Complex flow mechanisms in compound meandering channels with overbank flow

Turbulence and secondary flow measurements were undertaken using a two-comp onent laser-Doppler anemometer in meander channels with straight flood plai n banks. The most interesting feature of the compound meandering channel fl ow was found to be the behaviour of the secondary flow. The difference in d irection of rotation of the flow before and after inundation at a bend sect ion was confirmed by the detailed velocity measurements. In addition, by pe rforming the measurement over a half wavelength of meander, the originating and developing processes of the secondary flow were also clarified. In con trast to the centrifugal force for inbank flow, the interaction between the main channel flow and the hood plain flow in the cross-over region was fou nd to play an important role in developing a shear produced secondary flow in the overbank cases. New experimental evidence concerning the spatial dis tribution of Reynolds stress -rho<(uw)over bar>, -rho<(uv)over bar> and -rh o<(vw)over bar> are presented for sinuous compound meander channels. In suc h channels, large interfacial shear stresses were induced at around the ban kfull level, especially in the cross-over region, and were found to be larg er than the bed shear stress in magnitude. Particular importance is placed on -rho<(vw)over bar>, which is usually small compared with other stress co mponents, as the cause of the secondary flow in the lower layer. The influe nce of secondary flow on eddy viscosity was found also to be significant. T hese turbulence data are particularly useful in understanding the flow mech anisms that occur in meandering channels and in developing proper turbulenc e models for such flows.