A set of comparative experiments are employed to identify a predominant factor in the generation of large eddies on the water surface at the interface between the main channel and the floodplain in a composite channel flow. It is indicated that the large surface eddies are not phenomena whose action resembles boiling but tornadolike vortices generated by the local shear layer at the interface. Flow visualization technique showed that there existed a complicated three-dimensional structure in the form of inclined vortex tubes which behaved like longitudinal vortices near the top edge of the floodplain as well as periodically generated large surface eddies. The periodicity of those phenomena is explained as the preferred mode of the local shear layer with two-dimensional stability analysis. [ABSTRACT FROM AUTHOR]
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