Experimental Study Of Propagation Of Low Frequency Waves And Wave Blocking

In the sea, currents occur over a broad range of space-time-scales, especially at the entrances of tidal inlets and estuaries, where the currents are relatively strong. The wave propagation and transformation in coastal regions are often influenced by the currents. Based on laboratory experiments, this thesis studies the following two parts:Firstly, experiments were conducted in a wave flume to examine the low-frequency motion induced by bichromatic wave groups considering a range of amplitudes, group frequencies and modulation rates, because in coastal zone, low frequency waves have a large influence on the nearshore morphology and can induce the occurrence of low frequency resonance in harbors and the vibration of moored vessels. After the three decomposition methods of low frequency waves, i.e. the local array method, the colocated method and bispectral analysis were reviewed and compared, the local array method was adopted to decompose the incoming wave groups, and the cross-shore propagation of incoming bound long waves was investigated. In still water, the growth rate of the shoaling bound long waves over mild slope is proportional to h^-5/2. The variation of phase lag of the incident bound long waves behind the short-wave groups, which is caused by the energy transfer from the grouped short waves to the shoaling bound waves, has also been found in our experimental data analyzed with cross-correlation technique. It is also found that the opposing current makes the amplitude of the bound long wave increase, and that the stronger the current is, the larger the bound long wave is until the wave group breaks. On the other hand, it is found that under a following current, the amplitude of bound long wave generated by weakly modulated bichromatic wave group decreases but the bound long wave generated by fully modulated bichromatic wave group increases. In order to discuss this anomaly, the nonlinear interactions between the bound long waves and other wave components were analyzed with the wavelet-based bicoherence.Secondly, a series of experiments on waves being blocked by opposing currents are carried out. The experimental data of regular waves show that a longer wave requires a stronger current to block. It is observed that the dispersion of wave amplitude plays a critical role in wave blocking and can not be ignored, and that with the amplitude increasing, a frequency downshift which is ascribed to sideband instability, greatly affects the wave field and prevents the wave blocking. In the bichromatic wave tests, the higher frequency component is blocked while the lower one continues to propagate. As a result, the waves are transformed from being groupy to being monochromatic.