Field Observations of Swash Zone Currents Estimated with Near-Field Optical Remote Sensing

Currents in the swash zone on a sandy Atlantic coast beach estimated with near-field optical remote sensing by tracking breaking-wave-generated foam with particle image velocimetry (PIV) are similar to those measured with in situ acoustic Doppler velocimeters (ADVs). The observations were obtained from a 40-m-tall tower located about 60 m inland of the beach over a 2-month period, yielding 180 h of data during a wide range of incident wave and local foam conditions. A smaller, overlapping set of observations also was obtained from a drone. The remote sensing estimates of mean alongshore flows, and the magnitude and phase of cross-shore flows, are highly correlated with the in situ measurements. However, the remote sensing estimates tend to underestimate the in situ measurements of downrush flows and overestimate the uprush flows, with the differences becoming smaller as the amount of foam increases. Analysis with drone-based images suggests that higher image resolution improves agreement between remotely sensed and in situ measurements, particularly for time-averaged cross-shore flows, but a nadir view angle does not eliminate persistent cross-shore bias. The remote sensing estimates allow for swash zone currents to be estimated both across and along the swash zone with relatively high spatial resolution, and show strong cross-shore gradients, including direction reversals in alongshore currents. In addition, the remote sensing estimates indicate flow patterns associated with the formation of beach cusps during times of submergence when low-tide topographic surveys are not feasible.