  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.
