Foam patches left by waves breaking as they approach a smooth and gently sloping beach from a near-normal direction sometimes have the distinctly triangular shape that has been studied by Turner and Turner (2011). Explanations of the size of the angle at the apex of the triangles observed by Turner and Turner are suggested in terms of physical processes that determine the speed at which the point of breaking travels along a wave crest. These explanations differ from the entrainment model proposed by Turner and Turner (2011). The range of sizes of the apex angles can most likely be explained in terms of the directional spreading of waves approaching the surf zone.
Thorpe, S. A.. 2013. "Foam triangles." Journal of Marine Research 71, (5). https://elischolar.library.yale.edu/journal_of_marine_research/377