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Unseeded Large Scale PIV measurements accounting for capillary-gravity waves phase speed

Large Scale Particle Image Velocimetry (LSPIV) is widely recognized as a reliable method to measure water surface velocity field in open channels and rivers. LSPIV technique is based on a camera view that frames the water surface in a sequence, and image-processing methods to compute water surface displacements between consecutive frames. Using LSPIV, high flow velocities, as for example flood conditions, were accurately measured, whereas determinations of low flow velocities is more challenging, especially in absence of floating seeding transported by the flow velocity. In fact, in unseeded conditions, typical surface features dynamics must be taken into account: besides surface structures convected by the current, capillary-gravity waves travel in all directions, with their own dynamics. Discrimination between all these phenomena is here discussed, providing a new method to distinguish and to correct unseeded LSPIV measurements associated with wavy structures, accounting for their phase speed magnitude and direction. This has been done measuring wavenumber vectors by using the same images exploited for PIV analysis. All measurements are performed without any flow seeding and in total absence of suspended materials, using the specular reflection of the water surface as a key marker. Results obtained at low-flow regimes in a straight concrete-made rectangular-section channel and in a river are satisfying, especially if compared to those obtained from classic LSPIV application without discrimination and correction. Moreover, a novel simple and safe procedure to orthorectify images is here presented.