Surface image velocimetry: Aerial tracer particle distribution system and techniques for reducing environmental noise with coloured tracer particles

A novel aerial tracer particle distribution system has been developed. This system is mounted on an unmanned aerial vehicle (UAV) and flown upstream from where surface velocimetry measurements are conducted. Tracer particles can be rapidly deployed across the full channel width and distributed in a crescent shape (for larger channels), so that they reach the measurement cross section at a similar time. This enables surface velocimetry techniques to be applied in rivers and channels lacking sufficient natural tracer particles or surface features. Lack of tracers is a common problem during low flows, in lowland rivers, or in artificial channels. Techniques for colouring tracer particles with biodegradable dye have also been developed, along with methods for extracting them from Red Green Blue (RGB) imagery in the Hue Saturation Value (HSV) colour space. The use of coloured tracer particles enables flow measurements in situations where sunglint, surface waves, moving shadows or dappled lighting on riverbeds can interfere with and corrupt results using surface velocimetry techniques. These developments further expand the situations where surface velocimetry can be applied, as well as improving the accuracy of the results. The tracer particle distribution system has been successfully deployed in multiple rivers and canals in New Zealand and has enabled surface velocimetry measurements where they were not previously possible due to a lack of natural tracers.