1 INTRODUCTION

Rivers during flood are characterized by a compound cross-section (water overflows the banks to floodplains) and therefore understanding of the mechanisms governing the transport of various constituents in two-stage channels is a challenging task within environmental hydraulics. It has been recognized that the shear layer generated by the difference in velocities in the main channel and in the floodplains influences the turbulence structure and as a consequence it should impact on the transport of mass in a two-stage channel (Knight and Shiono, 1990; Knight et al., 1994; Shiono and Knight, 1991; Rowi?ski et al., 2002, Sofialdis and Prinos, 1999). From that studies it turns out that the intensity of turbulence increases significantly at the interface between shallow and deep areas of the channel as well as horizontal vortices develop in this area.

Researchers have realized that studies of the mixing processes in channels of irregular cross-section are of great importance, however most of the attempts have been based on numerical investigations (e.g. Nokes and Hughes, 1994; Shiono et al., 2003). Experimental investigations in this respect are still lacking. Among the best-known are the studies of Wood and Liang (1989) and recently of Shiono and Feng (2003) who simultaneously measured the tracer concentrations and the flow characteristics in a compound channel. The mentioned studies are all based on the...