In this study, a numerical simulation consisting of a two-dimensional model of Hakata Bay, Kyushu Island. Japan, was conducted to analyze the impact oflarge river discharges due to torrential rain. Since Fukuoka city, one of the largest cities in Japan, surrounds Hakata Bay, the bay has a very complex environment, including an artificial island (Island City) in its eastern region and small bay (Imazu Bay) where tideland is spread with rich ecosystems in its western region. Tank models were applied to calculate the discharges of the 38 rivers that flow into Hakata Bay, and a two-way nesting technique, "edge nesting," was developed and applied in the model to capture the influence of complex geographical features, such as the Island City area and the Imazu Bay area, which were nested with a high-resolution mesh. The model accurately reproduced the observed tidal current and salinity data. To simulate the behavior oflarge river discharges, a historical simulation period including a torrential rain event (163.5 mm/d) was input into the model. The results showed that after the rain event, low-salinity water covered the entire inner part of Hakata Bay, and freshwater covered the entire area of the tidal flats and flowed out from the mouth of Imazu Bay. The behavior of this large mass of fresh water was remarkably different from usual discharge river flow. These results indicate that the environment in the bay can be significantly changed by torrential rain events.