| Research Abstract |
In order to investigate on movement of fresh water discharged into Ariake Sea from the Chikugo River, in-situ measurements of Lagrangian currents by using drifters with GPS were conducted. The measurements were carried out on June 7 and July 21, 2006 and July 17 and 28, 2007, under spring and middle tide conditions and with discharge of the Chikugo River from medium wcale flash (1,678m^3/s) to normal stage(approximately 50m^3/s). Position of fresh water body, velocity, salinity, water temperature, water quality (turbidity, Chlorophyll-a, pH), wind velocity and direction were continuously measured for one tidal period (approx. 13hrs) or a half period (approx. 8hrs). As a result of these measurements, the followings were clarified: (i) Under weak wind condition, pressure gradient force is a dominant factor for currents in NS direction, while Coriolis force for currents in EW directions. (ii)Under weak wind condition, riverine water from the Chikugo River is transported to the western area for one tidal period. The excursion depends on river flow rate. Especially, under flush condition, it can move into Isahaya Bay for only one tidal period. (iii) Under strong southern wind condition, riverine water stagnates in the eastern area In addition, to measure eddy diffusivity in horizontal and vertical directions, in-situ measurements by using drifters and the turbulent microstructure profiler were conducted. Horizontal eddy diffusivity varied from 1 to 10^2m^2/s, while vertical one changed from 10^<-7> to 10^<-3>m^2/s. The former depends on local condition of river inflow, and so on, but the latter was affected by density stratification, tide, and tidal hour. We expect that these results will contribute to development of high accurate numerical model of currents and material transport in Ariake Sea.
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