At the mouth of Nanakita River, elongated sand spit can be frequently observed due to predominance of wave motion, and is occasionally flushed during big floods. For a river planning with the presence of elongated sand spit, the unsteady flushing process of sediment at the river entrance should be considered, instead of a simple assumption of constant water level at the downstream end as has been usually employed up to now. The present paper describes influence of flood hydrograph on the time variation of river stage and corresponding development of bed topography under the actions of both tide and high flood flows at the mouth of Nanakita River during a design flood. The water level thus obtained is utilized as downstream end water level for the river planning.
