On extreme spatial variations of surface slope for a spilling breaking water wave

The steep and complex crest structure of a spilling breaking wave was studi ed in the laboratory by repeated slope measurements at a fixed observation point of a sequence of virtually identical breaking waves. These waves were generated using a repeatable converging wave packet with maximum frequency of 1 Hz. A fixed laser beam was directed vertically downward from the air into the water, and the location of the refracted beam was measured with a photodiode array to provide the water surface slope as the wave passed the laser beam. A maximum slope of 74 degrees was consistently measured just fo rward of the breaking wave crest with the crest region itself showing less repeatability, which was probably due to parasitic capillary waves. Before reaching its maximum, the slope reveals a two-stage spatial variation. A mo derate rate of increase was first observed between 0 degrees and 44 degrees , then the wave slope increased sharply to 74 degrees; the horizontal trans lation of the crest cor responding to the interval for the slope change fro m 44 degrees to the observed maximum of 74 degrees was about 4.2 cm. The de crease of surface slope from the maximum to 0 degrees at the crest was subs equently observed over a 3.6-cm translation. The slope variation reported h ere refines the macroprofile of a spilling breaker suggested by other inves tigators. Were the laser beam used to illuminate from below, internal refle ction would occur for slopes greater than 51.5 degrees, thus eliminating re fraction and preventing steeper slopes from being measured.