Enhanced Exchange Flow During Spring Tide and Its Cause in the Sumjin River Estuary, Korea

Abstract

Knowledge of exchange flow and its variations is important, because it determines the distribution of salt, nutrient, pollutant, and suspended sediment in estuaries. Although exchange flow has been studied extensively, their 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 others have reported that it might be stronger during spring tides. Therefore, we investigated the spring-neap variation of an exchange flow and its cause based on intensive observational data and an analytical model applied to the Sumjin River estuary (SRE), Korea. 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.

Fortnightly variability of horizontal salinity gradient affects exchange flow in the Sumjin River estuary

“…Even when the salinity gradient was estimated by averaging the salinity at various water depths, there was almost no change. In the analytical model using the balance equation of pressure gradient and friction (Cho et al, 2020), the observed exchange flow was well realized when the horizontal length scale was set at a 1 km interval when calculating the salinity gradient. Therefore, we used the salinity from the surface layer to the bottom and set the horizontal length to 1 km to calculate the salinity gradient.…”

“…In a previous study of the Sumjin River estuary, the salinity gradient was the main determinant in the fortnightly variation of exchange flow in the lower estuary (Cho et al, 2020). To determine what mechanism caused the variation in the intensity of the exchange flow in the entire estuary, we compared the salinity gradient and vertical mixing effect, which are the main factors that determine the vertical shear of the exchange flow.…”

“…The subtidal salinity was calculated by averaging the flood and ebb observations. More detailed information regarding these observations is described in Cho et al (2020).…”

“…During spring tides, vertical momentum exchange is active owing to strong vertical mixing, and exchange flow is weakened; during the neap tide, exchange flow is strong owing to decreased vertical mixing, as observed in many estuaries, such as the Hudson River estuary (Geyer et al, 2000;Bowen and Geyer, 2003;Scully et al, 2009;MacCready and Geyer, 2010), Modaomen estuary (Gong et al, 2014), Peruıṕe River estuary (Andutta et al, 2013), and Curimatau River estuary (de Miranda et al, 2005). On the other hand, recent studies of short estuaries (i.e., those that are shorter than the dominant tidal wavelength) have reported that the salinity gradient plays a major role in the exchange flow variation, which is stronger during the spring tide (Becker et al, 2009;Cho et al, 2020). In the Cape Fear River estuary, stronger tidal forcing and associated mixing contribute to increased estuarine circulation due to a greater near-bottom horizontal salinity gradient during the high tidal range (Becker et al, 2009).…”

“…In the Cape Fear River estuary, stronger tidal forcing and associated mixing contribute to increased estuarine circulation due to a greater near-bottom horizontal salinity gradient during the high tidal range (Becker et al, 2009). In the Sumjin River estuary, the effect of the horizontal salinity gradient, which is six times higher during the spring tide compared with the neap tide, overwhelms the effect of vertical mixing, resulting in stronger exchange flow during the spring tide (Cho et al, 2020). However, as these studies were conducted only at specific points, the fortnightly variation of exchange flow over the whole short estuary remains unclear.…”