A preliminary assessment of the use of drones to quantify current velocities at tidal stream sites

Whenever considering new tidal stream sites; information on current characteristics is of paramount importance. This contribution describes a methodology to measure surface currents at tidal stream sites using drones and provides initial validation of the approach against velocities measured by GPS surface drifters. Drones have the potential to measure surface currents at high spatial resolution and low cost. An open-source toolbox, PIVlab, is used to apply particle image velocimetry (PIV) to nadir imagery that is georectified based on GPS position and gimbal heading information. The technique shows potential when PIV derived velocities are compared to velocities measured by the surface drifters. For drifter velocities over 0.8ms-1, there is reasonable correlation, although substantial variance, with mean absolute errors between 0.12ms^-1 and 0.24ms^-1. There is a poorer match with drifter velocities below 0.8ms-1; mean absolute errors are between 0.33ms^-1 and 0.34ms^-1, which considering the smaller velocities, give large percentage errors. It is hypothesised that improved image pre-processing would reduce these errors.