75 ISSN 1541-308X, Physics of Wave Phenomena, 2020, Vol. 28, No. 1, pp. 75–82. © Allerton Press, Inc., 2020. Wave Breaking Type as a Typical Sign of Nonlinear Wave Transformation Stage in Coastal Zone Ya. V. Saprykina a, *, S. Yu. Kuznetsov a , O. A. Kuznetsova a, b , I. V. Shugan a, c , and Yang-Yih Chen c a Shirshov Institute of Oceanology, Russian Academy of Science, Moscow, 117997 Russia b Zubov State Oceanographic Institute, Roshydromet, Moscow, 119034 Russia c National Sun Yat-sen University, Kaoshiung, 804 Taiwan *e-mail: saprykina@ocean.ru Received July 18, 2019; revised July 31, 2019; accepted August 2, 2019 Abstract– Experimental data demonstrate dependence of wave breaking type on wave amplitude-frequency- phase structure that is determined by nonlinear wave transformation. The spilling and plunging breaking waves are differed in the symmetry of the wave profile and the ratio of the amplitudes of the 1st and 2nd har- monics in breaking wave. The wave profile symmetry, in turn, is determined by the phase shift between these harmonics. In spilling breaking waves, it is close to zero, which corresponds to symmetrical waves. In plung- ing breaking waves, the phase shift is negative, which corresponds to the forward shifted 2nd harmonic. The ratio of the amplitudes of the 1st and 2nd harmonics in spilling breaking waves is less than that in plunging breaking waves. The periodic energy exchange between harmonics during near resonant triad wave interac- tions is the reason for the change in the types of wave breaking for waves propagating above a gentle inclined bottom. Due to the different structure, plunging breaking waves contribute to the erosion of the cross-shore underwater profile, while spilling breaking waves contribute to the accumulation of sand on it. DOI: 10.3103/S1541308X20010082 1. INTRODUCTION Waves propagating in the coastal zone of the sea transform with decreasing water depth, become steeper, interact with bottom and finally break. In the coastal zone, two types of wave breaking are most often observed: plunging and spilling. Plunging type is characterized by the formation of a visible roll at the forward wave front of the breaking wave and then the forward wave front turns up with a sharp release of a foam water jet. For the waves breaking by spilling type, this does not occur and during the breaking the crest of the wave in the form of the foam smoothly slides over the forward wave front surface. These two types of breaking waves are shown in Fig. 1а. Almost always, when there are several zones (or lines) of wave break- ing in the surf zone, it can be observed that the type of breaking can change as the waves approach the shore (Fig. 1b). It can also be observed that when the wave regime changes even though the wave breaking loca- tion remains approximately the same, wave breaking type can also change. Based on laboratory data [1] and field experiments [2], it was revealed that spilling and plunging breaking waves have different influence on a fine-sand beach profile evolution. Spilling breaking waves smooth out the underwater profile to an almost equilibrium shape, while plunging breaking waves cause the formation of an underwater bar. The wave propagation process in the coastal zone can be considered as a weakly nonlinear dispersive wave process with the generation of higher nonlinear harmonics at near resonant triad interactions. In near resonant interactions, both energy transfer from the main to higher harmonics and the reverse energy transfer from higher harmonics to the main can occur [3]. According to field experimental data, near reso- nant triad interactions between the 1st and 2nd wave harmonics lead to variability in an amplitude-fre- quency-phase content in waves and to the develop- ment of wave irregularities at shallow and intermediate water depths [4]. They are also the reason for the impossibility of unambiguous parametrization of the biphase (phase shift of the second harmonic against the main one) for all coastal zones, because it fluctu- ates in the range ( ) [5]. However, for breaking waves, the interval of the biphase change is ( ) and the biphase ϕ is linearly related to the coefficient of wave asymmetry (As) [6]: (1) -π +π 2, 2 -π +π 2, 6 ϕ= 1.25 , As UNDERWATER ACOUSTICS