The confluence of two rivers is prone to flooding, because the flow there is complex. This is particularly true at times of high discharge. This study investigates the flow at such locations. Field investigation was conducted at the confluence of the Atsubetsu and Biu rivers, which was the most severely damaged of any location in the Hidaka district during Typhoon Etau in 2003. Flood flow direction was estimated from the orientation of fallen trees, and water level was estimated from evidence of water marks on the walls of houses. A flume experiment was conducted on the confluence of two rivers. The flow velocity and riverbed elevation in the flume were measured precisely. Numerical analysis using 0-equation and k-ε turbulent models was performed, and the experimental results were compared with calculated results. The numerical analysis using the k-ε model agreed more closely with the experimental results than did the numerical analysis using the 0-equation model. Numerical models were applied to the confluence of the Atsubetsu and Biu rivers. The k-ε model was better than the 0-equation model at reproducing the results of onsite observation. It was shown that the model can be employed as a sophisticated tool to analyze the flow at the confluence of two rivers.
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