潮流による拡散の水理模型実験について

Abstract

The diffusion phenomena due to the tidal current in the shallow broad estuary arehere studied in a hydraulic model experiment, for which Ariake Bay in the Omuta areaof Kyushu, was used as the prototype. Only the tidal current is taken into account, theeffect of the other factors, the ocean current, density stratification, the wind, waves, andso 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 of1/2000 and 1/200 respectivery, was constructed, and a semidiurnal tide generated by anautomatically controlled pneumatic tide generator was provided for it. The water levelat 5 stations, the current ellipses at 4 stations and the flow pattern were measured andcompared with those in the prototype. The diffusion from an instantaneous point sourceand a continuous point source was investigated mainly by photographic method.Experiments have shown that the tide and the tidal current are accurately reproducedin the model. And the diffusion coefficient evaluated through the mixing of the waterfrom Chikugo River with the sea water is almost the same as in the prototype. How-ever, the diffusion coefficient evaluated through the rate of increase of the dye patch isabout 1/3 of that in the prototype. This may be due to the difference between the flowin the model and that in the prototype, the former belonging to the perfect turbulentregime and the latter to the transient regime. The diffusion coefficient in the model ata future stage, in which two areas of reclaimed land arc presented, becomes a littlelarger than that at the present stage. The diffusion from a continuous point source atthe mouth of Omuta River is also compared with the prototype.