In this study, numerical simulation of a two-dimensional convective-dispersive model in Hakata Bay, Japan, is performed to analyze the impact of major river discharges due to torrential rain in Fukuoka City. Tank models are applied to calculate river discharges, which are taken into consideration as river inflow in the hydrodynamic model of Hakata Bay. A two-way nesting "edge" technique is developed and applied in the model in order to consider the influence of narrow and complex geographical features. The area around "Island City" and Imazu Bay are calculated in high resolution. The resulting model has high reproducibility since the calculated river discharges, tidal current, and salinity show good agreement with observed data. To analyze the impact of large river discharges, the calculation period is set from 11 September 2002 to 21 September 2002 since there was torrential rain on September 16 in the given year in Fukuoka City (163.5 mm/d). The results show that low-salinity water covered the whole of the inner part of Hakata Bay, and water of lower salinity than outer sea water (<34.0 psu) spread out to the bay's mouth two days after the torrential rain event. Fresh water covered the entire area of Imazu Bay and flowed out from the mouth of the Bay after the torrential rain event. The behavior of fresh water after a few days of torrential rain was remarkably different from normal discharge river flow. These results indicate that the environment in Imazu Bay can be degraded severely by torrential rain. Therefore, countermeasures to protect ecosystems in Hakata Bay must be examined immediately.
Massive freshwater discharges due to heavy rainfall have recently caused negative changes to coastal waters, such as red tides and anoxic water masses. In this research, a salinity analysis was conducted using a three-dimensional σ-coordinate model to evaluate the impact of large freshwater discharges on the horizontal and vertical distribution of salinity. Specifically, Hakata Bay was examined after a heavy rainfall with a six-year return period, beginning 16 September 2002, a common recurrence interval. The data were calculated for 11-27 September 2002. Tank models were applied to calculate river inflow discharges, which were considered river inflow in the hydrodynamic and salinity diffusion model for Hakata Bay. Model validation results showed high reproducibility, and the calculated tidal current and salinity agreed well with observed data. Results also showed that: (1) salinity was less than 15.0 psu at the river mouths about one day after heavy rainfall; (2) low-salinity water (< 28.0 psu) spread across the surface of the inner part of the bay; and (3) salinity differences between the surface and bottom were large (approximately 4 psu) and lasted three days after the heavy rainfall. It was concluded that red tides and anoxic water masses could be induced in the inner part of the bay.
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