湾水振動におよぼす防波堤の効果
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概要
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The effects of a large breakwater with a constricted opening on the oscillations of bay water as a whole are examined on the basis of a very simple one-dimensional mathematical model. The bay is separated into two parts by a breakwater with a narrow opening and the dynamical condition at the opening of the breakwater is assumed to follow the wellknown relationship in hydraulics between the volume flux through the opening and the difference of water levels across the breakwater. Several different cases such as tidal oscillations, invasion of tsunamis, and storm surges are considered and the theoretical results are compared with those of model experiments as far as possible. It appears that a simple theoretical model presented here can explain with satisfaction the observed change of wave height at the head of the bay with respect to the change of the period of forcing functions and / or the area of the opening, as long as the opening is small. Thus, the most important factor in this kind of phenomena should be the following non-dimensional parameter: B ; for tidal oscillations, η*/K; for tsunamis and storm surges. For details, see § 4 and § 3 respectively. The amplitude and phase changes of the tidal oscillation inside the bay can be estimated as a function of B from Fig. 2. The effect of a breakwater on invading tsunamis of the solitary type is shown in Fig. 9. The effectiveness of a breakwater on storm surges is seen in Figs. 10-15. From these figures it is evident that the period of the oscillating water body in relation to the incoming wave period or the given period of external forces seems to be the decisive factor in determining the effectiveness of the barrier, in addition to the geometry of the bay, the area of the opening and the wave amplitude. The present theory is applicable to the cases where the opening is sufficiently small so that the non- linear effects predominate in the motion of water near the opening.
- 1963-07-30
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