Enhanced exchange flow during spring tide and its cause in the Sumjin River estuary, Korea

Investigating dynamics and the mechanisms of estuarine exchange flow is essential to understand distributions of salts, nutrients, pollutants, and suspended sediments in estuary. Although exchange flow has been studied extensively, its variations over the spring–neap tidal cycle remain unclear. Many studies have indicated that the exchange flow is weaker during spring tides than neap tides, but a few studies have reported that exchange flow might be stronger during spring tides. In the Sumjin River estuary (SRE), Korea, the spring–neap variation of exchange flow and its cause were investigated based on intensive observational data, an analytical and numerical model results. The observations revealed that exchange flow increased during the spring tide but decreased during the neap tide. The horizontal salinity gradient increased about seven times more during spring tide than during neap tide in the middle of the estuary. The analytical model implied that the exchange flow in the SRE is mainly driven by the horizontal salinity gradient. The large horizontal salinity gradient and tidal current amplitude ratio in the SRE between the spring and neap tides suggests that the horizontal pressure gradient driven by the salinity gradient overwhelms vertical mixing in driving exchange flow in the SRE. Numerical model results were analyzed to investigate the variation of horizontal salinity gradient. The horizontal and vertical salt flux determine salinity distribution which results in temporal variation of the horizontal salinity gradient over the spring-neap tidal cycle.