The Use of Wave-Averaged Movies for Measuring Nearshore Circulation

Nearshore currents are the prime drivers of sediment transport, hence are a key to understanding evolving nearshore morphology. While current patterns are sometimes simple, more often they include rip currents and alongshore-variable circulation and vary with time-dependencies on scales of 100’s of seconds in pulsing features or hours in response to changing forcing with tides. Spatial gradients are usually strong in both the cross-and longshore directions making sampling of the full circulation patterns with an array of fixed instruments very challenging. Remote sensing offers alternate approaches.

This paper explores the use of Wave-Averaged Movies (WAMs) to visualize and measure nearshore currents and circulation. The approach is based on tracking the movement of foam or other water turbidity features but first removes the dominant optical clutter of incident waves by averaging over a few wave periods, commonly over 20-60 seconds. Residual features are very common and appear to track with the flow as quasi-passive tracers. The paper will explore sampling strategies and quantification algorithms using recent data from the FRF at Duck, NC.