Comparison of Surface Current Measurement Techniques and Observations of Tidal Inlet Dynamics Using Unmanned Aerial Systems

Abstract Understanding mixing dynamics through barrier island inlets requires large-scale observations of tidal exchange near the bay–ocean boundary, including reliable measurements of surface currents. In this paper, we evaluate the ocean surface currents at Galveston Bay inlet and Freeport Harbor inlet, Texas, from a video sequence of surface waves collected from a small, unmanned aerial system (UAS). Surface current measurements from three different measurement techniques (UAS wave-based current mapping, particle image velocimetry, and optical flow) were compared with and without the presence of artificial floating tracer particles. When used, tracer particles of packaging peanuts were released using a payload-capable UAS platform. Particle image velocimetry (PIV) and optical flow methods estimated coherent surface currents consistent with the motion of uniform, well-distributed seeding particles drifting upon the spectral surface waves and the near-surface currents. Without seeding particles, the UAS wave-based current mapping technique captured the near-surface currents at tidal inlets. The UAS wave-based current mapping technique (Streßer et al.), which does not require particle seeding, is further validated using in situ measurements of current velocity from an acoustic Doppler current profiler (ADCP). The UAS wave-based surface current measurements agreed with the ADCP measurements within a root-mean-square error (RMSE) of 0.06 m s −1 and 16° in magnitude and direction, respectively, and a normalized RMSE (NRMSE) of 0.26 in magnitude. The surface currents near the jetty at Galveston Bay inlet were visualized with the resulting UAS wave-based current maps.