Measuring Instantaneous Velocity Fields Remotely Using a 2-D Power Spectral Density Technique

Surface currents are a fundamental hydrodynamic variable that is essential in estimating volumetric discharge, material, and sediment transport in riverine environments. Existing methods of remotely measuring currents are met with steep challenges when applied in the field. Here, we adapt a 2-D power spectral density (2-D PSD) technique for use in a riverine setting, where wind waves dominate the free surface. The technique relies on quantifying the Doppler shift of surface gravity waves, which are omnipresent on most free surfaces. Therefore, typical problems such as insufficient and uneven distribution of tracer material that frequently plague other remote methods in the field are not a concern with this method. We demonstrate the ability of the 2-D PSD technique to remotely measure surface and sub-surface mean currents. Moreover, because this technique analyzes image pairs, temporal statistics of the velocity field (i.e., time series, velocity spectra, and turbulence quantities) can easily be extracted. Given its capabilities, the 2-D PSD current measuring technique could profoundly impact the status quo of remote sensing of hydrodynamic variables in a variety of aquatic settings.