Influence of sluice gate operation on salinity stratification and hypoxia development in a brackish estuary dam

The water quality in the artificially made Saemangeum Lake, located in an estuary, is poor; here, we explored the causes of hypoxia development. The field survey for spatial and vertical distribution of salinity and dissolved oxygen (DO) was conducted three times at 6 sites in the Mankyung Basin during the period when the salinity stratification was developed, and continuous observations for temporal and vertical distribution of water temperature, salinity, DO, and flow direction and velocity were performed at a site in the downstream of the lake for about 40 days. Hypoxia was detected at the bottom layer of the survey sites, which were directly affected by seawater inflow. At these sites, the salinity stratification, one of the causes of hypoxia, was clearly observed. The Richardson number and potential energy anomaly (PEA) calculated from the continuous observation measurement results indicated that the water column was very stable and strongly stratified during the investigation period. The vertical salinity structure due to the stabilization of the water column and the bottom temperature above 20 °C caused hypoxia of the bottom layer when insufficient seawater inflow or non-operation of sluice gate. Due to the horizontal salinity gradient by the seawater flowing in through the sluice gate operation, a baroclinic force acts from the sluice gate to the upstream direction at the bottom layer. In contrast, a barotropic force acts from the upstream inlet toward the sluice gate at the surface layer. The directional flow of each water layer changes in opposite directions to form a repeated reversing current which prevents the supply of DO through the inflow of fresh seawater. Therefore, to improve the water quality of Saemangeum Lake, it is necessary to control the short/ long-term reversing current that obstructs DO supplementation from seawater flowing through the sluice gate. •Seawater inflow by sluice gate operation formed a strong salinity stratification.•Two-layer circulation serves to supply fresh DO to bottom layer during high seawater inflow.•Vertical salinity structure and high temperature resulted in hypoxia.•A repeated reversing current through sluice gate weakened dilution effect of seawater.