Effects of Detached Breakwaters on the Characteristics of a Directional Wave Field

• Main Authors: Huang Wei Po, 黃偉柏
• Other Authors: Chou C. R.
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/33992919314596141478

博士 === 國立海洋大學 === 河海工程學系 === 91 === Abstract The main objective of this thesis is to study the characteristics of the wave field around a detached breakwater. It is well known that wave fields here are highly complicated due to the presence of incident, reflected, refracted, and scattered waves. To achieve this goal, wave field characteristics around a detached breakwater, as well as within a harbour were studied through wave gauge measurements. However, the results of wave gauges are so-called point measurements, which can be representative only when the wave field is homogeneous. As an effort to study the wave fields around detached breakwaters more thoroughly, the technique of the remote sensing through a CCD (charge coupled device) camera was adopted for the second part of this study. Various methods have been proposed by researchers in the past to estimate the directional spread of a wave field. To assess their relative merits, numerical studies were first carried out. Three of the non-phase-locked methods were adopted for the purpose. They are, the Direct Fourier Transform Method, (DFT), the Maximum Likelihood Method (MLM), and the Extended Maximum Entropy Principle Method (EMEP). It is shown that, the last one outperforms the other two. Experiments were carried out in the multi-directional wave basin of the Ocean Engineering Laboratory, National Taiwan Ocean University. At first, experiments were carried out for the case where the breakwater is the sole factor that can affect the wave field, i.e., no topographical effects were taken into consideration. This is believed to simplify the complexity of the directional wave field. The results indicated that using the EMEP method, the time ratio of TL/S, where TL is the time required for the waves from the gauge to travel to the breakwater and back again, and S is the length of the record used in estimating the spectrum, can be reduced to as small as 0.15. This extends the results of previous researchers. Using the results, a relationship between the ratios of D/L, where D is the distance between the wave gauge and the breakwater, and L is wavelength corresponding to the peak frequency, and the square root of the outgoing to the incoming wave energies has been derived. An empirical equation between the sheltering coefficient, KD, and the directionality of the incident waves, was also proposed. It is believed that this can be used for engineering applications. These results were then applied for the experiments where the fishery harbour Ba-Do-Zhi (BDZ) was used as model. With a mean value of 0.10 for the relative errors, it is concluded that the empirical relation obtained from previous study is directly applicable to all other cases. In the second part of the study a CCD (charge coupled device) camera system was used. It was designated to assess the possibility of sensing wave fields remotely so as to study the wave field as a whole. By comparing with results measured with wave gauge arrays, a transfer function between the gray scales of the image sequences and the wave profile was obtained. The relative errors have a mean of 0.15 for estimated wave heights for the case of regular waves, and it is 0.005 for the associated wave periods. Estimations of the significant wave heights for the case with irregular waves yield a mean value of the relative errors of 0.11, and it is 0.06 for the significant wave periods. However, since studies of image processing are presently only at the initial stage in the Laboratory, no further studies of the image sequences in both the frequency and wavenumber domains were carried out. This makes a comparison with the results of part I of this thesis impossible.