The Saemangeum dike, 33-km long enclosing 40,100 ha, is one of the biggest costal constructions in mid-west estuarine area of Korea. After final closure of the dike in April 2006, coastal environments near the dike have been changing drastically. The hypoxia in upper layer and the oxygen deficient in lower layer occasionally occur during warm seasons because that tidal mixing is reduced by the dike construction. The prediction of deteriorated outflow through sluice gates is the major issue to manage marine environments outside of the dike. We estimate behavior of the sluice gates outflow by numerical model experiment. The outflow extends by the inertia of jet plume and diffuses with outer seawater by tidal mixing. The numerical model with real-time monitoring network can be helpful to mitigate the environmental impact by the outflow accompany with a displeased bubble.