Coastal Engineering Journal

The maximum nonbreaking wave height on a horizontal bed is examined through laboratory tests by changing the slope of an approach section from 1/5 to 1/80. As the slope becomes steep, the maximum wave height gradually decreases from the breaker height in water of constant depth: the maximum wave height on the slope 1/80 is about 94% and that on the slope 1/5 is about 80% of the breaker height in water of constant depth. The decrease is induced by the free harmonic waves generated at the junction point of the approach slope and the horizontal bed. Test results also yield a recommendation that the water depth on a horizontal bed be maintained equal to or greater than 10 cm in order to avoid scale effects on wave breaking phenomenon there.

Based on a large amount of published laboratory results, reliable models are developed for computing the average rate of energy dissipation in regular and irregular breaking waves. The average energy dissipation rate is assumed to be proportional to the difference between the local mean energy density and stable energy density. Wave height transformation is computed from the energy flux conservation law based on the linear wave theory. The models are examined and verified extensively for a variety of wave and bottom conditions, including small and large scale laboratory and field experiments. Reasonable good agreements are obtained between the measured and computed wave heights and root mean square wave heights.

Wave observations were carried out at the mouth of Danang Bay and off the Key Ha Cape in central Vietnam from April 1997 to February 1998 in a port development study by the Japan International Cooperation Agency. It is the first long-term measurement of waves in the west South China Sea by reliable wave measurement devices.

Wind waves due to typhoons as high as 9m in maximum height were successfully recorded, including those generated by Typhoons 9721 and 9726. Long period waves up to 16 seconds in significant period were revealed to exist. It is proved that the long period waves are swells from a large-scale typhoon remotely located in the west Pacific and propagated through the Luzon Strait to the central coast of Vietnam.

Wave hindcast calculations were made by a spectral method for the past 30 typhoons which affected the area from 1961 to 1997. Through statistical treatment of the hindcast waves, it is assessed that an offshore significant wave with a height of 9.7 m could be generated off Danang in a 50-year return period.

Seasonal wave climate is that the sea is rather calm, except the typhoon and northeast monsoon season specifically from September to November.

The dynamic response of the seabed to ocean surface waves is treated analytically on the basis of an elastic wave theory in a soil-water mixture. The seabed is modeled as the aggregate of a poro-elastic soil-skeleton with air-containing pore-water, from which a general solution is presented for a homogeneous bed of infinite thickness. Two kinds of compressive waves and one shear wave are shown to exist in the seabed and their phases and attenuation characteristics are clarified. Based on the propagation of these elastic waves, exact solutions for the dynamic response in pore-water pressure and displacements of porous soil-skeleton are derived in physically lucid form. It is found that the relative motion between the soil and water is produced by the second compressive wave, but not from other elastic waves. The present solution is shown to include the well-known quasi-static solution given by Yamamoto et al. (1978), as a limiting case. By comparing the present dynamic solution with that of quasi-static state, two important nondimensional parameters are presented to discuss the applicability of each solution.