Coastal Engineering Journal

Standard functional forms of directional wave spectrum are presented and compared. Frequency spectra of the JONSWAP, Wallops, and TMA are presented as the functions of significant wave height and spectral peak period. In combination with these frequency spectra, directional spreading functions of cos^2s(θ/2), cos^2lθ, exp[κ cosθ], sech² βθ, the wrapped normal, and the Mitsuyasu types are compared for their functional shapes, angular standard deviations, and half-peak spreading angles. Empirical relations between the spreading parameters of these directional functions are presented.

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.

The Fukushima Coast composed of a series of soft rock cliffs had suffered from severe erosion due to two factors: high wave attacks and weak rocks. This study attempts to quantify these factors, respectively, by the onshore component of wave energy flux at the breaking point, F, through the refraction analysis, and by Young's modulus, E, through the measurement of the elastic wave. The dimensional consideration yields a dimensionless parameter Π_C≡ qEL/F where q is the recession rate, L is the cliff height. On this coast, the value of Π_C=0.082 obtained from the best fit of the present data-set gives the recession characteristic q=0.082F/EL.

In this paper, a composite boundary element method (Composite BEM) is formulated and applied to study the reflection coefficient of a submerged vertical plate-type breakwater, which is modeled as a thin or non-thickness structure. In the theory, two-dimensional motion with the wave crests parallel to the submerged breakwater and the linearized irrotational flow are assumed. The accuracy of the solution obtained using the numerical technique is demonstrated by comparing the numerical values with those obtained from experiments and with other analytical solutions. In addition, numerical results of wave reflection for both a submerged rectangular dike and a vertical plate-type breakwater are presented and discussed.

An experimental study is carried out to investigate the transformation of oscillatory boundary layer from ordinary to depth-limited one by using an oscillating tunnel with triangular roughness pasted on the top and bottom surfaces. The velocity is measured by one-component LDV. It is proved that only longer period of oscillation is not sufficient to induce depth-limited properties. Instead, the ratio of boundary layer thickness to the distance from the bottom to the free surface must be used in order to distinguish a depth-limited oscillatory boundary layer from an ordinary one. The experimental data for mean velocity, phase difference, fluctuating velocity, friction coefficient and boundary layer thickness are presented in order to elaborate the structure of depth-limited oscillatory boundary layers on a rough bottom. On the basis of the present experiments, a modification has been made in an existing criterion to distinguish between ordinary and depth-limited wave boundary layers. A typical field situation is discussed to show the usefulness of the proposed criterion.