A High Accurate and Efficient 3D River Flow Model with A New Mode-Splitting Technique

In this study, we applied a highly accurate and efficient three-dimensional river flow simulation method that introduced the mode splitting method to the integrated flow composed of river high-flow and inundation flow. In the flood damage of the Kuma River in 2020, a large discharge flowed down, and after the inundation progressed, the inundation flow was combined with the river flow in the upper reaches of the Watari district. Therefore, the flow path forms a compound meandering river channel, the water surface width also fluctuating greatly in the streamwise direction. In addition, overflow and return-overflow occurred at the levee. Thus, extremely complicated flow phenomena occurred. Therefore, it is necessary to analyze a wide range, including long sections and inundation areas, while considering the three-dimensional flow nature, and the three¬dimensional river flow calculation using the mode splitting method is performed. Focusing on the flow structure, a high flow velocity region was formed on the outer bank side of the river channel until the overflow, whereas the high flow velocity region transitioned to the inner bank side, which is, the inundation side during the integrated flow of the river channel and the inundation. Those results indicate that the present model can seamlessly analyze a series of processes of river high-flow, inundation processes, and complex flows which composed of river high-flow and inundation flow over a wide area including long sections and flood plains, and can analyze complex three-dimensional flow structures.