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A Temporal and Spatial Analysis of Wave-Generated Foam Patterns in the Surf Zone

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Technical Report

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Naval Postgraduate School Monterey United States

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Aerial videos of the surf zone at Sand City, Monterey Bay, CA, were acquired using an unmanned aerial vehicle. Videos of 26 individual bores were converted to still images and were georectified, georeferenced, and post processed. The size, shape, and evolution of the wave generated foam patterns within the surf zone were analyzed. The results were tested against two existing hypotheses of foam pattern generation, Obliquely Descending Eddies ODEs and self-organization due to bubble rise. Three foam regions within the surf zone were recognized. The largest region, the foam mat, encompasses nearly the entire surf zone and is described as a mat of foam that develops obvious foam holes. The areas of the holes grew in area and elongated with time. The results were inconsistent with the two theories of foam hole generation. The fringe region is the most seaward foam region and is marked with circular foam rings that become larger in area and more distinct with time. The fringe region data are contrary to observations of ODEs but is consistent with the theory of self-organization due to bubble rise. The gap region, located between the plunge point and the splash up created by the bore collapse, is marked by horizontal foam tubes oriented in the cross-shore direction. The foam tubes are likely created in the convergent region between two counter-rotating vortices.

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