Coastal Engineering Journal

A large amount of C-Heavy oil spilled accidentally from a Russian oil tanker "Nakhodka" on 2 January 1997 in the sea of Japan and polluted over the coast of Hokuriku district of Japan. The present study described on marine hydrocarbon degrading bacteria inhabited in the drifted oil balls. It was clarified that biodegradation of the polluted heavy oil by microorganisms took place actively in the seashore of Kanazawa City. Four pure cultures having the high ability of oxidative degradation of long chain hydrocarbons in heavy oil were isolated and characterized.

The nearshore potential vorticity balance of Bowen and Holman (1989) is expanded to include the forcing from wave group induced radiation stresses. Model results suggest that the forcing from these radiation stresses can drive oscillations in the longshore current that have a spatial structure similar to linear shear instabilities of the longshore current. In addition, the forced response is nearly resonant when the forcing has scales (k,σ) similar to the linearly most unstable mode. Thus, we suggest that wave groups may provide an initial perturbation necessary for the generation of shear instabilities of longshore currents and also act as a source of vortical motions on beaches where linear instabilities are completely damped.

Data from the SUPERDUCK (1986) field experiment were analyzed for the presence of spatially coherent wave groups. The analysis confirms that wave groups with periods and longshore spatial structures comparable to the observed shear wave motions were sometimes present on this open coast. This indicates that wave groups with the required spatial and temporal structure to initiate the low frequency oscillations in the longshore current can exist.

A new method for estimating directional wave spectra from the Doppler spectra by HF radar is proposed. This method is developed by introducing a Bayesian approach, previously proposed by one of the authors (Hashimoto et al., 1987), as one of the most accurate and reliable methods for estimating directional wave spectra for in situ measurements. The principal advantage of the new method is that it can be applied without introducing empirical weighting coefficients. Applicability, validity and accuracy of the proposed method are demonstrated with numerically simulated data for various wave conditions.

A new analytical solution of tsunami which is obliquely incident on a continental slope is derived based on the linear shallow water wave theory. The model of a continental slope consists of a finite constant slope and a bottom of uniform depth and has a vertical wall at the shoreline. Because of the existence of the vertical wall, another function can be taken as one of the fundamental solutions on a slope in addition to the confluent hypergeometric function M. Based on the solution obtained, the characteristics of tsunami amplification on a continental slope are discussed in terms of the conditions of the incidence. The magnitude of the amplification can be predicted with several parameters of wave incidence, such as, the incident angle and the relative incident-wave length to the topography. Especially, when a tsunami is incident almost parallel to the slope, we found that there exists distinctive amplifications for particular parameters characterizing the incident wave. The effect of the existence of the vertical wall is also discussed. The characteristics of amplification also depend much on the position of the vertical wall.

Fundamental characteristics of tsunamis propagating over an oceanic ridge are investigated analytically based on the linear shallow water wave theory. The oceanic ridge model used has a triangular shape which is formed by the combination of a constant slope and a bottom of uniform depth. Wave pattern of on-ridge tsunamis varies depending on the obliquity of incident waves. Especially, standing waves are formed when the incident wave angle to the ridge exceeds the critical angle. Then no transmitted waves exist because of the evanescent waves. The present study is applied to the 1996 Irian Jaya tsunami which propagated over the South-Honshu ridge. The characteristics of on-ridge tsunamis observed at Chichijima and Hachijojima, which are close to the South-Honshu ridge, can be partly explained by using the solution derived in the present study.

A performance index is defined for linear wave dispersion property. The index measures the capability of a water wave model to simulate irregular wave transformation over general topography.

Optimization of this index leads to:

(1) the optimum vertical distribution functions for the nonlinear mild slope equations given by Isobe (1994) for different number of vertical distribution functions,

(2) the optimum values of wave numbers to be used for the fully dispersive nonlinear wave model given by Nadaoka et al. (1994) for different number of vertical distribution functions,

(3) the optimum values of the wave number to be used for the time dependent mild slope equation given by Kirby (1984).

All optimized parameters are given as functions of the dimensionless peak frequency of the energy spectrum. Comparisons among the optimized models are given.