Flow Interaction at Multistage Floodplains of Open Channel during High Flow: A Study of Mixing Length and Flow Structures

Understanding compound open channels with separate shallow floodplains and different bankfull heights around the deep main channel is of paramount importance. A practical design of channels and rivers in urban environments, such as multistage floodplains, could improve embankment stability and higher discharge capacity at various flow rates. The present study conducted a comprehensive experimental analysis to understand the effect of terrace-like multistage floodplains on different flow structures. The characteristic velocity profile helps to attain velocity ratios, which signifies stronger shear layer turbulence for shallow flow depth in multistage channels. The turbulence of the shear layer tends to increase in multistage compound channels because the mixing layer widths significantly widen toward the higher velocity side of the main channel. Furthermore, dimensionless shear implies the formation of coherent structures over higher-stage floodplains as maximum velocity in the main channel grows more effectively than in other floodplains. The study of flow through quasi-two-dimensional structures suggests various correlations between small-scale flow motions that influence the Reynolds shear stress caused by large flow structures. The multistage channel has a larger head loss due to turbulent diffusion on the main channel. Finally, multistage floodplains provide more overall discharge compensation at the first and second stages of floodplains when flow depth increases.