In order to improve the water quality in closed water body like a port , various breakwaters with the seawater exchange function are studied and developed. The seawater exchange function of this case is using the wave energy. On the other hand, for the seawater exchange system using the tidal difference, it is necessary to operate the gate and measure the water level .Then, authors designed the siphon type check valve. In this paper, the structure of this siphon type check valve is explained and the performance of this seawater exchange system involving this check valve is studied by a numerical analysis. From the results obtained by the numerical calculations, it was confirmed that this system with the siphon type check valve can apply to seawater exchange in a port .
A long term field observation was conducted on the secondary undulations locally called "Abiki" occurred in Urauchibay, Kami-Koshiki Island. A comprehensive observation deployment for the sea level, current and barometric pressure was taken covering not only inside and nearby Urauchi-bay but also offshore field in the East China Sea. Vigorous secondary undulations greater than 150cm were happened over 5 days during 82 days' observation period. Just before the Abiki events, the atmospheric disturbances were found to travel eastward or north-eastward at a speed around 140km/h, which was resonantly coupled with the ocean wave in the offshore site. The observed Abiki heights can roughly be explained by the forced resonance in the offshore and the amplification after entering the bay.
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