Research for the Opening width and End process on the artificial reefs

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From views of environment and seascape, artificial reefs are replacing detached breakwaters as coastal protection structures. The purpose of this study is to know the influence of narrowing the reef gap on the bottom current velocity. We conducted numerical analysis by using the fully nonlinear Boussinesq equations derived by Wei etal (1995) and taking account of the energy dissipation due to wave breaking. This method was applied to the actual field in Japan Sea. Three ratios of opening width, W to the top deck length of the artificial reef, Lr; Lr/W=3, 4, and 5 are examined. The return flow velocity at the opening was 1.9 and 1.8 times of that evaluated from the mass transport for Lr/W= 3 and 5, respectively. Increase of return flow velocity is quantitatively shown for narrowing the gap. Moreover, the circulation pattern inside the reefs is changed by the width of the gap. And since the responses of the bottom shear stress, which is the major force to transport the sea bottom sediment, to the bottom water particle velocity are different for steady currents and for oscillatory currents the decomposition of currents into two modes is necessary. The bottom shear stress was evaluated by the decomposed velocities together with the friction factor. The topographical change was computed by the sediment flux and budget equations. The simulated results were verified through the comparison with the experimental ones in the hydraulic laboratory. This method was also applied to the actual field facing Japan Sea and quantitatively reproduced the measured local scour at the offshore region of the reef opening. This application shows that one-storm waves with the duration of 24 hours can produce significant scour of the maximum depth, 3m.