Large scale PIV-measurements at the surface of shallow water flows

To measure the flow dynamics at the surface of shallow water flows over a large measuring field, a simple and reliable method has been developed using the advantages of Particle Image Velocimetry (PIV). Besides the determination of mean flow conditions and turbulent flow characteristics, this method makes it possible to track two-dimensional large coherent structures, which are the dominating flow phenomena in many shallow flow applications. As basic equipment, a commercial PIV software system has been used. The measurements are carried out at the water surface, which means that no laser light sheet is needed. Depending on the time scales of the flow and camera characteristics, it is even possible to work with a constant light source. A particle dispenser to provide a homogeneous distribution of particles on the water surface is also presented. Because floating particles have a strong tendency of sticking together, different types of particles and special coatings have been tested to reduce this problem. A laboratory application of this method is presented to analyze the effects of shallow dead-water zones on exchange processes in rivers where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics.