Takase Flood Diversion Channel is usually closed by a sluice gate to retain freshwater creating domestic water sources, and to prevent saltwater intrusion to the lake, at the end of the channel. Closed time of the sluice gate creates a good condition for sand bar formation at the channel entrance under the action of waves. Basing on the conditions of waves and water depth at the entrance of the Takase Flood Diversion Channel as well as combination with the classification of sand bar formation in the previous researches (Nguyen (2005), (2006) and Tanaka (2005)), a demarcation between emergence and submergence of sand bar formation was proposed via the dimensionless water depth at the mouth. This paper shows that emerged sand bar forms certainly at the channel entrance in the winter with a present river mouth depth. In order to limit influences of sand bar on the operation of the channel entrance, water depth at the entrance should be larger as a result sand bar forming under the water level, when sluice gate is opened, the flow in the channel could flush the bar easily.