Yi-Yu Kuo

Transfer function between wave height and wave pressure for progressive waves

Abstract Predicting the wave height from the subsurface wave pressure based on the pressure response factor and the correction factor has fundamental difficulties. The relationship between the measured wave height and the subsurface wave pressure was directly assessed in this study without resorting to the two parameters. Laboratory experiments were performed under various wave conditions. An empirical expression of the transfer function between wave height and wave pressure was obtained from these test data by dimensional analysis and regression analysis. In intermediate and deep water depth conditions, the transfer function was only a function of a nondimensional parameter which is composed of the angular frequency, the depth of the pressure sensor under the still water level and the acceleration of gravity. Finally, the empirical expression was compared with the experimental data and observational data from Hom-ma et al. (1966), Cavaleri (1978), Lee and Wang (1984), and Bishop and Donelan (1987). The empirical formula was found to be in sufficient correlation with these data.

COASTAL BATHYMETRY FROM THE WAVE SPECTRUM OF SPOT IMAGES

Two SPOT images were used to derive the surface wave spectra for mapping water depths in the coast zone. By the general dispersion relation of water wave, the surface wave changes its wavelength over varying water depth. The estimated depths from this wave spectrum bathymetric (WSB) method were further corrected to water depths relative to the mean lower low tide. The average error of this image-derived water depth is about 10% from the measured water depths. This WSB method takes advantages of the remote sensing technology to obtain the synoptical bathymetry of coastal waters safely, economically and quickly. It is especially suitable for the coastal zone west of Taiwan where the bathymetry changes yearly near some river mouths, the bottom slope is gentle enough, satisfying the requirement of WSB method, and the non-uniformity of water quality incapacitate the application of optical bathymetry method.

Directional spectrum analysis and statistics obtained from ERS-1 SAR wave images

Abstract This work examines ERS-1 (the first European Remote Sensing Satellite) SAR (Synthetic Aperture Radar) water surface wave images over Hualien of Taiwan, indicating that the variation of SAR signals in space domain is similar to in situ wave datas in time domain. Some statistical properties of SAR data are investigated. The Rayleigh distribution function closely corresponds with the histogram of wave heights, but the Gaussian one cannot for water surface displacements. Evidence reveals that SAR wave signals do not respond well to actual ocean waves effectively. As wave spectral analysis of available SAR data reveals, the appropriate sample size of SAR wave image, sampling average, and moving average should be taken carefully to accurately confirm directional power spectra. Moreover, SAR spectra are compared with in situ ones, confirming that peak frequencies correlate well and wave directions approximately agree with each other. Some differences between both spectral shapes remain somewhat unclear and require further study. Nevertheless, in this study, ERS-1 SAR power spectra verified the feasibility of deriving an appropriate dominant wave direction and peak frequency.

The study of wave pressure and uplift force on a submerged plate

Abstract Boundary element method (BEM) is used to study wave pressure acting on a vertical quay, as well as the uplift forces on a submerged plate. The plate, placed in front of the quay, is bored with holes. The results are then compared with the case of a plate without any perforation. The results showed that: for q/h = 0.45, where q is the depth of submergence of the plate and h is water depth, wave pressure acting on the vertical quay is very similar for plates with or without borings. However, with q/h = 0.20, wave pressure on the quay is changed according to wave periods, widths of the plate, the distance of the plate away from the quay, as well as the ratio of the holes to the plates. Uplift forces exerted on the plate are indeed smaller for plates with borings than without, and decrease with increasing porous ratio. Furthermore, the uplift forces will be the same for the same porous ratios, regardless of these ratios being caused by a single or a double perforation.

The nonlinear wave effects by a submerged horizontal plate to the vertical wall

Wave pressure reduction due to a submerged horizontal plate placed before a vertical wall was investigated both numerically as well as experimentally. Pressure-reduction due to the submerged plate for short period waves was clearly evident. For all the case studies, the optimal submergence depth was found to be equal to d = 0.20h rather than 0.45h, where h is the total water depth. However, at this depth, d = 0.20/;, the pressurereduction effect will vary according to wave conditions, the width of the plate, (l), and the distance of the plate away from the vertical wall (w). Furthermore, it was found that the wider the plates, the longer the waves that will be affected. Calculated reflection coefficients were found to be in satisfactory agreements with experiments for nonbreaking waves.