¹Department of Civil Engineering, Chung-Ang University Ansung, Kyungki-Do, KOREA

trtk@netian.com

hongsik@cau.ac.kr

²Department of Civil Engineering, Nihon University Chiyoda-Ku, Tokyo, JAPAN

takezawa@civil.cst.nihon-u.ac.jp

　

The wave height distribution inside a harbor is an important consideration in planning the layout of the facilities prior to construction. Many investigators have performed studies involving the prediction of the wave field in a harbor due to short period incident waves, using finite difference, finite element, or boundary element methods. In this study, a numerical model is presented for the prediction of the wave field due to the diffraction of directional random waves in a harbor of arbitrary shape with partially reflecting boundaries. The water depth is assumed uniform and the method is based upon the superposition of diffraction solutions for monochromatic waves obtained by a two-dimensional boundary integral equation approach. The incident wave conditions are specified using a discrete form of the Mitsuyasu directional spectrum. The present numerical model has been validated through comparisons with theoretical results and previous experimental data for random wave diffraction by offshore breakwaters and in a harbor. Good agreement was obtained in all cases. Based on these comparisons it is concluded that the present numerical model is an accurate and efficient tool to predict the random wave field inside a harbor or around a breakwater in many practical applications.

　

　

¹Department of Social Systems Engineering, Tottori University Tottori, JAPAN

　

²Information Processing Center, Tottori University Tottori, JAPAN

　

The present study deals with a very low frequency waves of the order of some hundred seconds radiated from a step configuration such as reef flat. Since wave breaks around an edge of the step in practical situations, waves are very small on the step even in a stormy offshore sea condition. However in some coral islands, severe surf beats attack to the coral beach (Hino et al. 1989). The problem of the first order wave system on this topography has studied by several authors such as Ijima (1971), for example. Those studies, however, are not able to cope with the "surf beat" on the reef flat. The present authors clarified the mechanism of the generation of very low frequency wave on the step configuration (Kimura et al, 2001). The model clarified that the discrepancy between the 2^nd order low frequency waves inside and offshore side of the step generates new free low frequency wave system in both sides. Especially, when the wave lengths of the new waves are 4L, 4/3L, 4/5L..., (L: width of the step), wave resonance takes place on the step. The new wave is amplified some 10 times at the resonance on the step.

　

The new wave is also radiated to the offshore direction. When the resonance takes place, the new waves in the offshore region are also amplified. Since the wave period is very long (several kilometers) and directional spreading is very small, the waves presumably bring the long period phenomena such as harbor resonance, slow vessel oscillation and etc in the neighboring coasts. The present study deals with the characteristics of the low frequency waves theoretically, which are radiated from the step configuration.

　

　

¹Department of Civil Engineering, Chung-Ang University Ansung, Kyungki-Do, KOREA

brain@post.cau.ac.kr

hongsik@cau.ac.kr

　

²Department of Civil Engineering, Nihon University Chiyoda-Ku, Tokyo, JAPAN

takezawa@civil.cst.nihon-u.ac.jp

　

Dredging technology is applicable to a wide variety of functions, which involve improvement of water flow to provide flood control benefits, removal of polluted sediments, beach nourishment and restoration, and altering seabed contours to modify wave climate. There has been considerable interest recently in the development of alternative breakwater concepts for use in regions where the use of a conventional breakwater is not possible due to coast or environmental constraints. These types of structures include floating bodies, flexible membrane-type barriers and pits (or trenches) in the seabed. It is this last category of breakwater, namely the submarine pit or trench that will be the subject of the present paper. In the present study, the diffraction of multidirectional random surface waves by one or more rectangular submarine pits is investigated theoretically. The incident wave conditions are specified using a discrete form of the Mitsuyasu directional spectrum. The water depth is assumed uniform and the method involves the superposition of diffraction solutions based on linearized shallow water wave theory obtained by a two-dimensional boundary integral approach. The present numerical model has been validated through comparisons with previous theoretical results for regular waves. Good agreement was obtained in all cases. Based on these comparisons it is concluded that the present numerical model is an accurate and efficient tool to predict the wave field around multiple submarine pits and navigation channels in many practical situations.