Benbow, C.A.; MacMahan, J.H., and Thornton, E.B., 2017. Analysis of surface foam holes associated with depth-limited breaking.

The behavior of surf zone foam holes, as observed at the surface and associated with depth-limited breaking, was investigated. Aerial imagery of the surf zone was obtained with a small, unmanned quadcopter that supported an integrated, high-resolution camera. The quadcopter is an ideal platform for acquiring images directly above the surf zone, a requirement to obtain the requisite resolution. The images were georectified so that size, shape, orientation, and evolution of the wave-generated foam patterns could be quantified. Three hypotheses are proposed for foam-hole generation: obliquely descending eddies (ODEs), self-organization because of bubble rise, and bottom-generated turbulent boils. The fringe region was the most seaward foam region and was marked with circular foam rings that increased in area and were more distinct with time. The fringe region data were consistent with both the self-organization because of bubble rise and turbulent boil mechanisms. The gap region, located between the plunge point and the splash-up created by the bore collapse, was marked by horizontal foam tubes oriented in the cross-shore direction. The foam tubes were likely created in the convergent region between two counter-rotating vortices. The largest region encompassed nearly the entire surf zone and was described as a mat of foam that developed obvious foam holes. The foam holes located in the outer surf zone, near the break point, initially decreased in size, consistent with ODEs before increasing in size and elongation. The foam holes located in the inner surf zone, increased in both size and elongation during a wave period. Because of increasing size with time, the foam-hole generation was attributed to turbulent boils. The rate of increase in foam-hole growth significantly decreased as the waves shoaled from the fringe region in the outer to the inner surf zone, suggesting that growth rate and size decreased with depth.