One of the longest coastal dykes in the world has been constructed in Saemangeum area in the southwest of Korean Peninsula (center location at 126.5E and 35.8N). A 33km-long dyke system connecting a few islands separates the open ocean from the landward water of 401 km2 area that is planned for reclamation for national coastal development.The construction of dyke has started in 1991 and finished closure in 2006. Until the final closure of dyke in 2006, the sea water has exchanged with the fresh waters discharging from Mangeong and Dongjin rivers, preserving the meso-tidal (tidal range > 4m) estuarine dynamics. However the closure of dyke has changed the tidal dynamics significantly and associated residual flow system has changed dramatically, yielding new pattern of coastal erosion and deposition problems. In this study, we investigate the change in residual circulation in the Saemangeum coastal water using field observation data and numerical simulation. For numerical simulation, a comprehensive three-dimensional model ROMS is used to simulate the circulation patterns of before-dyke and after-dyke environmental setting. The ROMS is allowing resolving the water movement on the tidal flat with accuracy of order of centimeter.