Complex flow mechanisms in compound meandering channels with overbank flow

Abstract

Turbulence and secondary flow measurements were undertaken using a two-component laser-Doppler anemometer in meander channels with straight flood plain banks. The most interesting feature of the compound meandering channel flow was found to be the behaviour of the secondary flow. The difference in direction of rotation of the flow before and after inundation at a bend section was confirmed by the detailed velocity measurements. In addition, by performing the measurement over a half wavelength of meander, the originating and developing processes of the secondary flow were also clarified. In contrast to the centrifugal force for inbank flow, the interaction between the main channel flow and the flood plain flow in the cross-over region was found to play an important role in developing a shear produced secondary flow in the overbank cases. New experimental evidence concerning the spatial distribution of Reynolds stress −ρuw, −ρuv and −ρvw are presented for sinuous compound meander channels. In such channels, large interfacial shear stresses were induced at around the bankfull level, especially in the cross-over region, and were found to be larger than the bed shear stress in magnitude. Particular importance is placed on −ρvw, which is usually small compared with other stress components, as the cause of the secondary flow in the lower layer. The influence of secondary flow on eddy viscosity was found also to be significant. These turbulence data are particularly useful in understanding the flow mechanisms that occur in meandering channels and in developing proper turbulence models for such flows.