Radar remote sensing of currents and waves in the nearshore zone
The relationship between microwave radar and optical video imaging of the nearshore region is studied. The remotely sensed data were used to estimate the longshore currents and the surf zone width.
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 study compares video PIV-derived 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 ≥ 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. Both systems observe a strong eddy-like mean flow pattern over 200 m section of coastline with the mean alongshore current changing direction at about the mid surf zone. The radar and PIV velocities at particular locations in the surf zone track each other well over a 6 hour period, showing strong modulations in the mean alongshore flow occurring on 10-20 minute time intervals.
The offshore region in the absence of sufficiently strong wind is sometimes barely visible, while the surf zone always appears very bright in radar backscatter images due to persistent surface roughness produced by breaking waves. The backscatter and coherence radar images were used in conjunction with edge detection filters to estimate the surf zone width from radar data. The surf zone width from video data is calculated using the time-stacking techniques. The comparison of surf zone width over 6 hours showed the rms difference of 8.8 m close to the radar location while the radar had the tendency to overestimate the distance for most of the run. The correlation of two measurements was high at 0.89. At locations farther than 600 m away from the radar the surf zone width rms differences were higher, up to 24 m, while correlation remained high. The differences are attributed to the estimate of the shoreline in radar images due to different scattering properties of wet and dry sand.
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 nearshore processes.
0544: Electrical engineering
0799: Remote sensing