This paper presents an experimental study on the relationship between wave breaking and the asymmetry of wave forms or profiles in shallow water. The model test of regular and irregular wave breaking is firstly carried out in a wave flume with a beach slope of 1/200. Then, by means of the analyses of the asymmetric wave forms near breaking, the relationship between the asymmetric parameters and the relative water depth is gotten by the Least-Square Method. Finally, the wave breaking indices based on the asymmetric wave parameters are obtained. The new obtained breaking indices for regular waves are the same as those proposed by Kjeldsen (1981), but greater than those for the irregular waves obtained in the present research. The study also shows that the breaking indices are affected by the relative water depth: the shallower the water depth is, the greater the value of criterion is.
Water wave breaking criteria are of great significance for coastal and offshore engineering. In these studies, the basic criteria are determined geometrically. Early in 1849, Stokes pointed out that waves can not exist in excess of the limiting steepness, (H/L)lim, which was calculated exactly by Mitchell in 1893, i.e., (H/L)lim =0.141, or (H/gT2)=0.027. Stokes further demonstrated that the limiting crest acquires a sharp corner with an included angle of 120°. Stokes' ideas are the start of the wave breaking research. Ochi and Tsai (1983) demonstrated that the value of (H/gT2)=0.021 may be used in laboratory conditions. According to Nelson's result, (H/d)b is a constant about 0.55 as the beach slope is near to 1/500. This is completely different from Goda's one. In allusion to Nelson's viewpoint, Goda and Morinobu (1997) reaffirmed the exactness of their formula after a series of physical model tests.