An experimental evaluation of the blocks in floodplain on hydraulic characteristics of flow in a meandering compound channel

Abstract

• Flow behavior in meandering channels with 2-sided blocks in floodplains examined experimentally. • The presence of blocks in floodplains significantly enhances Q mc / Q ). • In cases with blocks densities of 6.2% and less, the values of Manning's n decrease by increasing Dr. • For all cases of blocks densities, the maximum bed shear stress occures near the convex side of CS1. • With high blocks density, apparent lateral velocity gradients seen in central region of CS3-CS5. In this research, the main flow behaviors in the meandering compound channels with different densities of two side blocks on the floodplains were investigated by using laboratory experiments at the three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. The results show that the blocks density on the floodplain has a significant effect on the flow behavior in the main channel. So that the smooth channel efficiently conveys 100% more water on average than that conveyed in the case with maximum blocks density. For the case with smooth floodplains, the Q mc /Q ratios of main channel discharge to total discharge increases continuously by reducing the flow depth, reaching 47% for Dr = 0.29 (Dr is the relative depth). By contrast, Q mc /Q in the channel with maximum blocks density peaks at 70% (>50%), showing that the main channel controls the capacity of compound channel conveyance because the blocks occupy most of the cross-sectional area of the floodplain when Dr = 0.29. In cases with blocks densities of 6.2% and less in floodplain, the Manning n decreases by increasing the relative depth (Dr). In cases with blocks densities of 9.7% and 23.7%, the Manning n increases by increment the relative depth. In addition, in cross-over sections of the case with smooth floodplains, a steep gradient of velocity in the vertical direction appears from the dense contour lines around the bankfull level near the right-hand sidewall indicating that a considerable effect occurs from the upstream flow. Instead, for channel with maximum blocks density, an apparent lateral velocity gradient was located in the central region in cross-over sections. The maximum shear stress of the bed at the apex section for all channels occurs near the convex side and the amount of bed shear stress at all sections decreases with reducing the relative depth. In the cross-over sections, due to the increase of the flow angle between the main channel and the floodplain, the effect of the secondary flow is stronger and the flow velocity decreases more, so the Darcy–Weisbach coefficient increases more in this area.