Seiches in bays forming a coupled system

Abstract

The seiches occurring in two adjacent bays, Koaziro Bay and Moroiso Bay, situated in Miura Peninsula at the mouth of Tokyo Bay, are very regular, and the phenomenon of beat appears in their amplitudes of oscillation. Nakano once presented a theoretical explanation of this phenomenon, considering two rectangular bays of the same size and form as well as of uniform depth, by assuming that a kind of coupling takes place between the two bays through a portion of water flowing across the mouth of each bay. In the present paper, the validity of the theory has been proved in a series of hydraulic model experiments. The experiments have shown that, in the case of the two adjacent rectangular bays mentioned above, there are two distinct modes of oscillation viz. (i) the co-phasic oscillation and (ii) the contra-phasic oscillation, and since the frequencies of these oscillations are close to each other, when they come to interfere, the beat phenomenon occurs.

It has also been found that, when a tsunami wave enters the two bays, the mode of oscillation of the seiches occurring in them changes in the following manner: the co-phasic oscillation → the oscillation accompanied with the beat phenomenon → the contra-phasic oscillation. As regards this phenomenon, the authors have presented a possible explanation by considering the dissipation of energy due to wave scattering.