The work was designed to visualise the propagation of pollutants in the compound channel of a river stream by releasing small light floating solid particles into a turbulent flow in a way allowing to trace them with video cameras (Particle Image Velocimetry). Obviously because the turbulent velocity fluctuations are irregular, the results of each trail differ. On average, the particles wander apart from each other and the rate at which they wander apart can be related to the diffusion coefficient. Experimental investigations of that kind were carried out using a 16 x 2.08 m hydraulic model, and their results are discussed in detail in the present paper.

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