Summarizing of river mouth processing method based on comparative study on river mouth in Japan

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There have been several countermeasures against river mouth blockage due to high waves, such as training jetties, gate, culvert and artificial dredging. The relative advantages and disadvantages of each method are summarized based on a numerous field observations at various river mouths in Japan. Among these, it is noted that an artificial dredging method has quite less influence to adjacent coastal and estuarine environments. But, the presence of sand spit at a river mouth interrupts effluent flood flow into the sea. An appropriate flushing of the sand spit can reduce flooding intensity in the adjoining areas. Initial sand spit elevation influences initiation and development of flushing through a complex interaction of effluent flood discharge, tidal level and sediment transport processes. To investigate this complicated process, a numerical computation method is developed. After verification, it is used to investigate the influence of initial sand spit level on the time-variation of river stage and corresponding development of bed topography under the actions of both tide and high flood flow. Furthermore, sedimentation process around a river mouth is studied in connection with sand budget in a river basin and beach processes.