Experimental Studies on Breaking Wave Trains generated by Strong Winds on Shallow Water

When strong winds blow over shallow water for a long duration, wind induced currents are generated. At the same time wind waves are evolved on them untill the waves break such as white cups. The heigher the wind wave heights evolve, the more the breaking of waves of particular wave heights and periods resonant with the wind speed may become predominant. At this circumstance the wind acts on the waves as a driving force. As the result of these interaction between winds and waves, we may often observe progressive breaking waves without deformation of their profiles, The breaking phenomena are named the 'Breaking wave trains', The waves are vitally important in studying shore processes, as they deeply concern with near - shore currents and sand drifts in a surf zone. Hydraulic experiments, as the first step to find out properties of the breaking wave trains, were carried out with a recirculating wind wave tank which can reproduce wind - generated random waves accompanied with currents. Wave periods and heights of breaking wave trains correspond to those obtained from the peak frequencies of wind wave power spectra and significant wave heights. The wind - induced currents have an important role in the mass transport in the wave trains. Because of the strong currents, the wave field may be considered as a unidirectional flow, and there exists a critical depth which is nearly equal to the mean depth of the breaking wave trains in a reference frame moving with the wave trains at a speed of the wave celerity. The phase of the wave crest is, hydraulically speaking, subcritical, and that of the wave trough is supercritical in this steady flow system