Longshore Surface Currents Measured by Doppler Radar and Video PIV Techniques

Mean longshore surface currents within the surf zone were measured using two remote sensing techniques: microwave Doppler radar and optical video. Doppler radar relies on small-scale surface roughness that scatters the incident electromagnetic radiation so that velocities are obtained from the Doppler shift of the backscattered radiation. Video relies on texture and contrast of scattered sunlight from the sea surface, and velocity estimates are determined using particle imaging velocimetry (PIV). This paper compares video PIV and Doppler radar surface velocities over a 1-km alongshore by 0.5-km cross-shore area in the surf zone of a natural beach. The two surface velocity estimates are strongly correlated (R² ges 0.79) over much of the surf zone. Estimates differ at the outer edge of the surf where strong breaking is prevalent, with radar-estimated velocities as much as 50% below the video estimates. The radar and PIV velocities at particular locations in the surf zone track each other well over a 6-h period, showing strong modulations in the mean alongshore flow occurring on 10-20-min time intervals. In one case, both systems observe a strong eddy-like mean flow pattern over a 200-m section of coastline, with the mean alongshore current changing direction at about the mid surf zone. The good spatial and temporal agreement between the two remote measurement techniques, which rely on very different mechanisms, suggests that both are reasonably approximating the true mean longshore surface velocity.