Author(s): Q. Fraselle; N. Denies; D. Bousmar; Y. Zech
Keywords: Compound channels; Tracer injection; Conductivity; Solute distributions; Diffusion/dispersion processes; Floodplain deposition
Abstract: Sediment transport during overbank flows is a complex matter. The understanding and prediction of floodplain deposit pattern is of particular importance in many engineering applications. Such phenomena rely on mixing processes in compound channels, including convection, diffusion and dispersion but those processes are not fully understood yet. A new experimental study was initiated in order to investigate sediment exchanges between mainchannel and floodplains. Preliminary experiments were performed in a 10 m long compound channel flume. A tracer (NaCl solution) was injected at two successive positions in the main channel: on the symmetrical axis and in the shear layer between main channel and floodplains. Evolution of the tracer plume was appreciated through solute concentration measurements. Specially designed conductivity probes were mounted on an automatic positioning device. The probes were designed to avoid wall/free surface effects, using artefacts to limit the control volume. Results from the symmetrical injection are compared to a depth-averaged analytical transport model limited to the main channel. This model uses the analytical uniform flow solution of solute transport equation in rectangular channels, with steady source and assuming full mixing in vertical direction. When injected in the shear layer, the solute expands asymmetrically over the flood plains, mainly due to the strong secondary currents observed in compound channels.