HYDRAULIC MODEL EXPERIMENT ON THE DIFFUSION DUE TO THE TIDAL CURRENT

Abstract

The diffusion phenomena due to the tidal current in a shallow broad estuary are here studied in a hydraulic model experiment for which Ariake Bay in the Omuta area of Kyushu, was used as the prototype. Only the tidal current is taken into account, the effect of the other factors, the ocean current, density stratification, the wind, waves and so on, which may influence the diffusion in the estuary, is not considered. A model of the northern half of Ariake Bay, with a horizontal and vertical scale of 1/2000 and 1/200 respectively, was constructed, and a semidiurnal tide generated by an automatically controlled pneumatic tide generator was provided for it. The water level at 5 stations, the current ellipses at 4 stations and the flow pattern were measured and compared with those in the prototype. The diffusion from an instantaneous point source was investigated mainly by the photographic method. Experiments have shown that the tide and the tidal current are accurately reproduced in the model. However, the diffusion coefficient evaluated through the rate of increase of the dye patch is about 1/3 of that in the prototype. This may be due to the difference between the flow in the model and that in the prototype, the former belonging to the perfect turbulent regime and the latter to the transient regime.