Introduction:
During floods in many Rivers, the flow exits its main section and inundates the vicinity floodplains. The hydraulic flow in compound sections differs significantly from single channels. In such conditions, due to the change in flow section shape and the roughness difference between the main channel and floodplains, the flow structure in compound channels becomes highly complex (Yang et al., 2007; 2013). Vegetation in natural rivers is typically classified into rigid (trees) and flexible (grasses) plant. The stems of flexible plants can change shape with the movement of water flow, whereas the trunks of rigid trees remain unchanged against the flow without deformation (Terrier, 2010). The vegetation of floodplain is one of the main components that can influence the velocity distribution in compound channels (samadi Rahim et al., 2021; Hamidifar et al., 2016). this study, has investigated the flow field and the formation of secondary currents resulting from rigid and flexible vegetation in divergence compound channels with a mobile bed.
Methodology:
In order to investigate the effect of the vegetation of floodplain on the flow structure and the formation of secondary currents, solid cylindrical plastic rods with a diameter (D) of 10 mm were used to model the vegetation of trees and artificial grass with a height of 3 cm as flexible vegetation on They were placed in floodplains with a width of 0.36 cm. The distance between the rows of rods (lx) was considered constant and equal to 75 mm. While in order to change the vegetation density, the transverse distance between two rods (ly) was set at three distances of 50, 75 and 100 mm. In the main channel with a width of 0.24 meters and a height of 0.15 meters, siliceous sediments with a mean diameter (D50) of 1 mm created a mobile bed. In the floodplain, three divergence angles of 3.8, 5.7 and 11.3 degrees were set in such a way that in three relative depths of flow equal to 0.25, 0.35 and 0.45 and three densities of vegetation with the space ratio to 5, 7.5 and 10 were performed in a total of 36 experiments. The components of the local velocity of the flow in the middle section and at the end of the divergence of floodplain were measured by the 3D velocimeter of the Vectrino Profiler.
Results and Discussion:
In the non-prismatic compound channels with vegetation, the vertical isovelocity in the common area between the main channel and the floodplain indicates a strong velocity gradient in this area and confirms the formation of a free shear layer due to the Kelvin-Helmholtz phenomenon. Due to the presence of grass vegetation in the floodplain bed, the flow in floodplain is divided into two separate regions: the flow within the grass vegetation and the flow top of the grass vegetation. In divergent compound sections without vegetation, the transverse velocity of the flow in the main channel and the floodplain is significant and its positive. In the presence of vegetation, because the crest of the dune has reached the middle range of the divergence reach, the vertical component of the velocity in the main channel has become negative. As the distance from the floodplain bed increases, the drag force caused by the trees dominates and the contribution of the bed roughness in controlling the flow decreases. Therefore, the velocity profile in floodplain does not follow the logarithmic distribution and almost assumes an S shape. Vegetation has caused the formation of very weak secondary currents in the floodplain, especially in the reach of the end of the divergence. Also, the increase in vegetation density has caused the strength of eddies to decrease in the the main channel. The transverse gradient shows that the greatest effect of the secondary currents was in the common area between the main channel and floodplain, so that the increase in vegetation density confirms the formation of a stronger shear layer in this area. Vegetation on the floodplain weakens the secondary currents in this area. These results agree well with the studies of researchers such as Yang et al. (2007), Hamidifar et al. (2016) and Samadi Rahim et al. (2023). Also, the movement of eddies direction is constantly changing and the flow is very complicated. Therefore, local erosion is observed in the bed of the main channel and close to the common side wall, and on the other side, the height of the dune is increased. While in the main channel, there are two eddies in opposite directions.


Conclusion:
The Kelvin-Helmholtz instability causes the generation of a free shear layer in the intersection area. Also, the formation of points with a velocity close to zero on the floodplains shows the existence of excessive resistance of flexible and rigid vegetation compared to literature. The periodic positive and negative values of the transverse flow velocity in the floodplain range indicate the interference of Von Kàrmàn vortex streets in the space between the two vegetation elements. In the divergent compound channel with the vegetated floodplain, too weak secondary currents are generated, especially in the end of the divergence reach.