Impact of Large Fresh Water Discharges into Hakata Bay Due to Torrential Rain Using Nested Two-Dimensional Convective-Dispersive Model

Abstract

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.

Analysis of Salinity Behavior in Hakata Bay after Heavy Rainfall Using a Three-dimensional <i>σ</i>-Coordinate Model

“…It is important to evaluate the impact of heavy rains occurring frequently in semi-enclosed bays, so three-dimensional salinity modeling is essential. Fukuda et al (2017) conducted a two-dimensional numerical analysis of salinity behavior in Hakata Bay after a heavy rainfall. Though the study demonstrated a rough trend in the horizontal behavior of large freshwater discharge, it is necessary to also evaluate the vertical structure of salinity.…”