Abstract
Field measurements have been carried out at the mouth of the Nanakita River which flows into the Pacific Ocean. Since 1988, the river mouth was closed twice, owing to high seas. The wave incident direction was important as well as the wave height and period. The onshore sediment transport was more effective than the longshore sediment transport. The sand spit at the mouth was also flushed twice by floods. At the initial stage of floods, river water overflows with a depth of several ten centimeters, and the flushing was completed within 3 to 4 hours.
