Abstract
A technology of hastening the seawater exchange using deep seawater (DSW, known as clean and cold seawater) was tested in a case study on Rausu Fishing Port using a hydraulic model and a numerical simulation. The technology was proposed by the authors as follows in 2006. When cold seawater is continuously emitted in the state of a buoyant jet along the bottom of an anchorage in a port, the jet increases its width in the shape of a fan immediately by entraining the surroundingseawater. Although the jet flow velocity decreases with the increase in the entrainment velocity, the buoyant jet expands in the shape of a concentric circle. When the buoyant jet flows out through the lower layer of the port, outside freshseawater flows into the port through the upper layer in order to compensate the flux. At Rausu Fishing Port, Hokkaido, water quality was reported to be worsenedparticularly at the west anchorage in autumn, but 4, 560 ton/day of DSW has been pumped from a depth of 350m for industrial use since 2006. In the present study, a hydraulic model (1.5×8×1m, with topography in the port and its vicinity of 1/250 in scale) was used with a wave producer assuming the use of emission pipes with inner diameter 25 or 50 cm. The numerical simulations were conducted using a 3-dimension fluid analysis model in the case of emission of 5, 000 t/day of DSW (5°C, 33.3 psu) as a buoyant jet at an initial velocity of 1.18 m/s into the west anchorage (12°C, 33.5 psu representing the autumnal condition) toward its center. The amount of seawater exchange was calculated to be 2 to 12 times largerthan that obtained at the spring tide. The validity of the technology was discussed.
