This study focuses on the morphology changes of the sand spit due to stormy waves, especially due to overtopping waves and due to flood accompanied with those extreme waves. Firstly, continuous field surveys was conducted over a year around the Tenryu river mouth and the relationship between the morphology change and the offshore wave condition was analyzed. Then, focusing on the morphology change due to overtopping waves and its’ impact on the flood event, it was found that the preceding waves affected the collapse of sand spit during flood. Finally, based on the water level change behind the sand spit and time-series of rectified images, it was revealed that the long period wave was dominant factor of occurrence of overtopping waves and it affected both morphology change with onshore sediment transport and hydrodynamics through the intrusion into the river against the river flow.
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