CHANGE IN TOPOGRAPHY AND WATER LEVEL FLUCTUATION CAUSED BY CONSTRUCTION OF JETTY AT THE SHIRIBETSU RIVER MOUTH

At the Shiribetsu River mouth located in Hokkaido, Japan, water level rise due to wave set-up has frequently been observed every year from October to March, and it is reported that the magnitude of water level rise shows very close correlation with sand spit length at the river mouth. Ariver mouth jetty has been constructed at the right hand side of the river mouth during the period from July 1999 to October 1999. In this study, the effectiveness of the jetty construction is studied from aviewpoint of river mouth morphology and water level variation. It is confirmed that the prevention of longshore sediment movement caused by construction of the jetty led to stability of the river mouth topography. Furthermore, the water depth at the river entrance has increased, resulted in the reduction of water level rise in the river mouth caused by wave breaking.