Effects of Large-Scale Effluent of the Chikugo River due to 2020 Kyushu Floods on the Development of Hypoxia in the Ariake Sea

Abstract

 In 2020 Kyushu floods, the cumulative rainfall exceeded 500 mm in the entire Chikugo River basin. The water level measured by the Senoshita observatory located downstream of the river became the highest water level in the past years. The Chikugo River is one of the sources of freshwater into the Ariake Sea and plays a critical role in the marine environment and benthic ecosystem of the bay. This study evaluated the effect of large-scale effluent on the development of hypoxia (DO less than 3 mg/L) in the bay by numerical simulation of the hydrodynamic model and the ecosystem model. Meantime, we also compared with five other cases to explain the variation of hypoxia due to different flood patterns. The results indicated that historical extreme flood event in 2020 caused long duration of hypoxia lasting more than two months at Sta.B3 in Isahaya Bay and the most severe bottom hypoxia was occurred throughout the central and northern regions of the Ariake Sea. Furthermore, it also showed that the duration of hypoxia in 2020 was longer than in 2006 when the largest hypoxia had occurred since 2000.