The primary objective of the Four Major Rivers Restoration Project, the largest pannational river project in Korean history in terms of its spatial coverage and budget, is to secure water in river channels.This is achieved by large-scale channel dredging and constructing a number of weirs in low flow channels.Among the four major rivers, the Nakdong River is the focus of the project.The weirs and estuary barrage in the Nakdong River are partially gated.The gates are to be opened when a flood occurs to prevent water level rise.On the other hand, the water level of the weirs and estuary barrage should not fall far below the normal pool level at the end of the flood because it is required to secure water in the low flow channel.Gate operation strategies for weirs and estuary barrage are needed to satisfy those two conflicting objectives, and appropriate simulation models are essential for developing them.The purpose of this study is to develop an unsteady flow model for establishing gate operation strategies of the weirs in the Nakdong River during the flood periods.