At Shiribetsu River mouth located in Hokkaido, Japan, water level rise above tidal level can be observed every year during the period from October to March. Detailed analysis of tidal and wave climate data revealed that it is caused by high waves, i.e., wave set-up due to wave breaking. Furthermore, it is found that the magnitude of water level rise increased with the development of sand spit at the river mouth. To obtain the sand spit length, oblique photographs, taken from a small mountain near the river mouth every two weeks for these twenty years, are effectively utilized in this study.
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