A 33-km long sea dyke, enclosing a coastal zone of 401 km2, is being constructed to reclaim tidal flat in the Saemangeum area, mid-west of Korea. The dike construction inevitably affects the coastal environmental system that is primarily dominated by a combination of riverine and tidal regimes. An integrated preservation study on the marine environment was launched in 2002 to monitor and assess the coastal environment change during and after the dike construction. Radical changes in the coastal hydrodynamic regime such as reduction of tidal current off the dyke and tidal range in the inner area took place immediately after the complete closure of the dyke in April 2006. Subsequent changes of vertical stratification and oxygen deficiency occurred in the inner area even in the first summer after the dyke closure. We will introduce the outline of the multidisciplinary monitoring program and significant changes in the physical aspects.