The Shinmachi river flows in the center of Tokushima city and is composed of 7 tidal branches. The water pollutant level of this river is still high because of a large quantity of domestic wastewater. The pump system, which was constructed by Tokushima Prefectural Office for introducing clean water from the Yoshino river, has been in operation since 1980.
The aims of this paper are to ascertain the characteristics of flow in the Shinmachi river to valuate the amount of clean water introduced by the pump system and to investigate the effective operation of the pump system. By using field observation and a numerical analysis of an implicit type finite difference method, it was found that 70% of clean water from the Yoshino river flows into the Suketoh branch and the remain flows into the Shinmachi river. In the spring tide, clean water can be introduced most effectively when the pump system is activated only in the ebb tide.

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