THE GROUNDWATER FLOW MECHANISM IN THE OTA RIVER ESTUARY AND THE FORMATION OF HIGH TURBIDITY LAYER IN HIROSHIMA BAY

摘要

The purpose of this study is to analyze the mechanism of groundwater flow in the Ota River Estuary, and to clarify the relationship between the formation of a high turbidity layer and groundwater flow. It was found that the variation of groundwater flow depends on both river water level and seawater level, and their contributions to the groundwater level differ by time scales of the variation (seasonal or semi diurnal cycle). In neap tide, since the hydraulic gradient from the Hiroshima delta to the sea develops relatively, the groundwater flow can be prominent. Furthermore, since the seawater stratification in the flood season reduces the pressure head in the sea bottom, pore water in the bottom sediment is supplied from the deep sediment layer. Consequently, the increase of water content in sediment promotes the resuspension of sediment even if the disturbance by the seawater flow is weak near the bottom of Hiroshima Bay. In addition, the groundwater flow from the aquifer of the Hiroshima delta supplies warm fresh water to the riverbank of the Ota River floodway. The masonry revetment between the floodplain and the main channel helps the groundwater flow to keep a saline environment under the main channel of the riverbed.