We conducted numerical experiments on nonlinear tide and fine sediment transport in the Ariake Sea and the Yatsushiro Sea by Delft3D model. As a result of the present research the followings are clarified: i) Amplitude of S2 tide correlates inversely with M2 tide; ii) Maximum velocity of nonlinear tidal current is around 20cm/s near bay mouth area of both bays; iii) In the case excluding tidal flat sedimentation is enhanced; and iv) Variation of nodal factor f for M2 tide can change erosion/sedimentation characteristics around mouth of Isahaya Bay.
A flash flood forecasting model including a state-of-the-art data assimilation method was developed to provide a precise water stage forecast for flood emergency response. The model integrates a flash flood routing model (FFRM) coupled with an ensemble Kalman filter (EnKF) and an artificial neural network (ANN) submodel. In the model, the ANN forecasts river water stages at gauge stations first. Then, these are used as the initial and boundary conditions of the FFRM. The water stages, simulated from the FFRM, are then corrected by the EnKF for lead time. The model was applied to the Tanshui River watershed in northern Taiwan during past typhoons. The model forecasts almost covered the data observed during a typhoon period to within 95% confidence intervals. Compared with the use of FFRM without EnKF, the forecast water stages from the EnKF improved the accuracy at the conjunctions between upstream and downstream channels and the steep slope location.
The speciation of mercury (Hg) in Minamata Bay (Japan) was studied over a 2-year period (2006)(2007)(2008). Concentrations of dissolved total Hg, dissolved methylmercury (MeHg), particulate total Hg, and suspended solids were 0.43±0.14 ng/l (mean±standard deviation), 0.10±0.06 ng/l, 3.04±2.96 ng/l, and 5.94± 2.10 mg/l, respectively. Correlations between concentrations of particulate total Hg and suspended solids at four depths (surface: 0 m; mid-depth: −6 m, −10 m; and bottom +1 m layer) were only significant in the bottom +1 m layer. The mean dissolved MeHg concentration and the ratio of dissolved MeHg to dissolved total Hg were considerably higher in summer compared to other seasons. The data suggest that bottom sediment was not the sole source of MeHg, and that MeHg may be produced in the water column by the conversion of divalent Hg eluted from resuspended bottom sediment. The correlation between seawater characteristics such as salinity, temperature, dissolved oxygen (DO), and dissolved MeHg concentration indicates that Hg methylation could be influenced by the heterotrophic activity of microorganisms in the seawater. In particular, inverse correlations were observed between DO, salinity, and MeHg concentration. However, dissolved MeHg concentrations did not correlate with seawater characteristics such as pH or chlorophyll-a.
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