Heavy rain has concentrated in Nasu area in Tochigi prefecture on August 26',1998. The maximum precipitation of 607 mm in a day has been recorded at Nasuweather observatory on August 27~(th).This heavy rain had Kurokawa and Yosasa riversflood and a great number of houses have suffered from the flooding. In order toprevent threat of natural disaster, it is important to deepen the knowledge of floodingmechanism. From this point of view, numerical analysis has been performed forselected flow region of Kurokawa river with moving free surface by using algebraicReynolds stress model. The turbulent flow in actual flooding river is one of thecomplicated turbulent flows, because the flow behavior is influenced by many kinds offorces. Calculated results suggest that flooding is caused by accelerated flow near thefree surface which appears at upstream of sharp-turn open channel. This acceleratedflow is generated by the reduction of cross sectional area which is induced by theseparated flow near the river bed.
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