Observations of waves and currents near the surf zone

A major challenge in understanding nearshore wave/current systems lies in obtaining measurements that resolve both the waves and currents over appropriate spatial and temporal scales. In SandyDuck (1997), two "Phased-Array Doppler Sonars" (PADS) were deployed looking shoreward from sites 300 m apart. Each provides radial velocities over an area roughly 400 m radius by 90 degrees. Horizontal vector currents are estimated over the intersecting area. The result is analogous to CODAR, but on a finer scale: resolution of 5 to 20 m up to 400 m away, sampled every second. Deployments of current meters by collaborators at SandyDuck help in assessment of the usefulness and limitations of this approach. Correlations with these independent current measurements are very high, and the amplitude of the sonar response is consistent with a simple acoustic model for the bubble/bottom competition. Two limitations are: (1) severe breaking can fill the water column with bubbles, so the sound attenuates quickly, limiting the useful range. (2) In light conditions, there is competition between backscatter from within the water column versus the bottom. On very calm days the response can be dominated by bottom backscatter for depths less than 3 or 4 meters. In general, however, the measurements are usable over a wide range of conditions. Several compelling advantages derive from quasi-continuous coverage in both space and time, including the ability to estimate vorticity fields associated with the circulation, to locate nodes associated with edge waves and reflected waves as functions of frequency, and to estimate gradients of wave quantities. Such concurrent observation of waves and currents is ideal for wave/current interaction studies. Many applications for this technique can be envisioned, including (for example) the interaction of waves and currents in tidal inlets or between islands.