm. The points near the interface of floodplain and the main channel were measured with a distance of 5 mm due to the importance of the flow velocity changes in this area.
Results and Discussion
By investigation the results obtained from the changes in the ratio of the depth average velocity to the average velocity, it can be seen that for inbank flow in the channel in sections CS1, CS6 and CS7, the velocity at outer bend is higher than the velocity at the inner bend and center of the main channel. In sections CS2, CS3 and CS8, the velocity is maximum in the center of the main channel. In overbank flow, the ratio of the depth average velocity to the total average velocity has increased in the inner bend (sections CS1 and CS2) and this increase in velocity is inclined towards the center of the main channel. In addition, in sections CS3 and CS6, the velocity is highest in the center of the main channel. By increasing of relative depth, the intensity of depth velocities decreases and according to the obtained results, the amount of relative depth velocity in the flow of the main channel has increased by 40%. As the relative depth increases, the kinetic energy of the flow decreases and the results showed that the kinetic energy of the flow in the inbank flow is between 40% and 70% higher than the overbank flow. Maximum shear stress of the bed is inclined from the centerline of the main channel towards the outer bend wall as seen in sections CS1, CS2 and CS6. In the middle sections, the most changes in bed shear stress occur in the central area of the main channel bed, as seen in sections CS2, CS3 and CS8. In the overbank flow, the maximum bed shear stress has increased at the beginning of the flood plains and then decreased. Considering both inbank flow and overbank flow, it observed that the highest amount of wall shear stress occurs near the bed surface (1≤ z ≤8cm). In the case of middle sections, shear stress changes have occurred near the water level for overbank flow. By changing the depth from 0.12 m to relative depth 0.46, the maximum turbulence energy in the urban bank flow has decreased by 20%.
Conclusion
In the inbank flow with a depth of 0.12 m, the maximum kinetic energy occurred in the outer bend, and overbank flow with a relative depth of 0.46 and flow on the floodplains, the maximum kinetic energy occurs in the inner bend and the flow on the floodplain connected to the wall of the inner bend. Maximum depth velocity occurred in the center part and in the distance of 4 cm ≤ z ≤ 10 cm, which is more visible in the middle sections. In the case of overbank flow with relative depth, D_r=0.46, the maximum depth velocity changes occur between 10 cm ≤ z ≤ 20 cm.
Keywords
Secondary currents, Kinetic energy, Shear stress, depthe average velocity