Flushing time is a scientific concept in estuarine management, and it is used to represent time scales required to remove a parcel of freshwater from an upstream to the sea. The flushing time of the Ota diversion channel was calculated based on a series of experimental data, obtained using an innovative generation of measurement instrument, called Fluvial Acoustic Tomography (FAT). For this purpose two transducers were installed diagonally at both sides of the channel. Even in the tidal river with the periodic intrusion of salt wedges, the cross-sectional average velocities and salinity could be estimated from the travel time difference data. The results indicate that the flushing time, which is calculated by the volume of freshwater fraction divided by the freshwater inflow, ranges from several hours to 10 days.

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