BEHAVIOR OF FRESH WATER FROM THE CHIKUGO RIVER IN THE ARIAKE BAY

Abstract

Recently, the frequency of occurrence of red tide in the Ariake Bay tends to increase. The nutrients-rich fresh water from rivers is pointed out as a reason for the red tide in the Ariake Bay. Therefore, information on behavior of the fresh water in the sea area is very important to discuss the aquatic environment in the Ariake Bay. In this study, four field observations on surface currents were carried out by using drifters under the stratified condition in order to clarify the Lagrangian flow patterns of fresh water from the Chikugo River. As a result of this study, it is clarified that the westward excursion distance of the fresh water under calm conditions is considerably influenced by the river inflow rate, because the velocity at the mouth of the Chikugo River governs the magnitude of the Colioris force. On the other hand, the motion in meridional direction mainly depends on the tidal range. Findings of the present study imply that the fresh water from the Chikugo River can reach the Isahaya Bay within a tidal period under circumstances that the inflow rate is larger than that in this study.